Olbers' Paradox

Why is the sky dark at night?

This was a question that German astronomer Heinrich Wilhelm Olbers described in 1823. If the universe were static and infinite, then the night sky should be incomparably bright, as the light from an infinite number of stars would fill the horizon. The dimness of stars farther away would be countered by the fact that there would be more stars farther away from us (assuming a near-constant density of stars).

Of course, the paradox is resolved by a number of explanations. Here are a few:

• The universe has only a finite number of stars
• The distribution of stars is not uniform (e.g. many could be blocked by others stars)
• The universe is expanding
• The universe is young, so distant light hasn't reached us yet

Nevertheless, the sheer simplicity of Olbers' Paradox is profound. A dark night sky is something that we all take for granted, yet the darkness itself imposes certain restrictions on the universe.

A more detailed overview of Olbers' Paradox can be found on Wikipedia:
http://en.wikipedia.org/wiki/Olbers'_paradox