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Tonight, we're going to talk about something that sounds impossible.
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Albert Einstein, the man whose name has become synonymous with genius,
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the person who revolutionized our understanding of space, time, and
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reality itself made a mistake, not just any mistake, but
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what he himself called his biggest blunder. And here's what
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makes this story so fascinating. That mistake, that error in judgment,
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that Einstein regretted for years, might actually turn out to
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be one of the most important insights in all of cosmology.
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It might be the key to understanding why the universe
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behaves the way it does. Before we dive in. If
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you find this exploration interesting, a quick like or subscribe
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really helps the channel grow. It's a small thing for you,
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but it makes a huge difference for me. Now let's begin.
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When you think of Albert Einstein, you probably picture wild
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white hair, a rumpled sweater, maybe a chalkboard covered in equations.
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You think of genius, of brilliance, of a mind that
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could see things no one else could see, and you'd
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be right. Einstein changed everything. He showed us that time
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isn't constant, that space can bend, that mass and energy
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are the same thing, expressed in different ways. He gave
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us equations that predicted black holes, gravitational waves, and the
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expansion of the universe itself, decades before any of these
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things were actually observed. But Einstein was human, and humans
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make mistakes, even the brilliant ones, maybe especially the brilliant ones,
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because they're working at the edge of what's non pushing
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into territory where no one has been before. There are
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no maps, no guides, no textbook answers. You're making educated guesses,
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following your intuition, hoping you're right, but never entirely sure.
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In nineteen seventeen, Einstein published a paper that added something
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to his equations, a single term, a constant he called lambda,
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the cosmological constant. He added it for one reason, and
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one reason only, to keep the universe still, to prevent
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it from collapsing inward under its own gravity or expanding
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outward into the void. He wanted a static, unchanging universe,
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one that had always existed and would always exist, and
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to get that result he needed to add this extra
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term to balance things out. Twelve years later, in nineteen
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twenty nine, an astronomer named Edwin Hubble showed that the
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universe wasn't static at all. It was expanding. Galaxies were
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moving away from each other, The entire cosmos was growing
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larger with each passing moment. Einstein's cosmological constant wasn't necessary.
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The universe didn't need to be held still because it
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wasn't still. It was dynamic, changing, evolving. Einstein reportedly called
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this his biggest blunder. He'd let his philosophical preferences, his
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desire for a universe that was eternal and unchanging, cloud
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his scientific judgment. He'd added something to his equations not
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because the math demanded it, not because observations required it,
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but because he wanted a specific result, and he'd been wrong. Except,
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and this is where the story gets really interesting, he
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might not have been wrong after all. In the late
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nineteen nineties, astronomers made a shocking discovery. The expansion of
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the universe wasn't slowing down as everyone expected. It was
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speeding up. Something was pushing the universe apart, accelerating its expansion,
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and the best explanation any one could come up with
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was Einstein's cosmological constant, that term he'd added in nineteen
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seventeen and removed in embarrassment in nineteen twenty nine. It
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turned out to describe something real, something we now call
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dark energy. So Einstein's biggest mistake might actually be one
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of his greatest insights. A theoretical prediction made for the
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wrong reasons that turned out to describe a fundamental property
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of the universe. That's the story we're going to explore tonight,
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how Einstein came to make this mistake, why it seemed
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like a mistake for decades, and how modern cosmology has
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brought it back from the dead. Let's start at the beginning,
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with Einstein himself and the revolutionary ideas that changed physics forever.
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Albert Einstein was born in eighteen seventy nine in the
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German city of Ulm. He wasn't a child prodigy in
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the traditional sense. He didn't speak until he was relatively late,
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and his parents worried about him. He wasn't the best student,
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often clashing with teachers who valued memorization over understanding. He
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questioned everything, pushed back against authority, and generally made himself
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difficult to teach in the rigid German school system of
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the time. His family moved to Munich when he was
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an infant, and this is where he grew up. His father, Hermann,
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was an engineer and businessman who ran an electrical equipment
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company with Einstein's uncle. His mother, Pauline, was educated and cultured,
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encouraging young Albert's introes in music. Einstein learned to play
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the violin and maintained his love of music throughout his life,
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often saying that he thought in music and saw his
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life in terms of harmony. The late nineteenth century was
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a time of rapid change in Germany and throughout Europe.
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The Industrial Revolution was transforming societies. Cities were growing, New
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technologies like electricity and telephones were becoming common. It was
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an age of optimism and progress, a belief that human
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knowledge was advancing rapidly and that science would solve humanity's problems.
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But Einstein had something more valuable than obedience or wrote
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learning ability. He had an incredible imagination. He could visualize
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abstract concepts in ways that made them clear and understandable.
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He thought in pictures, in thought experiments, in scenarios he
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could play out in his mind. This ability to visualize
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the invisible would become his greatest strength. When Einstein was
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five years old, his father showed him a pocket compass.
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The boy was fascinated by the invisible force that made
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the needle always point north no matter how you turned
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the compass. This was his first encounter with the idea
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that there were hidden forces in nature, things you couldn't
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see but could detect through their effects. That compass made
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a deep impression on him, planting a seed that would
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grow into his lifelong quest to understand the hidden workings
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of the universe. There's a famous story about Einstein as
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a teenager imagining what it would be like to ride
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alongside a beam of light. If you could somehow match
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light speed and travel next to it, what would you see?
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Would the light wave appear frozen, stationery, or would something
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stranger happen? This simple thought experiment, this act of imagination
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planted seeds that would eventually grow into special relativity. He
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puzzled over this question for years. According to the physics
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of his time, According to Maxwell's equations of electromagnetism, which
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were the most successful physics theory of the late nineteenth century,
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light was a wave that traveled at a fixed speed
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through space. But if you could match that speed, if
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you could travel alongside the light, what would happen the
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light wave should appear stationary from your perspective frozen in time,
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but Maxwell's equations didn't allow for stationary light waves. Something
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didn't make sense. This paradox bothered Einstein throughout his teenage
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years and into his twenties. It suggested that either Maxwell's
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equations were wrong, which seemed unlikely given how successful they
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were at describing electors, tricity, magnetism, and light, or there
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was something fundamentally wrong with how people thought about space,
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time and motion. Einstein attended the Swiss Federal Polytechnic in Zurich,
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studying physics and mathematics. He was a good student, but
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not exceptional. He skipped classes that didn't interest him, preferring
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to study what he found fascinating rather than following the
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prescribed curriculum. This didn't endear him to his professors. When
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he graduated in nineteen hundred, none of his teachers would
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write him a recommendation for an academic position. This was
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a serious problem in the academic world of the early
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twentieth century. Personal recommendations from your professors were essential for
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getting your careers started. Einstein spent two years unemployed or underemployed,
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taking temporary teaching positions, tutoring students, doing whatever he could
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to make ends meet. It was a difficult, uncertain time.
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He'd invested years in his education, but the academic world
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seemed close to him because he'd alienated his professors by
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not conforming to their expectations. After university, Einstein struggled to
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find an academic position. He ended up working at a
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patent office in Bern, Switzerland. It was a good job,
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steady and respectable, but it wasn't what he'd dreamed of.
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He spent his days evaluating patent applications, determining whether inventions
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were truly novel or just rehashes of existing ideas. It
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was steady work, but it left him time time to think,
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time to read, time to work on physics in the
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evenings and weekends. In nineteen o five, Einstein's miracle year,
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he published four papers that changed physics forever. These weren't
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incremental improvements on existing theories. These were revolutionary ideas that
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overturned centuries of scientific understanding. The first paper explained the
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photoelectric effect, showing that light behaves as if it's made
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of particles, little packets of energy we now call photons.
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This work would eventually win him the Nobel Prize and
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help establish quantum mechanics, though Einstein himself would later have
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deep reservations about where quantum theory was heading. The second
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paper provided convincing evidence for the existence of atoms by
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explaining Brownian motion, the random jittering movement of tiny particles
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suspended in fluid. At the time, not all scientists believed
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atoms were real rather than just useful mathematical fictions. Einstein's
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paper helped settle that debate. The third paper introduce special relativity,
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showing that space and time are not separate absolute things,
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but are intertwined into a single entity called space time.
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Time can pass at different rates for different observers depending
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on their relative motion. This wasn't philosophy or speculation. It
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was mathematics backed by experimental evidence. The fourth paper contained
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the most famous equation in physics. E equals MC squared.
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Energy equals mass times the speed of light squared. This
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simple equation showed that mass and energy are equivalent, that
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matter is essentially frozen energy. It predicted nuclear reactions, explained
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where the Sun gets its power, and eventually led to
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both nuclear weapons and nuclear power. These four papers, any
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one of which would have been a career defining achievement,
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were all published in a single year by a twenty
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six year old patent clerk working outside the academic system.
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It's one of the most remarkable achievements in the history
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of science. Special relativity was revolutionary, but it had limitations.
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It only worked for objects moving at constant speeds relative
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to each other. It didn't handle acceleration or gravity. Einstein
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knew this was incomplete. For the next decade, he worked
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on extending his theory to include gravity and acceleration. The result,
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published in nineteen fifteen, was general relativity. General relativity is
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hard to understand mathematically. The equations are complex, involving advanced
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mathematics that most people, even most physicists, never study in detail.
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Einstein himself had to learn new mathematics, tenser calculus and
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differential geometry specifically to express his ideas. He worked with
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his friend Marcel Grossmann, a mathematician, to master these tools.
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It took him eight years from the initial idea to
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the final complete theory. Eight years of intense work, false starts, mistakes,
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and frustration, but the core idea is actually quite simple
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and can be understood without any math at all. Imagine
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space as a rubber sheet, stretched flat and smooth. Now
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place a heavy ball in the middle of the sheet.
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The ball makes a dent, a depression in the rubber
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The heavier the ball, the deeper the dent. Now roll
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a marble across the sheet. As it passes near the
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heavy ball, it curves toward it, following the contour of
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the depression. The marble isn't being pulled by some mysterious force,
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It's just following the natural shape of the curved sheet.
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That's how gravity works. In general relativity, massive objects like
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stars and planets don't pull on other objects with some
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invisible force reaching across space. Instead, they curve the space
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around them. Other objects move through that curved space, and
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their paths bend as a result. The Earth orbits the
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Sun not because the Sun is pulling on it, but
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because the Sun curves space around it, and the Earth
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is following the straightest possible path through that curved space. Now,
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this analogy has limitations. The rubber sheet is two dimensional
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and embedded in three dimensional space. Real space time is
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four dimensional, three dimensions of space and one of time,
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and it's not embedded in anything. It's the fundamental arena
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in which everything exists. There's no higher dimensional space that
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spacetime curves into. The curvature is intrinsic to spacetime itself. Also,
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the analogy shows spatial curvature, but not temporal curvature. In
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general relativity, time itself is affected by gravity. Time passes
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more slowly near massive objects. This isn't just clocks running slow,
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its actual time, the fundamental flow of causality. Two identical clocks,
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one near a massive object and one far away, will
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tick at different rates. The one near the massive object
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runs slower, and this has been measured with atomic clocks
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with extraordinary precision. Think about that for a moment. It's
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a completely different way of understanding gravity compared to what
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Isaac Newton proposed. Newton said gravity is a force, something
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that reaches out across empty space and pulls objects toward
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each other. Einstein said there's no force at all. There's
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just curved space, and objects move through that space following
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the natural contours of the curves. This might sound like
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just a different way of describing the same thing, but
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it's not. General relativity makes predictions that are different from
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Newton's theory, and every time we've tested those predictions, Einstein
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has been right and Newton has been wrong, or at
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least incomplete. Newton's theory says that if you suddenly removed
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the Sun, the Earth would immediately fly off in a
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straight line. The gravitational force would disappear instantly, and the
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Earth would no longer orbit. Einstein's theory says something different.
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If the Sun disappeared, a gravitational wave, a ripple in
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space time would propagate outward at the speed of light.
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The Earth wouldn't feel any change until that wave reached
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it eight minutes and twenty seconds later. Information about the
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Sun's disappearance would travel at light speed through space time,
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not instantaneously. General relativity predicted that light should bend when
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it passes near massive objects. In nineteen nineteen, during a
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solar eclipse, astronomers measured the positions of stars visible near
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the Sun's edge. The stars appeared slightly displaced from their
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usual positions because the Sun's mass curved the space around it,
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bending the light from those distant stars as it passed nearby.
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This observation made Einstein world famous. Overnight, the newspapers ran
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headlines about how space was curved and reality was different
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from what everyone had assumed. This wasn't just abstract physics.
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Light bending around massive objects, called gravitational lensing, has become
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one of astronomy's most powerful tools. When a massive galaxy
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or galaxy cluster sits between Earth and a more distant galaxy,
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the foreground object's gravity bends the light from the background galaxy,
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often creating multiple distorted images arranged in rings or arcs
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around the lens. Astronomers use these natural magnifying glasses to
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study galaxies that would otherwise be too faint and distant
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to observe. Some of the deepest images of the early
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universe come from studying gravitationally lensed galaxies. General relativity predicted
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that time should pass more slowly in strong gravitational fields.
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This has been tested with atomic clocks placed at different altitudes.
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A clock on a mountaintop runs slightly faster than an
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identical clocket sea level because it's farther from Earth's mass
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and therefore in a weaker gravitational field. The difference is tiny,
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only a few nanoseconds per day, but it's measurable and
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exactly matches Einstein's predictions. GPS satellites must account for this
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time dilation effect. The satellites orbit about twenty thousand kilometers
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or twelve thousand, four hundred miles above Earth's surface, where
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gravity is weaker than at ground level. This causes their
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atomic clocks to run faster than clocks on the ground
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by about forty five microseconds per day. Additionally, because the
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satellites are moving at high speeds, special relativity causes their
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clocks to run slower by about seven microseconds per day.
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The net effect is about thirty eight microseconds per day faster.
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This might sound insignificant, but GPS works by precisely timing
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radio signals. Light travels about thirty centimeters or twelve inches
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per nanosecond. If GPS didn't correct for relativistic time dilation,
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the errors would accumulate at a rate of about ten
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kilometers or six miles per day within hours. GPS navigation
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would become useless. Every time you use GPS to navigate,
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you're relying on Einstein's relativity to be accurate. Your phone
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is doing relativistic calculations to correct for time dilation. General
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relativity predicted gravitational waves ripples in spacetime itself, created when
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massive objects accelerate. These waves were finally detected directly in
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twenty fifteen, a full century after Einstein predicted them, when
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the Ligo experiment measured the incredibly tiny distortions in space
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caused by two black holes colliding over a billion light
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years away. The measurement was so precise it could detect
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changes in distance smaller than a proton, and it matched
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Einstein's predictions perfectly. Since that first detection, LIGO and its
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partner observatory Virgo have detected dozens of gravitational wave events.
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Most involve colliding black holes, but they've also colliding neutron stars.
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In twenty seventeen, a neutron star merger was detected both
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in gravitational waves and across the electromagnetic spectrum, from gamma
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rays to radio waves. This event confirmed theories about where
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heavy elements like gold and platinum come from. These elements
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are forged in neutron star collisions and scattered across space
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to eventually become part of new star systems and planets.
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Gravitational wave astronomy is now an established field, giving us
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a completely new way to observe the universe. Unlike light,
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which can be absorbed or scattered, gravitational waves pass through
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everything unimpeded. They carry information about events that produce no
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light at all, like the collision of two black holes
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in empty space. As detectors become more sensitive, we'll observe
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gravitational waves from the early universe itself, potentially seeing events
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that occurred when the cosmos was too young and hot
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for light to travel freely. So general relativity works. It's
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been tested countless times in countless ways, and it always
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gives the right answer. It's one of the most successful
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scientific theories ever developed. But when Einstein first worked out
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his equations, he ran into a problem, a problem that
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would lead to what he later called his biggest blunder.
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The equations of general relativity describe how mass and energy
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curve space and time, but their dynamic equations they describe
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how things change, how space responds to the presence of
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matter and energy. When Einstein applied his equations to the
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universe as a whole, he found something troubling. The universe,
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according to his equations, couldn't remain still. It had to
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be either expanding or contracting. If you filled space with
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stars and galaxies, gravity would pull everything together, causing a
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cosmic collapse, or if things were already moving apart, they'd
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continue separating, the universe growing larger forever. But this contradicted
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Einstein's philosophical view of the cosmos. He envisioned an eternal,
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steady state universe that had always existed and would always exist.
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This wasn't base on observations. It was an esthetic judgment,
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a belief that the cosmos should be balanced and timeless.
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Most astronomers shared this view in the early twentieth century.
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They believed the universe consisted only of the Milky Way Galaxy,
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a large but finite island of stars floating in infinite
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empty space. The fuzzy spiral nebulae visible through telescopes were
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thought to be gas clouds within our galaxy, not separate
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galaxies millions of light years away. If the cosmos was
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just our galaxy surrounded by void, then a steady state
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model made intuitive sense. Individual stars were born and died,
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but the overall structure remained roughly constant. There was no
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reason to think the entire universe was evolving in any
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fundamental way. So when Einstein's equations predicted an unstable universe
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that couldn't remain status. He faced a choice. He could
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accept what the math was telling him and conclude that
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the universe must be either expanding or contracting, or he
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could modify the equations to allow for a static solution.
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He chose the second option. In nineteen seventeen, he published
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a paper adding a new term to his field equations,
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the cosmological constant, represented by the Greek letter lambda. This
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constant represented a kind of repulsive force, something that pushed
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space apart, counteracting the attractive force of gravity. By carefully
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tuning this constant to exactly the right value, Einstein could
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balance the inward pull of gravity with the outward push
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of lambda, creating a universe that was static and stable.
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The mathematics behind this was elegant in its own way.
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General relativity describes how the curve of space time responds
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to the presence of mass and energy. The equations relate
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the geometry of space time on one side to the
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distribution of matter and energy on the other side. Einstein
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realized he could add an additional term to the geometry side,
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a term that represented a kind of intrinsic curvature of
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space that existed even in the absence of matter. This term,
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the cosmological constant, had interesting properties. Unlike normal matter or energy,
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which becomes more dilute as space expands, the cosmological constant
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stayed constant. It was the same everywhere at all times.
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It represented a kind of energy density that was a
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property of space itself. The more space you had, the
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more of this energy you had, but the density never changed.
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If you doubled the volume of space, you doubled the
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total amount of this energy, keeping the density constant. It's
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important to understand what Einstein did here. He didn't discover
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the cosmological constant by observing something in nature or deriving
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it from more fundamental principles. He added it to make
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his equations give him the answer he wanted. The math
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allowed for such a term. There was nothing technically wrong
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with including it, but its only justification was that it
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produced a static universe, which Einstein believed, on philosophical grounds,
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must be correct. This wasn't the first time a physicist
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had modified equations to match desired results. In the history
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of science. There are many examples of ad hoc modifications
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that later proved necessary, and many more that proved to
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be wrong. The trick is that you can't know in
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advance which category or modification falls into. You have to
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wait for observations to tell you whether you were right
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or wrong. Einstein was aware of the somewhat arbitrary nature
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of this addition. In his paper, he noted that the
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cosmological constant was the simplest possible addition that would allow
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a static solution. He also noted that there was no
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independent theoretical reason to include it. Its justification was purely
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that it solved what he saw as a problem with
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his original equations, their prediction of an unstable universe. Think
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about what the cosmological constant actually means. Its energy inherent
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in empty space itself, even in a perfect vacuum, completely
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devoid of matter and radiation. This constant says there's a
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kind of pressure pushing outward. It's not particles, and it's
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not waves. It's a fundamental property of space. The more
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volume you have, the more of this energy exists. But crucially,
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the density stays uniform everywhere in everyday life. Forces weaken
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with distance. Gravity fades as you move away from massive objects.
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Light dims the farther you are from its source. But
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the cosmological constant behaves differently. Double the volume of space,
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and you double this repulsive energy while maintaining constant density.
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Its utterly alien to ordinary experience. Einstein knew this addition
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was unusual. He was introducing something fundamentally new to physics
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without direct evidence, but he did it anyway because his
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equations demanded modification to achieve the steady state cosmos he
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believed must exist. For the next twelve years, the cosmological
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constant sat in Einstein's equations. Other physicists debated whether it
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was necessary or justified. Some argued it was an elegant
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solution to a real problem. Others thought it was an
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ad hoc fix, something added to force a desired result,
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rather than emerging naturally from the theory. Then everything changed.
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In the early nineteen twenties, an astronomer named Edwin Hubble
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began making observations that would revolutionize our understanding of the cosmos.
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Working with the one hundred inch telescope at Mount Wilson
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Observatory in California, which was the largest telescope in the
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world at the time, Hubble studied the fuzzy spiral nebulae
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that astronomers had been observing for decades. These spiral nebulae
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were mysterious objects. Through small telescopes, they appeared as faint,
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fuzzy patches of light scattered across the sky. Through larger telescopes,
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you could see they had spiral structure, arms of light
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swirling around a bright central core, but nobody knew what
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they were. Some astronomers thought they were gas clouds within
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our own galaxy, perhaps regions where new stars were forming.
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Others speculated they might be distant star systems beyond the
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Milky Way. The debate about the nature of spiral nebulae
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was one of the great controversies in astronomy in the
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early twentieth century. In nineteen twenty there was even a
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formal debate on the subject between two prominent astronomers, Harlow Shapley,
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who argued the spiral nebulae were part of the Milky Way,
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and Heber Curtis, who argued they were separate galaxies. Neither
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side had definitive proof, so the debate ended without clear resolution.
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Hubble changed everything by finding a way to measure the
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distances to these nebulae. He did this using a type
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of star called a Cepheid variable. These stars pulsate, growing
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brighter and dimmer in a regular cycle that can last
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anywhere from days to months. The American astronomer Henrietta Levitt
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had discovered something remarkable about Cepheid variables. Their period of pulsation,
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how long it takes them to go from bright to
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dim and back again, is directly related to their intrinsic brightness.
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A cepheid that pulsates slowly is intrinsically brighter than one
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that pulsates quickly. This meant you could use cepheid's as
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standard candles to measure cosmic distances. If you could identify
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a Cepheid variable in a distant object measure its period,
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you'd know how bright it actually was. By comparing its
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actual brightness to how bright it appeared from Earth, you
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could calculate how far away it must be. The dimmer
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it appeared, the farther way it was. Using a technique
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developed by Henrietta Levitt to measure cosmic distances, Hubble determined
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that these nebulae were far outside the Milky Way. They
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weren't gas clouds in our galaxy. They were separate galaxies,
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each containing billions of stars. Located millions of light years away.
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The universe was vastly larger than anyone had imagined. The
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first distance Hubble measured was to the Andromeda galaxy, which
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appeared in the sky as a faint, fuzzy oval visible
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to the naked eye on dark nights. Hubble found Cepheid
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variables in Andromeda and measured their periods. His calculations showed
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that Andromeda was about nine hundred thousand light years away.
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This was later revised upward when we better understood Cepheid variables,
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and we now know Andromeda is actually about two point
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five million light years of way. But even Hubble's original
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estimate was far enough to prove that Andromeda was well
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outside the Milky Way. It was a separate galaxy, not
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part of our own. This was revolutionary. The universe wasn't
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just the Milky Way. It was filled with galaxies, countless
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islands of stars scattered through space, each containing billions of suns.
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The cosmos was far larger and more complex than anyone
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had imagined. What had appeared to be a universe perhaps
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one hundred thousand light years across the size of our
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galaxy was actually vastly bigger, filled with galaxies separated by
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millions of light years. But Hubble didn't stop there. He
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also measured how these galaxies were moving. When you look
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at light from a distant object, you can determine whether
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it's moving toward you or away from you by examining
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its spectrum, rainbow of colors created when you split the
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light through a prism. If the object is moving toward you,
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the spectrum shifts toward the blue end of the rainbow.
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If it's moving away, it shifts toward the red end.
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This is called red shift for objects moving away and
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blue shift for objects moving toward. Think of it like
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the Doppler effect with sound. When an ambulance drives toward you,
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its siren sounds higher pitched than normal because the sound
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waves are compressed. When it drives away, the siren sounds
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lower pitched because the waves are stretched out. Light does
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the same thing. Motion toward you compresses the waves, shifting
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them toward blue. Motion away stretches them out, shifting them
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toward red. Hubble found that nearly all galaxies showed red
476
00:36:56.679 --> 00:37:01.000
shift they were moving away from us, and the farther
477
00:37:01.079 --> 00:37:06.079
away a galaxy was the faster it was receding a
478
00:37:06.119 --> 00:37:09.960
galaxy twice as far away was moving away twice as fast.
479
00:37:10.599 --> 00:37:14.320
Three times the distance meant three times the speed. There
480
00:37:14.400 --> 00:37:20.719
was a clear mathematical relationship between distance and recession velocity.
481
00:37:21.320 --> 00:37:26.320
This could only mean one thing. The universe was expanding.
482
00:37:26.960 --> 00:37:32.239
Space itself was growing larger, carrying galaxies with it. The
483
00:37:32.280 --> 00:37:36.800
farther apart two galaxies were the more space between them,
484
00:37:36.960 --> 00:37:40.119
and therefore the faster they appeared to separate as that
485
00:37:40.280 --> 00:37:44.000
space stretched. Think of dots drawn on a balloon's surface.
486
00:37:44.960 --> 00:37:48.320
As you inflate the balloon, the rubber stretches and the
487
00:37:48.360 --> 00:37:52.880
dots move apart. They're not sliding across the surface. The
488
00:37:52.920 --> 00:37:57.840
surface itself is expanding, carrying the dots along. That's what's
489
00:37:57.840 --> 00:38:01.719
happening with the universe. Space piece is the expanding medium,
490
00:38:02.400 --> 00:38:06.039
and galaxies ride along with it. In nineteen twenty nine,
491
00:38:06.559 --> 00:38:12.719
Hubbell published his findings. The universe was not static. It
492
00:38:12.840 --> 00:38:16.880
was expanding and had been expanding for a very long time.
493
00:38:17.480 --> 00:38:23.360
Einstein's cosmological constant, added specifically to prevent expansion or collapse,
494
00:38:23.880 --> 00:38:27.280
was unnecessary. The universe didn't need to be held still
495
00:38:27.719 --> 00:38:33.440
because it wasn't still. It was dynamic, changing, evolving When
496
00:38:33.519 --> 00:38:38.760
Einstein learned of Hubbell's discovery, he recognized immediately what it meant.
497
00:38:39.599 --> 00:38:44.000
He'd made an error. His philosophical preference for a steady
498
00:38:44.000 --> 00:38:49.039
state cosmos had overridden what the mathematics of general relativity
499
00:38:49.480 --> 00:38:53.199
had been trying to tell him. His original equations without
500
00:38:53.199 --> 00:38:57.639
the added constant, had predicted expansion all along. If he'd
501
00:38:57.679 --> 00:39:02.159
trusted the mathematics instead of four it to match his preconceptions,
502
00:39:02.760 --> 00:39:08.280
he would have anticipated the discovery before astronomers observed it. Instead,
503
00:39:08.679 --> 00:39:12.920
he'd modified his work to get the wrong answer. Einstein
504
00:39:13.079 --> 00:39:16.880
visited Mount Wilson Observatory in nineteen thirty one to see
505
00:39:16.920 --> 00:39:21.920
Hubble's work firsthand. He looked through the telescope, examined the data,
506
00:39:22.199 --> 00:39:27.360
and confirmed that, yes, the universe was definitely expanding. According
507
00:39:27.440 --> 00:39:31.239
to the Russian physicist George Gammo, who was present during
508
00:39:31.280 --> 00:39:37.760
some of these discussions, Einstein expressed deep regret about adding
509
00:39:37.840 --> 00:39:43.639
the cosmological constant. He'd inserted it without good justification, except
510
00:39:43.679 --> 00:39:47.840
to achieve an answer he preferred philosophically, and that answer
511
00:39:47.880 --> 00:39:53.599
turned out to be incorrect. Einstein removed the cosmological constant
512
00:39:53.679 --> 00:39:57.920
from his equations for the next few decades. It was
513
00:39:58.000 --> 00:40:02.760
largely forgotten, delegated to a footnote in the history of physics,
514
00:40:03.360 --> 00:40:07.760
a cautionary tale about how even great scientists can be
515
00:40:07.840 --> 00:40:11.800
led astray by their assumptions and preferences. But the story
516
00:40:11.800 --> 00:40:15.639
doesn't end there. The cosmological constant might have been added
517
00:40:15.639 --> 00:40:18.400
for the wrong reasons, but as we'll see, it turned
518
00:40:18.400 --> 00:40:23.000
out to describe something very real about the universe, something
519
00:40:23.039 --> 00:40:27.000
that would remain hidden for nearly seventy years before observations
520
00:40:27.079 --> 00:40:31.159
finally revealed its presence. Let's talk about what an expanding
521
00:40:31.280 --> 00:40:35.159
universe actually means, because this is one of the most
522
00:40:35.199 --> 00:40:39.960
commonly misunderstood concepts in cosmology. When we say the universe
523
00:40:40.079 --> 00:40:44.320
is expanding, we don't mean galaxies are exploding outward from
524
00:40:44.360 --> 00:40:48.119
some central point into empty space. There is no center,
525
00:40:48.840 --> 00:40:54.800
there is no edge. Space itself is growing larger everywhere simultaneously.
526
00:40:55.400 --> 00:40:59.760
Remember that balloon analogy. Every point on the expanding so
527
00:40:59.760 --> 00:41:04.320
sude moves away from every other point, yet no location
528
00:41:04.519 --> 00:41:10.320
is special. Each observer, wherever they are, sees the same phenomenon,
529
00:41:10.840 --> 00:41:16.079
all distant objects receding in every direction. The expansion is
530
00:41:16.519 --> 00:41:21.840
uniform throughout space. Now here's something that confuses people. If
531
00:41:21.880 --> 00:41:26.360
space is expanding. Why aren't we expanding? Why isn't Earth
532
00:41:26.400 --> 00:41:32.119
getting bigger? The answer is that local forces completely overwhelm
533
00:41:32.199 --> 00:41:38.599
the expansion. Atoms are bound by electromagnetic forces, Planets are
534
00:41:38.599 --> 00:41:44.039
held together by gravity and chemical bonds. Even galaxies resist
535
00:41:44.079 --> 00:41:49.239
the expansion through their own gravitational pull. The expansion only
536
00:41:49.280 --> 00:41:53.960
dominates at the largest scales, the distances between galaxy clusters,
537
00:41:54.119 --> 00:41:57.679
where no other forces can compete. The discovery that the
538
00:41:57.840 --> 00:42:03.559
universe was expanding immediately raised a profound question. If space
539
00:42:03.639 --> 00:42:07.199
is growing larger, that means it used to be smaller.
540
00:42:07.679 --> 00:42:11.159
Wind the clock backward far enough, and you reach a
541
00:42:11.199 --> 00:42:15.719
point where all of space was compressed into an infinitely small,
542
00:42:16.239 --> 00:42:21.480
infinitely dense point, a singularity. This was the moment of creation,
543
00:42:22.159 --> 00:42:25.800
the beginning of the universe itself. This idea was initially
544
00:42:25.800 --> 00:42:30.440
called the primeval atom by George Lemetre, a Belgian priest
545
00:42:30.519 --> 00:42:33.280
and physicist who was one of the first to work
546
00:42:33.320 --> 00:42:38.639
out the implications of an expanding universe. Later, the astronomer
547
00:42:38.719 --> 00:42:42.280
Fred Hoyle would mockingly call it the Big Bang, a
548
00:42:42.400 --> 00:42:47.119
name that stuck even though Hoyle himself never accepted the theory.
549
00:42:47.559 --> 00:42:50.719
The Big Bang theory says the universe began in an
550
00:42:50.840 --> 00:42:58.880
unimaginably hot, dense state about thirteen point eight billion years ago. Space, time, matter,
551
00:42:59.199 --> 00:43:03.800
and energy all came into existence at that moment. There
552
00:43:03.920 --> 00:43:09.119
was no before. Because time itself began with the Big Bang.
553
00:43:09.679 --> 00:43:15.679
Space expanded rapidly, cooling as it grew, allowing first subatomic particles,
554
00:43:16.000 --> 00:43:20.800
then atoms, then stars and galaxies to form. This raised
555
00:43:20.880 --> 00:43:25.960
immediate objections. Many scientists in the nineteen thirties and forties
556
00:43:26.199 --> 00:43:30.440
found the idea philosophically troubling. The universe having a beginning
557
00:43:30.880 --> 00:43:35.920
seemed to imply a creator, which made some scientists uncomfortable.
558
00:43:36.679 --> 00:43:41.480
It felt more like theology than physics. Hoyle and others
559
00:43:41.880 --> 00:43:47.239
proposed alternative theories where the universe was eternal, with new
560
00:43:47.280 --> 00:43:51.840
matter continuously being created to fill the gaps left by expansion.
561
00:43:52.400 --> 00:43:56.320
This steady state model preserved the philosophical appeal of a
562
00:43:56.360 --> 00:44:01.719
timeless cosmos while still matching Hubble's observation of expansion. The
563
00:44:01.760 --> 00:44:06.480
steady state theory had its appeal. It avoided the uncomfortable
564
00:44:06.599 --> 00:44:10.280
question of what came before the Big Bang. If the
565
00:44:10.400 --> 00:44:14.199
universe had always existed, there was no moment of creation
566
00:44:14.400 --> 00:44:20.000
to explain new matter would appear gradually throughout space, spontaneously
567
00:44:20.039 --> 00:44:24.719
popping into existence to maintain constant density. As expansion spread
568
00:44:24.760 --> 00:44:29.280
things out, the rate would be incredibly slow, about one
569
00:44:29.440 --> 00:44:33.440
hydrogen atom per cubic kilometer or cubic mile of space
570
00:44:33.480 --> 00:44:38.480
per year, far too slow to detect directly. For several decades,
571
00:44:39.199 --> 00:44:43.000
the steady state model and the Big Bang model competed.
572
00:44:43.960 --> 00:44:49.239
Both could explain the observed expansion. Both made testable predictions.
573
00:44:49.840 --> 00:44:54.239
The scientific community was divided. Some found the steady state
574
00:44:54.719 --> 00:45:00.800
more philosophically satisfying, others found the Big Bang more elegant mathematically.
575
00:45:01.559 --> 00:45:05.000
It was one of the great debates in twentieth century cosmology.
576
00:45:05.440 --> 00:45:10.639
The debate wasn't settled by philosophical arguments. It was settled
577
00:45:10.679 --> 00:45:15.599
by observation. If the Big Bang happened, the early universe
578
00:45:16.119 --> 00:45:20.760
must have been hot and dense. As it expanded and cooled.
579
00:45:21.239 --> 00:45:24.960
There should be leftover radiation from that early hot phase,
580
00:45:25.440 --> 00:45:29.239
now cooled by expansion to just a few degrees above
581
00:45:29.400 --> 00:45:35.639
absolute zero. This cosmic microwave background radiation should fill all
582
00:45:35.800 --> 00:45:41.159
of space uniformly. In nineteen sixty five, two radio astronomers
583
00:45:41.559 --> 00:45:45.719
named Arno Penzias and Robert Wilson were testing a sensitive
584
00:45:45.840 --> 00:45:49.599
radio antenna at Bell Labs in New Jersey. They kept
585
00:45:49.639 --> 00:45:54.360
picking up mysterious noise, a faint hiss that came from
586
00:45:54.360 --> 00:45:58.559
all directions equally. At first, they thought it was interference,
587
00:45:59.119 --> 00:46:01.840
perhaps from near by New York City, or even from
588
00:46:01.920 --> 00:46:05.679
pigeon droppings on their antenna. They cleaned the antenna thoroughly,
589
00:46:06.000 --> 00:46:11.400
even removing nesting pigeons, but the noise persisted. Eventually, they
590
00:46:11.480 --> 00:46:17.159
learned that physicists at nearby Princeton University had predicted exactly
591
00:46:17.239 --> 00:46:21.800
this kind of background radiation as a consequence of the
592
00:46:21.800 --> 00:46:26.840
Big Bang. The noise Penzius and Wilson detected wasn't interference.
593
00:46:27.440 --> 00:46:31.000
It was the afterglow of the Big Bang itself, radiation
594
00:46:31.119 --> 00:46:35.119
that had been traveling through space for over thirteen billion years,
595
00:46:35.480 --> 00:46:39.559
cooled by cosmic expansion from millions of degrees to just
596
00:46:39.599 --> 00:46:44.719
a few degrees above absolute zero. This discovery won Penzius
597
00:46:44.800 --> 00:46:48.840
and Wilson the Nobel Prize and essentially ended serious debate
598
00:46:49.119 --> 00:46:52.960
about whether the Big Bang had occurred. The steady state
599
00:46:53.039 --> 00:46:58.360
model couldn't explain this background radiation. The Big Bang predicted it,
600
00:46:58.960 --> 00:47:03.079
so by the nineteen sixties and seventies, the picture seemed clear.
