|MadSci Network: Astronomy|
Let's start off with a reference; read A Brief History of Time by Stephen Hawking. He discusses some of these matters, and has a far better grasp of them than does this particular Mad Scientist.
Let me take a shot at it, however. You can approach the "mulitiple universes" thing in several ways:
One is to realize that it is sort of out there on the edge of cosmological theorizing and that the amount of evidence is pretty small. That is, it may not even be right, so you can sit back and wait for more evidence while remaining highly skeptical of the whole affair.
Two, you might look at the Hubble expansion and realize that there may be lots of areas of "our own" universe that are, due to the expansion of space-time, forever cut off from us (i.e., expanding away at greater than the speed of light). Clearly science has a lot of trouble with this one, because such areas are, by definition, unobservable and thus in one philosophical sense beyond the realm of science (which is based on observation).
Three, take the philosophical view that there can be only one "everything", call that "the universe" and leave it at that. This is more of a philosophical than a scientific matter. Whether all parts of the one "universe" can communicate with each other, or whether one part happens to be expanding when viewed from within it, is not a material issue to this view.
On the matter of the expanding universe, though, you must realize that your thinking is (my interpretation based on your comments) constrained by a Euclidian view of space. The universe is not "expanding into" anything, so far as we know. It is expanding, but since we are "inside" of it, we can not see or observe (so far...people have proposed experiments looking for tightly bound dimensions) anything "outside." The analogy is made of the surface of a balloon. The surface of such a balloon is a finite, yet unbounded area (that is, a two-dimensional creature living on the surface can find no edges, beginning, or end to it). Blow up the balloon and that same two-dimensional creature will see its two-dimensional universe expanding. Whether our 4-dimensional space-time is expanding into some N- dimensional space (N > 4) is an active topic in current theory (e.g. string theories, etc) but is hardly a well-established theory with lots of supporting evidence.
[This subject is discussed at length in the sci.astro FAQ and references therein as well. Moderator]
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