An Expanding Universe
摘要
Shortly after completing the general theory of relativity, Einstein went on to apply his new theory to the universe as a whole. The structure of the universe beyond our Milky Way galaxyMilky Way galaxy was then completely unknown, so Einstein had to make some assumptions. Following Newton, he assumed that on average matter is uniformly distributed in the cosmos. There are of course local variations, with the density of stars higher in some places and lower in others. However on very large scales the universe is well approximated as being perfectly homogeneousHomogeneous .