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Olbers' paradox

(dark night sky paradox)
(argument using the dark of the sky regarding the nature of the universe)

Olbers' paradox is an observation regarding the darkness of the night sky, specifically, that if there were stars in every possible direction, as could be imagined in a universe that is infinitely large, with an infinite number of randomly-distributed stars (and infinitely old so the light could reach us), the night sky would not be dark: it would all be bright. The fact that the night sky is dark is taken as evidence of limits on the size and/or age of the universe and/or the number of stars: something has been changing, and the universe must be dynamic in some way.

The "paradox" is named for Heinrich Wilhelm Olbers, an early writer on the conception.