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Jul31-10, 05:45 PM
heusdens's Avatar
P: 1,620
Quote Quote by Schlofster View Post
I started this thread a while ago talking about Stephen Hawking's "No Boundary Condition".

I think that I finally got a handle on it, but now Lawrence Krauss has discussed here why he thinks that the universe is not closed, but flat, and he says by implication, 'infinite in spatial extent'.
(he also claims to have empirical evidence of the flatness of space-time on the largest scales)

I don't understand how he can reconcile this with the big bang (which he also seems to accept).
If the universe is infinite in spatial extent, at what point did it become infinite, because when the universe was 1 second old, it was not infinitely large (I think that this is the scientifically accepted view).
He is obviously a widely respected physicist, and I am not a physicist, so I expect that I just don't understand what he is saying.

Could anyone explain it to me, or can it not be expressed in natural language?

In the metaphore of the universe being a flat sheet of rubber (the 2D analpgue of the 3D universe) that expands uniformly in all directions, it is true that (in the mathematical sense, not in the physical sense) all space we can observe now can be brought back to a single 0D spot, but that does not claim to say that all of space was just that single 0D dot.

It seems that too many people make this same implication, which is just an assumption, but doesn't need to be right. It would (IMHO) make more sense to assume that space near the Big bang was already infinite, and thus is still infinite.

This certainly makes sense in the context of the inflationary scenario, in which in other parts of the universe, inflation keeps gong on, creating ever more expanding universe bubbles.