Space Forum / Space Science / January 2008

-> If when we see the blackness of the night time sky we are in fact seeing  
-> dust,
-> then there is surely enough matter in the universe to block us from seeing  
-> the distant galaxies.

-> But there is not.  
 
I tried to give the answer that is generally accepted by astronomers a
few days ago, but nobody took any notice...
 
If the universe were infinite in extent, and filled with stars,
galaxies, etc., with an averge density that is constant with distance,
and if it were not expanding, then the sky would be brilliantly bright.
The brightness of each star would be inversely proportional to the
square of its distance, but the number of stars at any given distance
would be proportional to the square of the distance. So the effects
would cancel out, and each "shell" of stars at any given distance from
us would contribute equally to the brightness of the sky. Since there's
an infinite number of shells, the sky would be as bright as the surface
of a star.
 
This is called "Olbers paradox", after the guy who first described it,
centuries ago.
 
Dust, etc., in intergalactic space would make no difference! The dust
would absorb energy until it is hot enough to radiate energy as fast as
it receives it, at which point it would be as bright as the brilliant
sky around it.
 
The solution, according to all astronomers nowadays, is based on the
expansion of the universe. Light from distant galaxies is redshifted,
which makes them less bright than they would be without the redshift.
Beyond about 12 billion light-years, the redshift is complete, and
galaxies further away than that cannot be seen. So the *observable*
universe is not infinite, so Olbers paradox does not apply.
 
                           dow