Why is the night sky dark? If the universe is static and infinitely old, with an infinite number of stars distributed homogeneously in an infinitely large space, then, whatever direction we look in the night sky, our line of sight should end at a star. The sky should be filled with light.

This puzzle is most often associated with the German astronomer Heinrich Wilhelm Olbers, but Edgar Allan Poe made a strikingly similar observation in his 1848 prose poem Eureka:

Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy — since there could be absolutely no point, in all that background, at which would not exist a star.

Poe suggested that the universe isn’t infinitely old: “The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.” We now know that the sky is dark because the universe is expanding, which increases the wavelength of visible light until it appears dark to our eyes.