AST101Lecture 25

Why is the Night Sky Dark?

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 d2

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 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– 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

• 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 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

• 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 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 is expanding,

There are two possible cosmologies• Steady State

– Homogeneous, isotropic, unchanging

• Big Bang– Finite and evolving