When physicists are asked about “parallel worlds” or ideas along those lines, they have to be careful to distinguish among different interpretations of that idea. There is the “multiverse” of inflationary cosmology, the “many worlds” or “branches of the wave function” of quantum mechanics, and “parallel branes” of string theory. Increasingly, however, people are wondering whether the first two concepts might actually represent the same underlying idea. (I think the branes are still a truly distinct notion.)At first blush it seems crazy — or at least that was my own initial reaction. When cosmologists talk about “the multiverse,” it’s a slightly poetic term. We really just mean different regions of spacetime, far away so that we can’t observe them, but nevertheless still part of what one might reasonably want to call “the universe.” In inflationary cosmology, however, these different regions can be relatively self-contained — “pocket universes,” as Alan Guth calls them. When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be very different; they can have different particles, different forces, even different numbers of dimensions. So there is a good reason to think about them as separate universes, even if they’re all part of the same underlying spacetime.
Yes, he’s the Sean Carroll who wrote “No God Needed: CalTech physicist responds to Uncommon Descent’s questions,” shortly to become our fifth most-read post.