Why night sky dark?

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tagitables
Posts: 10
Joined: Fri Jul 01, 2011 11:22 pm

Why night sky dark?

Postby tagitables » Fri Oct 14, 2011 10:52 pm

If there are so many (so many that it can be considered infinite) number of stars, then the night sky should be bright.
Even though the speed of light is finite, that still doesn't make sense.
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joe
Posts: 745
Joined: Fri Aug 29, 2003 11:57 am
Location: Sydney

Re: Why night sky dark?

Postby joe » Wed Oct 26, 2011 8:58 am

This is known as Olber's paradox. I think that very many of us have enjoyed discovering independently. It can be variously stated, e.g.

At radius r, the intensity due to a star goes as 1/r^2 but the number of stars goes as 4.pi.r^2.dr, so the sky should be bright

or

The universe should have reached thermal equilibrium so why isn't it several thousand K everywhere?

There are several implicit assumptions made in formulating this paradox:
- That the universe is large enough for the r^3 term to beat the 1/r^2 term
- That it is old enough for the r^3 term to beat the 1/r^2 term
- That it is not expanding
- That the density of stars doesn't decrease rapidly enough if one considers large enough volumes.

In fact it has a finite age (~ 13 Gyr) and is expanding. Its density decreases with increasing scale (galaxies, groups, clusters, superclusters ... and perhaps beyond.)

Expansion means that distant sources are red-shifted and so less bright, and those sufficiently far away are and will remain invisible.

Joe


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