chap 15 new-v8

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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Hubble’s Law V r = H x D Distant galaxies are receding from us with a speed proportional to distance Question for later: Does Hubble’s constant H change with time?? V r = H x D
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The Expanding Universe On large scales, galaxies are moving apart with velocity proportional to distance. It’s NOT that galaxies are moving apart through space. RATHER: Space is expanding, carrying the galaxies along!
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Expanding Space Analogy: A loaf of raisin bread where the dough is rising and expanding, taking the raisins with it.
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The Expanding Universe You have the same impression from any other galaxy as well. This does not mean that we are at the center of the universe! Observer 1 Observer 2
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The Age of the Universe Knowing the current rate of expansion, we can estimate the time it took for galaxies to move as far apart as they are today: Age of universe ~ D/V = 1/H V = H x D & D = V x Time Light we see left this galaxy when the universe was ~ 10 9 years old! Units: 3.09 x 10 19 km/MPc & 3.16 x 10 7 sec/year [] 12 10 Ty e a r s km / sec H MPc = ⎡⎤ ⎢⎥ ⎣⎦
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Looking Back Towards the Early Universe The more distant the, the further back into the past we are looking. What is the earliest thing we observe?
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Cosmic Microwave Background Radiation (CMB) The radiation from the very early phase of the universe should still be detectable today Black body radiation with average temperature of T = 2.73 K, and spatial fluctuations of + 0.001 K.
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chap 15 new-v8 - Olbers's Paradox Why is the sky dark at...

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