As I understand it, the big bang is based on the fact that all the galaxies are drifting apart in the universe. If we reverse this we end up with the big bang. But is this not based on the assumption that the universe has always been expanding? How do we know that 4 billion years ago the universe was not expanding? Has it been proving that this expansion has always been constant. I am aware that the expansion is speeding up but is there a chance that this is only the case in the present? Please keep it simple as I am no brainiac.