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Transcript: Another fundamental quantity of the big bang model is the density parameter. It’s defined as the ratio of the mean density of the universe to the density just needed to overcome the cosmic expansion. The density parameter is denoted by the Greek symbol capital omega with a subscript zero. If omega equals one the universe is flat. If omega is less than one the universe is open, and if omega is greater than one the universe is closed. Unlike the deceleration the density parameter is a purely local measurement. All that’s required is to take a large volume of space, typically fifty to a hundred megaparsecs in distance from the Milky Way, add up all the matter that’s contained, luminous and dark, divide by the volume, and compare to the critical density. The answer gives the sense of whether the universe will expand forever. The best current measurements indicate that omega is about 0.3 or only one-third of the amount of density needed to overcome the expansion. Based on these measurements the universe will expand forever.
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Amerika Serikat
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Transcript: The best traditional test of cosmic nucleosynthesis and the predictions of the big bang model is the helium abundance in the universe. In the model of the big bang the predicted helium abundance is relatively insensitive to the cosmic density, so astronomers need a measurement accurate…
Transcript: The cosmic abundance of light elements is a primary piece of evidence in favor of the big bang model. Stars are fusion factories, and main sequence stars create helium from hydrogen by the fusion process in their cores. However, a careful accounting of stellar fusion over the history …
Transcript: The problem with using helium 3, helium 4, or lithium 7 for testing cosmic nucleosynthesis in the big bang model is that all three of these are created in stars, and lithium can be destroyed in stars. So it’s difficult to estimate which of the nuclear abundances are primordial, origina…
Transcript: What is the physical meaning of the cosmological constant? Astronomers do not know and physicists are puzzled too, but they’ve both come up with the idea of dark energy. In physics the vacuum is not totally empty. Particles and anti-particles can be created from the vacuum, and the g…
Transcript: In the big bang model it’s possible to calculate the abundance of light elements created by fusion reactions in the early phases of the expansion. This is called cosmic nucleosynthesis. The calculations depend on only one fundamental parameter of the universe which is the density of p…
Sanggahan: Podcast dan karya seni yang disematkan pada halaman ini berasal dari Dr. Christopher D. Impey, Professor, Astronomy, yang merupakan milik pemiliknya dan tidak berafiliasi atau didukung oleh Listen Notes, Inc.
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