Olbers' Paradox

Olbers' paradox is the puzzle of why the night sky is dark rather than uniformly bright, given that in an infinite, unchanging universe filled evenly with stars, a line of sight extended far enough in any direction should eventually strike the surface of some star, meaning the entire night sky ought to appear as bright as the surface of a star rather than mostly dark. German astronomer Heinrich Wilhelm Olbers described the puzzle in an influential 1823 paper, for which the paradox is now named, although earlier astronomers, including Johannes Kepler in the early seventeenth century, had already raised closely related versions of the same puzzle. The paradox is now understood to be resolved primarily by the fact that the universe is not infinitely old and static but instead had a finite beginning at the Big Bang roughly 13.8 billion years ago, meaning light from sufficiently distant stars simply has not yet had time to reach Earth, combined with the fact that the universe's ongoing expansion redshifts the light from very distant, receding sources out of the visible range and reduces its intensity. Olbers' paradox is now presented as one of the classic historical arguments supporting a finite-age, expanding universe rather than an infinite, static one, and it remains a commonly used teaching example in introductory cosmology.