Why is the sky dark at night?
If the Universe were infinitely old, infinite in extent, and filled with stars, then every direction you looked would eventually end on the surface of a star, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers’ Paradox after Heinrich Wilhelm Olbers (1757–1840) who wrote about it in 1823–1826 (though it had been discussed earlier). A common suggestion for resolving the paradox is to consider interstellar dust, which blocks light by absorping it. However, absorption by interstellar dust does not circumvent this paradox, as dust reradiates whatever radiation it absorbs within a few minutes, which is much less than the age of the Universe. The resolution of Olbers’ paradox comes by recognizing that the Universe is not infinitely old and it is expanding. The latter effect reduces the accumulated energy radiated by distant stars. Either one of these effects acting alone would solve Olbers’ Paradox, but they both act at once.
