Olbers's Paradox

There is a simple, seemingly trivial question one can ask -- Why is the night sky dark?


Consider a shell of stars of thickness T and radius R.

How much light do we receive from this shell of stars?

The key point is that the amount of light we receive from the shell does not depend upon how far away is the shell. We receive the same amount of light from distant shells as we do from nearby shells. Hmmmmmm. So, if there are million such shells in the Universe, then we simply multiply the contribution of 1 shell by million to get the total amount of energy we receive from the Universe. Further, we should see this light at all times, even at night, since the shells completely surround us. This type of reasoning gave rise to Olbers's Paradox

Another View:

Another way to think about the problem is to compare the brightness of the nightsky to the brightness of the surface of the Sun. We know that the surface of the Sun blazes away at a temperature of 5,800 Kelvin and the nightsky is substantially less bright. To understand why this is a paradox consider the following:

The fraction of the shell blocked out by the stars in the shell does not depend upon the radius of the shell (how far away the shell lives) ===> Olbers's Paradox if the Universe is big enough.

Resolution of Olbers's Paradox

Okay, so what's the way out? Something must be wrong with one (or more) of the original assumptions, or some physics has not been considered. Possibilities:

It is interesting that in asking and answering the seemingly trivial question, "Why is the night sky dark?" one could have inferred that the Universe was expanding and that the Universe had a finite age (or at the least the stars and galaxies had finite ages).

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