HowardS said:
Hello. I can understand that there is no central point for the BB as it is the only point that exists. But it does not make sense that all of the universe's mass came into existence, from one point - whether that point IS the universe or is a point in eternal, empy space. You would have a continuous "sphere" of particles (mass), or equivalent energy, ejecting from this point, in all directions, for as long as it takes to make up the finite? amount of matter in the universe.
One thing to grasp is that existing physics breaks down at this singularity.
If you use good old general relativity, and extend it back as far as you can go, you get one great big "divide by zero" error, in which the total volume of the universe vanishes to zero and the density blows up to infinite.
It's usually just wrong to think of stuff expanding from a point, because this language conventionally means thinking of a point in space; and that's not what we mean.
Think of it like this. At a certain time, (t=1) all the universe was filled with a hot dense mix of stuff at enormous temperatures and density. And it was all expanding... just like galaxies are expanding now. That is, it was becoming less dense, as everything expands away from everything else. There's no center to this, mind. The whole universe, without any edge or boundary, is filled with this hot dense mix of stuff. As time passes the density drops, which is the same as saying there's more space between things than there was before. Except that the density was so incredibly high that the idea of space between things doesn't really work. Nevertheless, that's what expansion means. It means that density drops. Forget centers, forget boundaries, they have nothing to do with it. Just... density is falling.
But if you run the clock in reverse to look at the past, density is increasing, without any limit. It increases up to infinite density within a finite time in the past, at t=0. And physics breaks apart in such conditions.
Suppose that the universe is infinite. (Flat or negative curvature throughout.) If so, then it has ALWAYS been infinite. It's just that it used to be so phenomenally dense that everything we can currently see, out to billions of lights years distance, used to be all within a space of a basketball, or a pea, or an atom, or a proton, as you go back.
Just as there is presumably more of the universe beyond the horizon of visibility at present, so too there was back then more of the universe beyond the "basksetball/pea/atom" sized region that contained all the matter of the observable universe. The idea of the observable universe being the size of a pea in the past doesn't mean that there was an edge. It means only that everything we now can see used to be packed together into a small region within the whole infinite universe. And everything expanded.
I do mean
everything. There's no center to this expansion; no central point from which it starts.
There is rather a
singularity. A time in the past where classical physics would involve infinite density and zero size, for any region; and so classical physics breaks down. At any other time, we have a hot dense infinite universe filled with stuff, without a center and without a boundary... and it expands.
Felicitations -- sylas
PS. Never give up. Whether you are asking or explaining. This is not easy to grasp, and every new generation of students has the fun of finding it out. Ask away, don't be shy!
PPS. Dmitry67 gives a nice set of four points in the next post to try and keep in mind!