Where did the Big Bang scenario come from?
If space (and everything with it) is expanding now, then the universe must have been much denser in the past. That is, all the matter and energy (such as light) that we observe in the universe would have been compressed into a much smaller space in the past. Einstein’s theory of gravity enables us to run the “movie” of the universe backwards – i.e., to calculate the density that the universe must have had in the past. The result: any chunk of the universe we can observe – no matter how large – must have expanded from an infinitesimally small volume of space.
