So what is the Multiverse? It could mean one of three things: More “Universe” like our own that came from the same Big Bang, but isn’t observable. Or it could mean more Universes like our own that came from different Big Bangs, but that originated from the same, common inflationary state. Or it could mean a wide variety of Universes — some like our own and some different — with different constants and even laws governing them. The Multiverse may be finite in size and number of Universes, or it could be infinite. If you accept the Big Bang and modern cosmology, then the first one is definitely true. If you accept cosmic inflation (and there are good reasons to do so), then so is the second one. And if you accept certain models of String Theory or other unification ideas, the third one may be true as well. (But not necessarily!) And as for the finite-or-infinite question? That’s still something we don’t know; there’s a theorem that says inflation couldn’t have gone on forever, but there are loopholes that may allow it to have done exactly that, after all.
(via The Multiverse for non-scientists — Starts With A Bang! — Medium)