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Chalnoth
#13
Nov22-11, 07:26 AM
Sci Advisor
P: 4,804
Quote Quote by cephron View Post
Hey Chalnoth, I was under the impression (From the FAQ, and from recent discussion here) that according to current models, the universe would have no boundaries even if it were finite, but rather "wrap back in on itself" 3-sphere style (corresponding to "positive spatial curvature"). So, for clarity, is it certain that there are no boundaries, or do you mean it when you say, "we don't know", and would a bounded finite universe be consistent with current models? Thanks!
Well, I don't know of a way to completely rule out the concept of some sort of boundary, especially if we are rather vague as to what we mean by the boundary. It is certainly true that we don't observe any boundary. And it is also true that it isn't something that we can describe in our equations just yet (so far as I am aware). But just because we can't describe it mathematically just yet doesn't necessarily mean it can't exist. How likely it is, then, just depends upon what you think, "have a hard time describing it in the mathematics," means.

Finally, as to Cosmo Novice's statement about homogeneity and isotropy, the fact of the matter is that these assumptions are known to be not completely accurate. They are approximations to the true behavior, and we know that these assumptions break down on small scales. It is entirely conceivable that they also break down on scales much larger than the cosmological horizon.