bassplayer142 said:
I still don't get how the universe doesn't have a center where it all began. At the big bang everything was defined at one point right?
Not really. There's a really good article about this, probably aimed at just your level, in Scientific American here:
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147
As other posters have said, the Big Bang is an expansion
of space, not
in space. Think of it this way:
distances (between galaxies) are getting bigger everywhere. Things in the universe are all growing further & further apart, but there's apparent no edge or center: the universe looks basically the same everywhere as far as we can see. This picture (just look at the black dots, which would represent galaxies) gives you a good idea of what we mean:
http://www.astro.ucla.edu/~wright/cphotons.gif .
To make this a more mathematical explanation, imagine a one-dimensional toy universe with galaxies located at the points:
x_n = n t , \qquad n \mbox{ an integer}
where t is the time. So at t=1, there are galaxies at x=0, 1, 2, 3, ... and x=-1, -2, ... . By t=2, these galaxies have moved to x=0, 2, 4, 6, ... and x=-2, -4, ... This is an infinite universe, with an infinite number of galaxies in it. And it's expanding. The distance between nearby galaxies is clearly just
t.
In this toy universe, let's "look back" at what happens in this universe at very early times, back near t=0. At earlier & earlier times -- t=0.1, t=0.01, t=0.001, and so on -- the galaxies in the universe become closer & closer together. As we approach t=0, the density approaches infinity. But at this t=0 things look very weird. For any positive time, no matter how small, there are an infinite number of galaxies and space extends infinitely in either direction; but at t=0, all the galaxies are at position x_n = 0. Clearly t=0 is the "Big Bang' in our toy universe.
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Two things that bear mentioning: first, it's not clear how seriously we should take the point at t=0. It's not an 'ordinary' point like any other. You can think of it sort of as like the point at the pointy tip of a cone. Imagine the universe (space and time here!) as a cone laying sideways. The distance along the cone from the pointy end represents time. So at any fixed time (not 0), we take a crosswise slice of the cone there and see this universe looks like a circle (btw, it's also closed so if you go far enough in one direction you come back around.) But if we look at a crosswise slice of the cone at t=0 -- at the pointy end itself -- we see that at t=0, the universe doesn't look like a circle, but like a point! We've gone from a one-dimensional universe at positive t, to zero-dimensional universe at the Big Bang. This is just like what happens in the real universe. At t=0, all the galaxies are "at one point", but it doesn't make sense to ask "where" that point is in the universe... that would be just like asking "where along the circle" the pointy tip of the cone is located.
A second thing is that there's no reason at all to trust our current laws of physics to work all the way back to the Big Bang. In fact we pretty much know they
won't work at very early times. To truly figure out what happened before ~10^{-44} seconds, we need a theory of quantum gravity; extrapolating back with GR just won't cut it. So we don't have any idea what happened in that "first" tiny fraction of a second.. whether there was a true Big Bang, or a bounce like marcus mentioned, or something else entirely.
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Anyways, do read that SciAm article -- it is very helpful. The pages from Ned Wright's cosmology FAQ below might help you, too.
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=0009F0CA-C523-1213-852383414B7F0147
http://www.astro.ucla.edu/~wright/nocenter.html
http://www.astro.ucla.edu/~wright/photons_outrun.html