601
00:47:03.960 --> 00:47:07.440
The universe began with the Big Bang thirteen point eight
602
00:47:07.559 --> 00:47:12.559
billion years ago. It's been expanding ever since. Gravity has
603
00:47:12.639 --> 00:47:17.360
been slowing that expansion like a ball thrown upward, gradually
604
00:47:17.440 --> 00:47:22.519
slowing as Earth's gravity pulls on it. Eventually, one of
605
00:47:22.559 --> 00:47:25.760
two things would happen. Either there was enough matter in
606
00:47:25.760 --> 00:47:30.199
the universe that gravity would eventually stop the expansion and
607
00:47:30.360 --> 00:47:33.800
reverse it, causing the universe to collapse back in on
608
00:47:33.920 --> 00:47:37.639
itself in a big crunch, or there wasn't enough matter
609
00:47:38.719 --> 00:47:43.679
and the universe would expand forever, gradually slowing but never
610
00:47:43.800 --> 00:47:48.239
quite stopping. Astronomer's work to measure the universe's total mass
611
00:47:48.280 --> 00:47:52.880
density to determine which scenario was correct. Would we get
612
00:47:52.920 --> 00:47:56.920
a big crunch or eternal expansion. The fate of the
613
00:47:57.039 --> 00:48:02.440
universe hung on this measurement. Einstein Signe's cosmological constant was
614
00:48:02.480 --> 00:48:07.039
barely mentioned during these decades. It had been added to
615
00:48:07.119 --> 00:48:12.639
create a static universe. The universe wasn't static, therefore the
616
00:48:12.719 --> 00:48:18.760
constant was unnecessary, just an embarrassing mistake. Case closed. But
617
00:48:18.880 --> 00:48:22.639
physics is never that simple. Just when you think you
618
00:48:22.760 --> 00:48:27.119
understand something, Nature throws you a curveball, and in the
619
00:48:27.199 --> 00:48:31.679
nineteen nineties, cosmology got one of the biggest curveballs in
620
00:48:31.840 --> 00:48:36.159
scientific history. To understand what happened, we need to talk
621
00:48:36.199 --> 00:48:41.559
about supernovae. When certain massive stars reach the end of
622
00:48:41.599 --> 00:48:47.840
their lives, they explode in spectacular fashion, briefly outshining entire galaxies.
623
00:48:48.400 --> 00:48:53.519
These explosions, called supernovae, are among the most energetic events
624
00:48:53.559 --> 00:48:57.639
in the universe. For a few weeks, a single dying
625
00:48:57.719 --> 00:49:01.119
star can emit as much light as billions of normal
626
00:49:01.199 --> 00:49:07.599
stars combined. The word supernova means new superstar in Latin.
627
00:49:08.679 --> 00:49:12.320
Ancient and medieval astronomers occasionally saw what appeared to be
628
00:49:12.760 --> 00:49:16.960
new stars suddenly appearing in the sky, shining brightly for
629
00:49:17.000 --> 00:49:20.320
a few weeks or months before fading away. They call
630
00:49:20.400 --> 00:49:24.480
these novae new stars, though we now know they're not
631
00:49:24.599 --> 00:49:30.360
new stars being born, but old stars dying in catastrophic explosions.
632
00:49:31.000 --> 00:49:34.639
A supernova releases more energy in a few seconds than
633
00:49:34.679 --> 00:49:38.360
our Sun will emit in its entire ten billion year lifetime.
634
00:49:39.239 --> 00:49:42.760
The explosion is so violent that it can briefly outshine
635
00:49:42.800 --> 00:49:47.119
an entire galaxy containing hundreds of billions of stars. The
636
00:49:47.360 --> 00:49:51.599
energy released is comparable to all the energy the Sun
637
00:49:51.719 --> 00:49:57.039
will ever produce, compressed into a moment of unimaginable violence.
638
00:49:57.719 --> 00:50:01.239
When a star explodes as a supernova, it doesn't just
639
00:50:01.360 --> 00:50:05.280
release light and heat, It also forges heavy elements in
640
00:50:05.360 --> 00:50:11.800
its final moments. Elements heavier than iron, things like gold, silver, uranium,
641
00:50:11.920 --> 00:50:15.920
and platinum, can only be created in these extreme conditions,
642
00:50:16.639 --> 00:50:20.920
in the incredible temperatures and pressures of a supernova explosion.
643
00:50:21.360 --> 00:50:25.639
Every atom of gold in your jewelry, every atom of
644
00:50:25.719 --> 00:50:30.000
silver in your electronics, every atom of uranium in the
645
00:50:30.039 --> 00:50:33.920
Earth's crust was forged in a dying star billions of
646
00:50:34.000 --> 00:50:38.719
years ago. We are quite literally made of star dust,
647
00:50:39.679 --> 00:50:44.519
the remnants of ancient supernova scattered across space and eventually
648
00:50:44.599 --> 00:50:49.320
recycled into new stars, planets, and life. There are different
649
00:50:49.440 --> 00:50:53.400
types of supernova, depending on what kind of star explodes
650
00:50:53.639 --> 00:50:58.199
and how the explosion happens. The classification system is a
651
00:50:58.239 --> 00:51:02.400
bit complicated, based on which spectral lines appear in the
652
00:51:02.440 --> 00:51:09.239
supernova's light. Type I supernovae don't show hydrogen lines. Type
653
00:51:09.280 --> 00:51:15.320
two supernova do show hydrogen. Within type one, there are
654
00:51:15.400 --> 00:51:22.840
further subdivisions Type EAR show silicon, Type IB and ICK don't.
655
00:51:23.440 --> 00:51:29.159
One particular type called type ear supernovae is especially interesting
656
00:51:29.199 --> 00:51:35.320
to astronomers because these explosions are remarkably consistent. They always
657
00:51:35.360 --> 00:51:39.239
have about the same brightness at their peak. This makes
658
00:51:39.280 --> 00:51:44.639
them perfect standard candles for measuring cosmic distances. Type ear
659
00:51:44.840 --> 00:51:50.000
supernova occur in binary star systems where one star is
660
00:51:50.039 --> 00:51:53.840
a white dwarf, the dense remnant of a dead star.
661
00:51:54.800 --> 00:51:59.679
The white dwarf pulls matter from its companion star gradually
662
00:51:59.719 --> 00:52:04.159
gain mass. When the white dwarf reaches a critical mass
663
00:52:04.760 --> 00:52:07.440
about one point four times the mass of the Sun,
664
00:52:08.079 --> 00:52:12.880
something called the Chandraseca limit, it can no longer support itself.
665
00:52:13.400 --> 00:52:19.039
The entire star detonates in a thermonuclear explosion, destroying itself completely.
666
00:52:19.519 --> 00:52:23.800
Because type ear supernova always explode at the same mass,
667
00:52:24.360 --> 00:52:28.320
they always release roughly the same amount of energy. This
668
00:52:28.519 --> 00:52:33.239
means they all have about the same intrinsic brightness. When
669
00:52:33.239 --> 00:52:37.639
you see a type ear supernova explode, you know how
670
00:52:37.679 --> 00:52:41.239
bright it actually is. If it appears dim, it must
671
00:52:41.280 --> 00:52:45.239
be far away. If it appears bright, it must be
672
00:52:45.320 --> 00:52:50.440
relatively close. By measuring how bright it appears and comparing
673
00:52:50.519 --> 00:52:54.519
that to how bright you know it actually is, you
674
00:52:54.559 --> 00:52:59.119
can calculate its distance. Here's how it works. If you
675
00:52:59.239 --> 00:53:04.360
know how bright something actually is its intrinsic luminosity, and
676
00:53:04.400 --> 00:53:07.679
you can measure how bright it appears from Earth, you
677
00:53:07.719 --> 00:53:11.079
can calculate how far away it must be. It's the
678
00:53:11.119 --> 00:53:14.800
same principle as estimating distance to a light at night.
679
00:53:15.320 --> 00:53:17.400
If you see a light and you know it's a
680
00:53:17.519 --> 00:53:21.559
hundred what bulb, you can tell approximately how far away
681
00:53:21.599 --> 00:53:25.440
it is by how dim it appears. The dimmer it looks,
682
00:53:25.920 --> 00:53:29.840
the farther away it must be. Light spreads out as
683
00:53:29.840 --> 00:53:33.360
it travels through space. Imagine a light bulb at the
684
00:53:33.360 --> 00:53:37.119
center of a sphere. The light it emits spreads out
685
00:53:37.159 --> 00:53:41.480
evenly in all directions, covering the inside surface of the sphere.
686
00:53:41.880 --> 00:53:46.159
Now imagine a larger sphere. The same amount of light
687
00:53:46.760 --> 00:53:50.360
has to cover a bigger area, so the light at
688
00:53:50.400 --> 00:53:54.159
any given point is dimmer. The area of a sphere
689
00:53:54.239 --> 00:53:57.280
grows with the square of its radius, So if you
690
00:53:57.400 --> 00:54:01.000
double the distance, the light is spread over four times
691
00:54:01.039 --> 00:54:05.000
the area, making it four times dimmer. Triple the distance,
692
00:54:05.559 --> 00:54:09.000
and the light is nine times dimmer. This is called
693
00:54:09.000 --> 00:54:13.400
the inverse square law, and its fundamental to measuring cosmic distances.
694
00:54:14.000 --> 00:54:18.440
Type ere supernovae are like cosmic light bulbs, where we
695
00:54:18.519 --> 00:54:22.039
know the wattage. When we see one explode in a
696
00:54:22.079 --> 00:54:26.559
distant galaxy, we can measure its apparent brightness and calculate
697
00:54:26.599 --> 00:54:30.840
the galaxy's distance. This is more reliable than other distance
698
00:54:30.880 --> 00:54:34.960
measurement methods, which is why astronomers were so excited to
699
00:54:35.039 --> 00:54:39.559
use type ear supernovae to map the universe's expansion history.
700
00:54:40.039 --> 00:54:44.559
In the nineteen nineties, two competing research teams set out
701
00:54:44.599 --> 00:54:48.880
to use type ear supernovae to measure how much the
702
00:54:49.039 --> 00:54:55.079
universe's expansion was slowing down. Remember, everyone expected gravity to
703
00:54:55.159 --> 00:54:59.880
be slowing the expansion. The question was how much these
704
00:55:00.039 --> 00:55:02.639
measurements would determine whether we were headed for a big
705
00:55:02.679 --> 00:55:08.400
crunch or eternal expansion. Both teams, the Supernova Cosmology Project
706
00:55:08.760 --> 00:55:12.199
led by Saul pearl Mutter and the High Z Supernova
707
00:55:12.360 --> 00:55:16.880
Search Team led by Brian Schmidt and Adam Reis, used
708
00:55:16.960 --> 00:55:22.000
large telescopes to find and study distant supernovae. They looked
709
00:55:22.000 --> 00:55:25.599
at explosions that happened billions of years ago, so far
710
00:55:25.719 --> 00:55:28.760
away that their light was just reaching Earth now. By
711
00:55:28.760 --> 00:55:32.440
comparing these ancient explosions to more recent ones, they could
712
00:55:32.440 --> 00:55:35.960
measure how the expansion rate had changed. Over cosmic history.
713
00:55:36.360 --> 00:55:41.880
The results were shocking. The distant supernovae were dimmer than expected,
714
00:55:42.800 --> 00:55:47.039
not by a lot, but by a consistent measurable amount.
715
00:55:47.599 --> 00:55:50.760
They appeared about twenty five percent farther away than they
716
00:55:50.800 --> 00:55:54.239
should have been if the expansion was slowing down as predicted.
717
00:55:54.719 --> 00:55:59.360
This could only mean one thing. The expansion wasn't slowing down,
718
00:56:00.199 --> 00:56:05.400
was speeding up. The universe's expansion rate was accelerating, getting
719
00:56:05.519 --> 00:56:11.199
faster with time. This was completely unexpected. Gravity, the only
720
00:56:11.239 --> 00:56:15.800
force anyone knew about that acted on cosmic scales, should
721
00:56:15.840 --> 00:56:20.239
be slowing things down, not speeding them up. It would
722
00:56:20.280 --> 00:56:23.880
be like throwing a ball upward and watching it accelerate
723
00:56:23.920 --> 00:56:27.440
away from you instead of slowing down and falling back.
724
00:56:28.039 --> 00:56:33.239
Something was pushing the universe apart, overcoming gravity's attractive pull.
725
00:56:33.800 --> 00:56:37.639
The two teams announced their findings in nineteen ninety eight.
726
00:56:38.360 --> 00:56:44.280
The results were checked, rechecked, and verified. More supernovae were observed.
727
00:56:45.039 --> 00:56:49.920
Different teams used different methods. Everyone got the same answer,
728
00:56:50.760 --> 00:56:57.000
the expansion was accelerating. The three leaders of the two teams, Perlmutter, Schmidt,
729
00:56:57.239 --> 00:57:01.760
and Reese, won the Nobel Prize in Physics in twenty
730
00:57:01.800 --> 00:57:05.840
eleven for this discovery. But what was causing the acceleration.
731
00:57:06.880 --> 00:57:10.760
What force could push the entire universe apart overcoming the
732
00:57:10.800 --> 00:57:16.360
gravitational attraction of all the matter it contains. Physicists scrambled
733
00:57:16.360 --> 00:57:20.719
for explanations, and they found one in an unlikely place,
734
00:57:21.320 --> 00:57:27.280
in Einstein's discarded cosmological constant. Here's why it works. As
735
00:57:27.280 --> 00:57:32.880
the universe expands, normal matter's density decreases, the same atoms
736
00:57:32.920 --> 00:57:37.639
spread over Larger volumes mean weaker gravity, but the cosmological
737
00:57:37.719 --> 00:57:42.639
constant behaves differently as a property of space itself. More
738
00:57:42.719 --> 00:57:49.000
space means more total energy, while density remains constant. Early
739
00:57:49.079 --> 00:57:54.239
in cosmic history, matter was dense and gravity dominated. The
740
00:57:54.320 --> 00:57:59.599
constant existed but couldn't compete, But as expansion diluted the matter,
741
00:58:00.119 --> 00:58:05.400
gravity weakened. Eventually, about five billion years ago, the constant's
742
00:58:05.400 --> 00:58:10.119
influence became strong enough to take over. From that point forward,
743
00:58:10.679 --> 00:58:16.719
expansion began accelerating. This vacuum energy is now called dark energy.
744
00:58:17.559 --> 00:58:21.119
We label it dark because we can't observe it directly,
745
00:58:21.679 --> 00:58:26.280
only through its gravitational effects on the universe's expansion, and
746
00:58:26.360 --> 00:58:31.199
it comprises about sixty eight percent of all cosmic energy.
747
00:58:31.760 --> 00:58:36.760
Normal matter everything made of atoms accounts for only five percent.
748
00:58:37.559 --> 00:58:42.840
Another twenty seven percent is dark matter, invisible substance we
749
00:58:42.960 --> 00:58:49.400
detect through gravitational influence but haven't yet identified. Dark energy
750
00:58:49.920 --> 00:58:54.880
dominates the cosmos. Einstein added the cosmological constant to create
751
00:58:54.880 --> 00:58:58.800
a static universe. He thought he was making a mistake
752
00:58:58.880 --> 00:59:01.840
when he included it, and a bigger mistake by not
753
00:59:01.920 --> 00:59:07.000
trusting his original equations. But those equations with the cosmological
754
00:59:07.079 --> 00:59:11.559
constant included turn out to describe the actual universe we
755
00:59:11.679 --> 00:59:17.199
live in. The constant is real, the vacuum energy is real.
756
00:59:17.719 --> 00:59:22.320
Einstein's biggest blunder was actually a profound insight into the
757
00:59:22.400 --> 00:59:27.199
nature of reality. Think about how remarkable this is. Einstein,
758
00:59:27.480 --> 00:59:31.199
working with nothing but mathematics and physical intuition, predicted the
759
00:59:31.280 --> 00:59:34.880
existence of dark energy eighty years before it was discovered.
760
00:59:35.519 --> 00:59:38.719
He got the reasoning wrong. He thought this energy would
761
00:59:38.800 --> 00:59:42.920
keep the universe static, when in fact it makes it accelerate.
762
00:59:43.239 --> 00:59:48.039
But the fundamental insight that space itself possesses energy, that
763
00:59:48.119 --> 00:59:51.719
there's something pushing the universe apart that turned out to
764
00:59:51.760 --> 00:59:55.360
be correct. This is how science works at its best.
765
00:59:56.199 --> 01:00:00.920
You make predictions based on theory. Sometimes those predictions come
766
01:00:00.920 --> 01:00:05.679
from deep insight and sometimes from mistakes, but then you
767
01:00:05.800 --> 01:00:12.400
test them against reality. Observations and experiments are the final judge.
768
01:00:12.840 --> 01:00:18.760
Einstein's cosmological constant survived because it matched what we actually observe,
769
01:00:19.280 --> 01:00:22.639
not because Einstein wanted it to be true. Let's dig
770
01:00:22.719 --> 01:00:26.719
deeper into what dark energy actually is, because even though
771
01:00:26.719 --> 01:00:30.039
we've given it a name, we still don't really understand it.
772
01:00:30.840 --> 01:00:34.239
Dark energy is one of the greatest mysteries in modern physics.
773
01:00:34.760 --> 01:00:37.519
We know it exists because we can see its effects.
774
01:00:38.039 --> 01:00:42.239
The acceleration of the Universe's expansion is real and measurable,
775
01:00:43.000 --> 01:00:46.239
but we don't know what dark energy actually is at
776
01:00:46.239 --> 01:00:51.360
a fundamental level. The simplest explanation is that it's exactly
777
01:00:51.400 --> 01:00:57.239
what Einstein proposed, a cosmological constant, a fixed amount of
778
01:00:57.400 --> 01:01:01.199
energy in every cubic meter or que yard of space.
779
01:01:01.920 --> 01:01:05.119
This energy doesn't change with time or location. It's the
780
01:01:05.159 --> 01:01:10.159
same everywhere always. It's a property of space itself, like
781
01:01:10.239 --> 01:01:14.119
space has an intrinsic tendency to expand think of it
782
01:01:14.159 --> 01:01:19.920
this way. Every cubic meter of empty space, a volume
783
01:01:19.960 --> 01:01:24.280
about the size of a large refrigerator, contains a tiny
784
01:01:24.320 --> 01:01:29.920
amount of dark energy. This energy is incredibly small, about
785
01:01:29.920 --> 01:01:33.199
ten to the power of minus nine duels per cubic meter.
786
01:01:33.760 --> 01:01:37.400
That's one billionth of a duel in a cubic meter.
787
01:01:38.079 --> 01:01:42.239
A single triple a battery contains about five thousand duels
788
01:01:42.280 --> 01:01:45.480
of energy, So the dark energy in a cubic meter
789
01:01:45.599 --> 01:01:49.559
of space is about five trillion times less than the
790
01:01:49.679 --> 01:01:53.960
energy in a battery. But space is vast. There's a
791
01:01:54.000 --> 01:01:57.280
lot of cubic meters in the universe. When you add
792
01:01:57.360 --> 01:02:01.519
up all that tiny energy over all of space, it
793
01:02:01.559 --> 01:02:05.559
becomes the dominant form of energy in the cosmos. It's
794
01:02:05.719 --> 01:02:09.880
like having an enormous pile of pennies. Each penny is
795
01:02:09.960 --> 01:02:15.000
nearly worthless, but billions of pennies add up to real money.
796
01:02:15.360 --> 01:02:19.480
But where does this energy come from? One possibility comes
797
01:02:19.480 --> 01:02:25.559
from quantum mechanics. In quantum theory, even empty space isn't
798
01:02:25.639 --> 01:02:30.639
truly empty. The vacuum is filled with virtual particles constantly
799
01:02:30.679 --> 01:02:34.480
popping into existence and then annihilating each other in tiny
800
01:02:34.559 --> 01:02:39.239
fractions of a second. These quantum fluctuations should create a
801
01:02:39.320 --> 01:02:43.800
kind of vacuum energy, a background energy that fills all
802
01:02:43.840 --> 01:02:47.239
of space. Quantum mechanics says that you can't have a
803
01:02:47.320 --> 01:02:52.960
perfectly empty vacuum with exactly zero energy. The Heisenberg uncertainty principle,
804
01:02:53.800 --> 01:02:57.679
one of the fundamental laws of quantum mechanics, says that
805
01:02:57.719 --> 01:02:59.880
if you try to pin down the energy of a
806
01:03:00.239 --> 01:03:05.760
region of space to exactly zero, you create infinite uncertainty
807
01:03:06.119 --> 01:03:10.760
in when that energy exists, which violates the principle. So
808
01:03:10.880 --> 01:03:15.639
the vacuum must have some energy constantly fluctuating around a
809
01:03:15.679 --> 01:03:19.800
non zero average. When physicists try to calculate how much
810
01:03:19.880 --> 01:03:24.719
vacuum energy these quantum fluctuations should produce, they get an answer,
811
01:03:25.599 --> 01:03:30.800
but it's the wrong answer, spectacularly wrong. The calculated value
812
01:03:31.000 --> 01:03:33.719
is about ten to the power of one hundred twenty
813
01:03:33.800 --> 01:03:38.320
times larger than the observed value of dark energy. That's
814
01:03:38.400 --> 01:03:42.079
one followed by one hundred twenty zeros. It's one of
815
01:03:42.119 --> 01:03:45.800
the worst predictions in the history of physics, a mismatch
816
01:03:45.920 --> 01:03:49.880
so enormous it's hard to even comprehend to put this
817
01:03:49.960 --> 01:03:54.880
in perspective. If the predicted vacuum energy were correct, the
818
01:03:55.000 --> 01:03:59.559
universe would be expanding so fast that atoms couldn't hold together.
819
01:04:00.400 --> 01:04:05.000
Galaxies would be torn apart, stars would be ripped apart,
820
01:04:05.639 --> 01:04:11.480
Planets would be destroyed, life would be impossible. The fact
821
01:04:11.519 --> 01:04:14.599
that we exist at all tells us that the actual
822
01:04:14.719 --> 01:04:19.119
vacuum energy must be much much smaller than the naive
823
01:04:19.239 --> 01:04:25.519
quantum mechanical prediction. This is called the cosmological constant problem.
824
01:04:25.679 --> 01:04:28.880
Why is the observed vacuum energy so much smaller than
825
01:04:28.920 --> 01:04:33.360
the predicted value. Why isn't it zero? And why is
826
01:04:33.400 --> 01:04:37.480
it the specific value it is, which is just large
827
01:04:37.559 --> 01:04:42.360
enough to start dominating the universe's expansion a few billion
828
01:04:42.480 --> 01:04:47.360
years ago, coincidentally, right around the time complex life was
829
01:04:47.400 --> 01:04:51.519
developing on Earth. Some physicists think there must be some
830
01:04:51.840 --> 01:04:56.119
unknown mechanism that cancels out most of the quantum vacuum energy,
831
01:04:56.719 --> 01:05:00.639
leaving behind just the tiny amount we observe as dar energy.
832
01:05:01.119 --> 01:05:04.280
But we don't know what that mechanism is or why
833
01:05:04.320 --> 01:05:07.840
it would leave behind exactly the amount it does. It's
834
01:05:07.960 --> 01:05:11.480
like having a complicated calculation that should give you an
835
01:05:11.679 --> 01:05:18.039
enormous number, but through some mysterious process involving billions of cancelations,
836
01:05:18.400 --> 01:05:22.599
you end up with a tiny number. We don't understand
837
01:05:22.920 --> 01:05:28.159
the cancelation mechanism. Others propose that dark energy isn't constant
838
01:05:28.199 --> 01:05:32.840
at all. Maybe it changes with time, growing stronger or
839
01:05:32.880 --> 01:05:37.119
weaker as the universe evolves. This would be a different
840
01:05:37.320 --> 01:05:44.079
kind of energy, not Einstein's cosmological constant, and physicists call
841
01:05:44.119 --> 01:05:49.199
it quintessence, after the fifth element of ancient philosophy. If
842
01:05:49.280 --> 01:05:53.440
dark energy is quintessence, it could behave differently in the
843
01:05:53.480 --> 01:05:56.840
future than it does now, leading to different fates for
844
01:05:56.880 --> 01:06:00.760
the universe. Still, others suggest we might need to modify
845
01:06:00.840 --> 01:06:05.719
general relativity itself. Maybe Einstein's equations work perfectly on the
846
01:06:05.760 --> 01:06:08.960
scale of the Solar System but need correction when applied
847
01:06:09.000 --> 01:06:12.800
to the entire universe. Maybe there's an additional term or
848
01:06:12.840 --> 01:06:17.320
modification that would explain the acceleration without needing dark energy
849
01:06:17.400 --> 01:06:22.440
at all. There's even more exotic proposals. Some physicists suggest
850
01:06:22.760 --> 01:06:26.079
that dark energy might be connected to extra dimensions of
851
01:06:26.159 --> 01:06:31.199
space beyond the three we experience. String theory, a leading
852
01:06:31.280 --> 01:06:34.920
candidate for a theory of quantum gravity, predicts that space
853
01:06:35.000 --> 01:06:38.760
has ten or eleven dimensions, most of them curled up
854
01:06:38.800 --> 01:06:42.880
so small we can't detect them. Maybe dark energy leaks
855
01:06:42.880 --> 01:06:47.039
into our three dimensions from these extra dimensions. Maybe the
856
01:06:47.079 --> 01:06:52.320
behavior of gravity changes on cosmic scales because it's affected
857
01:06:52.400 --> 01:06:56.880
by these hidden dimensions. Other proposals suggest that our universe
858
01:06:57.159 --> 01:07:02.159
might be embedded in a larger structure ultiverse containing many universes.
859
01:07:02.639 --> 01:07:06.840
Maybe dark energy represents the influence of other universes on ours,
860
01:07:07.199 --> 01:07:11.960
a kind of pressure from outside pushing our universe to expand.
861
01:07:12.559 --> 01:07:16.719
This sounds like science fiction, but it's based on serious
862
01:07:16.840 --> 01:07:22.519
mathematical models explored by leading physicists. These are all active
863
01:07:22.679 --> 01:07:28.400
areas of research. Physicists are designing new experiments and observations
864
01:07:28.760 --> 01:07:33.119
to try to distinguish between these possibilities. They're measuring the
865
01:07:33.239 --> 01:07:38.039
universe's expansion rate more precisely, looking for any signs that
866
01:07:38.239 --> 01:07:42.760
dark energy is changing with time, and testing general relativity
867
01:07:43.079 --> 01:07:47.079
on larger and larger scales. The fate of the universe
868
01:07:47.400 --> 01:07:51.719
depends on dark energy's nature. If it's truly a cosmological
869
01:07:51.800 --> 01:07:57.360
constant that never changes, then the universe will continue accelerating forever.
870
01:07:57.800 --> 01:08:04.360
Galaxies will move apart faster and faster. Eventually, in hundreds
871
01:08:04.360 --> 01:08:08.159
of billions of years, galaxies outside our local group will
872
01:08:08.159 --> 01:08:11.960
be receding so fast that their light can never reach us.
873
01:08:12.480 --> 01:08:16.399
The universe will become dark and cold and empty. From
874
01:08:16.479 --> 01:08:21.359
any observer's perspective. If dark energy is quintessence that can
875
01:08:21.439 --> 01:08:25.760
change strength, Then the future is less certain. Maybe it
876
01:08:25.760 --> 01:08:29.000
will weaken eventually and the universe will coast along at
877
01:08:29.000 --> 01:08:33.920
a steady expansion rate. Maybe it will strengthen dramatically, tearing
878
01:08:33.960 --> 01:08:39.560
apart galaxies, then stars, then planets, then atoms themselves in
879
01:08:39.600 --> 01:08:43.760
a big rip. Or maybe it will reverse, becoming attractive
880
01:08:43.800 --> 01:08:47.319
instead of repulsive, and the universe will collapse in a
881
01:08:47.359 --> 01:08:51.680
big crunch. After all, we simply don't know yet. The
882
01:08:51.760 --> 01:08:55.640
observations we have so far are consistent with dark energy
883
01:08:56.000 --> 01:09:01.000
being a cosmological constant unchanging with time, but the error
884
01:09:01.039 --> 01:09:04.359
bars are still large enough that we can't rule out
885
01:09:04.479 --> 01:09:09.960
other possibilities. Future observations will narrow down the possibilities and
886
01:09:10.079 --> 01:09:13.560
hopefully give us a clearer picture of what dark energy
887
01:09:13.680 --> 01:09:17.279
really is and what it means for the universe's future.
888
01:09:17.800 --> 01:09:21.560
Let's talk about some of the other interpretations and implications
889
01:09:22.039 --> 01:09:25.600
of Einstein's so called blunder, because the story of the
890
01:09:25.640 --> 01:09:30.520
cosmological constant is really a story about how science works
891
01:09:31.239 --> 01:09:36.079
and how even mistakes can lead to progress. First, was
892
01:09:36.119 --> 01:09:42.399
this really Einstein's biggest mistake? Einstein himself apparently thought so,
893
01:09:42.840 --> 01:09:46.159
or at least that's what he told people later in life,
894
01:09:46.560 --> 01:09:50.640
But historians of science have pointed out that Einstein made
895
01:09:50.680 --> 01:09:55.680
other errors that were arguably more significant. For example, Einstein
896
01:09:55.760 --> 01:09:58.880
spent the last thirty years of his life trying to
897
01:09:58.960 --> 01:10:03.640
develop a u unified field theory, a single framework that
898
01:10:03.680 --> 01:10:08.199
would combine gravity and electromagnetism. He was trying to find
899
01:10:08.239 --> 01:10:11.560
a theory of everything, a set of equations that would
900
01:10:11.600 --> 01:10:15.560
describe all the forces of nature and all the particles
901
01:10:15.560 --> 01:10:20.439
of matter. He pursued this goal obsessively, filling note books
902
01:10:20.479 --> 01:10:26.039
with calculations and trying approach after approach. He never succeeded,
903
01:10:26.680 --> 01:10:29.920
and in hindsight, his approach was doomed from the start
904
01:10:30.159 --> 01:10:33.520
because he was trying to unify the wrong things. He
905
01:10:33.640 --> 01:10:38.520
was ignoring quantum mechanics, which he deeply mistrusted, and focusing
906
01:10:38.560 --> 01:10:42.920
only on classical fields. But the universe is quantum at
907
01:10:42.920 --> 01:10:47.960
its core. Any theory of everything must incorporate quantum mechanics.
908
01:10:48.479 --> 01:10:53.920
Einstein's refusal to fully embrace quantum theory, despite being one
909
01:10:53.920 --> 01:10:56.800
of its founders with his work on the photoelectric effect,
910
01:10:57.439 --> 01:11:00.239
meant he was looking in the wrong direction. Though those
911
01:11:00.279 --> 01:11:03.560
thirty years could have been spent on more productive research.
912
01:11:04.560 --> 01:11:09.239
Einstein could have contributed to quantum field theory or nuclear physics,
913
01:11:09.479 --> 01:11:13.479
or other areas where his insights might have been valuable. Instead,
914
01:11:14.279 --> 01:11:20.279
he pursued a dead end isolated from mainstream physics, increasingly
915
01:11:20.359 --> 01:11:24.880
seen as out of touch by younger physicists. From that perspective,
916
01:11:25.479 --> 01:11:29.399
his quest for a classical unified field theory was a
917
01:11:29.439 --> 01:11:34.319
bigger mistake than adding the cosmological constant. But there's a difference.
918
01:11:35.119 --> 01:11:39.479
The unified field theory work was simply wrong. It led
919
01:11:39.520 --> 01:11:44.720
nowhere and contributed nothing to future physics. The cosmological constant,
920
01:11:44.760 --> 01:11:47.840
on the other hand, turned out to describe something real
921
01:11:48.159 --> 01:11:52.439
about the universe, even if Einstein added it for wrong reasons.
922
01:11:53.079 --> 01:11:56.119
So while the unified field theory work might have been
923
01:11:56.159 --> 01:12:00.720
a bigger waste of Einstein's time and talent, the cosmological
924
01:12:00.880 --> 01:12:05.039
constant story is more interesting because of its resurrection in
925
01:12:05.119 --> 01:12:09.800
modern cosmology. It's also worth noting that the cosmological constant
926
01:12:10.159 --> 01:12:12.359
might not have been a mistake at all in the
927
01:12:12.399 --> 01:12:16.920
context of its time. In nineteen seventeen, there was no
928
01:12:17.039 --> 01:12:21.199
evidence the universe was expanding. Einstein was working from the
929
01:12:21.199 --> 01:12:26.319
best available information, which suggested a static cosmos. Adding a
930
01:12:26.439 --> 01:12:29.680
term to his equations to allow a static solution was
931
01:12:29.760 --> 01:12:34.239
perfectly reasonable given what was known then. The mistake, if
932
01:12:34.279 --> 01:12:37.119
there was one, was not removing the constant as soon
933
01:12:37.119 --> 01:12:42.359
as better observations became available. But even that's understandable. Scientific
934
01:12:42.399 --> 01:12:47.079
paradigms don't shift instantly when new data arrives. It takes
935
01:12:47.119 --> 01:12:51.319
time for evidence to accumulate, for alternative explanations to be
936
01:12:51.399 --> 01:12:56.520
ruled out, and for consensus to form. Einstein accepted Hubble's
937
01:12:56.520 --> 01:13:01.399
observations fairly quickly by the standards of scientific revel Another
938
01:13:01.520 --> 01:13:04.279
interesting aspect of this story is what it tells us
939
01:13:04.279 --> 01:13:09.520
about the relationship between mathematics and physics. Einstein's equations predicted
940
01:13:09.760 --> 01:13:13.880
an unstable universe. He modified them to get a static universe.
941
01:13:14.399 --> 01:13:17.920
The original equations turned out to be right, but the
942
01:13:17.960 --> 01:13:22.760
modified version also turned out to describe something real. This
943
01:13:23.159 --> 01:13:30.159
raises a philosophical question. Do mathematical equations describe reality or
944
01:13:30.199 --> 01:13:35.039
do they just provide useful models that approximately match observations.
945
01:13:35.920 --> 01:13:41.039
Physicists generally lean toward the first view. They believe the
946
01:13:41.159 --> 01:13:46.600
universe operates according to mathematical laws, and our equations, when correct,
947
01:13:47.039 --> 01:13:52.479
are discovering those laws. Rather than just modeling observations, but
948
01:13:52.560 --> 01:13:58.279
the cosmological constant story complicates this. Einstein added a term
949
01:13:58.359 --> 01:14:01.840
for non physical reasons to get an answer he wanted
950
01:14:02.520 --> 01:14:07.039
by pure mathematics. The term was allowed but not required,
951
01:14:07.640 --> 01:14:10.319
and yet it turned out to be necessary to describe
952
01:14:10.359 --> 01:14:14.840
the actual universe. Does this mean the mathematics somehow knew
953
01:14:15.000 --> 01:14:19.800
more than Einstein did or is it just a lucky coincidence.
954
01:14:20.359 --> 01:14:24.399
There's no clear answer, but it does suggest that mathematics
955
01:14:24.600 --> 01:14:28.600
has a way of surprising us, of containing truths. We
956
01:14:28.680 --> 01:14:34.720
don't immediately recognize. Einstein's equations in their full generality, including
957
01:14:34.800 --> 01:14:39.119
all the terms that are mathematically possible, described a universe
958
01:14:39.399 --> 01:14:43.479
that could be static with fine tuned lambda, or expanding
959
01:14:44.119 --> 01:14:50.720
or contracting or accelerating. The equations contained all these possibilities
960
01:14:51.760 --> 01:14:56.680
observations told us which possibility is real. Let's also consider
961
01:14:56.920 --> 01:15:00.960
what this means for scientific reasoning and how scientists should
962
01:15:01.000 --> 01:15:06.439
approach their work. Einstein let his philosophical preferences guide his
963
01:15:06.560 --> 01:15:12.640
modifications to general relativity. He wanted an eternal, unchanging universe,
964
01:15:13.279 --> 01:15:17.840
and he adjusted his equations to provide one. In the end,
965
01:15:18.439 --> 01:15:23.640
this was wrong. The universe isn't static. Does this mean
966
01:15:23.800 --> 01:15:29.560
scientists should never let philosophical or esthetic considerations guide their work.
