Decoupling

Transcript: As we trace events in the universe back toward the big bang, the first important epoch reached is a time about three hundred thousand years after the big bang. The universe by this time has cooled to a temperature of about three thousand Kelvin like the photosphere of a cool star. The density of matter and radiation has become low enough that photons no longer routinely interact with the charged particles that are there, protons and electrons. This has two fundamental consequences for the universe. First, the photons start to travel freely without interacting with matter. This is the era of decoupling. Photons decouple from matter, and it’s the era at which we see the cosmic microwave background radiation. Those photons have been flowing freely through the universe for eleven billion years since. The universe becomes transparent at this point. The second consequence is that protons and electrons combine to form stable neutral hydrogen atoms. This is called recombination. The two physical processes are connected.