Olbers' paradox

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Olbers' paradox

(ol -berz) Why is the sky dark at night? Heinrich Olbers in 1826, and earlier J.P.L. Chesaux in 1744, pointed out that an infinite and uniform Universe, both unchanging and static, would produce a night sky of the same surface brightness as the Sun: every line of sight would eventually strike a star, a typical example of which is the Sun. This theoretical argument is obviously in disagreement with observation. The observable Universe is, however, neither uniform, unchanging, nor static and does not extend infinitely back into the past. The paradox is then resolved because the redshift of extragalactic radiation (i.e. the diminution in its energy) and in particular the youth of our Universe make the background radiation field at optical wavelengths very low indeed – less than about 10–21 times the surface brightness of the Sun.

Olbers' paradox

[′ōl·bərz ′par·ə‚däks]
(astronomy)
If the universe were static, of infinite age, and the galaxies distributed isotropically, the distance attenuation of their light would be exactly balanced by the increase in number in successive spherical shells centered at the earth; hence the night sky would be of daylight brightness instead of dark.