967
01:15:30.399 --> 01:15:35.319
Should they only follow the math wherever it leads. Not necessarily,
968
01:15:35.760 --> 01:15:41.920
Physical intuition, esthetic judgment, and philosophical reasoning have played crucial
969
01:15:42.000 --> 01:15:46.880
roles in many scientific advances. Einstein himself relied heavily on
970
01:15:47.000 --> 01:15:52.359
thought experiments and intuitive reasoning to develop both special and
971
01:15:53.079 --> 01:15:57.640
general relativity. The lesson isn't that philosophical reasoning is bad.
972
01:15:58.319 --> 01:16:02.600
It's that observations are the ultimate arbiter. You can use
973
01:16:02.640 --> 01:16:06.640
whatever reasoning you want to develop theories and make predictions,
974
01:16:07.159 --> 01:16:10.399
but then you have to test those predictions against reality,
975
01:16:10.880 --> 01:16:14.439
and when reality disagrees with your preferences, you have to
976
01:16:14.560 --> 01:16:18.720
change your mind. Einstein did change his mind when Hubble's
977
01:16:18.760 --> 01:16:23.560
observations came in. He accepted that the universe was expanding
978
01:16:23.960 --> 01:16:28.359
and that his static model was wrong. That's good science.
979
01:16:29.039 --> 01:16:32.880
Where he perhaps aired was in adding the cosmological constant
980
01:16:32.920 --> 01:16:38.119
in the first place without observational justification, purely to get
981
01:16:38.199 --> 01:16:41.840
a desired result. The modern attitude in physics is generally
982
01:16:41.880 --> 01:16:46.840
more conservative about adding new terms or modifying successful theories.
983
01:16:47.359 --> 01:16:52.239
If your equations predict something that seems strange or counterintuitive,
984
01:16:52.920 --> 01:16:56.840
you should think very carefully before changing them. Often, the
985
01:16:56.960 --> 01:17:02.079
strange prediction turns out to be correct. Special relativity predicted
986
01:17:02.119 --> 01:17:06.760
time dilation, which seemed absurd when Einstein first proposed it.
987
01:17:07.279 --> 01:17:12.399
General relativity predicted black holes, which many physicists thought must
988
01:17:12.439 --> 01:17:17.960
be mathematical artifacts rather than real objects. Quantum mechanics predicted
989
01:17:18.000 --> 01:17:22.520
particles could be in multiple places at once, which violated
990
01:17:22.720 --> 01:17:26.960
common sense. All of these strange predictions turned out to
991
01:17:27.000 --> 01:17:31.920
be correct. So when your beautiful, successful theory predicts something
992
01:17:31.960 --> 01:17:35.640
you don't like, maybe the problem is with your intuition,
993
01:17:36.279 --> 01:17:40.600
not with the theory. Maybe the universe's stranger than you thought.
994
01:17:41.239 --> 01:17:46.600
That's the lesson of the cosmological constant. Einstein's intuition told
995
01:17:46.680 --> 01:17:51.720
him the universe should be static. His equations told him
996
01:17:52.199 --> 01:17:58.279
it couldn't be. He modified the equations. The equations were right.
997
01:17:58.880 --> 01:18:01.800
Let's zoom out and look at the bigger picture of
998
01:18:01.880 --> 01:18:05.279
what we've learned about the universe. Thanks to Einstein's work,
999
01:18:05.920 --> 01:18:10.880
both his successes and his mistakes, general relativity has become
1000
01:18:10.920 --> 01:18:16.000
the foundation of modern cosmology. Every major discovery about the
1001
01:18:16.119 --> 01:18:19.640
large scale structure and evolution of the universe has been
1002
01:18:19.680 --> 01:18:24.520
interpreted through the framework Einstein provided. We've learned that space
1003
01:18:24.560 --> 01:18:29.199
and time are not separate absolute things, but are woven
1004
01:18:29.279 --> 01:18:34.880
together into a single fabric called space time. This fabric
1005
01:18:35.119 --> 01:18:42.159
can bend, stretch, compress, and ripple. Massive objects create depressions
1006
01:18:42.159 --> 01:18:47.199
in space time, and we experience these depressions as gravity.
1007
01:18:47.760 --> 01:18:51.000
We've learned that the universe had a beginning, the Big Bang,
1008
01:18:51.680 --> 01:18:55.960
about thirteen point eight billion years ago. Before that moment,
1009
01:18:56.399 --> 01:19:00.680
there was no space, no time, no matter, no energy.
1010
01:19:01.159 --> 01:19:04.720
The Big Bang wasn't an explosion in space. It was
1011
01:19:04.760 --> 01:19:08.239
the beginning of space itself, the moment when space time
1012
01:19:08.279 --> 01:19:12.199
came into existence and began expanding. We've learned that this
1013
01:19:12.319 --> 01:19:16.479
expansion is accelerating, driven by dark energy that makes up
1014
01:19:16.520 --> 01:19:20.439
most of the universe's total energy content. We don't fully
1015
01:19:20.520 --> 01:19:23.960
understand what dark energy is, but we can measure its
1016
01:19:24.000 --> 01:19:28.479
effects and predict its influence on the universe's future. We've
1017
01:19:28.560 --> 01:19:31.840
learned that most of the matter in the universe is
1018
01:19:31.960 --> 01:19:37.039
dark matter, another form of invisible substance that doesn't interact
1019
01:19:37.039 --> 01:19:42.399
with light but creates gravitational effects. Galaxies are embedded in
1020
01:19:42.560 --> 01:19:46.560
huge halos of dark matter that outweigh the visible matter
1021
01:19:46.640 --> 01:19:50.800
by a factor of six to one. Without dark matter's
1022
01:19:50.840 --> 01:19:56.279
extra gravity, galaxies would fly apart, stars would not orbit
1023
01:19:56.560 --> 01:20:01.159
the way they do. We've learned about black holes, regions
1024
01:20:01.199 --> 01:20:05.039
where space time is curved so severely that not even
1025
01:20:05.199 --> 01:20:10.159
light can escape. We've observed stellar mass black holes created
1026
01:20:10.199 --> 01:20:15.119
by collapsing stars, supermassive black holes, millions or billions of
1027
01:20:15.199 --> 01:20:18.560
times the Sun's mass sitting at the centers of galaxies,
1028
01:20:19.119 --> 01:20:22.479
and even the merger of two black holes detected through
1029
01:20:22.560 --> 01:20:27.800
gravitational waves. We've learned about the cosmic microwave background, the
1030
01:20:27.840 --> 01:20:31.520
after glow of the Big Bang, a faint radiation that
1031
01:20:31.680 --> 01:20:35.640
fills all of space and provides a snap shot of
1032
01:20:35.720 --> 01:20:38.600
what the universe looked like when it was only three
1033
01:20:38.720 --> 01:20:43.600
hundred eighty thousand years old. By studying tiny variations in
1034
01:20:43.640 --> 01:20:47.119
this radiation, we can learn about the seeds that grew
1035
01:20:47.199 --> 01:20:51.880
into galaxies and the fundamental properties of the cosmos. All
1036
01:20:51.960 --> 01:20:56.199
of this understanding rests on the foundation Einstein built with
1037
01:20:56.359 --> 01:21:01.039
general relativity. His equations have been tested counts times in
1038
01:21:01.119 --> 01:21:05.880
countless ways, from the bending of starlight during eclipses to
1039
01:21:05.960 --> 01:21:10.439
the time dilation of GPS satellites to the gravitational waves
1040
01:21:10.439 --> 01:21:15.399
from colliding black holes. Every test has confirmed that Einstein
1041
01:21:15.600 --> 01:21:20.279
was right, and yet general relativity is not the final word.
1042
01:21:20.920 --> 01:21:24.239
We know it must be incomplete because it doesn't play
1043
01:21:24.359 --> 01:21:29.880
nicely with quantum mechanics. General relativity is a classical theory.
1044
01:21:30.439 --> 01:21:35.960
It describes spacetime as smooth and continuous. Quantum mechanics says
1045
01:21:36.000 --> 01:21:41.079
everything is discreete and probabilistic. At the smallest scales, these
1046
01:21:41.119 --> 01:21:44.439
two frameworks are best theories of the very large and
1047
01:21:44.479 --> 01:21:48.760
the very small contradict each other. When pushed to extremes
1048
01:21:49.359 --> 01:21:53.359
inside a black hole. At the singularity, where all the
1049
01:21:53.399 --> 01:21:58.920
masses compress to infinite density, both gravity and quantum effects
1050
01:21:59.359 --> 01:22:07.039
become import General relativity predicts infinite density an infinite spacetime curvature,
1051
01:22:07.479 --> 01:22:12.760
but quantum mechanics doesn't allow true infinities. Something has to give.
1052
01:22:13.279 --> 01:22:16.720
At the moment of the Big Bang, the entire universe
1053
01:22:16.880 --> 01:22:21.039
was compressed to a size smaller than an atom. Again,
1054
01:22:21.560 --> 01:22:26.279
both gravity and quantum effects matter. We can't use general
1055
01:22:26.359 --> 01:22:30.439
relativity alone to describe what happened at that moment because
1056
01:22:30.439 --> 01:22:35.359
we need quantum gravity. A theory that combines general relativity
1057
01:22:35.640 --> 01:22:39.920
with quantum mechanics in a consistent way. Physicists have been
1058
01:22:39.960 --> 01:22:46.279
working on quantum gravity for decades. String theory, loop quantum gravity,
1059
01:22:46.720 --> 01:22:50.439
and other approaches try to build a framework where space
1060
01:22:50.520 --> 01:22:54.680
time itself becomes quantum, where space and time are made
1061
01:22:54.720 --> 01:22:58.960
of discrete units rather than being continuous. But we don't
1062
01:22:58.960 --> 01:23:02.920
have a complete, tested theory yet. This is one of
1063
01:23:02.920 --> 01:23:07.439
the great unsolved problems in physics. When we do finally
1064
01:23:07.479 --> 01:23:10.560
develop a theory of quantum gravity, it might change our
1065
01:23:10.640 --> 01:23:15.720
understanding of dark energy and the cosmological constant. The vacuum
1066
01:23:15.800 --> 01:23:19.760
energy that makes up dark energy is inherently quantum in nature.
1067
01:23:20.199 --> 01:23:23.359
A complete theory should be able to calculate its value
1068
01:23:23.399 --> 01:23:27.760
from first principles, explaining why it is what it is,
1069
01:23:28.479 --> 01:23:33.399
rather than just measuring it. That calculation might resolve the
1070
01:23:33.439 --> 01:23:38.000
cosmological constant problem, or might reveal that dark energy is
1071
01:23:38.039 --> 01:23:42.520
something entirely different from what we currently think. So Einstein's
1072
01:23:42.600 --> 01:23:47.359
legacy in cosmology is twofold. He gave us the framework
1073
01:23:47.439 --> 01:23:51.199
we still use to understand the universe, and he left
1074
01:23:51.279 --> 01:23:55.319
us with puzzles that still haven't been solved. The cosmological
1075
01:23:55.399 --> 01:23:59.439
constant was his attempt to solve one puzzle his desire
1076
01:23:59.520 --> 01:24:04.159
for a stativerse. He removed it when observations showed that
1077
01:24:04.359 --> 01:24:08.359
wasn't the right puzzle to solve, Modern cosmology brought it
1078
01:24:08.479 --> 01:24:12.840
back to solve a different puzzle, the acceleration of cosmic expansion.
1079
01:24:13.560 --> 01:24:16.439
But the deeper puzzle of why dark energy has the
1080
01:24:16.520 --> 01:24:21.319
value it does remains unsolved. Let's talk about what all
1081
01:24:21.399 --> 01:24:25.079
this means for the future, both the future of cosmology
1082
01:24:25.119 --> 01:24:28.880
as a science and the future of the universe itself.
1083
01:24:29.920 --> 01:24:34.640
Where do we go from here? For cosmology, the immediate
1084
01:24:34.720 --> 01:24:39.159
goal is to better understand dark energy. Is it truly
1085
01:24:39.239 --> 01:24:43.479
constant or does it change with time? Is it the
1086
01:24:43.560 --> 01:24:48.800
vacuum energy predicted by quantum mechanics or something else Entirely
1087
01:24:49.279 --> 01:24:55.560
To answer these questions, astronomers are conducting increasingly precise observations
1088
01:24:55.920 --> 01:25:00.640
of the universe's expansion history. Projects like the Dark Energy
1089
01:25:00.680 --> 01:25:06.199
Survey and the upcoming Verrubin Observatory are mapping millions of galaxies,
1090
01:25:06.600 --> 01:25:10.920
measuring their distances and redshifts to trace how the expansion
1091
01:25:11.000 --> 01:25:15.520
rate has changed over time. The James Webb Space Telescope
1092
01:25:15.560 --> 01:25:18.279
is looking at some of the most distant galaxies in
1093
01:25:18.319 --> 01:25:22.279
the universe, seeing them as they were more than thirteen
1094
01:25:22.439 --> 01:25:26.720
billion years ago, when the universe was young. By comparing
1095
01:25:26.760 --> 01:25:31.640
the expansion rate at different times in cosmic history, astronomers
1096
01:25:31.680 --> 01:25:35.960
can determine whether dark energy has been getting stronger, weaker,
1097
01:25:36.520 --> 01:25:40.640
or staying the same. Current data suggests its constant, but
1098
01:25:40.720 --> 01:25:45.800
the uncertainties are still large. Another decade of observations should
1099
01:25:45.880 --> 01:25:50.199
narrow down the possibilities considerably. There are also efforts to
1100
01:25:50.319 --> 01:25:55.880
directly detect dark energy particles. If such particles exist, some
1101
01:25:55.960 --> 01:25:59.199
theories propose that dark energy might be carried by a
1102
01:25:59.279 --> 01:26:03.600
new type of article, similar to how electromagnetic force is
1103
01:26:03.680 --> 01:26:09.039
carried by photons. Experiments in underground laboratories are searching for
1104
01:26:09.119 --> 01:26:13.880
these hypothetical particles, though most physicists think they probably won't
1105
01:26:13.920 --> 01:26:17.840
find anything because dark energy doesn't seem to interact with
1106
01:26:17.960 --> 01:26:23.319
normal matter except through gravity. The most exciting possibility would
1107
01:26:23.319 --> 01:26:27.079
be finding evidence that general relativity needs to be modified
1108
01:26:27.079 --> 01:26:31.800
on cosmic scales. If observations showed that gravity behaves differently
1109
01:26:31.880 --> 01:26:35.079
in different parts of the universe or at different times,
1110
01:26:35.560 --> 01:26:40.880
that would be revolutionary. It would mean Einstein's theory, while
1111
01:26:40.960 --> 01:26:45.239
correct in our solar system and our galaxy, breaks down
1112
01:26:45.359 --> 01:26:49.039
when applied to the universe as a whole. That would
1113
01:26:49.079 --> 01:26:53.920
open up entirely new areas of physics to explore. As
1114
01:26:53.920 --> 01:26:58.039
for the universe's future. If dark energy is truly a
1115
01:26:58.119 --> 01:27:03.119
cosmological constant that never changes, the story is rather bleak,
1116
01:27:03.680 --> 01:27:08.199
at least from a human perspective. The universe will continue
1117
01:27:08.239 --> 01:27:13.800
expanding forever. Galaxies outside our local group are already receding
1118
01:27:13.840 --> 01:27:19.159
from us, and that recession is accelerating. Eventually, in about
1119
01:27:19.159 --> 01:27:24.279
one hundred billion years, all galaxies beyond our immediate neighborhood
1120
01:27:24.520 --> 01:27:29.119
will have receded beyond our cosmic horizon. Their light will
1121
01:27:29.159 --> 01:27:32.479
be stretched to such long wavelengths by the expansion of
1122
01:27:32.560 --> 01:27:36.039
space that we'll never see them again. Think about what
1123
01:27:36.119 --> 01:27:40.479
this means. An astronomer living in the distant future, one
1124
01:27:40.600 --> 01:27:44.319
hundred billion years from now, looking out into the universe
1125
01:27:44.560 --> 01:27:49.560
with the most powerful telescopes imaginable, will see something completely
1126
01:27:49.680 --> 01:27:53.640
different from what we see today. They'll see only our
1127
01:27:53.680 --> 01:27:57.520
local group of galaxies, the Milky Way, and perhaps a
1128
01:27:57.520 --> 01:28:00.680
few dozen nearby galaxies that have merge merged with us
1129
01:28:01.159 --> 01:28:06.720
or remain gravitationally bound to us. Beyond that, nothing, just
1130
01:28:06.920 --> 01:28:12.800
empty darkness, extending in all directions forever. To an astronomer
1131
01:28:13.039 --> 01:28:16.760
living one hundred billion years from now in the Milky Way,
1132
01:28:17.279 --> 01:28:19.960
or in one of the few nearby galaxies that will
1133
01:28:20.000 --> 01:28:23.840
have merged with us. By then, the observable universe will
1134
01:28:23.840 --> 01:28:28.720
appear to contain only our local group of galaxies surrounded
1135
01:28:29.039 --> 01:28:32.960
by empty space. They'll see no evidence of cosmic expansion
1136
01:28:33.039 --> 01:28:38.079
because everything observable will be gravitationally bound together. They'll have
1137
01:28:38.159 --> 01:28:40.239
no way to know that there are trillions of other
1138
01:28:40.319 --> 01:28:43.720
galaxies beyond their horizon. They won't be able to see
1139
01:28:43.760 --> 01:28:47.479
the cosmic microwave background radiation because it will have been
1140
01:28:47.520 --> 01:28:51.479
red shifted into invisibility. They won't be able to see
1141
01:28:51.520 --> 01:28:55.439
distant galaxies moving away. They won't be able to detect
1142
01:28:55.640 --> 01:28:59.079
any of the evidence that we currently use to understand
1143
01:28:59.119 --> 01:29:02.920
the Big Bang and the universe's history. From their perspective,
1144
01:29:03.279 --> 01:29:06.640
the universe will appear to consist of a single large
1145
01:29:06.720 --> 01:29:11.640
galaxy surrounded by infinite empty space. They might not even
1146
01:29:11.680 --> 01:29:14.720
be able to figure out that the universe had a beginning.
1147
01:29:15.279 --> 01:29:18.000
In fact, they might not even know that the Big
1148
01:29:18.039 --> 01:29:22.920
Bang happened. The cosmic microwave background radiation will have been
1149
01:29:22.960 --> 01:29:26.560
red shifted to such long wave lengths that it will
1150
01:29:26.640 --> 01:29:31.800
be undetectable. The evidence that our current generation of astronomers
1151
01:29:31.800 --> 01:29:35.800
has used to understand the Universe's history and structure will
1152
01:29:35.800 --> 01:29:41.239
simply be gone, carried beyond the horizon by the accelerating expansion.
1153
01:29:41.800 --> 01:29:45.159
This is a sobering thought. We live at a privileged
1154
01:29:45.199 --> 01:29:49.319
time in cosmic history. When the universe's nature and history
1155
01:29:49.479 --> 01:29:53.680
are observable past a certain point in the future, that
1156
01:29:53.840 --> 01:29:58.680
window closes. The universe becomes opaque to its own history.
1157
01:29:59.159 --> 01:30:02.439
Information about where we came from and what else exists
1158
01:30:02.479 --> 01:30:08.119
out there becomes permanently inaccessible. Future civilizations, if any exist,
1159
01:30:08.800 --> 01:30:11.840
will be unable to learn what we know about the cosmos,
1160
01:30:12.159 --> 01:30:15.319
simply because the evidence will have moved beyond their reach.
1161
01:30:15.840 --> 01:30:20.520
On even longer time scales, the universe becomes truly lonely.
1162
01:30:21.479 --> 01:30:24.920
Stars will continue forming from available gas for a few
1163
01:30:25.039 --> 01:30:30.119
tens of billions of years. Eventually, star formation will stop
1164
01:30:30.439 --> 01:30:33.800
when all the available gas has been converted to stars
1165
01:30:34.319 --> 01:30:37.479
or locked up in black holes and dead stellar remnants.
1166
01:30:38.279 --> 01:30:41.600
The last stars will burn out in about ten trillion
1167
01:30:41.680 --> 01:30:46.960
years or so. That's ten thousand billion years, about seven
1168
01:30:47.039 --> 01:30:50.199
hundred times longer than the current age of the universe.
1169
01:30:50.840 --> 01:30:56.359
After that, the universe will be dark, dead stars, brown dwarfs,
1170
01:30:56.560 --> 01:31:00.520
white dwarfs, neutrons, stars, and black holes will be all
1171
01:31:00.600 --> 01:31:06.520
that remains. Even these will eventually fade. White dwarfs will
1172
01:31:06.560 --> 01:31:10.399
cool to black dwarfs, dark cinders of carbon and oxygen
1173
01:31:10.800 --> 01:31:15.600
floating through space. Neutron stars will cool, though this takes
1174
01:31:15.600 --> 01:31:18.800
an incredibly long time because they're so dense and have
1175
01:31:18.920 --> 01:31:22.239
so little surface area to radiate away their heat, and
1176
01:31:22.319 --> 01:31:27.159
black holes will evaporate through Hawking radiation, though this process
1177
01:31:27.199 --> 01:31:32.079
takes an incomprehensibly long time. Stephen Hawking predicted in the
1178
01:31:32.199 --> 01:31:37.560
nineteen seventies that black holes aren't completely black. They emit
1179
01:31:37.760 --> 01:31:41.920
radiation due to quantum effects near the event horizon. This
1180
01:31:42.000 --> 01:31:47.479
Hawking radiation is incredibly faint and incredibly slow, but given
1181
01:31:47.600 --> 01:31:51.239
enough time, it will cause even the largest black holes
1182
01:31:51.680 --> 01:31:55.840
to evaporate completely. A solar mass black hole evaporates in
1183
01:31:55.880 --> 01:31:59.439
about ten to the power of sixty seven years. That's
1184
01:31:59.479 --> 01:32:03.840
one fall by sixty seven zeros. To put this in perspective,
1185
01:32:04.800 --> 01:32:09.640
the universe is currently only about ten to the power
1186
01:32:10.279 --> 01:32:14.960
of ten years old fourteen billion years. A solar mass
1187
01:32:15.039 --> 01:32:20.680
black hole lasts about ten thousand, trillion, trillion, trillion, trillion
1188
01:32:20.840 --> 01:32:24.800
trillion times longer than the current age of the universe.
1189
01:32:25.279 --> 01:32:28.359
A supermassive black hole, like the one at the center
1190
01:32:28.399 --> 01:32:32.840
of our galaxy takes even longer, around ten to the
1191
01:32:32.880 --> 01:32:37.119
power of one hundred years. These numbers are so large
1192
01:32:37.439 --> 01:32:42.439
they become meaningless. We're talking about time scales that are
1193
01:32:42.520 --> 01:32:45.640
to the age of the universe, as the age of
1194
01:32:45.680 --> 01:32:49.560
the universe is to a single second. By the time
1195
01:32:49.640 --> 01:32:54.119
all black holes have evaporated, the universe will be unimaginably
1196
01:32:54.159 --> 01:33:00.880
old and unimaginably empty. Eventually, even protons might decay, though
1197
01:33:00.920 --> 01:33:05.800
this is speculative. Some theories of particle physics predict that
1198
01:33:05.920 --> 01:33:10.119
protons are unstable with a lifetime of ten to the
1199
01:33:10.119 --> 01:33:14.000
power of forty years or so. We've never observed proton decay,
1200
01:33:14.520 --> 01:33:18.279
so if it happens, it must be incredibly rare. But
1201
01:33:18.359 --> 01:33:23.279
given enough time, if protons do decay, then eventually all
1202
01:33:23.359 --> 01:33:28.199
normal matter will dissolve into radiation. The universe will contain
1203
01:33:28.279 --> 01:33:32.079
nothing but a thin mist of photons and other fundamental particles,
1204
01:33:32.319 --> 01:33:37.279
growing more dilute as space continues expanding forever. This is
1205
01:33:37.359 --> 01:33:40.760
called the heat death of the universe. A state of
1206
01:33:40.800 --> 01:33:45.039
maximum entropy where no energy differences exist, no work can
1207
01:33:45.079 --> 01:33:50.600
be done, and nothing interesting ever happens. The universe doesn't
1208
01:33:50.760 --> 01:33:57.079
end with a bang. It fades into an eternal, unchanging darkness.
1209
01:33:57.159 --> 01:34:00.600
All the stars, all the galaxies, all the black coals,
1210
01:34:01.039 --> 01:34:04.520
or the structure we see today will have dissolved into
1211
01:34:04.520 --> 01:34:10.119
a uniform sea of particles spreading ever farther apart through
1212
01:34:10.119 --> 01:34:15.119
an eternally expanding space in this far future, in this
1213
01:34:15.479 --> 01:34:20.560
heat death scenario, the universe approaches a state where nothing
1214
01:34:20.680 --> 01:34:26.079
changes anymore. There are no temperature differences to drive heat flows,
1215
01:34:26.720 --> 01:34:31.520
there's no available energy to do work. Entropy, the measure
1216
01:34:31.560 --> 01:34:36.479
of disorder, has been maximized. Everything that could happen has happened.
1217
01:34:36.760 --> 01:34:41.079
Time itself becomes meaningless because nothing changes from one moment
1218
01:34:41.479 --> 01:34:45.920
to the next. That's if dark energy is constant. If
1219
01:34:45.920 --> 01:34:49.760
it's quintessence that grows stronger with time, the ending could
1220
01:34:49.840 --> 01:34:54.560
be more dramatic. If dark energy's repulsive force keeps growing,
1221
01:34:55.399 --> 01:35:00.319
it will eventually overcome not just cosmic expansion, but also
1222
01:35:00.439 --> 01:35:05.840
the forces holding galaxies together, then solar systems, then planets,
1223
01:35:06.159 --> 01:35:11.319
then atoms. In this big rip scenario, the universe's expansion
1224
01:35:11.439 --> 01:35:17.840
accelerates so rapidly that everything gets torn apart. First, galaxy
1225
01:35:17.920 --> 01:35:21.960
clusters unbind. The galaxies that are currently held together by
1226
01:35:22.000 --> 01:35:27.239
their mutual gravity drift apart as dark energy overcomes that gravity.
1227
01:35:27.760 --> 01:35:33.359
Then individual galaxies unbind and their stars scatter. Our Milky Way,
1228
01:35:33.640 --> 01:35:37.319
which has held together for over thirteen billion years, would
1229
01:35:37.359 --> 01:35:41.720
fly apart, its hundreds of billions of stars flung in
1230
01:35:41.840 --> 01:35:46.079
all directions. Then solar systems come apart as planets are
1231
01:35:46.119 --> 01:35:49.560
ripped away from their stars. The Earth would be torn
1232
01:35:49.600 --> 01:35:52.439
from its orbit around the Sun and flung into the
1233
01:35:52.479 --> 01:35:57.439
cold darkness of space. Then planets themselves are pulled apart
1234
01:35:57.479 --> 01:36:01.399
by tidal forces. The Earth would be stretched and torn,
1235
01:36:02.039 --> 01:36:07.880
first breaking into large fragments, then smaller pieces, then individual rocks,
1236
01:36:08.399 --> 01:36:13.760
then molecules, then atoms. Eventually, even atoms are ripped apart,
1237
01:36:14.479 --> 01:36:19.239
electrons torn away from nuclei. The electromagnetic force that holds
1238
01:36:19.319 --> 01:36:23.880
atoms together would be overcome by the acceleration of space itself.
1239
01:36:24.520 --> 01:36:28.279
Matter as we know it would cease to exist, reduced
1240
01:36:28.319 --> 01:36:33.760
to a plasma of individual particles. Finally, even atomic nuclei
1241
01:36:33.960 --> 01:36:37.680
might be torn apart, Protons and neutrons split into their
1242
01:36:37.680 --> 01:36:42.840
constituent quarks. The universe ends in a cataclysm. Where space
1243
01:36:42.920 --> 01:36:47.439
expands so fast that nothing can hold together anymore. The
1244
01:36:47.439 --> 01:36:50.359
Big rip is more dramatic than the heat death, but
1245
01:36:50.439 --> 01:36:54.720
it's also quicker. Instead of the slow fade over google
1246
01:36:54.800 --> 01:36:59.119
years that heat death entails, the big rip happens relatively
1247
01:36:59.199 --> 01:37:04.359
quickly once the acceleration becomes strong enough. Current estimates if
1248
01:37:04.359 --> 01:37:07.479
the Big Rip is real, suggest it would occur perhaps
1249
01:37:07.600 --> 01:37:11.600
twenty to fifty billion years from now, still a long
1250
01:37:11.680 --> 01:37:14.920
time by human standards, but nothing compared to the time
1251
01:37:14.960 --> 01:37:19.000
scales of heat death. Most physicists think the Big Rip
1252
01:37:19.279 --> 01:37:24.960
is unlikely. Current observations suggest dark energy is probably constant
1253
01:37:25.319 --> 01:37:29.479
or nearly constant, but we can't entirely rule it out yet.
1254
01:37:30.319 --> 01:37:34.680
The fate of the universe remains uncertain. There's also a
1255
01:37:34.760 --> 01:37:40.399
fourth possibility worth mentioning, though it's quite speculative. Some theories
1256
01:37:40.439 --> 01:37:43.560
suggest that the vacuum state of the universe might not
1257
01:37:43.760 --> 01:37:47.399
be stable. We might be living in a false vacuum,
1258
01:37:47.479 --> 01:37:50.920
a local minimum of energy that appears stable but isn't
1259
01:37:50.960 --> 01:37:55.479
truly the lowest possible energy state. If that's true, then
1260
01:37:55.520 --> 01:37:59.000
at some random point in the future, a quantum fluctuation
1261
01:37:59.119 --> 01:38:02.239
could trigger a train transition to the true vacuum state.
1262
01:38:02.920 --> 01:38:06.159
This would create a bubble of true vacuum that expands
1263
01:38:06.159 --> 01:38:09.399
at the speed of light, converting the false vacuum to
1264
01:38:09.479 --> 01:38:14.479
the true vacuum. Wherever it passes inside the bubble, the
1265
01:38:14.560 --> 01:38:19.920
laws of physics might be different. Particles might have different masses,
1266
01:38:20.359 --> 01:38:23.880
forces might have different strengths. Atoms as we know them
1267
01:38:24.319 --> 01:38:27.880
might not be able to exist. This bubble would destroy
1268
01:38:28.079 --> 01:38:33.199
everything it touches, rewriting the rules of physics as it expands.
1269
01:38:34.039 --> 01:38:37.720
There would be no warning, because it expands at light speed.
1270
01:38:38.479 --> 01:38:42.039
You wouldn't see it coming. One moment you exist, the
1271
01:38:42.079 --> 01:38:46.760
next moment you don't, converted into whatever configurations of matter
1272
01:38:46.880 --> 01:38:51.039
and energy are stable in the true vacuum. This sounds
1273
01:38:51.159 --> 01:38:55.399
like science fiction, but it's a real possibility allowed by
1274
01:38:55.520 --> 01:38:59.159
quantum field theory. We have no way of knowing whether
1275
01:38:59.239 --> 01:39:04.119
our vacuum is truly stable or just meta stable, appearing
1276
01:39:04.199 --> 01:39:07.239
stable but not actually being in the lowest energy state.
1277
01:39:07.760 --> 01:39:11.319
If it's meta stable, the universe could end at any moment,
1278
01:39:12.039 --> 01:39:14.720
though the probability of this happening in any given year
1279
01:39:15.199 --> 01:39:20.279
is astronomically small. Most physicists don't lose sleep over vacuum
1280
01:39:20.319 --> 01:39:23.399
decay because there's nothing we can do about it, and
1281
01:39:23.439 --> 01:39:27.039
no evidence that it's actually a threat. It's more of
1282
01:39:27.119 --> 01:39:31.680
a theoretical curiosity than a practical concern, but it does
1283
01:39:31.760 --> 01:39:35.640
illustrate how much we still don't know about the universe's
1284
01:39:35.720 --> 01:39:40.199
fundamental nature. Let's circle back to Einstein himself and what
1285
01:39:40.319 --> 01:39:45.560
his story teaches us about science and creativity. Einstein was
1286
01:39:45.600 --> 01:39:50.279
a revolutionary thinker who changed our understanding of reality more
1287
01:39:50.319 --> 01:39:55.159
profoundly than perhaps any other single individual in history. His
1288
01:39:55.279 --> 01:40:00.560
work on special relativity, general relativity, and quantum mechanics provided
1289
01:40:00.600 --> 01:40:04.319
the foundations for modern physics. But he was also human,
1290
01:40:04.800 --> 01:40:10.199
prone to mistakes, biases, and errors of judgment. He added
1291
01:40:10.439 --> 01:40:15.439
the cosmological constant because he preferred a static universe for
1292
01:40:15.520 --> 01:40:20.920
philosophical reasons. He spent decades pursuing a unified field theory
1293
01:40:21.439 --> 01:40:26.000
that led nowhere. Because he refused to fully embrace quantum mechanics,
1294
01:40:26.800 --> 01:40:31.319
he made errors and wrong turns, just like any other scientist.
1295
01:40:31.840 --> 01:40:37.319
Einstein's relationship with quantum mechanics is particularly interesting and tragic.
1296
01:40:37.800 --> 01:40:40.880
He was one of the founders of quantum theory, winning
1297
01:40:40.920 --> 01:40:44.840
the Nobel Prize for his explanation of the photoelectric effect,
1298
01:40:45.479 --> 01:40:49.680
which showed that light behaves as particles in certain situations.
1299
01:40:50.159 --> 01:40:54.119
But as quantum mechanics developed through the nineteen twenties, taking
1300
01:40:54.159 --> 01:40:58.800
on its modern form with wave functions, probability, and uncertainty,
1301
01:40:59.279 --> 01:41:04.199
Einstein and became increasingly uncomfortable with where the theory was heading.
1302
01:41:04.760 --> 01:41:08.000
He famously declared that God does not play dice with
1303
01:41:08.079 --> 01:41:13.279
the universe, expressing his discomfort with the probabilistic nature of
1304
01:41:13.359 --> 01:41:17.199
quantum mechanics. In the classical physics he grew up with,
1305
01:41:17.720 --> 01:41:22.039
everything was deterministic. If you knew the exact state of
1306
01:41:22.039 --> 01:41:25.600
a system at one moment, you could, in principle calculate
1307
01:41:25.640 --> 01:41:29.439
its state at any future moment. Quantum mechanics said no,
1308
01:41:30.239 --> 01:41:34.920
the best you can do is calculate probabilities. The universe
1309
01:41:35.560 --> 01:41:39.960
is fundamentally uncertain at its core. Einstein thought this couldn't
1310
01:41:39.960 --> 01:41:44.439
be the final word. He believed quantum mechanics must be incomplete,
1311
01:41:44.960 --> 01:41:49.359
that there must be deeper variables, hidden mechanisms that would
1312
01:41:49.439 --> 01:41:53.880
restore determinism and certainty if only we could discover them.
1313
01:41:54.439 --> 01:41:57.680
He spent enormous effort trying to prove quantum mechanics was
1314
01:41:57.720 --> 01:42:02.479
incomplete or contradictory divine in thought experiments meant to expose
1315
01:42:02.520 --> 01:42:07.279
its problems. The most famous of these was the EPR paradox,
1316
01:42:07.640 --> 01:42:11.600
named after Einstein, Podolski, and Rosen, who published it in
1317
01:42:11.680 --> 01:42:16.000
nineteen thirty five. The thought experiment showed that quantum mechanics
1318
01:42:16.039 --> 01:42:21.279
seemed to allow instantaneous influences between distant particles, what Einstein
1319
01:42:21.359 --> 01:42:26.640
mockingly called spooky action at a distance. He thought this
1320
01:42:26.760 --> 01:42:32.479
proved quantum mechanics must be wrong or incomplete, but experiments
1321
01:42:32.560 --> 01:42:37.159
later showed that Einstein was wrong. The spooky action at
1322
01:42:37.199 --> 01:42:42.279
a distance is real. Quantum entanglement, as it's now called,
1323
01:42:42.640 --> 01:42:47.840
has been demonstrated countless times. It's not just theoretical. It's
1324
01:42:47.880 --> 01:42:52.439
been measured, tested, and even used in practical applications like
1325
01:42:52.560 --> 01:42:58.359
quantum cryptography and quantum computing. Einstein's intuition that it couldn't
1326
01:42:58.399 --> 01:43:03.119
be real turned out to be mistaken. The universe really
1327
01:43:03.640 --> 01:43:08.479
is that strange. So Einstein spent the last thirty years
1328
01:43:08.520 --> 01:43:12.199
of his life pursuing a unified field theory, trying to
1329
01:43:12.279 --> 01:43:17.399
merge gravity and electromagnetism into a single framework while ignoring
1330
01:43:17.479 --> 01:43:22.439
quantum mechanics. Meanwhile, the physics community moved on without him.
