The Olbers paradox, described by the German astronomer Wilhelm Olbers in 1823 and earlier by Johannes Kepler, is the statement that in an infinite universe the night-sky should be bright. If the universe is assumed to be infinite, containing an infinite number of uniformly distributed luminous stars, then almost every line of sight should terminate eventually on the surface of a star. Because of this, almost every point in the sky should be as bright as the surface of a star. This reasoning was advanced to support the idea that the universe (or at least the part of it which contains stars) must be finite in extent, but this argument is incorrect.
The suggestion that light travelling from far away stars may be absorbed or blocked along the way by gas or dust or burned out stars does not resolve the paradox: the matter would reradiate the energy quickly, still leaving us with strong radiation (possibly not in the visible spectrum) from all directions, but this is not observed.
Another resolution that has been offered points to the fact that every star contains only a finite amount of matter and therefore shines only for a finite period of time, after which it runs out of fuel. However, the paradox stands if one assumes that stars are constantly being created randomly across the infinite universe, shine for a finite period, and die.
The paradox is resolvable in a variety of ways. If the universe has existed for only a finite amount of time, as the prevalent Big Bang theory holds, then only the light of finitely many stars has had a chance to reach us yet, and the paradox breaks down. Alternatively, if the universe is expanding and distant stars are receding from us (also a claim of the Big Bang theory), then their light is redshifted which diminishes their brightness, again resolving the paradox. Either effect alone would resolve the paradox, but according to the Big Bang theory, both are working together; the finiteness of time is the more important effect.
In fact, the darkness of the night sky is nowadays taken to be evidence in support of the Big Bang theory.
Another explanation, which does not rely on the Big Bang theory, was offered by Benoit Mandelbrot. It holds that the stars in the universe may not be uniformly distributed, but rather fractally like a Cantor dust, thus accounting for large dark areas. It is currently not known whether this is true or not.
- Relativity FAQ about Olbers' paradox
- Astronomy FAQ about Olbers' paradox
- Cosmology FAQ about Olbers' paradox
- Paul Wesson, Olber's paradox and the spectral intensity of the extragalactic background light, The Astrophysical Journal 367, pp. 399-406 (1991).
- Edward Harrison, Darkness at Night: A Riddle of the Universe, Harvard University Press, 1987