Lecture12 Cosmic Microwave Background Radiation

Lecture12 Cosmic Microwave Background Radiation - The...

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The Cosmic Microwave Background Radiation Hawley & Holcomb, chapter 14 4/24/09 1
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Where is the Big Bang? Pick a direction or a point on the sky. The farther you look, the longer in the past of the universe you see. If you look far enough, you might see the earliest moment, when the universe first formed. (Provided that the universe did have an earliest moment!) You would see this earliest moment, “the Big Bang,” when looking in any direction. 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 is transparent , 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 could not avoid bumping into (scattering off) electrons along the way; the universe was opaque to light. Consider looking through a transparent object that becomes less and less transparent with distance. Eventually, you will reach the end of transparency, which will appear as a surface. The surface you see in any direction is called “the last scattering surface”. 4/24/09 2
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Radiation in the Universe Obvious ingredients that fill the universe are: matter (atoms, etc.) and radiation (light). Throughout most of the history of the universe, the energy density in the universe is almost completely dominated by matter; the radiation contributes only a small fraction of the energy density. In the past, however, the radiation made a more substantial contribution. In fact, if we could see beyond the last scattering surface, we would notice that the energy density in radiation dominated the universe at very early times. Most of the radiation in the universe at present is in the Cosmic Microwave Background (CMB), which we cannot see because the wavelength is too long (much longer than the infrared wavelength), because the wavelength has been redshifted by a huge amount as the universe expanded. (Of course, a fraction of the radiation filling the universe that we can see comes from stars, accreting black holes, etc.) The CMB is radiation with very long wavelength. The wavelength has been stretched by the expansion of the universe. The original wavelength of Cosmic Microwave Background photons was much smaller than it is today. 4/24/09 3
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Why can’t we see the CMB? • We cannot see the CMB with a naked eye, even at night.
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This note was uploaded on 09/14/2009 for the course AST 309 taught by Professor Johnlacy during the Spring '08 term at University of Texas at Austin.

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Lecture12 Cosmic Microwave Background Radiation - The...

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