1331
01:43:22.840 --> 01:43:27.960
Younger physicists, developed quantum field theory, figured out the weak
1332
01:43:28.079 --> 01:43:33.079
and strong nuclear forces, discovered new particles, and built the
1333
01:43:33.159 --> 01:43:37.680
standard model of particle physics. All of this passed Einstein
1334
01:43:37.840 --> 01:43:41.000
by because he was working in isolation on a problem
1335
01:43:41.199 --> 01:43:44.079
that couldn't be solved with the tools he was using.
1336
01:43:44.520 --> 01:43:47.960
What made Einstein's special wasn't that he was always right.
1337
01:43:48.640 --> 01:43:53.399
It was his willingness to think differently, to question assumptions
1338
01:43:53.399 --> 01:43:56.720
that everyone else took for granted, and to follow his
1339
01:43:56.840 --> 01:44:00.640
mathematical and physical intuition, even when when it led to
1340
01:44:00.720 --> 01:44:06.880
bizarre conclusions. He imagined riding alongside light beams as a teenager,
1341
01:44:07.600 --> 01:44:12.079
he wondered what would happen if space and time weren't absolute.
1342
01:44:12.159 --> 01:44:15.479
He asked what gravity would look like if it wasn't
1343
01:44:15.520 --> 01:44:19.880
a force but a curvature of space time. These thought experiments,
1344
01:44:20.239 --> 01:44:25.159
these acts of imagination, these willingness to question everything, that's
1345
01:44:25.239 --> 01:44:29.439
what led to his greatest discoveries, and those same qualities
1346
01:44:29.960 --> 01:44:33.920
led to his mistakes. He questioned whether the universe had
1347
01:44:33.920 --> 01:44:38.319
to be dynamic. He imagined it could be static and eternal.
1348
01:44:39.319 --> 01:44:42.439
He followed that intuition and added a term to make
1349
01:44:42.479 --> 01:44:47.920
his equations match his vision. The lesson is that mistakes
1350
01:44:48.520 --> 01:44:52.199
are part of the process. You can't be at the
1351
01:44:52.239 --> 01:44:56.520
cutting edge of knowledge without sometimes being wrong. The key
1352
01:44:56.880 --> 01:45:01.439
is to let observations and experiments guide, to be willing
1353
01:45:01.479 --> 01:45:05.520
to change your mind when the evidence doesn't match your expectations,
1354
01:45:06.039 --> 01:45:09.479
and to keep pushing forward even when you make errors.
1355
01:45:09.920 --> 01:45:13.720
Einstein did all of this. He made mistakes, but he
1356
01:45:13.800 --> 01:45:18.600
corrected them when better information became available. He pursued dead ends,
1357
01:45:19.279 --> 01:45:23.159
but he never stopped thinking about fundamental questions, and some
1358
01:45:23.199 --> 01:45:27.560
of his mistakes, like the cosmological constant, turned out to
1359
01:45:27.600 --> 01:45:32.159
contain deep truths that he couldn't have anticipated. Science is
1360
01:45:32.279 --> 01:45:36.640
fundamentally a collective enterprise, even though we often focus on
1361
01:45:36.840 --> 01:45:42.560
individual genius. Einstein built on the work of Maxwell, Lorentz, Pointcarret,
1362
01:45:42.680 --> 01:45:45.840
and others who had been struggling with the same problems
1363
01:45:45.880 --> 01:45:51.119
he solved. Hubble built on Levitt's work on cepheid variables.
1364
01:45:52.119 --> 01:45:56.000
The supernova teams that discovered dark energy built on decades
1365
01:45:56.000 --> 01:46:02.079
of work by astronomers measuring distances and mapping galaxies. Every
1366
01:46:02.199 --> 01:46:05.920
discovery stands on the shoulders of what came before, but
1367
01:46:06.039 --> 01:46:10.159
science also needs individuals willing to take big risks to
1368
01:46:10.239 --> 01:46:14.800
propose radical ideas to question the foundations of what everyone
1369
01:46:14.880 --> 01:46:20.720
thinks they know. Einstein was exceptional at this. He had
1370
01:46:20.760 --> 01:46:24.319
the mathematical skill to work out the details of his ideas,
1371
01:46:24.960 --> 01:46:28.439
but more importantly, he had the physical intuition to know
1372
01:46:28.600 --> 01:46:32.279
which questions to ask, and the courage to follow wherever
1373
01:46:32.359 --> 01:46:37.880
those questions led. This is the nature of science. It's messy, uncertain,
1374
01:46:38.239 --> 01:46:41.479
full of wrong turns and dead ends, but it's also
1375
01:46:41.520 --> 01:46:46.760
self correcting. Bad ideas get weeded out by observation and experiment.
1376
01:46:47.359 --> 01:46:52.439
Good ideas survive and become stronger as more evidence accumulates,
1377
01:46:52.920 --> 01:46:55.840
and sometimes ideas that seemed bad turn out to be
1378
01:46:55.920 --> 01:46:59.600
good after all when viewed from a different perspective. The
1379
01:46:59.680 --> 01:47:04.079
cosmological constant story is a perfect example of this process.
1380
01:47:04.600 --> 01:47:09.560
Einstein added it for wrong reasons, it was removed as unnecessary.
1381
01:47:10.199 --> 01:47:14.000
It was brought back when new observations demanded it. Now
1382
01:47:14.119 --> 01:47:19.359
it's a central part of cosmology, describing the dominant component
1383
01:47:19.520 --> 01:47:24.520
of the universe's energy content. The story has come full circle,
1384
01:47:25.119 --> 01:47:29.000
but in an unexpected way that nobody could have predicted.
1385
01:47:29.560 --> 01:47:32.800
What does this tell us about the nature of physical
1386
01:47:32.920 --> 01:47:38.319
law and our ability to understand the universe? On one level,
1387
01:47:38.840 --> 01:47:43.640
it's humbling. Even Einstein, working at the peak of his powers,
1388
01:47:44.119 --> 01:47:48.439
couldn't foresee all the implications of his equations. He added
1389
01:47:48.840 --> 01:47:54.039
and removed the cosmological constant based on incomplete information and
1390
01:47:54.119 --> 01:47:59.960
philosophical preferences. He missed the opportunity to predict the universe's expansion.
1391
01:48:00.560 --> 01:48:04.680
But on another level, it's empowering. The fact that the
1392
01:48:04.760 --> 01:48:09.000
equations contained the right answer all along, that the mathematics
1393
01:48:09.079 --> 01:48:13.159
knew more than Einstein did, suggests that there really are
1394
01:48:13.399 --> 01:48:17.760
underlying laws of nature waiting to be discovered. The universe
1395
01:48:17.920 --> 01:48:23.680
operates according to mathematical principles. Our equations, when we get
1396
01:48:23.680 --> 01:48:28.239
them right, are discovering these principles, rather than just inventing
1397
01:48:28.319 --> 01:48:33.479
convenient fictions. General relativity has now been tested for over
1398
01:48:33.520 --> 01:48:38.600
a century. It's past every test. It's made predictions that
1399
01:48:38.680 --> 01:48:42.159
seemed absurd at the time but turned out to be correct.
1400
01:48:43.079 --> 01:48:49.119
Black holes exist, gravitational waves exist, time runs at different
1401
01:48:49.199 --> 01:48:54.920
rates in different gravitational fields, Light bends around massive objects.
1402
01:48:55.840 --> 01:49:01.600
The universe is expanding, and that expansion is excel All
1403
01:49:01.640 --> 01:49:07.319
of this follows from Einstein's equations. That doesn't mean general
1404
01:49:07.399 --> 01:49:12.039
relativity is the final word. We know it must be
1405
01:49:12.199 --> 01:49:17.600
incomplete because it doesn't include quantum mechanics. We know there
1406
01:49:17.600 --> 01:49:20.760
are puzzles it can't solve, like what happens at the
1407
01:49:20.800 --> 01:49:23.840
center of a black hole or what happened at the
1408
01:49:23.960 --> 01:49:28.560
moment of the Big Bang. But within its domain of applicability,
1409
01:49:29.079 --> 01:49:35.279
general relativity works. It describes reality. The mathematics maps onto
1410
01:49:35.319 --> 01:49:39.600
the physical world in a deep and precise way. This
1411
01:49:39.760 --> 01:49:42.520
is one of the great mysteries of physics and mathematics.
1412
01:49:43.079 --> 01:49:47.159
Why does mathematics work so well to describe nature? Why
1413
01:49:47.199 --> 01:49:51.680
do equations written on paper correspond so precisely to measurements
1414
01:49:51.720 --> 01:49:56.159
made with instruments. There's no obvious reason why the universe
1415
01:49:56.159 --> 01:50:01.279
should be mathematical, but it is galic Us' orbit according
1416
01:50:01.319 --> 01:50:09.119
to mathematical laws. Light follows mathematical paths. Particles interact according
1417
01:50:09.199 --> 01:50:16.520
to mathematical rules. Everything we've studied follows mathematical patterns. Eugene Wigner,
1418
01:50:17.039 --> 01:50:19.840
a physicist who won the Nobel Prize for his work
1419
01:50:19.880 --> 01:50:24.560
on quantum mechanics, wrote a famous essay called The Unreasonable
1420
01:50:24.640 --> 01:50:29.880
Effectiveness of Mathematics in the Natural Sciences. He marveled at
1421
01:50:29.880 --> 01:50:35.439
how mathematical concepts developed for purely abstract reasons often turn
1422
01:50:35.479 --> 01:50:41.039
out to be exactly what's needed to describe physical phenomena.
1423
01:50:41.439 --> 01:50:47.560
Complex numbers, group theory, differential geometry. All of these mathematical
1424
01:50:47.640 --> 01:50:52.920
tools were invented by mathematicians following their own esthetic sense
1425
01:50:53.279 --> 01:50:58.600
of what was interesting or elegant. Later physicists found these
1426
01:50:58.640 --> 01:51:02.600
tools were exactly what they needed to describe quantum mechanics,
1427
01:51:03.000 --> 01:51:07.600
particle physics, and relativity. Why should this be the case.
1428
01:51:08.640 --> 01:51:12.359
Why should the pure mathematics created by human minds match
1429
01:51:12.399 --> 01:51:16.640
the structure of external reality. It's a deep question without
1430
01:51:16.640 --> 01:51:20.479
a clear answer. Maybe our minds are somehow tuned by
1431
01:51:20.479 --> 01:51:24.920
evolution to think mathematically because that reflects how the universe
1432
01:51:25.039 --> 01:51:30.119
actually works. Maybe there's something fundamental about mathematical structure that
1433
01:51:30.239 --> 01:51:34.680
physics must follow. Or maybe we're selecting the mathematical frameworks
1434
01:51:34.720 --> 01:51:39.079
that work and ignoring the countless ones that don't, creating
1435
01:51:39.399 --> 01:51:44.840
an illusion of unreasonable effectiveness. Whatever the answer, the fact
1436
01:51:44.920 --> 01:51:50.479
remains that mathematics is our most powerful tool for understanding
1437
01:51:50.520 --> 01:51:58.359
the cosmos. Einstein's equations are mathematical. The cosmological constant is
1438
01:51:58.399 --> 01:52:03.119
a mathematical term. Dark energy is observed through measurements that
1439
01:52:03.159 --> 01:52:07.800
are then fit to mathematical models. All of our understanding
1440
01:52:07.840 --> 01:52:12.279
of the universe flows through this mathematical language. Let's talk
1441
01:52:12.279 --> 01:52:15.199
about one more aspect of this story that's worth reflecting,
1442
01:52:15.239 --> 01:52:21.239
on the role of technology in driving scientific discovery. Hubble's
1443
01:52:21.439 --> 01:52:27.840
observations that revealed the expanding universe were only possible because
1444
01:52:27.880 --> 01:52:32.119
of the one hundred inch telescope at Mount Wilson, which
1445
01:52:32.239 --> 01:52:35.960
was the largest and most powerful telescope in the world
1446
01:52:36.039 --> 01:52:40.239
at the time. Without that technology, without the ability to
1447
01:52:40.279 --> 01:52:45.279
see faint distant galaxies and measure their spectra precisely, Hubble
1448
01:52:45.479 --> 01:52:50.520
couldn't have made his discovery. Similarly, the supernova observations that
1449
01:52:50.600 --> 01:52:56.439
revealed dark energy required large telescopes, sensitive digital cameras, and
1450
01:52:56.560 --> 01:53:01.039
powerful computers to process the data. The detection of gravitational
1451
01:53:01.079 --> 01:53:05.159
waves required the Ligo experiment, a marvel of engineering that
1452
01:53:05.199 --> 01:53:08.840
can measure distances changing by less than the width of
1453
01:53:08.880 --> 01:53:13.279
a proton across arms four kilometers or two point five
1454
01:53:13.359 --> 01:53:18.600
miles long. The cosmic microwave background was mapped by satellites
1455
01:53:18.680 --> 01:53:24.760
like COBE, WMAP and PLANK that measured temperature variations of
1456
01:53:24.800 --> 01:53:28.079
a few millions of a degree across the sky. Every
1457
01:53:28.439 --> 01:53:33.279
major advance in cosmology has been enabled by technological progress.
1458
01:53:34.079 --> 01:53:38.600
Better telescopes let us see farther and more clearly. Better
1459
01:53:38.680 --> 01:53:43.600
detectors let us measure fainter signals. Better computers let us
1460
01:53:43.680 --> 01:53:49.119
analyze more data and run more sophisticated simulations. The discoveries
1461
01:53:49.159 --> 01:53:54.600
don't come from technology alone. You need ideas, theories, questions
1462
01:53:54.920 --> 01:53:58.680
to guide what you're looking for, but the technology is
1463
01:53:58.720 --> 01:54:02.960
what lets you actually look. This creates an interesting dynamic.
1464
01:54:03.640 --> 01:54:10.439
Theorists like Einstein develop mathematical frameworks that make predictions. Observers
1465
01:54:10.439 --> 01:54:15.720
and experimentalists build the instruments to test those predictions. When
1466
01:54:15.760 --> 01:54:21.880
observations confirm theory, both are strengthened. When observations contradict theory,
1467
01:54:22.359 --> 01:54:26.199
both have to adapt. Theory guides what to look for,
1468
01:54:26.479 --> 01:54:31.760
but observations determine what's actually true. The cosmological constant sat
1469
01:54:31.800 --> 01:54:37.119
in Einstein's equations for decades before technology advanced enough to
1470
01:54:37.239 --> 01:54:40.960
detect its effects. The super and Over observations of the
1471
01:54:41.039 --> 01:54:46.039
nineteen nineties required digital cameras that didn't exist in Einstein's time.
1472
01:54:46.600 --> 01:54:49.840
If he'd lived to see these observations, he would have
1473
01:54:49.920 --> 01:54:54.039
learned that his blunder was actually correct, that the constant
1474
01:54:54.119 --> 01:54:58.680
he added and removed, really did describe something fundamental about
1475
01:54:58.720 --> 01:55:02.800
the universe. This back and forth between theory and observation,
1476
01:55:03.439 --> 01:55:08.680
between mathematics and measurement, between prediction and conformation is the
1477
01:55:08.720 --> 01:55:14.960
engine that drives physics forward. Neither theory nor observation alone
1478
01:55:15.520 --> 01:55:21.239
is sufficient. You need both working together, challenging each other,
1479
01:55:22.000 --> 01:55:26.479
refining understanding through iteration. As we look to the future,
1480
01:55:27.119 --> 01:55:30.720
the next generation of telescopes and experiments will test our
1481
01:55:30.800 --> 01:55:37.039
current understanding even more rigorously. The VERA Rubin Observatory will
1482
01:55:37.079 --> 01:55:42.760
survey the entire visible sky every few nights, discovering millions
1483
01:55:42.840 --> 01:55:47.119
of new objects and tracking how they change with time.
1484
01:55:47.640 --> 01:55:51.319
The James Webb Space Telescope is already finding the most
1485
01:55:51.359 --> 01:55:54.720
distant galaxies in the universe, seeing them as they were
1486
01:55:54.760 --> 01:55:58.319
in the first few hundred million years after the Big Bang.
1487
01:55:58.840 --> 01:56:03.600
The Square kilometre Array, when complete, will be the world's
1488
01:56:03.720 --> 01:56:09.479
largest radio telescope, studying everything from the cosmic microwave background
1489
01:56:09.920 --> 01:56:13.479
to the formation of the first stars to the distribution
1490
01:56:13.600 --> 01:56:18.319
of neutral hydrogen throughout the universe. The thirty meter Telescope
1491
01:56:18.560 --> 01:56:22.640
and the extremely Large Telescope will have mirrors so large
1492
01:56:23.000 --> 01:56:26.760
they can see Earth like planets around nearby stars and
1493
01:56:26.880 --> 01:56:30.399
study the atmospheres of those planets for signs of life.
1494
01:56:30.960 --> 01:56:35.840
Each of these instruments will test our understanding of cosmology,
1495
01:56:36.520 --> 01:56:41.880
dark energy, and general relativity in new ways. They might
1496
01:56:41.960 --> 01:56:47.600
confirm that dark energy is truly a cosmological constant. They
1497
01:56:47.680 --> 01:56:51.399
might reveal that it changes with time. They might find
1498
01:56:51.439 --> 01:56:56.000
evidence that general relativity breaks down on the largest scales.
1499
01:56:57.039 --> 01:57:02.359
Or they might discover something entirely unacted, something nobody has
1500
01:57:02.399 --> 01:57:07.720
even imagined yet. That's the exciting part about science. You
1501
01:57:07.800 --> 01:57:12.720
build your best theories based on current knowledge. You make predictions,
1502
01:57:13.159 --> 01:57:18.359
you test them, and sometimes the universe surprises you. Sometimes
1503
01:57:18.359 --> 01:57:21.640
you find something you weren't looking for, something that doesn't
1504
01:57:21.640 --> 01:57:26.039
fit any existing framework, something that forces you to rethink everything.
1505
01:57:26.439 --> 01:57:32.359
Dark energy was one of those surprises. Nobody expected the
1506
01:57:32.520 --> 01:57:37.680
universe's expansion to be accelerating. The supernova teams were trying
1507
01:57:37.680 --> 01:57:41.720
to measure how much the expansion was slowing down, not
1508
01:57:41.800 --> 01:57:45.560
whether it was speeding up. The discovery was a complete shock,
1509
01:57:46.319 --> 01:57:51.000
forcing cosmologists to revise their understanding of the universe's composition
1510
01:57:51.119 --> 01:57:55.000
and fate. There will be more surprises. We don't know
1511
01:57:55.159 --> 01:57:59.239
what they'll be or when they'll come. But history shows
1512
01:57:59.279 --> 01:58:01.720
that every every time we look at the universe in
1513
01:58:01.800 --> 01:58:06.399
a new way, with new instruments or new techniques, we
1514
01:58:06.479 --> 01:58:11.640
find something unexpected. The universe is more strange, more beautiful,
1515
01:58:12.000 --> 01:58:17.520
and more surprising than we imagine. Einstein's cosmological constant teaches
1516
01:58:17.600 --> 01:58:21.640
us to stay humble. Even the greatest minds can miss
1517
01:58:21.640 --> 01:58:26.159
things or get things wrong. But it also teaches us
1518
01:58:26.560 --> 01:58:32.359
to stay curious. The universe has secrets waiting to be discovered.
1519
01:58:32.840 --> 01:58:38.359
Mathematics gives us a language to describe those secrets, Observations
1520
01:58:38.439 --> 01:58:42.680
test whether our descriptions are correct, and technology gives us
1521
01:58:42.720 --> 01:58:46.279
new eyes to see what was previously hidden. In the end,
1522
01:58:46.760 --> 01:58:51.119
Einstein's biggest mistake wasn't really a mistake at all. It
1523
01:58:51.239 --> 01:58:55.319
was a step in the long process of understanding the cosmos.
1524
01:58:55.840 --> 01:58:59.079
He added a term to his equations for wrong reasons,
1525
01:58:59.680 --> 01:59:02.840
but the the term turned out to be necessary for
1526
01:59:03.000 --> 01:59:06.520
right reasons. He removed it when he thought it was wrong,
1527
01:59:07.199 --> 01:59:11.520
but it came back when observations demanded it. The story
1528
01:59:11.640 --> 01:59:19.359
has ambiguity, uncertainty, and surprise, just like science itself. As
1529
01:59:19.399 --> 01:59:22.760
we close, let's think about what this means for us,
1530
01:59:23.439 --> 01:59:26.479
for people living in the early twenty first century, trying
1531
01:59:26.520 --> 01:59:30.600
to understand our place in the cosmos. We live at
1532
01:59:30.640 --> 01:59:34.479
a remarkable moment in history. For the first time, our
1533
01:59:34.600 --> 01:59:39.840
species can see almost the entire observable universe. We can
1534
01:59:39.920 --> 01:59:43.239
look back in time to when the first stars formed.
1535
01:59:43.920 --> 01:59:47.760
We can map the distribution of galaxies across billions of
1536
01:59:47.880 --> 01:59:52.760
light years. We can measure the fundamental parameters that determine
1537
01:59:52.800 --> 01:59:57.319
the universe's composition and fate. We've learned that the universe
1538
01:59:57.600 --> 02:00:01.840
is larger, older, and stranger than anyone imagined just a
1539
02:00:01.960 --> 02:00:06.399
century ago. We've learned that most of what exists is
1540
02:00:06.439 --> 02:00:11.159
invisible to us, dark matter and dark energy that we
1541
02:00:11.239 --> 02:00:15.880
detect only through their gravitational effects. We've learned that space
1542
02:00:15.920 --> 02:00:21.079
itself is dynamic, expanding and carrying galaxies with it. We've
1543
02:00:21.159 --> 02:00:24.560
learned that time is not absolute, that it runs at
1544
02:00:24.560 --> 02:00:29.079
different rates depending on motion and gravity. The story of
1545
02:00:29.119 --> 02:00:33.199
how we learned all this is remarkable in itself. It
1546
02:00:33.359 --> 02:00:38.399
involved countless scientists working over generations, each contributing a piece
1547
02:00:38.439 --> 02:00:44.000
to the puzzle. Some, like Einstein and Hubble, made revolutionary
1548
02:00:44.039 --> 02:00:50.439
discoveries that changed everything. Others made smaller but still essential contributions,
1549
02:00:51.000 --> 02:00:57.920
developing techniques, building instruments, making careful observations that constrained theories,
1550
02:00:58.439 --> 02:01:03.720
and tested predictions. The cosmological constant story shows us that
1551
02:01:03.800 --> 02:01:07.960
science isn't a straight line from ignorance to knowledge. It's
1552
02:01:08.000 --> 02:01:12.479
a winding path with many side trails, dead ends, and
1553
02:01:12.720 --> 02:01:21.119
unexpected turns. Einstein added the constant, then removed it. Decades passed,
1554
02:01:21.720 --> 02:01:26.760
then observations demanded its return. But now it meant something different,
1555
02:01:27.439 --> 02:01:32.159
described something different. The journey from Einstein's static universe to
1556
02:01:32.239 --> 02:01:37.159
our accelerating universe took almost a century and involved thousands
1557
02:01:37.159 --> 02:01:41.960
of scientists. All of this understanding rests on Einstein's work.
1558
02:01:43.039 --> 02:01:47.119
General Relativity provides the framework we use to interpret our
1559
02:01:47.159 --> 02:01:53.960
observations and understand the cosmos. The cosmological constant, Einstein's blunder
1560
02:01:54.000 --> 02:01:57.359
that wasn't really a blunder, describes the dominant form of
1561
02:01:57.560 --> 02:02:01.520
energy in the universe. Every time you hear about dark energy,
1562
02:02:01.680 --> 02:02:06.039
or the accelerating expansion, or the ultimate fate of the cosmos,
1563
02:02:06.880 --> 02:02:12.880
your hearing about Einstein's legacy. When Einstein published General Relativity
1564
02:02:13.039 --> 02:02:16.840
in nineteen fifteen, he couldn't have imagined where it would lead.
1565
02:02:17.760 --> 02:02:22.399
He was solving a specific problem. How to incorporate gravity
1566
02:02:22.479 --> 02:02:25.680
into his theory of space time. He wasn't trying to
1567
02:02:25.760 --> 02:02:30.199
describe the entire universe, or predict dark energy or explain
1568
02:02:30.319 --> 02:02:34.439
the Big Bang. These applications of his theory came later,
1569
02:02:34.840 --> 02:02:38.600
as other physicists and astronomers built on his foundation. This
1570
02:02:38.720 --> 02:02:42.359
is typical of great scientific theories. They often turn out
1571
02:02:42.399 --> 02:02:45.640
to be more powerful and more general than their creators imagined.
1572
02:02:46.199 --> 02:02:50.680
Newton's laws of motion were developed to explain planetary orbits,
1573
02:02:50.960 --> 02:02:56.439
but they also describe billiard balls, pendulums, and rocket trajectories.
1574
02:02:57.039 --> 02:03:04.000
Maxwell's equations of electromagnetism developed to unify electricity and magnetism,
1575
02:03:04.239 --> 02:03:10.000
but they also predicted radio waves, which Maxwell himself didn't anticipate.
1576
02:03:10.520 --> 02:03:14.800
Darwin's theory of evolution by natural selection was developed to
1577
02:03:14.920 --> 02:03:19.560
explain the diversity of life, but it also provides insights
1578
02:03:19.600 --> 02:03:25.439
into psychology, sociology, and even computer science through genetic algorithms.
1579
02:03:26.000 --> 02:03:32.479
Einstein's general relativity has proven remarkably robust and versatile. It
1580
02:03:32.560 --> 02:03:35.920
describes the orbit of mercury around the Sun with its
1581
02:03:36.000 --> 02:03:40.920
tiny deviations from Newton's predictions. It describes the bending of
1582
02:03:41.000 --> 02:03:46.600
light around massive objects. It describes gravitational waves from colliding
1583
02:03:46.640 --> 02:03:51.119
black holes billions of light years away. It describes the
1584
02:03:51.159 --> 02:03:55.159
expansion of the entire universe and predicts its ultimate fate,
1585
02:03:55.680 --> 02:04:00.560
all from a single elegant mathematical framework that I. Einstein
1586
02:04:00.720 --> 02:04:05.520
worked out by thinking deeply about the nature of gravity, space,
1587
02:04:06.039 --> 02:04:09.800
and time. But we're also at the beginning of understanding,
1588
02:04:09.880 --> 02:04:13.159
not the end. We don't know what dark energy really is.
1589
02:04:13.479 --> 02:04:16.479
We don't know what dark matter is. We don't know
1590
02:04:16.520 --> 02:04:21.399
how to reconcile general relativity with quantum mechanics. We don't
1591
02:04:21.439 --> 02:04:24.520
know what happened at the exact moment of the Big Bang.
1592
02:04:25.319 --> 02:04:28.359
We don't know if there are other universes beyond our own.
1593
02:04:28.880 --> 02:04:31.000
We don't know whether the laws of physics could have
1594
02:04:31.039 --> 02:04:34.600
been different, or why they're the specific laws we observe.
1595
02:04:35.319 --> 02:04:40.520
These mysteries aren't failures of science. They're the frontier, the
1596
02:04:40.680 --> 02:04:43.960
edge of current knowledge, where the next generation of discoveries
1597
02:04:44.039 --> 02:04:49.880
will be made. Every answer raises new questions, every solution
1598
02:04:50.319 --> 02:04:55.840
reveals new puzzles. That's how science progresses. We solve one
1599
02:04:55.920 --> 02:05:00.399
mystery only to find three more hiding behind it. Consider
1600
02:05:00.439 --> 02:05:04.119
what we've learned just in the last few decades. In
1601
02:05:04.239 --> 02:05:08.760
nineteen ninety we knew of no planets outside our Solar system.
1602
02:05:09.000 --> 02:05:14.079
Now we've confirmed over five thousand exoplanets, with thousands more
1603
02:05:14.119 --> 02:05:20.840
candidates waiting. In nineteen ninety, gravitational waves were still theoretical predictions.
1604
02:05:21.600 --> 02:05:25.439
Now we detect them regularly from colliding black holes and
1605
02:05:25.520 --> 02:05:31.720
neutron stars. In nineteen ninety, dark energy was unknown. Now
1606
02:05:31.760 --> 02:05:36.119
we know it's the dominant component of the universe. What
1607
02:05:36.279 --> 02:05:39.640
will we learn in the next few decades? What discover
1608
02:05:39.800 --> 02:05:42.600
is a weight that we can't even imagine. The James
1609
02:05:42.600 --> 02:05:46.479
Web Space Telescope is already finding galaxies in the early
1610
02:05:46.640 --> 02:05:52.199
universe that challenge our understanding of galaxy formation. Future telescopes
1611
02:05:52.239 --> 02:05:56.960
will see even farther and more clearly. New experiments will
1612
02:05:56.960 --> 02:06:02.039
test fundamental physics in new regimes. We might discover what
1613
02:06:02.159 --> 02:06:08.039
dark matter actually is. We might detect quantum gravitational effects.
1614
02:06:08.640 --> 02:06:12.119
We might find life on other worlds. These are the
1615
02:06:12.279 --> 02:06:17.640
great questions of our time. Future generations will have better
1616
02:06:17.720 --> 02:06:21.239
answers than we do, just as we have better answers
1617
02:06:21.399 --> 02:06:27.720
than Einstein did. The process continues. Each generation builds on
1618
02:06:27.800 --> 02:06:34.079
what came before, correcting mistakes, refining understanding, pushing further into
1619
02:06:34.119 --> 02:06:38.359
the unknown. Einstein showed us that even mistakes can be
1620
02:06:38.399 --> 02:06:42.439
productive if you approach them with honesty and willingness to
1621
02:06:42.640 --> 02:06:46.920
change your mind when evidence demands it. He added, the
1622
02:06:46.960 --> 02:06:52.479
cosmological constant removed it, and eventually it came back because
1623
02:06:52.520 --> 02:06:57.399
the universe itself insisted on its reality. The story is
1624
02:06:57.520 --> 02:07:01.199
messy and human, but it led to found insights about
1625
02:07:01.239 --> 02:07:06.800
the nature of reality. His legacy extends beyond specific discoveries
1626
02:07:07.239 --> 02:07:12.239
or equations. He demonstrated that the universe can be understood
1627
02:07:12.640 --> 02:07:17.720
through mathematics and reason. Reality is often stranger than we imagine.
1628
02:07:18.000 --> 02:07:21.760
Common sense can mislead us, and only careful observation and
1629
02:07:21.840 --> 02:07:27.880
rigorous theory reveal the truth. Intellectual courage is essential for progress.
1630
02:07:28.479 --> 02:07:31.800
You must be willing to question everything, even the most
1631
02:07:31.840 --> 02:07:37.560
basic assumptions, to make revolutionary discoveries. Einstein also showed us
1632
02:07:37.600 --> 02:07:42.800
the limits of individual genius. Even the greatest mind can
1633
02:07:42.880 --> 02:07:49.800
miss things, make mistakes, let biases, cloud judgment. Science needs
1634
02:07:49.800 --> 02:07:56.039
individual brilliance, but it also needs collective effort, peer review, replication,
1635
02:07:56.520 --> 02:08:01.119
and the willingness to change course when evidence demands. No
1636
02:08:01.399 --> 02:08:05.760
single person, no matter how smart, can see everything or
1637
02:08:05.880 --> 02:08:12.000
know everything. We need diverse perspectives, different approaches and many
1638
02:08:12.039 --> 02:08:16.000
people working on problems from different angles. So the next
1639
02:08:16.039 --> 02:08:19.159
time you look up at the night sky and think
1640
02:08:19.199 --> 02:08:24.359
about the universe, remember this story. Remember that the cosmos
1641
02:08:24.840 --> 02:08:30.399
is stranger than we imagine. Remember that the universe is expanding,
1642
02:08:30.760 --> 02:08:35.359
accelerating into a future we're only beginning to understand. And
1643
02:08:35.479 --> 02:08:40.079
remember that the equations describing all of this, the mathematical
1644
02:08:40.199 --> 02:08:44.680
language that lets us comprehend the incomprehensibly large and complex,
1645
02:08:45.359 --> 02:08:49.119
were largely given to us by one man thinking deeply
1646
02:08:49.199 --> 02:08:54.479
about the nature of gravity, space, and time. Einstein's genius
1647
02:08:55.000 --> 02:08:57.760
wasn't that he never made mistakes. It was that he
1648
02:08:57.840 --> 02:09:02.600
had the courage to propose rever polutionary ideas, the skill
1649
02:09:02.720 --> 02:09:06.840
to work out their mathematical implications, and the honesty to
1650
02:09:06.920 --> 02:09:11.520
admit when observations proved him wrong. He changed how we
1651
02:09:11.560 --> 02:09:17.760
think about space, time, matter, energy, and gravity. He showed
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02:09:17.840 --> 02:09:22.439
us that the universe operates according to elegant mathematical laws
1653
02:09:22.800 --> 02:09:28.159
that we can discover and understand. The cosmological constant represents
1654
02:09:28.199 --> 02:09:34.039
both misjudgment and prescient insight added to preserve an incorrect
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02:09:34.079 --> 02:09:37.199
model of the cosmos. That same term turned out to
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02:09:37.279 --> 02:09:42.479
describe the dominant energy driving everything we observe. Dark energy
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02:09:42.560 --> 02:09:47.720
determines the universe's accelerating expansion and will shape its ultimate fate.
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02:09:48.600 --> 02:09:52.479
Einstein couldn't have known this in nineteen seventeen, but his
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02:09:52.600 --> 02:09:58.000
mathematics contain the truth even when he misunderstood its implications.
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02:09:58.560 --> 02:10:03.000
This teaches us humility. Even the smartest people can be wrong.
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02:10:03.760 --> 02:10:09.000
Even revolutionary theories can contain hidden truths their creators don't understand.
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02:10:09.560 --> 02:10:14.880
Science progresses not because individual scientists are infallible, but because
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the scientific method eventually sorts truth from error through observation
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02:10:20.680 --> 02:10:26.199
and experiment. It also teaches us persistence. Einstein labored on
1665
02:10:26.319 --> 02:10:31.720
general relativity for almost a decade, trying different approaches, learning
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new mathematics, pushing through obstacles. Discovery rarely comes easily, and
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02:10:38.119 --> 02:10:43.800
it teaches us openness. When Hubble's observations revealed expansion, Einstein
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02:10:43.880 --> 02:10:49.199
accepted it despite his philosophical preferences. He acknowledged error publicly.
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That willingness to revise understanding in the face of evidence,
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to abandon cherished ideas when reality disagrees is essential to science.
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02:10:58.640 --> 02:11:01.279
Thank you for joining me on this journey through cosmology
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02:11:01.319 --> 02:11:05.279
and history. If you found this exploration valuable. A like
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02:11:05.439 --> 02:11:09.159
or subscribe would mean a lot. And remember, in the
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02:11:09.279 --> 02:11:13.319
vast darkness of the cosmos, even the greatest minds stumble.
1675
02:11:13.840 --> 02:11:19.359
But those stumbles, those errors, those blunders, they're all part
1676
02:11:19.439 --> 02:11:23.800
of humanity's long struggle to understand where we are, how
1677
02:11:23.800 --> 02:11:27.079
we got here, and where we're going. The universe is
1678
02:11:27.159 --> 02:11:30.720
expanding into a future we can barely imagine, and we're
1679
02:11:30.760 --> 02:11:35.920
going along for the ride, asking questions, making mistakes, and
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02:11:36.000 --> 02:11:39.560
slowly gradually learning the truth. Good Night,
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00:00:00.160 --> 00:00:04.599
Tonight, we're going to talk about something that sounds impossible.
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Albert Einstein, the man whose name has become synonymous with genius,
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the person who revolutionized our understanding of space, time, and
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reality itself made a mistake, not just any mistake, but
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what he himself called his biggest blunder. And here's what
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makes this story so fascinating. That mistake, that error in judgment,
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that Einstein regretted for years, might actually turn out to
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be one of the most important insights in all of cosmology.
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It might be the key to understanding why the universe
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00:00:47.280 --> 00:00:51.520
behaves the way it does. Before we dive in. If
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you find this exploration interesting, a quick like or subscribe
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really helps the channel grow. It's a small thing for you,
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00:01:00.320 --> 00:01:04.319
but it makes a huge difference for me. Now let's begin.
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When you think of Albert Einstein, you probably picture wild
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white hair, a rumpled sweater, maybe a chalkboard covered in equations.
