1. AST101 Lecture 25 Why is the Night Sky Dark?

2. 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!

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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