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Originally Posted by d*d
You can't just chuck infinity in, the models of the universe described only work for finite universes, this is the way the big bang theory works to explain the nature of our universe - I ain't saying it's definately case but it works better than just supposing the universe is infinate because it's so big
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First, the universe can be both flat and finite. The only flat, boundryless surface embedable in 3 dimensional space is the infinite plane -- so it is a good example to use to describe what is going on. I guess I could say "the universe is like a sheet of paper with opposite sides identified" (mmm, donut), but that seems overly complex. One doesn't need topology to understand this concept.
If you can understand how an infinite plane can expand by a factor of two, without any centre, and without expanding "into" anything, it might help you understand how the universe could do the same thing. Even if the universe doesn't look like an infinite plane, the image is still useful.
Secondly, as far as I am aware, there is no reason to believe the universe isn't infinite in size. Attempts to determine the diameter of the universe using microwave background radiation came up with "bigger than what we can see".
You can have a 'big bang' that starts with an infinite sized universe, involves space stretching rapidly, and ends up with an infinite sized universe which is 'bigger' than the original universe in a local way. It looks just like ours does, as a bonus.