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You think of genius, of brilliance, of a mind that
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could see things no one else could see, and you'd
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be right. Einstein changed everything. He showed us that time
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isn't constant, that space can bend, that mass and energy
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are the same thing, expressed in different ways. He gave
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us equations that predicted black holes, gravitational waves, and the
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expansion of the universe itself, decades before any of these
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things were actually observed. But Einstein was human, and humans
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make mistakes, even the brilliant ones, maybe especially the brilliant ones,
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because they're working at the edge of what's non pushing
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into territory where no one has been before. There are
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no maps, no guides, no textbook answers. You're making educated guesses,
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following your intuition, hoping you're right, but never entirely sure.
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In nineteen seventeen, Einstein published a paper that added something
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to his equations, a single term, a constant he called lambda,
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the cosmological constant. He added it for one reason, and
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one reason only, to keep the universe still, to prevent
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it from collapsing inward under its own gravity or expanding
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outward into the void. He wanted a static, unchanging universe,
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one that had always existed and would always exist, and
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to get that result he needed to add this extra
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term to balance things out. Twelve years later, in nineteen
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twenty nine, an astronomer named Edwin Hubble showed that the
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universe wasn't static at all. It was expanding. Galaxies were
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moving away from each other, The entire cosmos was growing
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larger with each passing moment. Einstein's cosmological constant wasn't necessary.
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The universe didn't need to be held still because it
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wasn't still. It was dynamic, changing, evolving. Einstein reportedly called
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this his biggest blunder. He'd let his philosophical preferences, his
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desire for a universe that was eternal and unchanging, cloud
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his scientific judgment. He'd added something to his equations not
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because the math demanded it, not because observations required it,
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but because he wanted a specific result, and he'd been wrong. Except,
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and this is where the story gets really interesting, he
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might not have been wrong after all. In the late
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nineteen nineties, astronomers made a shocking discovery. The expansion of
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the universe wasn't slowing down as everyone expected. It was
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speeding up. Something was pushing the universe apart, accelerating its expansion,
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and the best explanation any one could come up with
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was Einstein's cosmological constant, that term he'd added in nineteen
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seventeen and removed in embarrassment in nineteen twenty nine. It
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turned out to describe something real, something we now call
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dark energy. So Einstein's biggest mistake might actually be one
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of his greatest insights. A theoretical prediction made for the
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wrong reasons that turned out to describe a fundamental property
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of the universe. That's the story we're going to explore tonight,
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how Einstein came to make this mistake, why it seemed
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like a mistake for decades, and how modern cosmology has
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brought it back from the dead. Let's start at the beginning,
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with Einstein himself and the revolutionary ideas that changed physics forever.
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Albert Einstein was born in eighteen seventy nine in the
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German city of Ulm. He wasn't a child prodigy in
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the traditional sense. He didn't speak until he was relatively late,
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and his parents worried about him. He wasn't the best student,
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often clashing with teachers who valued memorization over understanding. He
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questioned everything, pushed back against authority, and generally made himself
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difficult to teach in the rigid German school system of
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the time. His family moved to Munich when he was
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an infant, and this is where he grew up. His father, Hermann,
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was an engineer and businessman who ran an electrical equipment
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company with Einstein's uncle. His mother, Pauline, was educated and cultured,
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encouraging young Albert's introes in music. Einstein learned to play
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the violin and maintained his love of music throughout his life,
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often saying that he thought in music and saw his
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life in terms of harmony. The late nineteenth century was
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a time of rapid change in Germany and throughout Europe.
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The Industrial Revolution was transforming societies. Cities were growing, New
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technologies like electricity and telephones were becoming common. It was
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an age of optimism and progress, a belief that human
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knowledge was advancing rapidly and that science would solve humanity's problems.
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But Einstein had something more valuable than obedience or wrote
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learning ability. He had an incredible imagination. He could visualize
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abstract concepts in ways that made them clear and understandable.
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He thought in pictures, in thought experiments, in scenarios he
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could play out in his mind. This ability to visualize
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the invisible would become his greatest strength. When Einstein was
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five years old, his father showed him a pocket compass.
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The boy was fascinated by the invisible force that made
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the needle always point north no matter how you turned
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the compass. This was his first encounter with the idea
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that there were hidden forces in nature, things you couldn't
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see but could detect through their effects. That compass made
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a deep impression on him, planting a seed that would
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grow into his lifelong quest to understand the hidden workings
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of the universe. There's a famous story about Einstein as
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a teenager imagining what it would be like to ride
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alongside a beam of light. If you could somehow match
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light speed and travel next to it, what would you see?
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Would the light wave appear frozen, stationery, or would something
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stranger happen? This simple thought experiment, this act of imagination
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planted seeds that would eventually grow into special relativity. He
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puzzled over this question for years. According to the physics
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of his time, According to Maxwell's equations of electromagnetism, which
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were the most successful physics theory of the late nineteenth century,
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light was a wave that traveled at a fixed speed
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through space. But if you could match that speed, if
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you could travel alongside the light, what would happen the
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light wave should appear stationary from your perspective frozen in time,
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but Maxwell's equations didn't allow for stationary light waves. Something
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didn't make sense. This paradox bothered Einstein throughout his teenage
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years and into his twenties. It suggested that either Maxwell's
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equations were wrong, which seemed unlikely given how successful they
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were at describing electors, tricity, magnetism, and light, or there
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was something fundamentally wrong with how people thought about space,
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time and motion. Einstein attended the Swiss Federal Polytechnic in Zurich,
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studying physics and mathematics. He was a good student, but
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not exceptional. He skipped classes that didn't interest him, preferring
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to study what he found fascinating rather than following the
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prescribed curriculum. This didn't endear him to his professors. When
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he graduated in nineteen hundred, none of his teachers would
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write him a recommendation for an academic position. This was
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a serious problem in the academic world of the early
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twentieth century. Personal recommendations from your professors were essential for
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getting your careers started. Einstein spent two years unemployed or underemployed,
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taking temporary teaching positions, tutoring students, doing whatever he could
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to make ends meet. It was a difficult, uncertain time.
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He'd invested years in his education, but the academic world
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seemed close to him because he'd alienated his professors by
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not conforming to their expectations. After university, Einstein struggled to
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find an academic position. He ended up working at a
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patent office in Bern, Switzerland. It was a good job,
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steady and respectable, but it wasn't what he'd dreamed of.
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He spent his days evaluating patent applications, determining whether inventions
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were truly novel or just rehashes of existing ideas. It
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was steady work, but it left him time time to think,
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time to read, time to work on physics in the
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evenings and weekends. In nineteen o five, Einstein's miracle year,
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he published four papers that changed physics forever. These weren't
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incremental improvements on existing theories. These were revolutionary ideas that
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overturned centuries of scientific understanding. The first paper explained the
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photoelectric effect, showing that light behaves as if it's made
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of particles, little packets of energy we now call photons.
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This work would eventually win him the Nobel Prize and
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help establish quantum mechanics, though Einstein himself would later have
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deep reservations about where quantum theory was heading. The second
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paper provided convincing evidence for the existence of atoms by
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explaining Brownian motion, the random jittering movement of tiny particles
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suspended in fluid. At the time, not all scientists believed
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atoms were real rather than just useful mathematical fictions. Einstein's
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paper helped settle that debate. The third paper introduce special relativity,
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showing that space and time are not separate absolute things,
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but are intertwined into a single entity called space time.
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Time can pass at different rates for different observers depending
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on their relative motion. This wasn't philosophy or speculation. It
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was mathematics backed by experimental evidence. The fourth paper contained
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the most famous equation in physics. E equals MC squared.
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Energy equals mass times the speed of light squared. This
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simple equation showed that mass and energy are equivalent, that
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matter is essentially frozen energy. It predicted nuclear reactions, explained
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where the Sun gets its power, and eventually led to
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both nuclear weapons and nuclear power. These four papers, any
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one of which would have been a career defining achievement,
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were all published in a single year by a twenty
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six year old patent clerk working outside the academic system.
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It's one of the most remarkable achievements in the history
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of science. Special relativity was revolutionary, but it had limitations.
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It only worked for objects moving at constant speeds relative
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to each other. It didn't handle acceleration or gravity. Einstein
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knew this was incomplete. For the next decade, he worked
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on extending his theory to include gravity and acceleration. The result,
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published in nineteen fifteen, was general relativity. General relativity is
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hard to understand mathematically. The equations are complex, involving advanced
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mathematics that most people, even most physicists, never study in detail.
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Einstein himself had to learn new mathematics, tenser calculus and
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differential geometry specifically to express his ideas. He worked with
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his friend Marcel Grossmann, a mathematician, to master these tools.
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It took him eight years from the initial idea to
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the final complete theory. Eight years of intense work, false starts, mistakes,
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and frustration, but the core idea is actually quite simple
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and can be understood without any math at all. Imagine
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space as a rubber sheet, stretched flat and smooth. Now
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place a heavy ball in the middle of the sheet.
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The ball makes a dent, a depression in the rubber
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The heavier the ball, the deeper the dent. Now roll
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a marble across the sheet. As it passes near the
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heavy ball, it curves toward it, following the contour of
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the depression. The marble isn't being pulled by some mysterious force,
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It's just following the natural shape of the curved sheet.
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That's how gravity works. In general relativity, massive objects like
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stars and planets don't pull on other objects with some
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invisible force reaching across space. Instead, they curve the space
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around them. Other objects move through that curved space, and
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their paths bend as a result. The Earth orbits the
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Sun not because the Sun is pulling on it, but
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because the Sun curves space around it, and the Earth
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is following the straightest possible path through that curved space. Now,
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this analogy has limitations. The rubber sheet is two dimensional
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and embedded in three dimensional space. Real space time is
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four dimensional, three dimensions of space and one of time,
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and it's not embedded in anything. It's the fundamental arena
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in which everything exists. There's no higher dimensional space that
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spacetime curves into. The curvature is intrinsic to spacetime itself. Also,
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the analogy shows spatial curvature, but not temporal curvature. In
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general relativity, time itself is affected by gravity. Time passes
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more slowly near massive objects. This isn't just clocks running slow,
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its actual time, the fundamental flow of causality. Two identical clocks,
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one near a massive object and one far away, will
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tick at different rates. The one near the massive object
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runs slower, and this has been measured with atomic clocks
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with extraordinary precision. Think about that for a moment. It's
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a completely different way of understanding gravity compared to what
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Isaac Newton proposed. Newton said gravity is a force, something
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that reaches out across empty space and pulls objects toward
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each other. Einstein said there's no force at all. There's
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just curved space, and objects move through that space following
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the natural contours of the curves. This might sound like
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just a different way of describing the same thing, but
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it's not. General relativity makes predictions that are different from
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Newton's theory, and every time we've tested those predictions, Einstein
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has been right and Newton has been wrong, or at
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least incomplete. Newton's theory says that if you suddenly removed
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the Sun, the Earth would immediately fly off in a
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straight line. The gravitational force would disappear instantly, and the
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Earth would no longer orbit. Einstein's theory says something different.
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If the Sun disappeared, a gravitational wave, a ripple in
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space time would propagate outward at the speed of light.
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The Earth wouldn't feel any change until that wave reached
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it eight minutes and twenty seconds later. Information about the
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Sun's disappearance would travel at light speed through space time,
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not instantaneously. General relativity predicted that light should bend when
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it passes near massive objects. In nineteen nineteen, during a
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solar eclipse, astronomers measured the positions of stars visible near
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the Sun's edge. The stars appeared slightly displaced from their
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usual positions because the Sun's mass curved the space around it,
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bending the light from those distant stars as it passed nearby.
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This observation made Einstein world famous. Overnight, the newspapers ran
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headlines about how space was curved and reality was different
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from what everyone had assumed. This wasn't just abstract physics.
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Light bending around massive objects, called gravitational lensing, has become
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one of astronomy's most powerful tools. When a massive galaxy
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or galaxy cluster sits between Earth and a more distant galaxy,
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the foreground object's gravity bends the light from the background galaxy,
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often creating multiple distorted images arranged in rings or arcs
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around the lens. Astronomers use these natural magnifying glasses to
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study galaxies that would otherwise be too faint and distant
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to observe. Some of the deepest images of the early
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universe come from studying gravitationally lensed galaxies. General relativity predicted
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that time should pass more slowly in strong gravitational fields.
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This has been tested with atomic clocks placed at different altitudes.
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A clock on a mountaintop runs slightly faster than an
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identical clocket sea level because it's farther from Earth's mass
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and therefore in a weaker gravitational field. The difference is tiny,
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only a few nanoseconds per day, but it's measurable and
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exactly matches Einstein's predictions. GPS satellites must account for this
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time dilation effect. The satellites orbit about twenty thousand kilometers
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or twelve thousand, four hundred miles above Earth's surface, where
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gravity is weaker than at ground level. This causes their
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atomic clocks to run faster than clocks on the ground
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by about forty five microseconds per day. Additionally, because the
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satellites are moving at high speeds, special relativity causes their
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clocks to run slower by about seven microseconds per day.
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The net effect is about thirty eight microseconds per day faster.
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This might sound insignificant, but GPS works by precisely timing
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radio signals. Light travels about thirty centimeters or twelve inches
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per nanosecond. If GPS didn't correct for relativistic time dilation,
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the errors would accumulate at a rate of about ten
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kilometers or six miles per day within hours. GPS navigation
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would become useless. Every time you use GPS to navigate,
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you're relying on Einstein's relativity to be accurate. Your phone
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is doing relativistic calculations to correct for time dilation. General
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relativity predicted gravitational waves ripples in spacetime itself, created when
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massive objects accelerate. These waves were finally detected directly in
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twenty fifteen, a full century after Einstein predicted them, when
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the Ligo experiment measured the incredibly tiny distortions in space
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caused by two black holes colliding over a billion light
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years away. The measurement was so precise it could detect
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changes in distance smaller than a proton, and it matched
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Einstein's predictions perfectly. Since that first detection, LIGO and its
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partner observatory Virgo have detected dozens of gravitational wave events.
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Most involve colliding black holes, but they've also colliding neutron stars.
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In twenty seventeen, a neutron star merger was detected both
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in gravitational waves and across the electromagnetic spectrum, from gamma
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rays to radio waves. This event confirmed theories about where
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heavy elements like gold and platinum come from. These elements
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are forged in neutron star collisions and scattered across space
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to eventually become part of new star systems and planets.
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Gravitational wave astronomy is now an established field, giving us
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a completely new way to observe the universe. Unlike light,
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which can be absorbed or scattered, gravitational waves pass through
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everything unimpeded. They carry information about events that produce no
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light at all, like the collision of two black holes
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in empty space. As detectors become more sensitive, we'll observe
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gravitational waves from the early universe itself, potentially seeing events
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that occurred when the cosmos was too young and hot
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for light to travel freely. So general relativity works. It's
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been tested countless times in countless ways, and it always
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gives the right answer. It's one of the most successful
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scientific theories ever developed. But when Einstein first worked out
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his equations, he ran into a problem, a problem that
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would lead to what he later called his biggest blunder.
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The equations of general relativity describe how mass and energy
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curve space and time, but their dynamic equations they describe
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how things change, how space responds to the presence of
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matter and energy. When Einstein applied his equations to the
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universe as a whole, he found something troubling. The universe,
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according to his equations, couldn't remain still. It had to
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be either expanding or contracting. If you filled space with
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stars and galaxies, gravity would pull everything together, causing a
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cosmic collapse, or if things were already moving apart, they'd
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continue separating, the universe growing larger forever. But this contradicted
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Einstein's philosophical view of the cosmos. He envisioned an eternal,
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steady state universe that had always existed and would always exist.
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This wasn't base on observations. It was an esthetic judgment,
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a belief that the cosmos should be balanced and timeless.
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Most astronomers shared this view in the early twentieth century.
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They believed the universe consisted only of the Milky Way Galaxy,
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a large but finite island of stars floating in infinite
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empty space. The fuzzy spiral nebulae visible through telescopes were
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thought to be gas clouds within our galaxy, not separate
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galaxies millions of light years away. If the cosmos was
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just our galaxy surrounded by void, then a steady state
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model made intuitive sense. Individual stars were born and died,
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but the overall structure remained roughly constant. There was no
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reason to think the entire universe was evolving in any
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fundamental way. So when Einstein's equations predicted an unstable universe
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that couldn't remain status. He faced a choice. He could
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accept what the math was telling him and conclude that
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the universe must be either expanding or contracting, or he
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could modify the equations to allow for a static solution.
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He chose the second option. In nineteen seventeen, he published
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a paper adding a new term to his field equations,
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the cosmological constant, represented by the Greek letter lambda. This
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constant represented a kind of repulsive force, something that pushed
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space apart, counteracting the attractive force of gravity. By carefully
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tuning this constant to exactly the right value, Einstein could
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balance the inward pull of gravity with the outward push
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of lambda, creating a universe that was static and stable.
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The mathematics behind this was elegant in its own way.
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General relativity describes how the curve of space time responds
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to the presence of mass and energy. The equations relate
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the geometry of space time on one side to the
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distribution of matter and energy on the other side. Einstein
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realized he could add an additional term to the geometry side,
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a term that represented a kind of intrinsic curvature of
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space that existed even in the absence of matter. This term,
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the cosmological constant, had interesting properties. Unlike normal matter or energy,
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which becomes more dilute as space expands, the cosmological constant
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stayed constant. It was the same everywhere at all times.
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It represented a kind of energy density that was a
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property of space itself. The more space you had, the
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more of this energy you had, but the density never changed.
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If you doubled the volume of space, you doubled the
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total amount of this energy, keeping the density constant. It's
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important to understand what Einstein did here. He didn't discover
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the cosmological constant by observing something in nature or deriving
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it from more fundamental principles. He added it to make
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his equations give him the answer he wanted. The math
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allowed for such a term. There was nothing technically wrong
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with including it, but its only justification was that it
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produced a static universe, which Einstein believed, on philosophical grounds,
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must be correct. This wasn't the first time a physicist
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had modified equations to match desired results. In the history
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of science. There are many examples of ad hoc modifications
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that later proved necessary, and many more that proved to
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be wrong. The trick is that you can't know in
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advance which category or modification falls into. You have to
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wait for observations to tell you whether you were right
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or wrong. Einstein was aware of the somewhat arbitrary nature
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of this addition. In his paper, he noted that the
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cosmological constant was the simplest possible addition that would allow
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a static solution. He also noted that there was no
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independent theoretical reason to include it. Its justification was purely
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that it solved what he saw as a problem with
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his original equations, their prediction of an unstable universe. Think
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about what the cosmological constant actually means. Its energy inherent
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in empty space itself, even in a perfect vacuum, completely
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devoid of matter and radiation. This constant says there's a
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kind of pressure pushing outward. It's not particles, and it's
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not waves. It's a fundamental property of space. The more
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volume you have, the more of this energy exists. But crucially,
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the density stays uniform everywhere in everyday life. Forces weaken
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with distance. Gravity fades as you move away from massive objects.
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Light dims the farther you are from its source. But
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the cosmological constant behaves differently. Double the volume of space,
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and you double this repulsive energy while maintaining constant density.
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Its utterly alien to ordinary experience. Einstein knew this addition
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was unusual. He was introducing something fundamentally new to physics
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without direct evidence, but he did it anyway because his
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equations demanded modification to achieve the steady state cosmos he
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believed must exist. For the next twelve years, the cosmological
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constant sat in Einstein's equations. Other physicists debated whether it
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was necessary or justified. Some argued it was an elegant
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solution to a real problem. Others thought it was an
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ad hoc fix, something added to force a desired result,
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rather than emerging naturally from the theory. Then everything changed.
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In the early nineteen twenties, an astronomer named Edwin Hubble
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began making observations that would revolutionize our understanding of the cosmos.
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Working with the one hundred inch telescope at Mount Wilson
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Observatory in California, which was the largest telescope in the
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world at the time, Hubble studied the fuzzy spiral nebulae
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that astronomers had been observing for decades. These spiral nebulae
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were mysterious objects. Through small telescopes, they appeared as faint,
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fuzzy patches of light scattered across the sky. Through larger telescopes,
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you could see they had spiral structure, arms of light
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swirling around a bright central core, but nobody knew what
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they were. Some astronomers thought they were gas clouds within
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our own galaxy, perhaps regions where new stars were forming.
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Others speculated they might be distant star systems beyond the
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Milky Way. The debate about the nature of spiral nebulae
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was one of the great controversies in astronomy in the
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early twentieth century. In nineteen twenty there was even a
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formal debate on the subject between two prominent astronomers, Harlow Shapley,
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who argued the spiral nebulae were part of the Milky Way,
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and Heber Curtis, who argued they were separate galaxies. Neither
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side had definitive proof, so the debate ended without clear resolution.
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Hubble changed everything by finding a way to measure the
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distances to these nebulae. He did this using a type
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of star called a Cepheid variable. These stars pulsate, growing
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brighter and dimmer in a regular cycle that can last
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anywhere from days to months. The American astronomer Henrietta Levitt
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had discovered something remarkable about Cepheid variables. Their period of pulsation,
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how long it takes them to go from bright to
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dim and back again, is directly related to their intrinsic brightness.
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A cepheid that pulsates slowly is intrinsically brighter than one
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that pulsates quickly. This meant you could use cepheid's as
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standard candles to measure cosmic distances. If you could identify
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a Cepheid variable in a distant object measure its period,
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you'd know how bright it actually was. By comparing its
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actual brightness to how bright it appeared from Earth, you
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could calculate how far away it must be. The dimmer
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it appeared, the farther way it was. Using a technique
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developed by Henrietta Levitt to measure cosmic distances, Hubble determined
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that these nebulae were far outside the Milky Way. They
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weren't gas clouds in our galaxy. They were separate galaxies,
440
00:34:16.679 --> 00:34:21.239
each containing billions of stars. Located millions of light years away.
441
00:34:21.960 --> 00:34:26.000
The universe was vastly larger than anyone had imagined. The
442
00:34:26.039 --> 00:34:31.079
first distance Hubble measured was to the Andromeda galaxy, which
443
00:34:31.119 --> 00:34:35.119
appeared in the sky as a faint, fuzzy oval visible
444
00:34:35.159 --> 00:34:39.199
to the naked eye on dark nights. Hubble found Cepheid
445
00:34:39.320 --> 00:34:44.800
variables in Andromeda and measured their periods. His calculations showed
446
00:34:44.800 --> 00:34:48.960
that Andromeda was about nine hundred thousand light years away.
447
00:34:49.440 --> 00:34:54.360
This was later revised upward when we better understood Cepheid variables,
448
00:34:54.960 --> 00:34:58.519
and we now know Andromeda is actually about two point
449
00:34:58.559 --> 00:35:02.519
five million light years of way. But even Hubble's original
450
00:35:02.639 --> 00:35:06.440
estimate was far enough to prove that Andromeda was well
451
00:35:06.440 --> 00:35:10.880
outside the Milky Way. It was a separate galaxy, not
452
00:35:11.079 --> 00:35:16.079
part of our own. This was revolutionary. The universe wasn't
453
00:35:16.119 --> 00:35:20.480
just the Milky Way. It was filled with galaxies, countless
454
00:35:20.559 --> 00:35:25.679
islands of stars scattered through space, each containing billions of suns.
455
00:35:26.360 --> 00:35:29.920
The cosmos was far larger and more complex than anyone
456
00:35:29.960 --> 00:35:33.360
had imagined. What had appeared to be a universe perhaps
457
00:35:33.480 --> 00:35:37.400
one hundred thousand light years across the size of our
458
00:35:37.440 --> 00:35:43.199
galaxy was actually vastly bigger, filled with galaxies separated by
459
00:35:43.320 --> 00:35:47.719
millions of light years. But Hubble didn't stop there. He
460
00:35:47.800 --> 00:35:51.119
also measured how these galaxies were moving. When you look
461
00:35:51.159 --> 00:35:54.400
at light from a distant object, you can determine whether
462
00:35:54.440 --> 00:35:58.079
it's moving toward you or away from you by examining
463
00:35:58.079 --> 00:36:02.440
its spectrum, rainbow of colors created when you split the
464
00:36:02.519 --> 00:36:06.079
light through a prism. If the object is moving toward you,
465
00:36:06.679 --> 00:36:09.920
the spectrum shifts toward the blue end of the rainbow.
466
00:36:10.840 --> 00:36:14.719
If it's moving away, it shifts toward the red end.
467
00:36:15.360 --> 00:36:19.679
This is called red shift for objects moving away and
468
00:36:19.800 --> 00:36:24.239
blue shift for objects moving toward. Think of it like
469
00:36:24.320 --> 00:36:29.400
the Doppler effect with sound. When an ambulance drives toward you,
470
00:36:29.760 --> 00:36:33.760
its siren sounds higher pitched than normal because the sound
471
00:36:33.800 --> 00:36:38.320
waves are compressed. When it drives away, the siren sounds
472
00:36:38.400 --> 00:36:42.360
lower pitched because the waves are stretched out. Light does
473
00:36:42.440 --> 00:36:47.239
the same thing. Motion toward you compresses the waves, shifting
474
00:36:47.280 --> 00:36:52.119
them toward blue. Motion away stretches them out, shifting them
475
00:36:52.159 --> 00:36:56.639
toward red. Hubble found that nearly all galaxies showed red
476
00:36:56.679 --> 00:37:01.000
shift they were moving away from us, and the farther
477
00:37:01.079 --> 00:37:06.079
away a galaxy was the faster it was receding a
478
00:37:06.119 --> 00:37:09.960
galaxy twice as far away was moving away twice as fast.
479
00:37:10.599 --> 00:37:14.320
Three times the distance meant three times the speed. There
480
00:37:14.400 --> 00:37:20.719
was a clear mathematical relationship between distance and recession velocity.
481
00:37:21.320 --> 00:37:26.320
This could only mean one thing. The universe was expanding.
482
00:37:26.960 --> 00:37:32.239
Space itself was growing larger, carrying galaxies with it. The
483
00:37:32.280 --> 00:37:36.800
farther apart two galaxies were the more space between them,
484
00:37:36.960 --> 00:37:40.119
and therefore the faster they appeared to separate as that
485
00:37:40.280 --> 00:37:44.000
space stretched. Think of dots drawn on a balloon's surface.
486
00:37:44.960 --> 00:37:48.320
As you inflate the balloon, the rubber stretches and the
487
00:37:48.360 --> 00:37:52.880
dots move apart. They're not sliding across the surface. The
488
00:37:52.920 --> 00:37:57.840
surface itself is expanding, carrying the dots along. That's what's
489
00:37:57.840 --> 00:38:01.719
happening with the universe. Space piece is the expanding medium,
490
00:38:02.400 --> 00:38:06.039
and galaxies ride along with it. In nineteen twenty nine,
491
00:38:06.559 --> 00:38:12.719
Hubbell published his findings. The universe was not static. It
492
00:38:12.840 --> 00:38:16.880
was expanding and had been expanding for a very long time.
493
00:38:17.480 --> 00:38:23.360
Einstein's cosmological constant, added specifically to prevent expansion or collapse,
494
00:38:23.880 --> 00:38:27.280
was unnecessary. The universe didn't need to be held still
495
00:38:27.719 --> 00:38:33.440
because it wasn't still. It was dynamic, changing, evolving When
496
00:38:33.519 --> 00:38:38.760
Einstein learned of Hubbell's discovery, he recognized immediately what it meant.
497
00:38:39.599 --> 00:38:44.000
He'd made an error. His philosophical preference for a steady
498
00:38:44.000 --> 00:38:49.039
state cosmos had overridden what the mathematics of general relativity
499
00:38:49.480 --> 00:38:53.199
had been trying to tell him. His original equations without
500
00:38:53.199 --> 00:38:57.639
the added constant, had predicted expansion all along. If he'd
501
00:38:57.679 --> 00:39:02.159
trusted the mathematics instead of four it to match his preconceptions,
502
00:39:02.760 --> 00:39:08.280
he would have anticipated the discovery before astronomers observed it. Instead,
503
00:39:08.679 --> 00:39:12.920
he'd modified his work to get the wrong answer. Einstein
504
00:39:13.079 --> 00:39:16.880
visited Mount Wilson Observatory in nineteen thirty one to see
505
00:39:16.920 --> 00:39:21.920
Hubble's work firsthand. He looked through the telescope, examined the data,
506
00:39:22.199 --> 00:39:27.360
and confirmed that, yes, the universe was definitely expanding. According
507
00:39:27.440 --> 00:39:31.239
to the Russian physicist George Gammo, who was present during
508
00:39:31.280 --> 00:39:37.760
some of these discussions, Einstein expressed deep regret about adding
509
00:39:37.840 --> 00:39:43.639
the cosmological constant. He'd inserted it without good justification, except
510
00:39:43.679 --> 00:39:47.840
to achieve an answer he preferred philosophically, and that answer
511
00:39:47.880 --> 00:39:53.599
turned out to be incorrect. Einstein removed the cosmological constant
512
00:39:53.679 --> 00:39:57.920
from his equations for the next few decades. It was
513
00:39:58.000 --> 00:40:02.760
largely forgotten, delegated to a footnote in the history of physics,
514
00:40:03.360 --> 00:40:07.760
a cautionary tale about how even great scientists can be
515
00:40:07.840 --> 00:40:11.800
led astray by their assumptions and preferences. But the story
516
00:40:11.800 --> 00:40:15.639
doesn't end there. The cosmological constant might have been added
517
00:40:15.639 --> 00:40:18.400
for the wrong reasons, but as we'll see, it turned
518
00:40:18.400 --> 00:40:23.000
out to describe something very real about the universe, something
519
00:40:23.039 --> 00:40:27.000
that would remain hidden for nearly seventy years before observations
520
00:40:27.079 --> 00:40:31.159
finally revealed its presence. Let's talk about what an expanding
521
00:40:31.280 --> 00:40:35.159
universe actually means, because this is one of the most
522
00:40:35.199 --> 00:40:39.960
commonly misunderstood concepts in cosmology. When we say the universe
523
00:40:40.079 --> 00:40:44.320
is expanding, we don't mean galaxies are exploding outward from
524
00:40:44.360 --> 00:40:48.119
some central point into empty space. There is no center,
525
00:40:48.840 --> 00:40:54.800
there is no edge. Space itself is growing larger everywhere simultaneously.
526
00:40:55.400 --> 00:40:59.760
Remember that balloon analogy. Every point on the expanding so
527
00:40:59.760 --> 00:41:04.320
sude moves away from every other point, yet no location
528
00:41:04.519 --> 00:41:10.320
is special. Each observer, wherever they are, sees the same phenomenon,
529
00:41:10.840 --> 00:41:16.079
all distant objects receding in every direction. The expansion is
530
00:41:16.519 --> 00:41:21.840
uniform throughout space. Now here's something that confuses people. If
531
00:41:21.880 --> 00:41:26.360
space is expanding. Why aren't we expanding? Why isn't Earth
532
00:41:26.400 --> 00:41:32.119
getting bigger? The answer is that local forces completely overwhelm
533
00:41:32.199 --> 00:41:38.599
the expansion. Atoms are bound by electromagnetic forces, Planets are
534
00:41:38.599 --> 00:41:44.039
held together by gravity and chemical bonds. Even galaxies resist
535
00:41:44.079 --> 00:41:49.239
the expansion through their own gravitational pull. The expansion only
536
00:41:49.280 --> 00:41:53.960
dominates at the largest scales, the distances between galaxy clusters,
537
00:41:54.119 --> 00:41:57.679
where no other forces can compete. The discovery that the
538
00:41:57.840 --> 00:42:03.559
universe was expanding immediately raised a profound question. If space
539
00:42:03.639 --> 00:42:07.199
is growing larger, that means it used to be smaller.
540
00:42:07.679 --> 00:42:11.159
Wind the clock backward far enough, and you reach a
541
00:42:11.199 --> 00:42:15.719
point where all of space was compressed into an infinitely small,
542
00:42:16.239 --> 00:42:21.480
infinitely dense point, a singularity. This was the moment of creation,
543
00:42:22.159 --> 00:42:25.800
the beginning of the universe itself. This idea was initially
544
00:42:25.800 --> 00:42:30.440
called the primeval atom by George Lemetre, a Belgian priest
545
00:42:30.519 --> 00:42:33.280
and physicist who was one of the first to work
546
00:42:33.320 --> 00:42:38.639
out the implications of an expanding universe. Later, the astronomer
547
00:42:38.719 --> 00:42:42.280
Fred Hoyle would mockingly call it the Big Bang, a
548
00:42:42.400 --> 00:42:47.119
name that stuck even though Hoyle himself never accepted the theory.
549
00:42:47.559 --> 00:42:50.719
The Big Bang theory says the universe began in an
550
00:42:50.840 --> 00:42:58.880
unimaginably hot, dense state about thirteen point eight billion years ago. Space, time, matter,
551
00:42:59.199 --> 00:43:03.800
and energy all came into existence at that moment. There
552
00:43:03.920 --> 00:43:09.119
was no before. Because time itself began with the Big Bang.
553
00:43:09.679 --> 00:43:15.679
Space expanded rapidly, cooling as it grew, allowing first subatomic particles,
554
00:43:16.000 --> 00:43:20.800
then atoms, then stars and galaxies to form. This raised
555
00:43:20.880 --> 00:43:25.960
immediate objections. Many scientists in the nineteen thirties and forties
556
00:43:26.199 --> 00:43:30.440
found the idea philosophically troubling. The universe having a beginning
557
00:43:30.880 --> 00:43:35.920
seemed to imply a creator, which made some scientists uncomfortable.
558
00:43:36.679 --> 00:43:41.480
It felt more like theology than physics. Hoyle and others
559
00:43:41.880 --> 00:43:47.239
proposed alternative theories where the universe was eternal, with new
560
00:43:47.280 --> 00:43:51.840
matter continuously being created to fill the gaps left by expansion.
561
00:43:52.400 --> 00:43:56.320
This steady state model preserved the philosophical appeal of a
562
00:43:56.360 --> 00:44:01.719
timeless cosmos while still matching Hubble's observation of expansion. The
563
00:44:01.760 --> 00:44:06.480
steady state theory had its appeal. It avoided the uncomfortable
564
00:44:06.599 --> 00:44:10.280
question of what came before the Big Bang. If the
565
00:44:10.400 --> 00:44:14.199
universe had always existed, there was no moment of creation
566
00:44:14.400 --> 00:44:20.000
to explain new matter would appear gradually throughout space, spontaneously
567
00:44:20.039 --> 00:44:24.719
popping into existence to maintain constant density. As expansion spread
568
00:44:24.760 --> 00:44:29.280
things out, the rate would be incredibly slow, about one
569
00:44:29.440 --> 00:44:33.440
hydrogen atom per cubic kilometer or cubic mile of space
570
00:44:33.480 --> 00:44:38.480
per year, far too slow to detect directly. For several decades,
571
00:44:39.199 --> 00:44:43.000
the steady state model and the Big Bang model competed.
572
00:44:43.960 --> 00:44:49.239
Both could explain the observed expansion. Both made testable predictions.
573
00:44:49.840 --> 00:44:54.239
The scientific community was divided. Some found the steady state
574
00:44:54.719 --> 00:45:00.800
more philosophically satisfying, others found the Big Bang more elegant mathematically.
575
00:45:01.559 --> 00:45:05.000
It was one of the great debates in twentieth century cosmology.
576
00:45:05.440 --> 00:45:10.639
The debate wasn't settled by philosophical arguments. It was settled
577
00:45:10.679 --> 00:45:15.599
by observation. If the Big Bang happened, the early universe
578
00:45:16.119 --> 00:45:20.760
must have been hot and dense. As it expanded and cooled.
579
00:45:21.239 --> 00:45:24.960
There should be leftover radiation from that early hot phase,
580
00:45:25.440 --> 00:45:29.239
now cooled by expansion to just a few degrees above
581
00:45:29.400 --> 00:45:35.639
absolute zero. This cosmic microwave background radiation should fill all
582
00:45:35.800 --> 00:45:41.159
of space uniformly. In nineteen sixty five, two radio astronomers
583
00:45:41.559 --> 00:45:45.719
named Arno Penzias and Robert Wilson were testing a sensitive
584
00:45:45.840 --> 00:45:49.599
radio antenna at Bell Labs in New Jersey. They kept
585
00:45:49.639 --> 00:45:54.360
picking up mysterious noise, a faint hiss that came from
586
00:45:54.360 --> 00:45:58.559
all directions equally. At first, they thought it was interference,
587
00:45:59.119 --> 00:46:01.840
perhaps from near by New York City, or even from
588
00:46:01.920 --> 00:46:05.679
pigeon droppings on their antenna. They cleaned the antenna thoroughly,
589
00:46:06.000 --> 00:46:11.400
even removing nesting pigeons, but the noise persisted. Eventually, they
590
00:46:11.480 --> 00:46:17.159
learned that physicists at nearby Princeton University had predicted exactly
591
00:46:17.239 --> 00:46:21.800
this kind of background radiation as a consequence of the
592
00:46:21.800 --> 00:46:26.840
Big Bang. The noise Penzius and Wilson detected wasn't interference.
