If the universe was infinitely old, as we used to assume, then it must be filled with an infinite number of stars and galaxies. So why is the night sky not completely lit up by the light from these stars? This question was first asked by the nineteenth century astronomer Heinrich Olbers, and the answer is, because of the age of the universe, not all of the photons have had enough time to reach us yet. The amount of universe we are able to see is called the observable universe, and according to a pair of astrophysicists, it just got...