Ah, so, that's another problem. There's no obvious way to describe the transition between "singularity" and "not a singularity." Look at the graph of 1/x. There

Ah, so, that's another problem. There's no obvious way to describe the transition between "singularity" and "not a singularity." Look at the graph of 1/x. There's a singularity at x=0. You can't describe the slope between any part of the equation and the singularity, since the derivative is also singular at the origin. At x=0, you're actually outside the domain of the function and the slope between x=0 and x=a is undefined. So what I mean is, it's not even clear that "the singularity began to expand" is a meaningful statement at all. Spacetime came into existence (as far as we can tell) with the big bang, and ordinary ideas of an "event" require a before and after. So far as anyone can tell, there might not have been a before at all, so talking about an event happening at "time zero" is... tricky. The Big Bang theory concerns itself with all instants that the universe had some finite extent. The equations go singular if you look back far enough, but it's possible there wasn't even a singularity, and our math is wrong: the universe was very, very dense, but not infinitely dense, and then expanded. We don't know.