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Lecture12 Cosmic Microwave Background Radiation - The...

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The Cosmic MicrowaveBackground RadiationHawley & Holcomb, chapter 144/24/091
Where is the Big Bang?Pick a direction or a point on the sky.The farther you look, the longer in thepast of the universe you see.If you look far enough, you might see the earliest moment, when theuniverse first formed.(Provided that the universe did have an earliestmoment!)You would see this earliest moment, “the Big Bang,” when looking in anydirection.The beginning of the universe is everywhere around us.The obvious reason we can see into the past of the universe is that currently,the universe istransparent, but it was not always so.When the distances in the universe were 1,000 times smaller than at present,it was so dense (a billion times denser than now on average) that light couldnot avoid bumping into (scattering off) electrons along the way; the universewas opaque to light.Consider looking through a transparent object that becomes less and lesstransparent with distance.Eventually, you will reach the end oftransparency, which will appear as a surface.The surface you see in anydirection is called “the last scattering surface”.4/24/092
Radiation in the UniverseObvious ingredients that fill the universe are: matter (atoms, etc.) andradiation (light).Throughout most of the history of the universe, theenergy density in the universe is almost completely dominated by matter;the radiation contributes only a small fraction of the energy density. Inthe past, however, the radiation made a more substantial contribution.Infact, if we could see beyond the last scattering surface, we would noticethat the energy density in radiation dominated the universe at very earlytimes.Most of the radiation in the universe at present is in the CosmicMicrowave Background (CMB), which we cannot see because thewavelength is too long (much longer than the infrared wavelength),because the wavelength has been redshifted by a huge amount as theuniverse expanded.(Of course, a fraction of the radiation filling theuniverse that wecansee comes from stars, accreting black holes, etc.)The CMB is radiation with very long wavelength.The wavelength hasbeen stretched by the expansion of the universe.The originalwavelength of Cosmic Microwave Background photons was muchsmaller than it is today.

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Term
Spring
Professor
JohnLacy
Tags
The Big Bang, Cosmic microwave background radiation, CMB, Cosmic Microwave Background

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