593
00:46:27.440 --> 00:46:31.000
It was the afterglow of the Big Bang itself, radiation
594
00:46:31.119 --> 00:46:35.119
that had been traveling through space for over thirteen billion years,
595
00:46:35.480 --> 00:46:39.559
cooled by cosmic expansion from millions of degrees to just
596
00:46:39.599 --> 00:46:44.719
a few degrees above absolute zero. This discovery won Penzius
597
00:46:44.800 --> 00:46:48.840
and Wilson the Nobel Prize and essentially ended serious debate
598
00:46:49.119 --> 00:46:52.960
about whether the Big Bang had occurred. The steady state
599
00:46:53.039 --> 00:46:58.360
model couldn't explain this background radiation. The Big Bang predicted it,
600
00:46:58.960 --> 00:47:03.079
so by the nineteen sixties and seventies, the picture seemed clear.
601
00:47:03.960 --> 00:47:07.440
The universe began with the Big Bang thirteen point eight
602
00:47:07.559 --> 00:47:12.559
billion years ago. It's been expanding ever since. Gravity has
603
00:47:12.639 --> 00:47:17.360
been slowing that expansion like a ball thrown upward, gradually
604
00:47:17.440 --> 00:47:22.519
slowing as Earth's gravity pulls on it. Eventually, one of
605
00:47:22.559 --> 00:47:25.760
two things would happen. Either there was enough matter in
606
00:47:25.760 --> 00:47:30.199
the universe that gravity would eventually stop the expansion and
607
00:47:30.360 --> 00:47:33.800
reverse it, causing the universe to collapse back in on
608
00:47:33.920 --> 00:47:37.639
itself in a big crunch, or there wasn't enough matter
609
00:47:38.719 --> 00:47:43.679
and the universe would expand forever, gradually slowing but never
610
00:47:43.800 --> 00:47:48.239
quite stopping. Astronomer's work to measure the universe's total mass
611
00:47:48.280 --> 00:47:52.880
density to determine which scenario was correct. Would we get
612
00:47:52.920 --> 00:47:56.920
a big crunch or eternal expansion. The fate of the
613
00:47:57.039 --> 00:48:02.440
universe hung on this measurement. Einstein Signe's cosmological constant was
614
00:48:02.480 --> 00:48:07.039
barely mentioned during these decades. It had been added to
615
00:48:07.119 --> 00:48:12.639
create a static universe. The universe wasn't static, therefore the
616
00:48:12.719 --> 00:48:18.760
constant was unnecessary, just an embarrassing mistake. Case closed. But
617
00:48:18.880 --> 00:48:22.639
physics is never that simple. Just when you think you
618
00:48:22.760 --> 00:48:27.119
understand something, Nature throws you a curveball, and in the
619
00:48:27.199 --> 00:48:31.679
nineteen nineties, cosmology got one of the biggest curveballs in
620
00:48:31.840 --> 00:48:36.159
scientific history. To understand what happened, we need to talk
621
00:48:36.199 --> 00:48:41.559
about supernovae. When certain massive stars reach the end of
622
00:48:41.599 --> 00:48:47.840
their lives, they explode in spectacular fashion, briefly outshining entire galaxies.
623
00:48:48.400 --> 00:48:53.519
These explosions, called supernovae, are among the most energetic events
624
00:48:53.559 --> 00:48:57.639
in the universe. For a few weeks, a single dying
625
00:48:57.719 --> 00:49:01.119
star can emit as much light as billions of normal
626
00:49:01.199 --> 00:49:07.599
stars combined. The word supernova means new superstar in Latin.
627
00:49:08.679 --> 00:49:12.320
Ancient and medieval astronomers occasionally saw what appeared to be
628
00:49:12.760 --> 00:49:16.960
new stars suddenly appearing in the sky, shining brightly for
629
00:49:17.000 --> 00:49:20.320
a few weeks or months before fading away. They call
630
00:49:20.400 --> 00:49:24.480
these novae new stars, though we now know they're not
631
00:49:24.599 --> 00:49:30.360
new stars being born, but old stars dying in catastrophic explosions.
632
00:49:31.000 --> 00:49:34.639
A supernova releases more energy in a few seconds than
633
00:49:34.679 --> 00:49:38.360
our Sun will emit in its entire ten billion year lifetime.
634
00:49:39.239 --> 00:49:42.760
The explosion is so violent that it can briefly outshine
635
00:49:42.800 --> 00:49:47.119
an entire galaxy containing hundreds of billions of stars. The
636
00:49:47.360 --> 00:49:51.599
energy released is comparable to all the energy the Sun
637
00:49:51.719 --> 00:49:57.039
will ever produce, compressed into a moment of unimaginable violence.
638
00:49:57.719 --> 00:50:01.239
When a star explodes as a supernova, it doesn't just
639
00:50:01.360 --> 00:50:05.280
release light and heat, It also forges heavy elements in
640
00:50:05.360 --> 00:50:11.800
its final moments. Elements heavier than iron, things like gold, silver, uranium,
641
00:50:11.920 --> 00:50:15.920
and platinum, can only be created in these extreme conditions,
642
00:50:16.639 --> 00:50:20.920
in the incredible temperatures and pressures of a supernova explosion.
643
00:50:21.360 --> 00:50:25.639
Every atom of gold in your jewelry, every atom of
644
00:50:25.719 --> 00:50:30.000
silver in your electronics, every atom of uranium in the
645
00:50:30.039 --> 00:50:33.920
Earth's crust was forged in a dying star billions of
646
00:50:34.000 --> 00:50:38.719
years ago. We are quite literally made of star dust,
647
00:50:39.679 --> 00:50:44.519
the remnants of ancient supernova scattered across space and eventually
648
00:50:44.599 --> 00:50:49.320
recycled into new stars, planets, and life. There are different
649
00:50:49.440 --> 00:50:53.400
types of supernova, depending on what kind of star explodes
650
00:50:53.639 --> 00:50:58.199
and how the explosion happens. The classification system is a
651
00:50:58.239 --> 00:51:02.400
bit complicated, based on which spectral lines appear in the
652
00:51:02.440 --> 00:51:09.239
supernova's light. Type I supernovae don't show hydrogen lines. Type
653
00:51:09.280 --> 00:51:15.320
two supernova do show hydrogen. Within type one, there are
654
00:51:15.400 --> 00:51:22.840
further subdivisions Type EAR show silicon, Type IB and ICK don't.
655
00:51:23.440 --> 00:51:29.159
One particular type called type ear supernovae is especially interesting
656
00:51:29.199 --> 00:51:35.320
to astronomers because these explosions are remarkably consistent. They always
657
00:51:35.360 --> 00:51:39.239
have about the same brightness at their peak. This makes
658
00:51:39.280 --> 00:51:44.639
them perfect standard candles for measuring cosmic distances. Type ear
659
00:51:44.840 --> 00:51:50.000
supernova occur in binary star systems where one star is
660
00:51:50.039 --> 00:51:53.840
a white dwarf, the dense remnant of a dead star.
661
00:51:54.800 --> 00:51:59.679
The white dwarf pulls matter from its companion star gradually
662
00:51:59.719 --> 00:52:04.159
gain mass. When the white dwarf reaches a critical mass
663
00:52:04.760 --> 00:52:07.440
about one point four times the mass of the Sun,
664
00:52:08.079 --> 00:52:12.880
something called the Chandraseca limit, it can no longer support itself.
665
00:52:13.400 --> 00:52:19.039
The entire star detonates in a thermonuclear explosion, destroying itself completely.
666
00:52:19.519 --> 00:52:23.800
Because type ear supernova always explode at the same mass,
667
00:52:24.360 --> 00:52:28.320
they always release roughly the same amount of energy. This
668
00:52:28.519 --> 00:52:33.239
means they all have about the same intrinsic brightness. When
669
00:52:33.239 --> 00:52:37.639
you see a type ear supernova explode, you know how
670
00:52:37.679 --> 00:52:41.239
bright it actually is. If it appears dim, it must
671
00:52:41.280 --> 00:52:45.239
be far away. If it appears bright, it must be
672
00:52:45.320 --> 00:52:50.440
relatively close. By measuring how bright it appears and comparing
673
00:52:50.519 --> 00:52:54.519
that to how bright you know it actually is, you
674
00:52:54.559 --> 00:52:59.119
can calculate its distance. Here's how it works. If you
675
00:52:59.239 --> 00:53:04.360
know how bright something actually is its intrinsic luminosity, and
676
00:53:04.400 --> 00:53:07.679
you can measure how bright it appears from Earth, you
677
00:53:07.719 --> 00:53:11.079
can calculate how far away it must be. It's the
678
00:53:11.119 --> 00:53:14.800
same principle as estimating distance to a light at night.
679
00:53:15.320 --> 00:53:17.400
If you see a light and you know it's a
680
00:53:17.519 --> 00:53:21.559
hundred what bulb, you can tell approximately how far away
681
00:53:21.599 --> 00:53:25.440
it is by how dim it appears. The dimmer it looks,
682
00:53:25.920 --> 00:53:29.840
the farther away it must be. Light spreads out as
683
00:53:29.840 --> 00:53:33.360
it travels through space. Imagine a light bulb at the
684
00:53:33.360 --> 00:53:37.119
center of a sphere. The light it emits spreads out
685
00:53:37.159 --> 00:53:41.480
evenly in all directions, covering the inside surface of the sphere.
686
00:53:41.880 --> 00:53:46.159
Now imagine a larger sphere. The same amount of light
687
00:53:46.760 --> 00:53:50.360
has to cover a bigger area, so the light at
688
00:53:50.400 --> 00:53:54.159
any given point is dimmer. The area of a sphere
689
00:53:54.239 --> 00:53:57.280
grows with the square of its radius, So if you
690
00:53:57.400 --> 00:54:01.000
double the distance, the light is spread over four times
691
00:54:01.039 --> 00:54:05.000
the area, making it four times dimmer. Triple the distance,
692
00:54:05.559 --> 00:54:09.000
and the light is nine times dimmer. This is called
693
00:54:09.000 --> 00:54:13.400
the inverse square law, and its fundamental to measuring cosmic distances.
694
00:54:14.000 --> 00:54:18.440
Type ere supernovae are like cosmic light bulbs, where we
695
00:54:18.519 --> 00:54:22.039
know the wattage. When we see one explode in a
696
00:54:22.079 --> 00:54:26.559
distant galaxy, we can measure its apparent brightness and calculate
697
00:54:26.599 --> 00:54:30.840
the galaxy's distance. This is more reliable than other distance
698
00:54:30.880 --> 00:54:34.960
measurement methods, which is why astronomers were so excited to
699
00:54:35.039 --> 00:54:39.559
use type ear supernovae to map the universe's expansion history.
700
00:54:40.039 --> 00:54:44.559
In the nineteen nineties, two competing research teams set out
701
00:54:44.599 --> 00:54:48.880
to use type ear supernovae to measure how much the
702
00:54:49.039 --> 00:54:55.079
universe's expansion was slowing down. Remember, everyone expected gravity to
703
00:54:55.159 --> 00:54:59.880
be slowing the expansion. The question was how much these
704
00:55:00.039 --> 00:55:02.639
measurements would determine whether we were headed for a big
705
00:55:02.679 --> 00:55:08.400
crunch or eternal expansion. Both teams, the Supernova Cosmology Project
706
00:55:08.760 --> 00:55:12.199
led by Saul pearl Mutter and the High Z Supernova
707
00:55:12.360 --> 00:55:16.880
Search Team led by Brian Schmidt and Adam Reis, used
708
00:55:16.960 --> 00:55:22.000
large telescopes to find and study distant supernovae. They looked
709
00:55:22.000 --> 00:55:25.599
at explosions that happened billions of years ago, so far
710
00:55:25.719 --> 00:55:28.760
away that their light was just reaching Earth now. By
711
00:55:28.760 --> 00:55:32.440
comparing these ancient explosions to more recent ones, they could
712
00:55:32.440 --> 00:55:35.960
measure how the expansion rate had changed. Over cosmic history.
713
00:55:36.360 --> 00:55:41.880
The results were shocking. The distant supernovae were dimmer than expected,
714
00:55:42.800 --> 00:55:47.039
not by a lot, but by a consistent measurable amount.
715
00:55:47.599 --> 00:55:50.760
They appeared about twenty five percent farther away than they
716
00:55:50.800 --> 00:55:54.239
should have been if the expansion was slowing down as predicted.
717
00:55:54.719 --> 00:55:59.360
This could only mean one thing. The expansion wasn't slowing down,
718
00:56:00.199 --> 00:56:05.400
was speeding up. The universe's expansion rate was accelerating, getting
719
00:56:05.519 --> 00:56:11.199
faster with time. This was completely unexpected. Gravity, the only
720
00:56:11.239 --> 00:56:15.800
force anyone knew about that acted on cosmic scales, should
721
00:56:15.840 --> 00:56:20.239
be slowing things down, not speeding them up. It would
722
00:56:20.280 --> 00:56:23.880
be like throwing a ball upward and watching it accelerate
723
00:56:23.920 --> 00:56:27.440
away from you instead of slowing down and falling back.
724
00:56:28.039 --> 00:56:33.239
Something was pushing the universe apart, overcoming gravity's attractive pull.
725
00:56:33.800 --> 00:56:37.639
The two teams announced their findings in nineteen ninety eight.
726
00:56:38.360 --> 00:56:44.280
The results were checked, rechecked, and verified. More supernovae were observed.
727
00:56:45.039 --> 00:56:49.920
Different teams used different methods. Everyone got the same answer,
728
00:56:50.760 --> 00:56:57.000
the expansion was accelerating. The three leaders of the two teams, Perlmutter, Schmidt,
729
00:56:57.239 --> 00:57:01.760
and Reese, won the Nobel Prize in Physics in twenty
730
00:57:01.800 --> 00:57:05.840
eleven for this discovery. But what was causing the acceleration.
731
00:57:06.880 --> 00:57:10.760
What force could push the entire universe apart overcoming the
732
00:57:10.800 --> 00:57:16.360
gravitational attraction of all the matter it contains. Physicists scrambled
733
00:57:16.360 --> 00:57:20.719
for explanations, and they found one in an unlikely place,
734
00:57:21.320 --> 00:57:27.280
in Einstein's discarded cosmological constant. Here's why it works. As
735
00:57:27.280 --> 00:57:32.880
the universe expands, normal matter's density decreases, the same atoms
736
00:57:32.920 --> 00:57:37.639
spread over Larger volumes mean weaker gravity, but the cosmological
737
00:57:37.719 --> 00:57:42.639
constant behaves differently as a property of space itself. More
738
00:57:42.719 --> 00:57:49.000
space means more total energy, while density remains constant. Early
739
00:57:49.079 --> 00:57:54.239
in cosmic history, matter was dense and gravity dominated. The
740
00:57:54.320 --> 00:57:59.599
constant existed but couldn't compete, But as expansion diluted the matter,
741
00:58:00.119 --> 00:58:05.400
gravity weakened. Eventually, about five billion years ago, the constant's
742
00:58:05.400 --> 00:58:10.119
influence became strong enough to take over. From that point forward,
743
00:58:10.679 --> 00:58:16.719
expansion began accelerating. This vacuum energy is now called dark energy.
744
00:58:17.559 --> 00:58:21.119
We label it dark because we can't observe it directly,
745
00:58:21.679 --> 00:58:26.280
only through its gravitational effects on the universe's expansion, and
746
00:58:26.360 --> 00:58:31.199
it comprises about sixty eight percent of all cosmic energy.
747
00:58:31.760 --> 00:58:36.760
Normal matter everything made of atoms accounts for only five percent.
748
00:58:37.559 --> 00:58:42.840
Another twenty seven percent is dark matter, invisible substance we
749
00:58:42.960 --> 00:58:49.400
detect through gravitational influence but haven't yet identified. Dark energy
750
00:58:49.920 --> 00:58:54.880
dominates the cosmos. Einstein added the cosmological constant to create
751
00:58:54.880 --> 00:58:58.800
a static universe. He thought he was making a mistake
752
00:58:58.880 --> 00:59:01.840
when he included it, and a bigger mistake by not
753
00:59:01.920 --> 00:59:07.000
trusting his original equations. But those equations with the cosmological
754
00:59:07.079 --> 00:59:11.559
constant included turn out to describe the actual universe we
755
00:59:11.679 --> 00:59:17.199
live in. The constant is real, the vacuum energy is real.
756
00:59:17.719 --> 00:59:22.320
Einstein's biggest blunder was actually a profound insight into the
757
00:59:22.400 --> 00:59:27.199
nature of reality. Think about how remarkable this is. Einstein,
758
00:59:27.480 --> 00:59:31.199
working with nothing but mathematics and physical intuition, predicted the
759
00:59:31.280 --> 00:59:34.880
existence of dark energy eighty years before it was discovered.
760
00:59:35.519 --> 00:59:38.719
He got the reasoning wrong. He thought this energy would
761
00:59:38.800 --> 00:59:42.920
keep the universe static, when in fact it makes it accelerate.
762
00:59:43.239 --> 00:59:48.039
But the fundamental insight that space itself possesses energy, that
763
00:59:48.119 --> 00:59:51.719
there's something pushing the universe apart that turned out to
764
00:59:51.760 --> 00:59:55.360
be correct. This is how science works at its best.
765
00:59:56.199 --> 01:00:00.920
You make predictions based on theory. Sometimes those predictions come
766
01:00:00.920 --> 01:00:05.679
from deep insight and sometimes from mistakes, but then you
767
01:00:05.800 --> 01:00:12.400
test them against reality. Observations and experiments are the final judge.
768
01:00:12.840 --> 01:00:18.760
Einstein's cosmological constant survived because it matched what we actually observe,
769
01:00:19.280 --> 01:00:22.639
not because Einstein wanted it to be true. Let's dig
770
01:00:22.719 --> 01:00:26.719
deeper into what dark energy actually is, because even though
771
01:00:26.719 --> 01:00:30.039
we've given it a name, we still don't really understand it.
772
01:00:30.840 --> 01:00:34.239
Dark energy is one of the greatest mysteries in modern physics.
773
01:00:34.760 --> 01:00:37.519
We know it exists because we can see its effects.
774
01:00:38.039 --> 01:00:42.239
The acceleration of the Universe's expansion is real and measurable,
775
01:00:43.000 --> 01:00:46.239
but we don't know what dark energy actually is at
776
01:00:46.239 --> 01:00:51.360
a fundamental level. The simplest explanation is that it's exactly
777
01:00:51.400 --> 01:00:57.239
what Einstein proposed, a cosmological constant, a fixed amount of
778
01:00:57.400 --> 01:01:01.199
energy in every cubic meter or que yard of space.
779
01:01:01.920 --> 01:01:05.119
This energy doesn't change with time or location. It's the
780
01:01:05.159 --> 01:01:10.159
same everywhere always. It's a property of space itself, like
781
01:01:10.239 --> 01:01:14.119
space has an intrinsic tendency to expand think of it
782
01:01:14.159 --> 01:01:19.920
this way. Every cubic meter of empty space, a volume
783
01:01:19.960 --> 01:01:24.280
about the size of a large refrigerator, contains a tiny
784
01:01:24.320 --> 01:01:29.920
amount of dark energy. This energy is incredibly small, about
785
01:01:29.920 --> 01:01:33.199
ten to the power of minus nine duels per cubic meter.
786
01:01:33.760 --> 01:01:37.400
That's one billionth of a duel in a cubic meter.
787
01:01:38.079 --> 01:01:42.239
A single triple a battery contains about five thousand duels
788
01:01:42.280 --> 01:01:45.480
of energy, So the dark energy in a cubic meter
789
01:01:45.599 --> 01:01:49.559
of space is about five trillion times less than the
790
01:01:49.679 --> 01:01:53.960
energy in a battery. But space is vast. There's a
791
01:01:54.000 --> 01:01:57.280
lot of cubic meters in the universe. When you add
792
01:01:57.360 --> 01:02:01.519
up all that tiny energy over all of space, it
793
01:02:01.559 --> 01:02:05.559
becomes the dominant form of energy in the cosmos. It's
794
01:02:05.719 --> 01:02:09.880
like having an enormous pile of pennies. Each penny is
795
01:02:09.960 --> 01:02:15.000
nearly worthless, but billions of pennies add up to real money.
796
01:02:15.360 --> 01:02:19.480
But where does this energy come from? One possibility comes
797
01:02:19.480 --> 01:02:25.559
from quantum mechanics. In quantum theory, even empty space isn't
798
01:02:25.639 --> 01:02:30.639
truly empty. The vacuum is filled with virtual particles constantly
799
01:02:30.679 --> 01:02:34.480
popping into existence and then annihilating each other in tiny
800
01:02:34.559 --> 01:02:39.239
fractions of a second. These quantum fluctuations should create a
801
01:02:39.320 --> 01:02:43.800
kind of vacuum energy, a background energy that fills all
802
01:02:43.840 --> 01:02:47.239
of space. Quantum mechanics says that you can't have a
803
01:02:47.320 --> 01:02:52.960
perfectly empty vacuum with exactly zero energy. The Heisenberg uncertainty principle,
804
01:02:53.800 --> 01:02:57.679
one of the fundamental laws of quantum mechanics, says that
805
01:02:57.719 --> 01:02:59.880
if you try to pin down the energy of a
806
01:03:00.239 --> 01:03:05.760
region of space to exactly zero, you create infinite uncertainty
807
01:03:06.119 --> 01:03:10.760
in when that energy exists, which violates the principle. So
808
01:03:10.880 --> 01:03:15.639
the vacuum must have some energy constantly fluctuating around a
809
01:03:15.679 --> 01:03:19.800
non zero average. When physicists try to calculate how much
810
01:03:19.880 --> 01:03:24.719
vacuum energy these quantum fluctuations should produce, they get an answer,
811
01:03:25.599 --> 01:03:30.800
but it's the wrong answer, spectacularly wrong. The calculated value
812
01:03:31.000 --> 01:03:33.719
is about ten to the power of one hundred twenty
813
01:03:33.800 --> 01:03:38.320
times larger than the observed value of dark energy. That's
814
01:03:38.400 --> 01:03:42.079
one followed by one hundred twenty zeros. It's one of
815
01:03:42.119 --> 01:03:45.800
the worst predictions in the history of physics, a mismatch
816
01:03:45.920 --> 01:03:49.880
so enormous it's hard to even comprehend to put this
817
01:03:49.960 --> 01:03:54.880
in perspective. If the predicted vacuum energy were correct, the
818
01:03:55.000 --> 01:03:59.559
universe would be expanding so fast that atoms couldn't hold together.
819
01:04:00.400 --> 01:04:05.000
Galaxies would be torn apart, stars would be ripped apart,
820
01:04:05.639 --> 01:04:11.480
Planets would be destroyed, life would be impossible. The fact
821
01:04:11.519 --> 01:04:14.599
that we exist at all tells us that the actual
822
01:04:14.719 --> 01:04:19.119
vacuum energy must be much much smaller than the naive
823
01:04:19.239 --> 01:04:25.519
quantum mechanical prediction. This is called the cosmological constant problem.
824
01:04:25.679 --> 01:04:28.880
Why is the observed vacuum energy so much smaller than
825
01:04:28.920 --> 01:04:33.360
the predicted value. Why isn't it zero? And why is
826
01:04:33.400 --> 01:04:37.480
it the specific value it is, which is just large
827
01:04:37.559 --> 01:04:42.360
enough to start dominating the universe's expansion a few billion
828
01:04:42.480 --> 01:04:47.360
years ago, coincidentally, right around the time complex life was
829
01:04:47.400 --> 01:04:51.519
developing on Earth. Some physicists think there must be some
830
01:04:51.840 --> 01:04:56.119
unknown mechanism that cancels out most of the quantum vacuum energy,
831
01:04:56.719 --> 01:05:00.639
leaving behind just the tiny amount we observe as dar energy.
832
01:05:01.119 --> 01:05:04.280
But we don't know what that mechanism is or why
833
01:05:04.320 --> 01:05:07.840
it would leave behind exactly the amount it does. It's
834
01:05:07.960 --> 01:05:11.480
like having a complicated calculation that should give you an
835
01:05:11.679 --> 01:05:18.039
enormous number, but through some mysterious process involving billions of cancelations,
836
01:05:18.400 --> 01:05:22.599
you end up with a tiny number. We don't understand
837
01:05:22.920 --> 01:05:28.159
the cancelation mechanism. Others propose that dark energy isn't constant
838
01:05:28.199 --> 01:05:32.840
at all. Maybe it changes with time, growing stronger or
839
01:05:32.880 --> 01:05:37.119
weaker as the universe evolves. This would be a different
840
01:05:37.320 --> 01:05:44.079
kind of energy, not Einstein's cosmological constant, and physicists call
841
01:05:44.119 --> 01:05:49.199
it quintessence, after the fifth element of ancient philosophy. If
842
01:05:49.280 --> 01:05:53.440
dark energy is quintessence, it could behave differently in the
843
01:05:53.480 --> 01:05:56.840
future than it does now, leading to different fates for
844
01:05:56.880 --> 01:06:00.760
the universe. Still, others suggest we might need to modify
845
01:06:00.840 --> 01:06:05.719
general relativity itself. Maybe Einstein's equations work perfectly on the
846
01:06:05.760 --> 01:06:08.960
scale of the Solar System but need correction when applied
847
01:06:09.000 --> 01:06:12.800
to the entire universe. Maybe there's an additional term or
848
01:06:12.840 --> 01:06:17.320
modification that would explain the acceleration without needing dark energy
849
01:06:17.400 --> 01:06:22.440
at all. There's even more exotic proposals. Some physicists suggest
850
01:06:22.760 --> 01:06:26.079
that dark energy might be connected to extra dimensions of
851
01:06:26.159 --> 01:06:31.199
space beyond the three we experience. String theory, a leading
852
01:06:31.280 --> 01:06:34.920
candidate for a theory of quantum gravity, predicts that space
853
01:06:35.000 --> 01:06:38.760
has ten or eleven dimensions, most of them curled up
854
01:06:38.800 --> 01:06:42.880
so small we can't detect them. Maybe dark energy leaks
855
01:06:42.880 --> 01:06:47.039
into our three dimensions from these extra dimensions. Maybe the
856
01:06:47.079 --> 01:06:52.320
behavior of gravity changes on cosmic scales because it's affected
857
01:06:52.400 --> 01:06:56.880
by these hidden dimensions. Other proposals suggest that our universe
858
01:06:57.159 --> 01:07:02.159
might be embedded in a larger structure ultiverse containing many universes.
859
01:07:02.639 --> 01:07:06.840
Maybe dark energy represents the influence of other universes on ours,
860
01:07:07.199 --> 01:07:11.960
a kind of pressure from outside pushing our universe to expand.
861
01:07:12.559 --> 01:07:16.719
This sounds like science fiction, but it's based on serious
862
01:07:16.840 --> 01:07:22.519
mathematical models explored by leading physicists. These are all active
863
01:07:22.679 --> 01:07:28.400
areas of research. Physicists are designing new experiments and observations
864
01:07:28.760 --> 01:07:33.119
to try to distinguish between these possibilities. They're measuring the
865
01:07:33.239 --> 01:07:38.039
universe's expansion rate more precisely, looking for any signs that
866
01:07:38.239 --> 01:07:42.760
dark energy is changing with time, and testing general relativity
867
01:07:43.079 --> 01:07:47.079
on larger and larger scales. The fate of the universe
868
01:07:47.400 --> 01:07:51.719
depends on dark energy's nature. If it's truly a cosmological
869
01:07:51.800 --> 01:07:57.360
constant that never changes, then the universe will continue accelerating forever.
870
01:07:57.800 --> 01:08:04.360
Galaxies will move apart faster and faster. Eventually, in hundreds
871
01:08:04.360 --> 01:08:08.159
of billions of years, galaxies outside our local group will
872
01:08:08.159 --> 01:08:11.960
be receding so fast that their light can never reach us.
873
01:08:12.480 --> 01:08:16.399
The universe will become dark and cold and empty. From
874
01:08:16.479 --> 01:08:21.359
any observer's perspective. If dark energy is quintessence that can
875
01:08:21.439 --> 01:08:25.760
change strength, Then the future is less certain. Maybe it
876
01:08:25.760 --> 01:08:29.000
will weaken eventually and the universe will coast along at
877
01:08:29.000 --> 01:08:33.920
a steady expansion rate. Maybe it will strengthen dramatically, tearing
878
01:08:33.960 --> 01:08:39.560
apart galaxies, then stars, then planets, then atoms themselves in
879
01:08:39.600 --> 01:08:43.760
a big rip. Or maybe it will reverse, becoming attractive
880
01:08:43.800 --> 01:08:47.319
instead of repulsive, and the universe will collapse in a
881
01:08:47.359 --> 01:08:51.680
big crunch. After all, we simply don't know yet. The
882
01:08:51.760 --> 01:08:55.640
observations we have so far are consistent with dark energy
883
01:08:56.000 --> 01:09:01.000
being a cosmological constant unchanging with time, but the error
884
01:09:01.039 --> 01:09:04.359
bars are still large enough that we can't rule out
885
01:09:04.479 --> 01:09:09.960
other possibilities. Future observations will narrow down the possibilities and
886
01:09:10.079 --> 01:09:13.560
hopefully give us a clearer picture of what dark energy
887
01:09:13.680 --> 01:09:17.279
really is and what it means for the universe's future.
888
01:09:17.800 --> 01:09:21.560
Let's talk about some of the other interpretations and implications
889
01:09:22.039 --> 01:09:25.600
of Einstein's so called blunder, because the story of the
890
01:09:25.640 --> 01:09:30.520
cosmological constant is really a story about how science works
891
01:09:31.239 --> 01:09:36.079
and how even mistakes can lead to progress. First, was
892
01:09:36.119 --> 01:09:42.399
this really Einstein's biggest mistake? Einstein himself apparently thought so,
893
01:09:42.840 --> 01:09:46.159
or at least that's what he told people later in life,
894
01:09:46.560 --> 01:09:50.640
But historians of science have pointed out that Einstein made
895
01:09:50.680 --> 01:09:55.680
other errors that were arguably more significant. For example, Einstein
896
01:09:55.760 --> 01:09:58.880
spent the last thirty years of his life trying to
897
01:09:58.960 --> 01:10:03.640
develop a u unified field theory, a single framework that
898
01:10:03.680 --> 01:10:08.199
would combine gravity and electromagnetism. He was trying to find
899
01:10:08.239 --> 01:10:11.560
a theory of everything, a set of equations that would
900
01:10:11.600 --> 01:10:15.560
describe all the forces of nature and all the particles
901
01:10:15.560 --> 01:10:20.439
of matter. He pursued this goal obsessively, filling note books
902
01:10:20.479 --> 01:10:26.039
with calculations and trying approach after approach. He never succeeded,
903
01:10:26.680 --> 01:10:29.920
and in hindsight, his approach was doomed from the start
904
01:10:30.159 --> 01:10:33.520
because he was trying to unify the wrong things. He
905
01:10:33.640 --> 01:10:38.520
was ignoring quantum mechanics, which he deeply mistrusted, and focusing
906
01:10:38.560 --> 01:10:42.920
only on classical fields. But the universe is quantum at
907
01:10:42.920 --> 01:10:47.960
its core. Any theory of everything must incorporate quantum mechanics.
908
01:10:48.479 --> 01:10:53.920
Einstein's refusal to fully embrace quantum theory, despite being one
909
01:10:53.920 --> 01:10:56.800
of its founders with his work on the photoelectric effect,
910
01:10:57.439 --> 01:11:00.239
meant he was looking in the wrong direction. Though those
911
01:11:00.279 --> 01:11:03.560
thirty years could have been spent on more productive research.
912
01:11:04.560 --> 01:11:09.239
Einstein could have contributed to quantum field theory or nuclear physics,
913
01:11:09.479 --> 01:11:13.479
or other areas where his insights might have been valuable. Instead,
914
01:11:14.279 --> 01:11:20.279
he pursued a dead end isolated from mainstream physics, increasingly
915
01:11:20.359 --> 01:11:24.880
seen as out of touch by younger physicists. From that perspective,
916
01:11:25.479 --> 01:11:29.399
his quest for a classical unified field theory was a
917
01:11:29.439 --> 01:11:34.319
bigger mistake than adding the cosmological constant. But there's a difference.
918
01:11:35.119 --> 01:11:39.479
The unified field theory work was simply wrong. It led
919
01:11:39.520 --> 01:11:44.720
nowhere and contributed nothing to future physics. The cosmological constant,
920
01:11:44.760 --> 01:11:47.840
on the other hand, turned out to describe something real
921
01:11:48.159 --> 01:11:52.439
about the universe, even if Einstein added it for wrong reasons.
922
01:11:53.079 --> 01:11:56.119
So while the unified field theory work might have been
923
01:11:56.159 --> 01:12:00.720
a bigger waste of Einstein's time and talent, the cosmological
924
01:12:00.880 --> 01:12:05.039
constant story is more interesting because of its resurrection in
925
01:12:05.119 --> 01:12:09.800
modern cosmology. It's also worth noting that the cosmological constant
926
01:12:10.159 --> 01:12:12.359
might not have been a mistake at all in the
927
01:12:12.399 --> 01:12:16.920
context of its time. In nineteen seventeen, there was no
928
01:12:17.039 --> 01:12:21.199
evidence the universe was expanding. Einstein was working from the
929
01:12:21.199 --> 01:12:26.319
best available information, which suggested a static cosmos. Adding a
930
01:12:26.439 --> 01:12:29.680
term to his equations to allow a static solution was
931
01:12:29.760 --> 01:12:34.239
perfectly reasonable given what was known then. The mistake, if
932
01:12:34.279 --> 01:12:37.119
there was one, was not removing the constant as soon
933
01:12:37.119 --> 01:12:42.359
as better observations became available. But even that's understandable. Scientific
934
01:12:42.399 --> 01:12:47.079
paradigms don't shift instantly when new data arrives. It takes
935
01:12:47.119 --> 01:12:51.319
time for evidence to accumulate, for alternative explanations to be
936
01:12:51.399 --> 01:12:56.520
ruled out, and for consensus to form. Einstein accepted Hubble's
937
01:12:56.520 --> 01:13:01.399
observations fairly quickly by the standards of scientific revel Another
938
01:13:01.520 --> 01:13:04.279
interesting aspect of this story is what it tells us
939
01:13:04.279 --> 01:13:09.520
about the relationship between mathematics and physics. Einstein's equations predicted
940
01:13:09.760 --> 01:13:13.880
an unstable universe. He modified them to get a static universe.
941
01:13:14.399 --> 01:13:17.920
The original equations turned out to be right, but the
942
01:13:17.960 --> 01:13:22.760
modified version also turned out to describe something real. This
943
01:13:23.159 --> 01:13:30.159
raises a philosophical question. Do mathematical equations describe reality or
944
01:13:30.199 --> 01:13:35.039
do they just provide useful models that approximately match observations.
945
01:13:35.920 --> 01:13:41.039
Physicists generally lean toward the first view. They believe the
946
01:13:41.159 --> 01:13:46.600
universe operates according to mathematical laws, and our equations, when correct,
947
01:13:47.039 --> 01:13:52.479
are discovering those laws. Rather than just modeling observations, but
948
01:13:52.560 --> 01:13:58.279
the cosmological constant story complicates this. Einstein added a term
949
01:13:58.359 --> 01:14:01.840
for non physical reasons to get an answer he wanted
950
01:14:02.520 --> 01:14:07.039
by pure mathematics. The term was allowed but not required,
951
01:14:07.640 --> 01:14:10.319
and yet it turned out to be necessary to describe
952
01:14:10.359 --> 01:14:14.840
the actual universe. Does this mean the mathematics somehow knew
953
01:14:15.000 --> 01:14:19.800
more than Einstein did or is it just a lucky coincidence.
954
01:14:20.359 --> 01:14:24.399
There's no clear answer, but it does suggest that mathematics
955
01:14:24.600 --> 01:14:28.600
has a way of surprising us, of containing truths. We
956
01:14:28.680 --> 01:14:34.720
don't immediately recognize. Einstein's equations in their full generality, including
957
01:14:34.800 --> 01:14:39.119
all the terms that are mathematically possible, described a universe
958
01:14:39.399 --> 01:14:43.479
that could be static with fine tuned lambda, or expanding
959
01:14:44.119 --> 01:14:50.720
or contracting or accelerating. The equations contained all these possibilities
960
01:14:51.760 --> 01:14:56.680
observations told us which possibility is real. Let's also consider
961
01:14:56.920 --> 01:15:00.960
what this means for scientific reasoning and how scientists should
962
01:15:01.000 --> 01:15:06.439
approach their work. Einstein let his philosophical preferences guide his
963
01:15:06.560 --> 01:15:12.640
modifications to general relativity. He wanted an eternal, unchanging universe,
964
01:15:13.279 --> 01:15:17.840
and he adjusted his equations to provide one. In the end,
965
01:15:18.439 --> 01:15:23.640
this was wrong. The universe isn't static. Does this mean
966
01:15:23.800 --> 01:15:29.560
scientists should never let philosophical or esthetic considerations guide their work.
