Comment Re:Look and calculate all you want (Score 1) 149
The reason scientists believe there must've been a big bang, or something very like it, is because of entropy. Time only flows in one direction, and as far as we can tell, there is no good reason why it should. Anyways, one way of figuring out which way time is flowing is the direction where entropy, or disorder, increases. I.e. you see an apple, fresh and organized, rot into indistinct mush, but never the opposite, though it is perfectly possible under the laws of physics, just not plausible. So as time flows, the disorder of the universe increases.
However, another property of disorder in a system is that a disordered system is *much* more likely than an ordered one. Thus, if we are currently in a somewhat ordered system, it is actually more like that the past up to now was more disordered than the present, and the future will become more disordered. Thus, we cannot truly tell which direction the past lies, and cannot trust our past experience. If this was the case, no science, or even normal living would be very nice. You would have no way of knowing what happened every time you do something, as past experience would not be a meaningful way of predicting things. Thus, oxygen might suddenly not enable us to burn glucose and we would die.
Thus, for us to be able to trust past experience, the past must be more ordered than the present. And as one goes back in time, there must be a time where there was absolute order (order cannot increase infinitly). This is what we call the big bang. As it would be perfectly ordered, it cannot become more ordered and thus time cannot decrease and has a beginning. The issue of size has to do with how all space is expanding, and that every galaxy is moving away from us a speed directly proportional to its distance from us, indicating that they all originated from the same point. (Galaxies have little actual speed through space, space is actually expanding between them, moving them apart. Like dots on a balloon.)
I'm not sure how well I explained it, but that's the gist of it, as I understand.
