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The Milky Way lights up the sky, but Olbers' Paradox suggests that the whole of the night sky should be bright. (Peresanz/


Background radiation from galaxies, and Olbers' Paradox

From Edmond Halley onwards various astronomers and philosophers have asked the question which has become known as Olbers’ Paradox: why is the sky dark at night? In an infinite universe filled with galaxies, every line of sight should end in a star. The sky should be an overlapping mass of stars, each as bright as the Sun. So why is the whole sky not as bright as the surface of the Sun?

This paradox is resolved in an expanding universe of finite age, because the summations discussed above do not then extend to infinity. We now have excellent observations of the background radiation from galaxies and this provides powerful constraints on the evolutionary history of galaxies.

Incidentally the sky is not actually that dark. The microwave background radiation (see chapter 11), although invisible to the eye, is about as bright as the Milky Way.


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