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chap 15 new-v8 - Olbers's Paradox Why is the sky dark at...

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Olbers’s Paradox Why is the sky dark at night? The night sky should be as bright as the surface of a typical star! If the universe is uniform, isotropic, infinite, and eternal, then every line of sight should end on the surface of a star at some point. Since the no. stars per unit area projected onto the celestial sphere increases as r 2 , and their apparent brightness decreases as 1/r 2 , then Solution to Olbers’s Paradox: If the universe is not eternal, i.e., it had a beginning, then we can only see light from galaxies that has had time to travel to us since the beginning of the universe. The observable universe is finite!
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The Cosmological Principle Considering the largest scales in the universe, we make the following fundamental assumptions: 1) Homogeneity: On the largest scales, after correcting for look-back time , the local universe has the same physical properties throughout the universe. Every region has the same physical properties (mass density, expansion rate, visible vs. dark matter, etc.) 2) Isotropy: On the largest scales, after correcting for look-back time , the local universe has the same physical properties in any direction. You should see similar large-scale structure in any direction. 3) Universality: The laws of physics that govern the universe are the same everywhere in the universe, but could have evolved with time .
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