Jan_15 - The Expanding Universe Tuesday, January 15 Thomas...

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The Expanding Universe Tuesday, January 15
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Thomas Digges (16 th century) proposed an infinite infinite universe.
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Infinite universe: hard to reconcile with appearance of the night sky. Night sky is dark dark , with stars (small in angular size) scattered across it.
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“The night sky is dark”: called Olbers’ paradox Olbers’ paradox , after astronomer who discussed the subject in 1823. Why is the darkness of the night sky paradoxical?
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If stars were stuck on a celestial sphere or dome, darkness would not not be paradoxical. Only a finite number of stars on the celestial sphere.
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In an infinite universe with infinite number of stars, paradox arises. How bright do we expect the sky to be in such a universe?
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Suppose there are n n stars per cubic parsec of the universe. Suppose that an average star is a sphere with radius R R . ASSUMPTIONS: ASSUMPTIONS: For Sun, R = 7×10 For Sun, R = 7×10  km = 2×10  km = 2×10 pc pc In Sun’s neighborhood, n = 1/pc In Sun’s neighborhood, n = 1/pc
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You are  here r   = radius of  shell t = thickness  of shell
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the spherical shell? Area = 4 π r 2 What’s the volume volume of the spherical shell? Volume ≈ area × thickness ≈ 4 π r
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Jan_15 - The Expanding Universe Tuesday, January 15 Thomas...

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