AST 101 Lecture 25 Why is the Night

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AST 101 Lecture 25 Why is the Night Sky Dark?

AST 101 Lecture 25 Why is the Night Sky Dark?

Olber’s Paradox Suppose the universe is infinite • In whatever direction you look, you

Olber’s Paradox Suppose the universe is infinite • In whatever direction you look, you will see a star • The brightness of an individual star falls by the inverse square law: I ~ d-2 • The number of stars increases as d 2 The night sky should be as bright as the surface of the Sun!

Solutions to Olber’s Paradox • dust/absorption – Dust does absorb visible light – But

Solutions to Olber’s Paradox • dust/absorption – Dust does absorb visible light – But the energy has to go somewhere – The universe would heat up and come to equilibrium - at the brightness of a stellar surface

Solutions to Olber’s Paradox • dust/absorption • universe is not infinite in space –

Solutions to Olber’s Paradox • dust/absorption • universe is not infinite in space – A finite universe contains a finite amount of energy – The brightness is the energy density

Solutions to Olber’s Paradox • dust/absorption • universe is not infinite in space •

Solutions to Olber’s Paradox • dust/absorption • universe is not infinite in space • universe is not infinite in time – An infinite universe with a non-infinite age will not yet be in equilibrium

Solutions to Olber’s Paradox • • dust/absorption universe is not infinite in space universe

Solutions to Olber’s Paradox • • dust/absorption universe is not infinite in space universe is not infinite in time universe is infinite, but evolves – It may not be in equilibrium – It may not have had stars in the past

Solutions to Olber’s Paradox • • • dust/absorption universe is not infinite in space

Solutions to Olber’s Paradox • • • dust/absorption universe is not infinite in space universe is not infinite in time universe is infinite, but evolves expansion of universe – The part of the universe we can see is finite

Implications of Hubble’s Law At some distance, the recessional velocity exceeds c. • This

Implications of Hubble’s Law At some distance, the recessional velocity exceeds c. • This limits the observable universe • At large distances, we look far back in time. • If the universe evolves, this has consequences

Cosmological Philosophy Given that – The night sky is dark, and – The universe

Cosmological Philosophy Given that – The night sky is dark, and – The universe is expanding, There are two possible cosmologies • Steady State – Homogeneous, isotropic, unchanging • Big Bang – Finite and evolving

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