SOME BEAUTIFUL, ELEGANT, AND COMPELLING PASSAGES FROM THE Chapter I “A Brief History Of Relativity”
General relativity completely changed the discussion of the origin and fate of the universe. A static universe could have existed forever or could have been created in its present form at some time in the past. However, if galaxies are moving apart now, it means that they must have been closer together in the past. About fifteen billion years ago, they would all have been on top of each other and the density would have been very large. This state was called the "primeval atom" by the Catholic priest Georges Lemaitre, who was the first to investigate the origin of the universe that we now call the big bang.
Einstein seems never to have taken the big bang seriously. He apparently thought that the simple model of a uniformly expanding universe would break down if one followed the motions of the galaxies back in time and that the small sideways velocities of the galaxies would cause them to miss each other. He thought the universe might have had a previous contracting phase, with a bounce into the present expansion at a fairly moderate density. However, we now know that in order for nuclear reactions in the early universe to produce the amounts of light elements we observe around us, the density must have been at least ten tons per cubic inch and the temperature ten billion degrees. Further, observations of the microwave background indicate that the density was probably once a trillion trillion trillion trillion trillion trillion (1 with 72 zeros after it) tons per cubic inch. We also now know that Einstein's general theory of relativity does not allow the universe to bounce from a contracting phase to the present expansion. As will be discussed in Chapter 2, Roger Penrose and I were able to show that general relativity predicts that the universe began in the big bang. So Einstein's theory does imply that time has a beginning, although he was never happy with the idea.
