How do cosmologists know that the universe is expanding? Well, how would anyone determine that the universe is expanding? If the objects which make up the universe are moving away from you, then you would conclude that the universe is expanding.

The objects which are the "building blocks" of the universe, and are observed by astronomers to move away from us are called galaxies. A galaxy is a huge collection of stars (about 100 billion stars), with each galaxy appearing to be an "island" in the vastness of space. Our own Sun is part of a galaxy called the Milky Way.

Scientific evidence that the universe is expanding requires (a) proof that galaxies are moving away from our galaxy (the Milky Way), and (b) proof that the distance between our galaxy and other galaxies is related to their speed away from us.

How far has the universe expanded since its beginning? Comparing the 10 light year distance to our nearest stars to the diameters of the structures formed gives clues to the answer. Milky Way: 105 light years, Local Group: 4×106 light years, Galaxy Clusters (Virgo, Coma): 25×106 light years, Local Supercluster: 150×106 light years, Universe: 15×109 light years.


South Carolina State University, 01/24/2015