We've finally reached the topic of cosmology , the study of the universe taken as a whole. Astronomers who study stars or galaxies have the luxury of being able to choose from billions of stars or billions of galaxies. Unfortunately, there's only one universe to study. In today's lecture, I will concentrate on the spatial properties of the universe as it exists today, and leave the time evolution of the universe for future lectures. Stars are much denser than the interstellar medium.
Galaxies are much denser than the intergalactic medium.
Even superclusters are denser than the voids between them. 100 Mpc is the same as the average density of any other sphere of the same size. For instance, we assume that Kepler's Third Law applies to a binary galaxies millions of light years away just as well as it applies to planets within the Solar System. Combine the assumptions of homogeneity, isotropy, and universality, and you have the cosmological principle . The cosmological principle, if true, implies that the universe cannot have an edge or a center.