Transcript AST101_lect_25

AST101
Lecture 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
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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
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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