Sean Carroll in the Financial Times:
It’s hard to decide what is more existentially challenging — the idea that the universe has lasted for ever, or the alternative that it had a beginning. Is it more disturbing to contemplate a cosmos that stretches eternally behind us, or to imagine a time before which there was no time at all?
You would be forgiven for thinking that the question of which scenario is worth worrying about the most had been settled over the course of the 20th century, with the triumph of the Big Bang model. The universe we find ourselves in is expanding — distant galaxies are moving away from us, which physicists interpret as the stretching of space itself.
Playing the cosmic movie in reverse, we encounter a universe that was increasingly hot and densely packed the further back in time we look. Ultimately, the equations of Albert Einstein’s general theory of relativity imply that about 13.8 billion years ago matter was infinitely hot and dense, and the expansion rate was infinitely fast. We label that hypothetical moment the Big Bang.