967
01:15:30.399 --> 01:15:35.319
Should they only follow the math wherever it leads. Not necessarily,
968
01:15:35.760 --> 01:15:41.920
Physical intuition, esthetic judgment, and philosophical reasoning have played crucial
969
01:15:42.000 --> 01:15:46.880
roles in many scientific advances. Einstein himself relied heavily on
970
01:15:47.000 --> 01:15:52.359
thought experiments and intuitive reasoning to develop both special and
971
01:15:53.079 --> 01:15:57.640
general relativity. The lesson isn't that philosophical reasoning is bad.
972
01:15:58.319 --> 01:16:02.600
It's that observations are the ultimate arbiter. You can use
973
01:16:02.640 --> 01:16:06.640
whatever reasoning you want to develop theories and make predictions,
974
01:16:07.159 --> 01:16:10.399
but then you have to test those predictions against reality,
975
01:16:10.880 --> 01:16:14.439
and when reality disagrees with your preferences, you have to
976
01:16:14.560 --> 01:16:18.720
change your mind. Einstein did change his mind when Hubble's
977
01:16:18.760 --> 01:16:23.560
observations came in. He accepted that the universe was expanding
978
01:16:23.960 --> 01:16:28.359
and that his static model was wrong. That's good science.
979
01:16:29.039 --> 01:16:32.880
Where he perhaps aired was in adding the cosmological constant
980
01:16:32.920 --> 01:16:38.119
in the first place without observational justification, purely to get
981
01:16:38.199 --> 01:16:41.840
a desired result. The modern attitude in physics is generally
982
01:16:41.880 --> 01:16:46.840
more conservative about adding new terms or modifying successful theories.
983
01:16:47.359 --> 01:16:52.239
If your equations predict something that seems strange or counterintuitive,
984
01:16:52.920 --> 01:16:56.840
you should think very carefully before changing them. Often, the
985
01:16:56.960 --> 01:17:02.079
strange prediction turns out to be correct. Special relativity predicted
986
01:17:02.119 --> 01:17:06.760
time dilation, which seemed absurd when Einstein first proposed it.
987
01:17:07.279 --> 01:17:12.399
General relativity predicted black holes, which many physicists thought must
988
01:17:12.439 --> 01:17:17.960
be mathematical artifacts rather than real objects. Quantum mechanics predicted
989
01:17:18.000 --> 01:17:22.520
particles could be in multiple places at once, which violated
990
01:17:22.720 --> 01:17:26.960
common sense. All of these strange predictions turned out to
991
01:17:27.000 --> 01:17:31.920
be correct. So when your beautiful, successful theory predicts something
992
01:17:31.960 --> 01:17:35.640
you don't like, maybe the problem is with your intuition,
993
01:17:36.279 --> 01:17:40.600
not with the theory. Maybe the universe's stranger than you thought.
994
01:17:41.239 --> 01:17:46.600
That's the lesson of the cosmological constant. Einstein's intuition told
995
01:17:46.680 --> 01:17:51.720
him the universe should be static. His equations told him
996
01:17:52.199 --> 01:17:58.279
it couldn't be. He modified the equations. The equations were right.
997
01:17:58.880 --> 01:18:01.800
Let's zoom out and look at the bigger picture of
998
01:18:01.880 --> 01:18:05.279
what we've learned about the universe. Thanks to Einstein's work,
999
01:18:05.920 --> 01:18:10.880
both his successes and his mistakes, general relativity has become
1000
01:18:10.920 --> 01:18:16.000
the foundation of modern cosmology. Every major discovery about the
1001
01:18:16.119 --> 01:18:19.640
large scale structure and evolution of the universe has been
1002
01:18:19.680 --> 01:18:24.520
interpreted through the framework Einstein provided. We've learned that space
1003
01:18:24.560 --> 01:18:29.199
and time are not separate absolute things, but are woven
1004
01:18:29.279 --> 01:18:34.880
together into a single fabric called space time. This fabric
1005
01:18:35.119 --> 01:18:42.159
can bend, stretch, compress, and ripple. Massive objects create depressions
1006
01:18:42.159 --> 01:18:47.199
in space time, and we experience these depressions as gravity.
1007
01:18:47.760 --> 01:18:51.000
We've learned that the universe had a beginning, the Big Bang,
1008
01:18:51.680 --> 01:18:55.960
about thirteen point eight billion years ago. Before that moment,
1009
01:18:56.399 --> 01:19:00.680
there was no space, no time, no matter, no energy.
1010
01:19:01.159 --> 01:19:04.720
The Big Bang wasn't an explosion in space. It was
1011
01:19:04.760 --> 01:19:08.239
the beginning of space itself, the moment when space time
1012
01:19:08.279 --> 01:19:12.199
came into existence and began expanding. We've learned that this
1013
01:19:12.319 --> 01:19:16.479
expansion is accelerating, driven by dark energy that makes up
1014
01:19:16.520 --> 01:19:20.439
most of the universe's total energy content. We don't fully
1015
01:19:20.520 --> 01:19:23.960
understand what dark energy is, but we can measure its
1016
01:19:24.000 --> 01:19:28.479
effects and predict its influence on the universe's future. We've
1017
01:19:28.560 --> 01:19:31.840
learned that most of the matter in the universe is
1018
01:19:31.960 --> 01:19:37.039
dark matter, another form of invisible substance that doesn't interact
1019
01:19:37.039 --> 01:19:42.399
with light but creates gravitational effects. Galaxies are embedded in
1020
01:19:42.560 --> 01:19:46.560
huge halos of dark matter that outweigh the visible matter
1021
01:19:46.640 --> 01:19:50.800
by a factor of six to one. Without dark matter's
1022
01:19:50.840 --> 01:19:56.279
extra gravity, galaxies would fly apart, stars would not orbit
1023
01:19:56.560 --> 01:20:01.159
the way they do. We've learned about black holes, regions
1024
01:20:01.199 --> 01:20:05.039
where space time is curved so severely that not even
1025
01:20:05.199 --> 01:20:10.159
light can escape. We've observed stellar mass black holes created
1026
01:20:10.199 --> 01:20:15.119
by collapsing stars, supermassive black holes, millions or billions of
1027
01:20:15.199 --> 01:20:18.560
times the Sun's mass sitting at the centers of galaxies,
1028
01:20:19.119 --> 01:20:22.479
and even the merger of two black holes detected through
1029
01:20:22.560 --> 01:20:27.800
gravitational waves. We've learned about the cosmic microwave background, the
1030
01:20:27.840 --> 01:20:31.520
after glow of the Big Bang, a faint radiation that
1031
01:20:31.680 --> 01:20:35.640
fills all of space and provides a snap shot of
1032
01:20:35.720 --> 01:20:38.600
what the universe looked like when it was only three
1033
01:20:38.720 --> 01:20:43.600
hundred eighty thousand years old. By studying tiny variations in
1034
01:20:43.640 --> 01:20:47.119
this radiation, we can learn about the seeds that grew
1035
01:20:47.199 --> 01:20:51.880
into galaxies and the fundamental properties of the cosmos. All
1036
01:20:51.960 --> 01:20:56.199
of this understanding rests on the foundation Einstein built with
1037
01:20:56.359 --> 01:21:01.039
general relativity. His equations have been tested counts times in
1038
01:21:01.119 --> 01:21:05.880
countless ways, from the bending of starlight during eclipses to
1039
01:21:05.960 --> 01:21:10.439
the time dilation of GPS satellites to the gravitational waves
1040
01:21:10.439 --> 01:21:15.399
from colliding black holes. Every test has confirmed that Einstein
1041
01:21:15.600 --> 01:21:20.279
was right, and yet general relativity is not the final word.
1042
01:21:20.920 --> 01:21:24.239
We know it must be incomplete because it doesn't play
1043
01:21:24.359 --> 01:21:29.880
nicely with quantum mechanics. General relativity is a classical theory.
1044
01:21:30.439 --> 01:21:35.960
It describes spacetime as smooth and continuous. Quantum mechanics says
1045
01:21:36.000 --> 01:21:41.079
everything is discreete and probabilistic. At the smallest scales, these
1046
01:21:41.119 --> 01:21:44.439
two frameworks are best theories of the very large and
1047
01:21:44.479 --> 01:21:48.760
the very small contradict each other. When pushed to extremes
1048
01:21:49.359 --> 01:21:53.359
inside a black hole. At the singularity, where all the
1049
01:21:53.399 --> 01:21:58.920
masses compress to infinite density, both gravity and quantum effects
1050
01:21:59.359 --> 01:22:07.039
become import General relativity predicts infinite density an infinite spacetime curvature,
1051
01:22:07.479 --> 01:22:12.760
but quantum mechanics doesn't allow true infinities. Something has to give.
1052
01:22:13.279 --> 01:22:16.720
At the moment of the Big Bang, the entire universe
1053
01:22:16.880 --> 01:22:21.039
was compressed to a size smaller than an atom. Again,
1054
01:22:21.560 --> 01:22:26.279
both gravity and quantum effects matter. We can't use general
1055
01:22:26.359 --> 01:22:30.439
relativity alone to describe what happened at that moment because
1056
01:22:30.439 --> 01:22:35.359
we need quantum gravity. A theory that combines general relativity
1057
01:22:35.640 --> 01:22:39.920
with quantum mechanics in a consistent way. Physicists have been
1058
01:22:39.960 --> 01:22:46.279
working on quantum gravity for decades. String theory, loop quantum gravity,
1059
01:22:46.720 --> 01:22:50.439
and other approaches try to build a framework where space
1060
01:22:50.520 --> 01:22:54.680
time itself becomes quantum, where space and time are made
1061
01:22:54.720 --> 01:22:58.960
of discrete units rather than being continuous. But we don't
1062
01:22:58.960 --> 01:23:02.920
have a complete, tested theory yet. This is one of
1063
01:23:02.920 --> 01:23:07.439
the great unsolved problems in physics. When we do finally
1064
01:23:07.479 --> 01:23:10.560
develop a theory of quantum gravity, it might change our
1065
01:23:10.640 --> 01:23:15.720
understanding of dark energy and the cosmological constant. The vacuum
1066
01:23:15.800 --> 01:23:19.760
energy that makes up dark energy is inherently quantum in nature.
1067
01:23:20.199 --> 01:23:23.359
A complete theory should be able to calculate its value
1068
01:23:23.399 --> 01:23:27.760
from first principles, explaining why it is what it is,
1069
01:23:28.479 --> 01:23:33.399
rather than just measuring it. That calculation might resolve the
1070
01:23:33.439 --> 01:23:38.000
cosmological constant problem, or might reveal that dark energy is
1071
01:23:38.039 --> 01:23:42.520
something entirely different from what we currently think. So Einstein's
1072
01:23:42.600 --> 01:23:47.359
legacy in cosmology is twofold. He gave us the framework
1073
01:23:47.439 --> 01:23:51.199
we still use to understand the universe, and he left
1074
01:23:51.279 --> 01:23:55.319
us with puzzles that still haven't been solved. The cosmological
1075
01:23:55.399 --> 01:23:59.439
constant was his attempt to solve one puzzle his desire
1076
01:23:59.520 --> 01:24:04.159
for a stativerse. He removed it when observations showed that
1077
01:24:04.359 --> 01:24:08.359
wasn't the right puzzle to solve, Modern cosmology brought it
1078
01:24:08.479 --> 01:24:12.840
back to solve a different puzzle, the acceleration of cosmic expansion.
1079
01:24:13.560 --> 01:24:16.439
But the deeper puzzle of why dark energy has the
1080
01:24:16.520 --> 01:24:21.319
value it does remains unsolved. Let's talk about what all
1081
01:24:21.399 --> 01:24:25.079
this means for the future, both the future of cosmology
1082
01:24:25.119 --> 01:24:28.880
as a science and the future of the universe itself.
1083
01:24:29.920 --> 01:24:34.640
Where do we go from here? For cosmology, the immediate
1084
01:24:34.720 --> 01:24:39.159
goal is to better understand dark energy. Is it truly
1085
01:24:39.239 --> 01:24:43.479
constant or does it change with time? Is it the
1086
01:24:43.560 --> 01:24:48.800
vacuum energy predicted by quantum mechanics or something else Entirely
1087
01:24:49.279 --> 01:24:55.560
To answer these questions, astronomers are conducting increasingly precise observations
1088
01:24:55.920 --> 01:25:00.640
of the universe's expansion history. Projects like the Dark Energy
1089
01:25:00.680 --> 01:25:06.199
Survey and the upcoming Verrubin Observatory are mapping millions of galaxies,
1090
01:25:06.600 --> 01:25:10.920
measuring their distances and redshifts to trace how the expansion
1091
01:25:11.000 --> 01:25:15.520
rate has changed over time. The James Webb Space Telescope
1092
01:25:15.560 --> 01:25:18.279
is looking at some of the most distant galaxies in
1093
01:25:18.319 --> 01:25:22.279
the universe, seeing them as they were more than thirteen
1094
01:25:22.439 --> 01:25:26.720
billion years ago, when the universe was young. By comparing
1095
01:25:26.760 --> 01:25:31.640
the expansion rate at different times in cosmic history, astronomers
1096
01:25:31.680 --> 01:25:35.960
can determine whether dark energy has been getting stronger, weaker,
1097
01:25:36.520 --> 01:25:40.640
or staying the same. Current data suggests its constant, but
1098
01:25:40.720 --> 01:25:45.800
the uncertainties are still large. Another decade of observations should
1099
01:25:45.880 --> 01:25:50.199
narrow down the possibilities considerably. There are also efforts to
1100
01:25:50.319 --> 01:25:55.880
directly detect dark energy particles. If such particles exist, some
1101
01:25:55.960 --> 01:25:59.199
theories propose that dark energy might be carried by a
1102
01:25:59.279 --> 01:26:03.600
new type of article, similar to how electromagnetic force is
1103
01:26:03.680 --> 01:26:09.039
carried by photons. Experiments in underground laboratories are searching for
1104
01:26:09.119 --> 01:26:13.880
these hypothetical particles, though most physicists think they probably won't
1105
01:26:13.920 --> 01:26:17.840
find anything because dark energy doesn't seem to interact with
1106
01:26:17.960 --> 01:26:23.319
normal matter except through gravity. The most exciting possibility would
1107
01:26:23.319 --> 01:26:27.079
be finding evidence that general relativity needs to be modified
1108
01:26:27.079 --> 01:26:31.800
on cosmic scales. If observations showed that gravity behaves differently
1109
01:26:31.880 --> 01:26:35.079
in different parts of the universe or at different times,
1110
01:26:35.560 --> 01:26:40.880
that would be revolutionary. It would mean Einstein's theory, while
1111
01:26:40.960 --> 01:26:45.239
correct in our solar system and our galaxy, breaks down
1112
01:26:45.359 --> 01:26:49.039
when applied to the universe as a whole. That would
1113
01:26:49.079 --> 01:26:53.920
open up entirely new areas of physics to explore. As
1114
01:26:53.920 --> 01:26:58.039
for the universe's future. If dark energy is truly a
1115
01:26:58.119 --> 01:27:03.119
cosmological constant that never changes, the story is rather bleak,
1116
01:27:03.680 --> 01:27:08.199
at least from a human perspective. The universe will continue
1117
01:27:08.239 --> 01:27:13.800
expanding forever. Galaxies outside our local group are already receding
1118
01:27:13.840 --> 01:27:19.159
from us, and that recession is accelerating. Eventually, in about
1119
01:27:19.159 --> 01:27:24.279
one hundred billion years, all galaxies beyond our immediate neighborhood
1120
01:27:24.520 --> 01:27:29.119
will have receded beyond our cosmic horizon. Their light will
1121
01:27:29.159 --> 01:27:32.479
be stretched to such long wavelengths by the expansion of
1122
01:27:32.560 --> 01:27:36.039
space that we'll never see them again. Think about what
1123
01:27:36.119 --> 01:27:40.479
this means. An astronomer living in the distant future, one
1124
01:27:40.600 --> 01:27:44.319
hundred billion years from now, looking out into the universe
1125
01:27:44.560 --> 01:27:49.560
with the most powerful telescopes imaginable, will see something completely
1126
01:27:49.680 --> 01:27:53.640
different from what we see today. They'll see only our
1127
01:27:53.680 --> 01:27:57.520
local group of galaxies, the Milky Way, and perhaps a
1128
01:27:57.520 --> 01:28:00.680
few dozen nearby galaxies that have merge merged with us
1129
01:28:01.159 --> 01:28:06.720
or remain gravitationally bound to us. Beyond that, nothing, just
1130
01:28:06.920 --> 01:28:12.800
empty darkness, extending in all directions forever. To an astronomer
1131
01:28:13.039 --> 01:28:16.760
living one hundred billion years from now in the Milky Way,
1132
01:28:17.279 --> 01:28:19.960
or in one of the few nearby galaxies that will
1133
01:28:20.000 --> 01:28:23.840
have merged with us. By then, the observable universe will
1134
01:28:23.840 --> 01:28:28.720
appear to contain only our local group of galaxies surrounded
1135
01:28:29.039 --> 01:28:32.960
by empty space. They'll see no evidence of cosmic expansion
1136
01:28:33.039 --> 01:28:38.079
because everything observable will be gravitationally bound together. They'll have
1137
01:28:38.159 --> 01:28:40.239
no way to know that there are trillions of other
1138
01:28:40.319 --> 01:28:43.720
galaxies beyond their horizon. They won't be able to see
1139
01:28:43.760 --> 01:28:47.479
the cosmic microwave background radiation because it will have been
1140
01:28:47.520 --> 01:28:51.479
red shifted into invisibility. They won't be able to see
1141
01:28:51.520 --> 01:28:55.439
distant galaxies moving away. They won't be able to detect
1142
01:28:55.640 --> 01:28:59.079
any of the evidence that we currently use to understand
1143
01:28:59.119 --> 01:29:02.920
the Big Bang and the universe's history. From their perspective,
1144
01:29:03.279 --> 01:29:06.640
the universe will appear to consist of a single large
1145
01:29:06.720 --> 01:29:11.640
galaxy surrounded by infinite empty space. They might not even
1146
01:29:11.680 --> 01:29:14.720
be able to figure out that the universe had a beginning.
1147
01:29:15.279 --> 01:29:18.000
In fact, they might not even know that the Big
1148
01:29:18.039 --> 01:29:22.920
Bang happened. The cosmic microwave background radiation will have been
1149
01:29:22.960 --> 01:29:26.560
red shifted to such long wave lengths that it will
1150
01:29:26.640 --> 01:29:31.800
be undetectable. The evidence that our current generation of astronomers
1151
01:29:31.800 --> 01:29:35.800
has used to understand the Universe's history and structure will
1152
01:29:35.800 --> 01:29:41.239
simply be gone, carried beyond the horizon by the accelerating expansion.
1153
01:29:41.800 --> 01:29:45.159
This is a sobering thought. We live at a privileged
1154
01:29:45.199 --> 01:29:49.319
time in cosmic history. When the universe's nature and history
1155
01:29:49.479 --> 01:29:53.680
are observable past a certain point in the future, that
1156
01:29:53.840 --> 01:29:58.680
window closes. The universe becomes opaque to its own history.
1157
01:29:59.159 --> 01:30:02.439
Information about where we came from and what else exists
1158
01:30:02.479 --> 01:30:08.119
out there becomes permanently inaccessible. Future civilizations, if any exist,
1159
01:30:08.800 --> 01:30:11.840
will be unable to learn what we know about the cosmos,
1160
01:30:12.159 --> 01:30:15.319
simply because the evidence will have moved beyond their reach.
1161
01:30:15.840 --> 01:30:20.520
On even longer time scales, the universe becomes truly lonely.
1162
01:30:21.479 --> 01:30:24.920
Stars will continue forming from available gas for a few
1163
01:30:25.039 --> 01:30:30.119
tens of billions of years. Eventually, star formation will stop
1164
01:30:30.439 --> 01:30:33.800
when all the available gas has been converted to stars
1165
01:30:34.319 --> 01:30:37.479
or locked up in black holes and dead stellar remnants.
1166
01:30:38.279 --> 01:30:41.600
The last stars will burn out in about ten trillion
1167
01:30:41.680 --> 01:30:46.960
years or so. That's ten thousand billion years, about seven
1168
01:30:47.039 --> 01:30:50.199
hundred times longer than the current age of the universe.
1169
01:30:50.840 --> 01:30:56.359
After that, the universe will be dark, dead stars, brown dwarfs,
1170
01:30:56.560 --> 01:31:00.520
white dwarfs, neutrons, stars, and black holes will be all
1171
01:31:00.600 --> 01:31:06.520
that remains. Even these will eventually fade. White dwarfs will
1172
01:31:06.560 --> 01:31:10.399
cool to black dwarfs, dark cinders of carbon and oxygen
1173
01:31:10.800 --> 01:31:15.600
floating through space. Neutron stars will cool, though this takes
1174
01:31:15.600 --> 01:31:18.800
an incredibly long time because they're so dense and have
1175
01:31:18.920 --> 01:31:22.239
so little surface area to radiate away their heat, and
1176
01:31:22.319 --> 01:31:27.159
black holes will evaporate through Hawking radiation, though this process
1177
01:31:27.199 --> 01:31:32.079
takes an incomprehensibly long time. Stephen Hawking predicted in the
1178
01:31:32.199 --> 01:31:37.560
nineteen seventies that black holes aren't completely black. They emit
1179
01:31:37.760 --> 01:31:41.920
radiation due to quantum effects near the event horizon. This
1180
01:31:42.000 --> 01:31:47.479
Hawking radiation is incredibly faint and incredibly slow, but given
1181
01:31:47.600 --> 01:31:51.239
enough time, it will cause even the largest black holes
1182
01:31:51.680 --> 01:31:55.840
to evaporate completely. A solar mass black hole evaporates in
1183
01:31:55.880 --> 01:31:59.439
about ten to the power of sixty seven years. That's
1184
01:31:59.479 --> 01:32:03.840
one fall by sixty seven zeros. To put this in perspective,
1185
01:32:04.800 --> 01:32:09.640
the universe is currently only about ten to the power
1186
01:32:10.279 --> 01:32:14.960
of ten years old fourteen billion years. A solar mass
1187
01:32:15.039 --> 01:32:20.680
black hole lasts about ten thousand, trillion, trillion, trillion, trillion
1188
01:32:20.840 --> 01:32:24.800
trillion times longer than the current age of the universe.
1189
01:32:25.279 --> 01:32:28.359
A supermassive black hole, like the one at the center
1190
01:32:28.399 --> 01:32:32.840
of our galaxy takes even longer, around ten to the
1191
01:32:32.880 --> 01:32:37.119
power of one hundred years. These numbers are so large
1192
01:32:37.439 --> 01:32:42.439
they become meaningless. We're talking about time scales that are
1193
01:32:42.520 --> 01:32:45.640
to the age of the universe, as the age of
1194
01:32:45.680 --> 01:32:49.560
the universe is to a single second. By the time
1195
01:32:49.640 --> 01:32:54.119
all black holes have evaporated, the universe will be unimaginably
1196
01:32:54.159 --> 01:33:00.880
old and unimaginably empty. Eventually, even protons might decay, though
1197
01:33:00.920 --> 01:33:05.800
this is speculative. Some theories of particle physics predict that
1198
01:33:05.920 --> 01:33:10.119
protons are unstable with a lifetime of ten to the
1199
01:33:10.119 --> 01:33:14.000
power of forty years or so. We've never observed proton decay,
1200
01:33:14.520 --> 01:33:18.279
so if it happens, it must be incredibly rare. But
1201
01:33:18.359 --> 01:33:23.279
given enough time, if protons do decay, then eventually all
1202
01:33:23.359 --> 01:33:28.199
normal matter will dissolve into radiation. The universe will contain
1203
01:33:28.279 --> 01:33:32.079
nothing but a thin mist of photons and other fundamental particles,
1204
01:33:32.319 --> 01:33:37.279
growing more dilute as space continues expanding forever. This is
1205
01:33:37.359 --> 01:33:40.760
called the heat death of the universe. A state of
1206
01:33:40.800 --> 01:33:45.039
maximum entropy where no energy differences exist, no work can
1207
01:33:45.079 --> 01:33:50.600
be done, and nothing interesting ever happens. The universe doesn't
1208
01:33:50.760 --> 01:33:57.079
end with a bang. It fades into an eternal, unchanging darkness.
1209
01:33:57.159 --> 01:34:00.600
All the stars, all the galaxies, all the black coals,
1210
01:34:01.039 --> 01:34:04.520
or the structure we see today will have dissolved into
1211
01:34:04.520 --> 01:34:10.119
a uniform sea of particles spreading ever farther apart through
1212
01:34:10.119 --> 01:34:15.119
an eternally expanding space in this far future, in this
1213
01:34:15.479 --> 01:34:20.560
heat death scenario, the universe approaches a state where nothing
1214
01:34:20.680 --> 01:34:26.079
changes anymore. There are no temperature differences to drive heat flows,
1215
01:34:26.720 --> 01:34:31.520
there's no available energy to do work. Entropy, the measure
1216
01:34:31.560 --> 01:34:36.479
of disorder, has been maximized. Everything that could happen has happened.
1217
01:34:36.760 --> 01:34:41.079
Time itself becomes meaningless because nothing changes from one moment
1218
01:34:41.479 --> 01:34:45.920
to the next. That's if dark energy is constant. If
1219
01:34:45.920 --> 01:34:49.760
it's quintessence that grows stronger with time, the ending could
1220
01:34:49.840 --> 01:34:54.560
be more dramatic. If dark energy's repulsive force keeps growing,
1221
01:34:55.399 --> 01:35:00.319
it will eventually overcome not just cosmic expansion, but also
1222
01:35:00.439 --> 01:35:05.840
the forces holding galaxies together, then solar systems, then planets,
1223
01:35:06.159 --> 01:35:11.319
then atoms. In this big rip scenario, the universe's expansion
1224
01:35:11.439 --> 01:35:17.840
accelerates so rapidly that everything gets torn apart. First, galaxy
1225
01:35:17.920 --> 01:35:21.960
clusters unbind. The galaxies that are currently held together by
1226
01:35:22.000 --> 01:35:27.239
their mutual gravity drift apart as dark energy overcomes that gravity.
1227
01:35:27.760 --> 01:35:33.359
Then individual galaxies unbind and their stars scatter. Our Milky Way,
1228
01:35:33.640 --> 01:35:37.319
which has held together for over thirteen billion years, would
1229
01:35:37.359 --> 01:35:41.720
fly apart, its hundreds of billions of stars flung in
1230
01:35:41.840 --> 01:35:46.079
all directions. Then solar systems come apart as planets are
1231
01:35:46.119 --> 01:35:49.560
ripped away from their stars. The Earth would be torn
1232
01:35:49.600 --> 01:35:52.439
from its orbit around the Sun and flung into the
1233
01:35:52.479 --> 01:35:57.439
cold darkness of space. Then planets themselves are pulled apart
1234
01:35:57.479 --> 01:36:01.399
by tidal forces. The Earth would be stretched and torn,
1235
01:36:02.039 --> 01:36:07.880
first breaking into large fragments, then smaller pieces, then individual rocks,
1236
01:36:08.399 --> 01:36:13.760
then molecules, then atoms. Eventually, even atoms are ripped apart,
1237
01:36:14.479 --> 01:36:19.239
electrons torn away from nuclei. The electromagnetic force that holds
1238
01:36:19.319 --> 01:36:23.880
atoms together would be overcome by the acceleration of space itself.
1239
01:36:24.520 --> 01:36:28.279
Matter as we know it would cease to exist, reduced
1240
01:36:28.319 --> 01:36:33.760
to a plasma of individual particles. Finally, even atomic nuclei
1241
01:36:33.960 --> 01:36:37.680
might be torn apart, Protons and neutrons split into their
1242
01:36:37.680 --> 01:36:42.840
constituent quarks. The universe ends in a cataclysm. Where space
1243
01:36:42.920 --> 01:36:47.439
expands so fast that nothing can hold together anymore. The
1244
01:36:47.439 --> 01:36:50.359
Big rip is more dramatic than the heat death, but
1245
01:36:50.439 --> 01:36:54.720
it's also quicker. Instead of the slow fade over google
1246
01:36:54.800 --> 01:36:59.119
years that heat death entails, the big rip happens relatively
1247
01:36:59.199 --> 01:37:04.359
quickly once the acceleration becomes strong enough. Current estimates if
1248
01:37:04.359 --> 01:37:07.479
the Big Rip is real, suggest it would occur perhaps
1249
01:37:07.600 --> 01:37:11.600
twenty to fifty billion years from now, still a long
1250
01:37:11.680 --> 01:37:14.920
time by human standards, but nothing compared to the time
1251
01:37:14.960 --> 01:37:19.000
scales of heat death. Most physicists think the Big Rip
1252
01:37:19.279 --> 01:37:24.960
is unlikely. Current observations suggest dark energy is probably constant
1253
01:37:25.319 --> 01:37:29.479
or nearly constant, but we can't entirely rule it out yet.
1254
01:37:30.319 --> 01:37:34.680
The fate of the universe remains uncertain. There's also a
1255
01:37:34.760 --> 01:37:40.399
fourth possibility worth mentioning, though it's quite speculative. Some theories
1256
01:37:40.439 --> 01:37:43.560
suggest that the vacuum state of the universe might not
1257
01:37:43.760 --> 01:37:47.399
be stable. We might be living in a false vacuum,
1258
01:37:47.479 --> 01:37:50.920
a local minimum of energy that appears stable but isn't
1259
01:37:50.960 --> 01:37:55.479
truly the lowest possible energy state. If that's true, then
1260
01:37:55.520 --> 01:37:59.000
at some random point in the future, a quantum fluctuation
1261
01:37:59.119 --> 01:38:02.239
could trigger a train transition to the true vacuum state.
1262
01:38:02.920 --> 01:38:06.159
This would create a bubble of true vacuum that expands
1263
01:38:06.159 --> 01:38:09.399
at the speed of light, converting the false vacuum to
1264
01:38:09.479 --> 01:38:14.479
the true vacuum. Wherever it passes inside the bubble, the
1265
01:38:14.560 --> 01:38:19.920
laws of physics might be different. Particles might have different masses,
1266
01:38:20.359 --> 01:38:23.880
forces might have different strengths. Atoms as we know them
1267
01:38:24.319 --> 01:38:27.880
might not be able to exist. This bubble would destroy
1268
01:38:28.079 --> 01:38:33.199
everything it touches, rewriting the rules of physics as it expands.
1269
01:38:34.039 --> 01:38:37.720
There would be no warning, because it expands at light speed.
1270
01:38:38.479 --> 01:38:42.039
You wouldn't see it coming. One moment you exist, the
1271
01:38:42.079 --> 01:38:46.760
next moment you don't, converted into whatever configurations of matter
1272
01:38:46.880 --> 01:38:51.039
and energy are stable in the true vacuum. This sounds
1273
01:38:51.159 --> 01:38:55.399
like science fiction, but it's a real possibility allowed by
1274
01:38:55.520 --> 01:38:59.159
quantum field theory. We have no way of knowing whether
1275
01:38:59.239 --> 01:39:04.119
our vacuum is truly stable or just meta stable, appearing
1276
01:39:04.199 --> 01:39:07.239
stable but not actually being in the lowest energy state.
1277
01:39:07.760 --> 01:39:11.319
If it's meta stable, the universe could end at any moment,
1278
01:39:12.039 --> 01:39:14.720
though the probability of this happening in any given year
1279
01:39:15.199 --> 01:39:20.279
is astronomically small. Most physicists don't lose sleep over vacuum
1280
01:39:20.319 --> 01:39:23.399
decay because there's nothing we can do about it, and
1281
01:39:23.439 --> 01:39:27.039
no evidence that it's actually a threat. It's more of
1282
01:39:27.119 --> 01:39:31.680
a theoretical curiosity than a practical concern, but it does
1283
01:39:31.760 --> 01:39:35.640
illustrate how much we still don't know about the universe's
1284
01:39:35.720 --> 01:39:40.199
fundamental nature. Let's circle back to Einstein himself and what
1285
01:39:40.319 --> 01:39:45.560
his story teaches us about science and creativity. Einstein was
1286
01:39:45.600 --> 01:39:50.279
a revolutionary thinker who changed our understanding of reality more
1287
01:39:50.319 --> 01:39:55.159
profoundly than perhaps any other single individual in history. His
1288
01:39:55.279 --> 01:40:00.560
work on special relativity, general relativity, and quantum mechanics provided
1289
01:40:00.600 --> 01:40:04.319
the foundations for modern physics. But he was also human,
1290
01:40:04.800 --> 01:40:10.199
prone to mistakes, biases, and errors of judgment. He added
1291
01:40:10.439 --> 01:40:15.439
the cosmological constant because he preferred a static universe for
1292
01:40:15.520 --> 01:40:20.920
philosophical reasons. He spent decades pursuing a unified field theory
1293
01:40:21.439 --> 01:40:26.000
that led nowhere. Because he refused to fully embrace quantum mechanics,
1294
01:40:26.800 --> 01:40:31.319
he made errors and wrong turns, just like any other scientist.
1295
01:40:31.840 --> 01:40:37.319
Einstein's relationship with quantum mechanics is particularly interesting and tragic.
1296
01:40:37.800 --> 01:40:40.880
He was one of the founders of quantum theory, winning
1297
01:40:40.920 --> 01:40:44.840
the Nobel Prize for his explanation of the photoelectric effect,
1298
01:40:45.479 --> 01:40:49.680
which showed that light behaves as particles in certain situations.
1299
01:40:50.159 --> 01:40:54.119
But as quantum mechanics developed through the nineteen twenties, taking
1300
01:40:54.159 --> 01:40:58.800
on its modern form with wave functions, probability, and uncertainty,
1301
01:40:59.279 --> 01:41:04.199
Einstein and became increasingly uncomfortable with where the theory was heading.
1302
01:41:04.760 --> 01:41:08.000
He famously declared that God does not play dice with
1303
01:41:08.079 --> 01:41:13.279
the universe, expressing his discomfort with the probabilistic nature of
1304
01:41:13.359 --> 01:41:17.199
quantum mechanics. In the classical physics he grew up with,
1305
01:41:17.720 --> 01:41:22.039
everything was deterministic. If you knew the exact state of
1306
01:41:22.039 --> 01:41:25.600
a system at one moment, you could, in principle calculate
1307
01:41:25.640 --> 01:41:29.439
its state at any future moment. Quantum mechanics said no,
1308
01:41:30.239 --> 01:41:34.920
the best you can do is calculate probabilities. The universe
1309
01:41:35.560 --> 01:41:39.960
is fundamentally uncertain at its core. Einstein thought this couldn't
1310
01:41:39.960 --> 01:41:44.439
be the final word. He believed quantum mechanics must be incomplete,
1311
01:41:44.960 --> 01:41:49.359
that there must be deeper variables, hidden mechanisms that would
1312
01:41:49.439 --> 01:41:53.880
restore determinism and certainty if only we could discover them.
1313
01:41:54.439 --> 01:41:57.680
He spent enormous effort trying to prove quantum mechanics was
1314
01:41:57.720 --> 01:42:02.479
incomplete or contradictory divine in thought experiments meant to expose
1315
01:42:02.520 --> 01:42:07.279
its problems. The most famous of these was the EPR paradox,
1316
01:42:07.640 --> 01:42:11.600
named after Einstein, Podolski, and Rosen, who published it in
1317
01:42:11.680 --> 01:42:16.000
nineteen thirty five. The thought experiment showed that quantum mechanics
1318
01:42:16.039 --> 01:42:21.279
seemed to allow instantaneous influences between distant particles, what Einstein
1319
01:42:21.359 --> 01:42:26.640
mockingly called spooky action at a distance. He thought this
1320
01:42:26.760 --> 01:42:32.479
proved quantum mechanics must be wrong or incomplete, but experiments
1321
01:42:32.560 --> 01:42:37.159
later showed that Einstein was wrong. The spooky action at
1322
01:42:37.199 --> 01:42:42.279
a distance is real. Quantum entanglement, as it's now called,
1323
01:42:42.640 --> 01:42:47.840
has been demonstrated countless times. It's not just theoretical. It's
1324
01:42:47.880 --> 01:42:52.439
been measured, tested, and even used in practical applications like
1325
01:42:52.560 --> 01:42:58.359
quantum cryptography and quantum computing. Einstein's intuition that it couldn't
1326
01:42:58.399 --> 01:43:03.119
be real turned out to be mistaken. The universe really
1327
01:43:03.640 --> 01:43:08.479
is that strange. So Einstein spent the last thirty years
1328
01:43:08.520 --> 01:43:12.199
of his life pursuing a unified field theory, trying to
1329
01:43:12.279 --> 01:43:17.399
merge gravity and electromagnetism into a single framework while ignoring
1330
01:43:17.479 --> 01:43:22.439
quantum mechanics. Meanwhile, the physics community moved on without him.
