Galaxy

At the dawn of the 20th century, astronomers still were not sure how large the universe was. One prevailing view was that the Universe had existed forever, was rather small, and the stars that we can see in the night sky filled its entire space. By comparing the intensity of more distant stars to our own sun, it was thought that the most distant stars were only a few thousand light years away.

(This argument was compelling, because if the Universe had existed forever in time, and was infinitely large and filled with stars, then everywhere you looked there should be stars, and the entire sky should shine with the light of these countless stars [Olber's paradox].)

Under this model, however, the nature of the "spiral nebula" that astronomers had seen through telescopes for many centuries were a mystery. Some astronomers believed that these were in fact "island universes," collections of countless stars that were much more distant than the stars we saw in our night sky. This idea was debated, and usually dismissed, for many years, until Edwin Hubble pointed the largest telescope that had been built at his time (Mt. Wilson?) toward the largest spiral nebula, in Andromeda. Hubble was able to resolve individual stars in his images, and he realized that some of them varied in a regular way that was identical to a peculiar set of stars that were much closer, the Cepheid variables. The period of the variation in the intensity of Cepheid variables was proportional to their absolute intensity, so Hubble was able to determine the distance to the Andromeda spiral nebula. He found that it was millions of light years away. This was a shocking result, because it expanded the size of the known universe by orders of magnitude.

Having realized that these spiral nebula were galaxies of stars, and that our home was just one of many of these Galaxies, Hubble and others went on to study the structure of this now-larger universe. By taking spectra of distant galaxies, he found that most of them were moving away from Earth: emission and absorption lines of recognizable elements in the spectra were Doppler-shifted toward the red. Moreover, the amount of the redshift was roughly proportional to the intensity of the galaxy. Hubble proposed a "law" that the more distant a galaxy was, the faster it was moving away from our own Galaxy. Remarkably, this hypothesis found an explanation a solution to the equations of Einstein's theory of General Relativity (the Friedman-Roberston-Walker solution). The conclusion that was soon widely-accepted was that the fabric of space was expanding, and carrying galaxies along with it.

(This resolved Obler's paradox --- the light from distant galaxies would never reach us, because space was expanding faster than the light could travel.)

Of course, this raised the question, What is the universe expanding from?