Edwin Hubble on the beginning of the universe

In the '29, Edwin Hubble observed that wherever you look, distant galaxies are moving rapidly away from us. In other words, the universe is expanding. This means that at earlier time objects would have been closer together. In fact, it seemed there was a time, about 10 or 20 thousand million years ago, when they were all at exactly the same place and when, therefore, the density of the universe was infinite. This discovery brought the question of the beginning of the universe into the realm of science. 

Hubble's observation suggested there was a time when the universe was infinitesimally small and infinitely dense. Under such conditions, all the laws of science, and therefore the ability to predict the future, would break down. If there were events earlier than this time, then they could not affect what happens at the present time. Their existence can be ignored because it would have no observational consequences. Times had a beginning at the Big Bang simply because earlier times would not be defined.