1331
01:43:22.840 --> 01:43:27.960
Younger physicists, developed quantum field theory, figured out the weak
1332
01:43:28.079 --> 01:43:33.079
and strong nuclear forces, discovered new particles, and built the
1333
01:43:33.159 --> 01:43:37.680
standard model of particle physics. All of this passed Einstein
1334
01:43:37.840 --> 01:43:41.000
by because he was working in isolation on a problem
1335
01:43:41.199 --> 01:43:44.079
that couldn't be solved with the tools he was using.
1336
01:43:44.520 --> 01:43:47.960
What made Einstein's special wasn't that he was always right.
1337
01:43:48.640 --> 01:43:53.399
It was his willingness to think differently, to question assumptions
1338
01:43:53.399 --> 01:43:56.720
that everyone else took for granted, and to follow his
1339
01:43:56.840 --> 01:44:00.640
mathematical and physical intuition, even when when it led to
1340
01:44:00.720 --> 01:44:06.880
bizarre conclusions. He imagined riding alongside light beams as a teenager,
1341
01:44:07.600 --> 01:44:12.079
he wondered what would happen if space and time weren't absolute.
1342
01:44:12.159 --> 01:44:15.479
He asked what gravity would look like if it wasn't
1343
01:44:15.520 --> 01:44:19.880
a force but a curvature of space time. These thought experiments,
1344
01:44:20.239 --> 01:44:25.159
these acts of imagination, these willingness to question everything, that's
1345
01:44:25.239 --> 01:44:29.439
what led to his greatest discoveries, and those same qualities
1346
01:44:29.960 --> 01:44:33.920
led to his mistakes. He questioned whether the universe had
1347
01:44:33.920 --> 01:44:38.319
to be dynamic. He imagined it could be static and eternal.
1348
01:44:39.319 --> 01:44:42.439
He followed that intuition and added a term to make
1349
01:44:42.479 --> 01:44:47.920
his equations match his vision. The lesson is that mistakes
1350
01:44:48.520 --> 01:44:52.199
are part of the process. You can't be at the
1351
01:44:52.239 --> 01:44:56.520
cutting edge of knowledge without sometimes being wrong. The key
1352
01:44:56.880 --> 01:45:01.439
is to let observations and experiments guide, to be willing
1353
01:45:01.479 --> 01:45:05.520
to change your mind when the evidence doesn't match your expectations,
1354
01:45:06.039 --> 01:45:09.479
and to keep pushing forward even when you make errors.
1355
01:45:09.920 --> 01:45:13.720
Einstein did all of this. He made mistakes, but he
1356
01:45:13.800 --> 01:45:18.600
corrected them when better information became available. He pursued dead ends,
1357
01:45:19.279 --> 01:45:23.159
but he never stopped thinking about fundamental questions, and some
1358
01:45:23.199 --> 01:45:27.560
of his mistakes, like the cosmological constant, turned out to
1359
01:45:27.600 --> 01:45:32.159
contain deep truths that he couldn't have anticipated. Science is
1360
01:45:32.279 --> 01:45:36.640
fundamentally a collective enterprise, even though we often focus on
1361
01:45:36.840 --> 01:45:42.560
individual genius. Einstein built on the work of Maxwell, Lorentz, Pointcarret,
1362
01:45:42.680 --> 01:45:45.840
and others who had been struggling with the same problems
1363
01:45:45.880 --> 01:45:51.119
he solved. Hubble built on Levitt's work on cepheid variables.
1364
01:45:52.119 --> 01:45:56.000
The supernova teams that discovered dark energy built on decades
1365
01:45:56.000 --> 01:46:02.079
of work by astronomers measuring distances and mapping galaxies. Every
1366
01:46:02.199 --> 01:46:05.920
discovery stands on the shoulders of what came before, but
1367
01:46:06.039 --> 01:46:10.159
science also needs individuals willing to take big risks to
1368
01:46:10.239 --> 01:46:14.800
propose radical ideas to question the foundations of what everyone
1369
01:46:14.880 --> 01:46:20.720
thinks they know. Einstein was exceptional at this. He had
1370
01:46:20.760 --> 01:46:24.319
the mathematical skill to work out the details of his ideas,
1371
01:46:24.960 --> 01:46:28.439
but more importantly, he had the physical intuition to know
1372
01:46:28.600 --> 01:46:32.279
which questions to ask, and the courage to follow wherever
1373
01:46:32.359 --> 01:46:37.880
those questions led. This is the nature of science. It's messy, uncertain,
1374
01:46:38.239 --> 01:46:41.479
full of wrong turns and dead ends, but it's also
1375
01:46:41.520 --> 01:46:46.760
self correcting. Bad ideas get weeded out by observation and experiment.
1376
01:46:47.359 --> 01:46:52.439
Good ideas survive and become stronger as more evidence accumulates,
1377
01:46:52.920 --> 01:46:55.840
and sometimes ideas that seemed bad turn out to be
1378
01:46:55.920 --> 01:46:59.600
good after all when viewed from a different perspective. The
1379
01:46:59.680 --> 01:47:04.079
cosmological constant story is a perfect example of this process.
1380
01:47:04.600 --> 01:47:09.560
Einstein added it for wrong reasons, it was removed as unnecessary.
1381
01:47:10.199 --> 01:47:14.000
It was brought back when new observations demanded it. Now
1382
01:47:14.119 --> 01:47:19.359
it's a central part of cosmology, describing the dominant component
1383
01:47:19.520 --> 01:47:24.520
of the universe's energy content. The story has come full circle,
1384
01:47:25.119 --> 01:47:29.000
but in an unexpected way that nobody could have predicted.
1385
01:47:29.560 --> 01:47:32.800
What does this tell us about the nature of physical
1386
01:47:32.920 --> 01:47:38.319
law and our ability to understand the universe? On one level,
1387
01:47:38.840 --> 01:47:43.640
it's humbling. Even Einstein, working at the peak of his powers,
1388
01:47:44.119 --> 01:47:48.439
couldn't foresee all the implications of his equations. He added
1389
01:47:48.840 --> 01:47:54.039
and removed the cosmological constant based on incomplete information and
1390
01:47:54.119 --> 01:47:59.960
philosophical preferences. He missed the opportunity to predict the universe's expansion.
1391
01:48:00.560 --> 01:48:04.680
But on another level, it's empowering. The fact that the
1392
01:48:04.760 --> 01:48:09.000
equations contained the right answer all along, that the mathematics
1393
01:48:09.079 --> 01:48:13.159
knew more than Einstein did, suggests that there really are
1394
01:48:13.399 --> 01:48:17.760
underlying laws of nature waiting to be discovered. The universe
1395
01:48:17.920 --> 01:48:23.680
operates according to mathematical principles. Our equations, when we get
1396
01:48:23.680 --> 01:48:28.239
them right, are discovering these principles, rather than just inventing
1397
01:48:28.319 --> 01:48:33.479
convenient fictions. General relativity has now been tested for over
1398
01:48:33.520 --> 01:48:38.600
a century. It's past every test. It's made predictions that
1399
01:48:38.680 --> 01:48:42.159
seemed absurd at the time but turned out to be correct.
1400
01:48:43.079 --> 01:48:49.119
Black holes exist, gravitational waves exist, time runs at different
1401
01:48:49.199 --> 01:48:54.920
rates in different gravitational fields, Light bends around massive objects.
1402
01:48:55.840 --> 01:49:01.600
The universe is expanding, and that expansion is excel All
1403
01:49:01.640 --> 01:49:07.319
of this follows from Einstein's equations. That doesn't mean general
1404
01:49:07.399 --> 01:49:12.039
relativity is the final word. We know it must be
1405
01:49:12.199 --> 01:49:17.600
incomplete because it doesn't include quantum mechanics. We know there
1406
01:49:17.600 --> 01:49:20.760
are puzzles it can't solve, like what happens at the
1407
01:49:20.800 --> 01:49:23.840
center of a black hole or what happened at the
1408
01:49:23.960 --> 01:49:28.560
moment of the Big Bang. But within its domain of applicability,
1409
01:49:29.079 --> 01:49:35.279
general relativity works. It describes reality. The mathematics maps onto
1410
01:49:35.319 --> 01:49:39.600
the physical world in a deep and precise way. This
1411
01:49:39.760 --> 01:49:42.520
is one of the great mysteries of physics and mathematics.
1412
01:49:43.079 --> 01:49:47.159
Why does mathematics work so well to describe nature? Why
1413
01:49:47.199 --> 01:49:51.680
do equations written on paper correspond so precisely to measurements
1414
01:49:51.720 --> 01:49:56.159
made with instruments. There's no obvious reason why the universe
1415
01:49:56.159 --> 01:50:01.279
should be mathematical, but it is galic Us' orbit according
1416
01:50:01.319 --> 01:50:09.119
to mathematical laws. Light follows mathematical paths. Particles interact according
1417
01:50:09.199 --> 01:50:16.520
to mathematical rules. Everything we've studied follows mathematical patterns. Eugene Wigner,
1418
01:50:17.039 --> 01:50:19.840
a physicist who won the Nobel Prize for his work
1419
01:50:19.880 --> 01:50:24.560
on quantum mechanics, wrote a famous essay called The Unreasonable
1420
01:50:24.640 --> 01:50:29.880
Effectiveness of Mathematics in the Natural Sciences. He marveled at
1421
01:50:29.880 --> 01:50:35.439
how mathematical concepts developed for purely abstract reasons often turn
1422
01:50:35.479 --> 01:50:41.039
out to be exactly what's needed to describe physical phenomena.
1423
01:50:41.439 --> 01:50:47.560
Complex numbers, group theory, differential geometry. All of these mathematical
1424
01:50:47.640 --> 01:50:52.920
tools were invented by mathematicians following their own esthetic sense
1425
01:50:53.279 --> 01:50:58.600
of what was interesting or elegant. Later physicists found these
1426
01:50:58.640 --> 01:51:02.600
tools were exactly what they needed to describe quantum mechanics,
1427
01:51:03.000 --> 01:51:07.600
particle physics, and relativity. Why should this be the case.
1428
01:51:08.640 --> 01:51:12.359
Why should the pure mathematics created by human minds match
1429
01:51:12.399 --> 01:51:16.640
the structure of external reality. It's a deep question without
1430
01:51:16.640 --> 01:51:20.479
a clear answer. Maybe our minds are somehow tuned by
1431
01:51:20.479 --> 01:51:24.920
evolution to think mathematically because that reflects how the universe
1432
01:51:25.039 --> 01:51:30.119
actually works. Maybe there's something fundamental about mathematical structure that
1433
01:51:30.239 --> 01:51:34.680
physics must follow. Or maybe we're selecting the mathematical frameworks
1434
01:51:34.720 --> 01:51:39.079
that work and ignoring the countless ones that don't, creating
1435
01:51:39.399 --> 01:51:44.840
an illusion of unreasonable effectiveness. Whatever the answer, the fact
1436
01:51:44.920 --> 01:51:50.479
remains that mathematics is our most powerful tool for understanding
1437
01:51:50.520 --> 01:51:58.359
the cosmos. Einstein's equations are mathematical. The cosmological constant is
1438
01:51:58.399 --> 01:52:03.119
a mathematical term. Dark energy is observed through measurements that
1439
01:52:03.159 --> 01:52:07.800
are then fit to mathematical models. All of our understanding
1440
01:52:07.840 --> 01:52:12.279
of the universe flows through this mathematical language. Let's talk
1441
01:52:12.279 --> 01:52:15.199
about one more aspect of this story that's worth reflecting,
1442
01:52:15.239 --> 01:52:21.239
on the role of technology in driving scientific discovery. Hubble's
1443
01:52:21.439 --> 01:52:27.840
observations that revealed the expanding universe were only possible because
1444
01:52:27.880 --> 01:52:32.119
of the one hundred inch telescope at Mount Wilson, which
1445
01:52:32.239 --> 01:52:35.960
was the largest and most powerful telescope in the world
1446
01:52:36.039 --> 01:52:40.239
at the time. Without that technology, without the ability to
1447
01:52:40.279 --> 01:52:45.279
see faint distant galaxies and measure their spectra precisely, Hubble
1448
01:52:45.479 --> 01:52:50.520
couldn't have made his discovery. Similarly, the supernova observations that
1449
01:52:50.600 --> 01:52:56.439
revealed dark energy required large telescopes, sensitive digital cameras, and
1450
01:52:56.560 --> 01:53:01.039
powerful computers to process the data. The detection of gravitational
1451
01:53:01.079 --> 01:53:05.159
waves required the Ligo experiment, a marvel of engineering that
1452
01:53:05.199 --> 01:53:08.840
can measure distances changing by less than the width of
1453
01:53:08.880 --> 01:53:13.279
a proton across arms four kilometers or two point five
1454
01:53:13.359 --> 01:53:18.600
miles long. The cosmic microwave background was mapped by satellites
1455
01:53:18.680 --> 01:53:24.760
like COBE, WMAP and PLANK that measured temperature variations of
1456
01:53:24.800 --> 01:53:28.079
a few millions of a degree across the sky. Every
1457
01:53:28.439 --> 01:53:33.279
major advance in cosmology has been enabled by technological progress.
1458
01:53:34.079 --> 01:53:38.600
Better telescopes let us see farther and more clearly. Better
1459
01:53:38.680 --> 01:53:43.600
detectors let us measure fainter signals. Better computers let us
1460
01:53:43.680 --> 01:53:49.119
analyze more data and run more sophisticated simulations. The discoveries
1461
01:53:49.159 --> 01:53:54.600
don't come from technology alone. You need ideas, theories, questions
1462
01:53:54.920 --> 01:53:58.680
to guide what you're looking for, but the technology is
1463
01:53:58.720 --> 01:54:02.960
what lets you actually look. This creates an interesting dynamic.
1464
01:54:03.640 --> 01:54:10.439
Theorists like Einstein develop mathematical frameworks that make predictions. Observers
1465
01:54:10.439 --> 01:54:15.720
and experimentalists build the instruments to test those predictions. When
1466
01:54:15.760 --> 01:54:21.880
observations confirm theory, both are strengthened. When observations contradict theory,
1467
01:54:22.359 --> 01:54:26.199
both have to adapt. Theory guides what to look for,
1468
01:54:26.479 --> 01:54:31.760
but observations determine what's actually true. The cosmological constant sat
1469
01:54:31.800 --> 01:54:37.119
in Einstein's equations for decades before technology advanced enough to
1470
01:54:37.239 --> 01:54:40.960
detect its effects. The super and Over observations of the
1471
01:54:41.039 --> 01:54:46.039
nineteen nineties required digital cameras that didn't exist in Einstein's time.
1472
01:54:46.600 --> 01:54:49.840
If he'd lived to see these observations, he would have
1473
01:54:49.920 --> 01:54:54.039
learned that his blunder was actually correct, that the constant
1474
01:54:54.119 --> 01:54:58.680
he added and removed, really did describe something fundamental about
1475
01:54:58.720 --> 01:55:02.800
the universe. This back and forth between theory and observation,
1476
01:55:03.439 --> 01:55:08.680
between mathematics and measurement, between prediction and conformation is the
1477
01:55:08.720 --> 01:55:14.960
engine that drives physics forward. Neither theory nor observation alone
1478
01:55:15.520 --> 01:55:21.239
is sufficient. You need both working together, challenging each other,
1479
01:55:22.000 --> 01:55:26.479
refining understanding through iteration. As we look to the future,
1480
01:55:27.119 --> 01:55:30.720
the next generation of telescopes and experiments will test our
1481
01:55:30.800 --> 01:55:37.039
current understanding even more rigorously. The VERA Rubin Observatory will
1482
01:55:37.079 --> 01:55:42.760
survey the entire visible sky every few nights, discovering millions
1483
01:55:42.840 --> 01:55:47.119
of new objects and tracking how they change with time.
1484
01:55:47.640 --> 01:55:51.319
The James Webb Space Telescope is already finding the most
1485
01:55:51.359 --> 01:55:54.720
distant galaxies in the universe, seeing them as they were
1486
01:55:54.760 --> 01:55:58.319
in the first few hundred million years after the Big Bang.
1487
01:55:58.840 --> 01:56:03.600
The Square kilometre Array, when complete, will be the world's
1488
01:56:03.720 --> 01:56:09.479
largest radio telescope, studying everything from the cosmic microwave background
1489
01:56:09.920 --> 01:56:13.479
to the formation of the first stars to the distribution
1490
01:56:13.600 --> 01:56:18.319
of neutral hydrogen throughout the universe. The thirty meter Telescope
1491
01:56:18.560 --> 01:56:22.640
and the extremely Large Telescope will have mirrors so large
1492
01:56:23.000 --> 01:56:26.760
they can see Earth like planets around nearby stars and
1493
01:56:26.880 --> 01:56:30.399
study the atmospheres of those planets for signs of life.
1494
01:56:30.960 --> 01:56:35.840
Each of these instruments will test our understanding of cosmology,
1495
01:56:36.520 --> 01:56:41.880
dark energy, and general relativity in new ways. They might
1496
01:56:41.960 --> 01:56:47.600
confirm that dark energy is truly a cosmological constant. They
1497
01:56:47.680 --> 01:56:51.399
might reveal that it changes with time. They might find
1498
01:56:51.439 --> 01:56:56.000
evidence that general relativity breaks down on the largest scales.
1499
01:56:57.039 --> 01:57:02.359
Or they might discover something entirely unacted, something nobody has
1500
01:57:02.399 --> 01:57:07.720
even imagined yet. That's the exciting part about science. You
1501
01:57:07.800 --> 01:57:12.720
build your best theories based on current knowledge. You make predictions,
1502
01:57:13.159 --> 01:57:18.359
you test them, and sometimes the universe surprises you. Sometimes
1503
01:57:18.359 --> 01:57:21.640
you find something you weren't looking for, something that doesn't
1504
01:57:21.640 --> 01:57:26.039
fit any existing framework, something that forces you to rethink everything.
1505
01:57:26.439 --> 01:57:32.359
Dark energy was one of those surprises. Nobody expected the
1506
01:57:32.520 --> 01:57:37.680
universe's expansion to be accelerating. The supernova teams were trying
1507
01:57:37.680 --> 01:57:41.720
to measure how much the expansion was slowing down, not
1508
01:57:41.800 --> 01:57:45.560
whether it was speeding up. The discovery was a complete shock,
1509
01:57:46.319 --> 01:57:51.000
forcing cosmologists to revise their understanding of the universe's composition
1510
01:57:51.119 --> 01:57:55.000
and fate. There will be more surprises. We don't know
1511
01:57:55.159 --> 01:57:59.239
what they'll be or when they'll come. But history shows
1512
01:57:59.279 --> 01:58:01.720
that every every time we look at the universe in
1513
01:58:01.800 --> 01:58:06.399
a new way, with new instruments or new techniques, we
1514
01:58:06.479 --> 01:58:11.640
find something unexpected. The universe is more strange, more beautiful,
1515
01:58:12.000 --> 01:58:17.520
and more surprising than we imagine. Einstein's cosmological constant teaches
1516
01:58:17.600 --> 01:58:21.640
us to stay humble. Even the greatest minds can miss
1517
01:58:21.640 --> 01:58:26.159
things or get things wrong. But it also teaches us
1518
01:58:26.560 --> 01:58:32.359
to stay curious. The universe has secrets waiting to be discovered.
1519
01:58:32.840 --> 01:58:38.359
Mathematics gives us a language to describe those secrets, Observations
1520
01:58:38.439 --> 01:58:42.680
test whether our descriptions are correct, and technology gives us
1521
01:58:42.720 --> 01:58:46.279
new eyes to see what was previously hidden. In the end,
1522
01:58:46.760 --> 01:58:51.119
Einstein's biggest mistake wasn't really a mistake at all. It
1523
01:58:51.239 --> 01:58:55.319
was a step in the long process of understanding the cosmos.
1524
01:58:55.840 --> 01:58:59.079
He added a term to his equations for wrong reasons,
1525
01:58:59.680 --> 01:59:02.840
but the the term turned out to be necessary for
1526
01:59:03.000 --> 01:59:06.520
right reasons. He removed it when he thought it was wrong,
1527
01:59:07.199 --> 01:59:11.520
but it came back when observations demanded it. The story
1528
01:59:11.640 --> 01:59:19.359
has ambiguity, uncertainty, and surprise, just like science itself. As
1529
01:59:19.399 --> 01:59:22.760
we close, let's think about what this means for us,
1530
01:59:23.439 --> 01:59:26.479
for people living in the early twenty first century, trying
1531
01:59:26.520 --> 01:59:30.600
to understand our place in the cosmos. We live at
1532
01:59:30.640 --> 01:59:34.479
a remarkable moment in history. For the first time, our
1533
01:59:34.600 --> 01:59:39.840
species can see almost the entire observable universe. We can
1534
01:59:39.920 --> 01:59:43.239
look back in time to when the first stars formed.
1535
01:59:43.920 --> 01:59:47.760
We can map the distribution of galaxies across billions of
1536
01:59:47.880 --> 01:59:52.760
light years. We can measure the fundamental parameters that determine
1537
01:59:52.800 --> 01:59:57.319
the universe's composition and fate. We've learned that the universe
1538
01:59:57.600 --> 02:00:01.840
is larger, older, and stranger than anyone imagined just a
1539
02:00:01.960 --> 02:00:06.399
century ago. We've learned that most of what exists is
1540
02:00:06.439 --> 02:00:11.159
invisible to us, dark matter and dark energy that we
1541
02:00:11.239 --> 02:00:15.880
detect only through their gravitational effects. We've learned that space
1542
02:00:15.920 --> 02:00:21.079
itself is dynamic, expanding and carrying galaxies with it. We've
1543
02:00:21.159 --> 02:00:24.560
learned that time is not absolute, that it runs at
1544
02:00:24.560 --> 02:00:29.079
different rates depending on motion and gravity. The story of
1545
02:00:29.119 --> 02:00:33.199
how we learned all this is remarkable in itself. It
1546
02:00:33.359 --> 02:00:38.399
involved countless scientists working over generations, each contributing a piece
1547
02:00:38.439 --> 02:00:44.000
to the puzzle. Some, like Einstein and Hubble, made revolutionary
1548
02:00:44.039 --> 02:00:50.439
discoveries that changed everything. Others made smaller but still essential contributions,
1549
02:00:51.000 --> 02:00:57.920
developing techniques, building instruments, making careful observations that constrained theories,
1550
02:00:58.439 --> 02:01:03.720
and tested predictions. The cosmological constant story shows us that
1551
02:01:03.800 --> 02:01:07.960
science isn't a straight line from ignorance to knowledge. It's
1552
02:01:08.000 --> 02:01:12.479
a winding path with many side trails, dead ends, and
1553
02:01:12.720 --> 02:01:21.119
unexpected turns. Einstein added the constant, then removed it. Decades passed,
1554
02:01:21.720 --> 02:01:26.760
then observations demanded its return. But now it meant something different,
1555
02:01:27.439 --> 02:01:32.159
described something different. The journey from Einstein's static universe to
1556
02:01:32.239 --> 02:01:37.159
our accelerating universe took almost a century and involved thousands
1557
02:01:37.159 --> 02:01:41.960
of scientists. All of this understanding rests on Einstein's work.
1558
02:01:43.039 --> 02:01:47.119
General Relativity provides the framework we use to interpret our
1559
02:01:47.159 --> 02:01:53.960
observations and understand the cosmos. The cosmological constant, Einstein's blunder
1560
02:01:54.000 --> 02:01:57.359
that wasn't really a blunder, describes the dominant form of
1561
02:01:57.560 --> 02:02:01.520
energy in the universe. Every time you hear about dark energy,
1562
02:02:01.680 --> 02:02:06.039
or the accelerating expansion, or the ultimate fate of the cosmos,
1563
02:02:06.880 --> 02:02:12.880
your hearing about Einstein's legacy. When Einstein published General Relativity
1564
02:02:13.039 --> 02:02:16.840
in nineteen fifteen, he couldn't have imagined where it would lead.
1565
02:02:17.760 --> 02:02:22.399
He was solving a specific problem. How to incorporate gravity
1566
02:02:22.479 --> 02:02:25.680
into his theory of space time. He wasn't trying to
1567
02:02:25.760 --> 02:02:30.199
describe the entire universe, or predict dark energy or explain
1568
02:02:30.319 --> 02:02:34.439
the Big Bang. These applications of his theory came later,
1569
02:02:34.840 --> 02:02:38.600
as other physicists and astronomers built on his foundation. This
1570
02:02:38.720 --> 02:02:42.359
is typical of great scientific theories. They often turn out
1571
02:02:42.399 --> 02:02:45.640
to be more powerful and more general than their creators imagined.
1572
02:02:46.199 --> 02:02:50.680
Newton's laws of motion were developed to explain planetary orbits,
1573
02:02:50.960 --> 02:02:56.439
but they also describe billiard balls, pendulums, and rocket trajectories.
1574
02:02:57.039 --> 02:03:04.000
Maxwell's equations of electromagnetism developed to unify electricity and magnetism,
1575
02:03:04.239 --> 02:03:10.000
but they also predicted radio waves, which Maxwell himself didn't anticipate.
1576
02:03:10.520 --> 02:03:14.800
Darwin's theory of evolution by natural selection was developed to
1577
02:03:14.920 --> 02:03:19.560
explain the diversity of life, but it also provides insights
1578
02:03:19.600 --> 02:03:25.439
into psychology, sociology, and even computer science through genetic algorithms.
1579
02:03:26.000 --> 02:03:32.479
Einstein's general relativity has proven remarkably robust and versatile. It
1580
02:03:32.560 --> 02:03:35.920
describes the orbit of mercury around the Sun with its
1581
02:03:36.000 --> 02:03:40.920
tiny deviations from Newton's predictions. It describes the bending of
1582
02:03:41.000 --> 02:03:46.600
light around massive objects. It describes gravitational waves from colliding
1583
02:03:46.640 --> 02:03:51.119
black holes billions of light years away. It describes the
1584
02:03:51.159 --> 02:03:55.159
expansion of the entire universe and predicts its ultimate fate,
1585
02:03:55.680 --> 02:04:00.560
all from a single elegant mathematical framework that I. Einstein
1586
02:04:00.720 --> 02:04:05.520
worked out by thinking deeply about the nature of gravity, space,
1587
02:04:06.039 --> 02:04:09.800
and time. But we're also at the beginning of understanding,
1588
02:04:09.880 --> 02:04:13.159
not the end. We don't know what dark energy really is.
1589
02:04:13.479 --> 02:04:16.479
We don't know what dark matter is. We don't know
1590
02:04:16.520 --> 02:04:21.399
how to reconcile general relativity with quantum mechanics. We don't
1591
02:04:21.439 --> 02:04:24.520
know what happened at the exact moment of the Big Bang.
1592
02:04:25.319 --> 02:04:28.359
We don't know if there are other universes beyond our own.
1593
02:04:28.880 --> 02:04:31.000
We don't know whether the laws of physics could have
1594
02:04:31.039 --> 02:04:34.600
been different, or why they're the specific laws we observe.
1595
02:04:35.319 --> 02:04:40.520
These mysteries aren't failures of science. They're the frontier, the
1596
02:04:40.680 --> 02:04:43.960
edge of current knowledge, where the next generation of discoveries
1597
02:04:44.039 --> 02:04:49.880
will be made. Every answer raises new questions, every solution
1598
02:04:50.319 --> 02:04:55.840
reveals new puzzles. That's how science progresses. We solve one
1599
02:04:55.920 --> 02:05:00.399
mystery only to find three more hiding behind it. Consider
1600
02:05:00.439 --> 02:05:04.119
what we've learned just in the last few decades. In
1601
02:05:04.239 --> 02:05:08.760
nineteen ninety we knew of no planets outside our Solar system.
1602
02:05:09.000 --> 02:05:14.079
Now we've confirmed over five thousand exoplanets, with thousands more
1603
02:05:14.119 --> 02:05:20.840
candidates waiting. In nineteen ninety, gravitational waves were still theoretical predictions.
1604
02:05:21.600 --> 02:05:25.439
Now we detect them regularly from colliding black holes and
1605
02:05:25.520 --> 02:05:31.720
neutron stars. In nineteen ninety, dark energy was unknown. Now
1606
02:05:31.760 --> 02:05:36.119
we know it's the dominant component of the universe. What
1607
02:05:36.279 --> 02:05:39.640
will we learn in the next few decades? What discover
1608
02:05:39.800 --> 02:05:42.600
is a weight that we can't even imagine. The James
1609
02:05:42.600 --> 02:05:46.479
Web Space Telescope is already finding galaxies in the early
1610
02:05:46.640 --> 02:05:52.199
universe that challenge our understanding of galaxy formation. Future telescopes
1611
02:05:52.239 --> 02:05:56.960
will see even farther and more clearly. New experiments will
1612
02:05:56.960 --> 02:06:02.039
test fundamental physics in new regimes. We might discover what
1613
02:06:02.159 --> 02:06:08.039
dark matter actually is. We might detect quantum gravitational effects.
1614
02:06:08.640 --> 02:06:12.119
We might find life on other worlds. These are the
1615
02:06:12.279 --> 02:06:17.640
great questions of our time. Future generations will have better
1616
02:06:17.720 --> 02:06:21.239
answers than we do, just as we have better answers
1617
02:06:21.399 --> 02:06:27.720
than Einstein did. The process continues. Each generation builds on
1618
02:06:27.800 --> 02:06:34.079
what came before, correcting mistakes, refining understanding, pushing further into
1619
02:06:34.119 --> 02:06:38.359
the unknown. Einstein showed us that even mistakes can be
1620
02:06:38.399 --> 02:06:42.439
productive if you approach them with honesty and willingness to
1621
02:06:42.640 --> 02:06:46.920
change your mind when evidence demands it. He added, the
1622
02:06:46.960 --> 02:06:52.479
cosmological constant removed it, and eventually it came back because
1623
02:06:52.520 --> 02:06:57.399
the universe itself insisted on its reality. The story is
1624
02:06:57.520 --> 02:07:01.199
messy and human, but it led to found insights about
1625
02:07:01.239 --> 02:07:06.800
the nature of reality. His legacy extends beyond specific discoveries
1626
02:07:07.239 --> 02:07:12.239
or equations. He demonstrated that the universe can be understood
1627
02:07:12.640 --> 02:07:17.720
through mathematics and reason. Reality is often stranger than we imagine.
1628
02:07:18.000 --> 02:07:21.760
Common sense can mislead us, and only careful observation and
1629
02:07:21.840 --> 02:07:27.880
rigorous theory reveal the truth. Intellectual courage is essential for progress.
1630
02:07:28.479 --> 02:07:31.800
You must be willing to question everything, even the most
1631
02:07:31.840 --> 02:07:37.560
basic assumptions, to make revolutionary discoveries. Einstein also showed us
1632
02:07:37.600 --> 02:07:42.800
the limits of individual genius. Even the greatest mind can
1633
02:07:42.880 --> 02:07:49.800
miss things, make mistakes, let biases, cloud judgment. Science needs
1634
02:07:49.800 --> 02:07:56.039
individual brilliance, but it also needs collective effort, peer review, replication,
1635
02:07:56.520 --> 02:08:01.119
and the willingness to change course when evidence demands. No
1636
02:08:01.399 --> 02:08:05.760
single person, no matter how smart, can see everything or
1637
02:08:05.880 --> 02:08:12.000
know everything. We need diverse perspectives, different approaches and many
1638
02:08:12.039 --> 02:08:16.000
people working on problems from different angles. So the next
1639
02:08:16.039 --> 02:08:19.159
time you look up at the night sky and think
1640
02:08:19.199 --> 02:08:24.359
about the universe, remember this story. Remember that the cosmos
1641
02:08:24.840 --> 02:08:30.399
is stranger than we imagine. Remember that the universe is expanding,
1642
02:08:30.760 --> 02:08:35.359
accelerating into a future we're only beginning to understand. And
1643
02:08:35.479 --> 02:08:40.079
remember that the equations describing all of this, the mathematical
1644
02:08:40.199 --> 02:08:44.680
language that lets us comprehend the incomprehensibly large and complex,
1645
02:08:45.359 --> 02:08:49.119
were largely given to us by one man thinking deeply
1646
02:08:49.199 --> 02:08:54.479
about the nature of gravity, space, and time. Einstein's genius
1647
02:08:55.000 --> 02:08:57.760
wasn't that he never made mistakes. It was that he
1648
02:08:57.840 --> 02:09:02.600
had the courage to propose rever polutionary ideas, the skill
1649
02:09:02.720 --> 02:09:06.840
to work out their mathematical implications, and the honesty to
1650
02:09:06.920 --> 02:09:11.520
admit when observations proved him wrong. He changed how we
1651
02:09:11.560 --> 02:09:17.760
think about space, time, matter, energy, and gravity. He showed
1652
02:09:17.840 --> 02:09:22.439
us that the universe operates according to elegant mathematical laws
1653
02:09:22.800 --> 02:09:28.159
that we can discover and understand. The cosmological constant represents
1654
02:09:28.199 --> 02:09:34.039
both misjudgment and prescient insight added to preserve an incorrect
1655
02:09:34.079 --> 02:09:37.199
model of the cosmos. That same term turned out to
1656
02:09:37.279 --> 02:09:42.479
describe the dominant energy driving everything we observe. Dark energy
1657
02:09:42.560 --> 02:09:47.720
determines the universe's accelerating expansion and will shape its ultimate fate.
1658
02:09:48.600 --> 02:09:52.479
Einstein couldn't have known this in nineteen seventeen, but his
1659
02:09:52.600 --> 02:09:58.000
mathematics contain the truth even when he misunderstood its implications.
1660
02:09:58.560 --> 02:10:03.000
This teaches us humility. Even the smartest people can be wrong.
1661
02:10:03.760 --> 02:10:09.000
Even revolutionary theories can contain hidden truths their creators don't understand.
1662
02:10:09.560 --> 02:10:14.880
Science progresses not because individual scientists are infallible, but because
1663
02:10:14.920 --> 02:10:20.079
the scientific method eventually sorts truth from error through observation
1664
02:10:20.680 --> 02:10:26.199
and experiment. It also teaches us persistence. Einstein labored on
1665
02:10:26.319 --> 02:10:31.720
general relativity for almost a decade, trying different approaches, learning
1666
02:10:31.760 --> 02:10:38.000
new mathematics, pushing through obstacles. Discovery rarely comes easily, and
1667
02:10:38.119 --> 02:10:43.800
it teaches us openness. When Hubble's observations revealed expansion, Einstein
1668
02:10:43.880 --> 02:10:49.199
accepted it despite his philosophical preferences. He acknowledged error publicly.
1669
02:10:49.640 --> 02:10:52.720
That willingness to revise understanding in the face of evidence,
1670
02:10:53.119 --> 02:10:58.199
to abandon cherished ideas when reality disagrees is essential to science.
1671
02:10:58.640 --> 02:11:01.279
Thank you for joining me on this journey through cosmology
1672
02:11:01.319 --> 02:11:05.279
and history. If you found this exploration valuable. A like
1673
02:11:05.439 --> 02:11:09.159
or subscribe would mean a lot. And remember, in the
1674
02:11:09.279 --> 02:11:13.319
vast darkness of the cosmos, even the greatest minds stumble.
1675
02:11:13.840 --> 02:11:19.359
But those stumbles, those errors, those blunders, they're all part
1676
02:11:19.439 --> 02:11:23.800
of humanity's long struggle to understand where we are, how
1677
02:11:23.800 --> 02:11:27.079
we got here, and where we're going. The universe is
1678
02:11:27.159 --> 02:11:30.720
expanding into a future we can barely imagine, and we're
1679
02:11:30.760 --> 02:11:35.920
going along for the ride, asking questions, making mistakes, and
1680
02:11:36.000 --> 02:11:39.560
slowly gradually learning the truth. Good Night,
