Alex Vilenkin

According to the theory of eternal inflation, there's a multiverse consisting of an exponentially expanding sea of high-energy "false vacuum." This sea continually gives rise to embedded, expanding, true-vacuum "bubble universes," each of which from the *inside* seems to have sprung from a Big Bang and to be spatially *infinite*.
Our own observable universe (OU) is a finite (albeit currently about 10^10 or 10^11 light-years in size) part of one bubble universe. Different regions of the bubble that are vastly larger than the OU will have different laws of physics. (This is relevant for the apparent "fine-tuning" of the physical constants [which the intelligent-design crowd is so eager to misinterpret] and the validity of some versions of the anthropic principle.)
Anything physically conceivable must occur an infinite number of times somewhere in our bubble (not to mention in all the other bubbles). E.g. an exact duplicate of Earth and you and me might exist as "close" as 10^(10^123) light-years away! (That's a number so big that writing it out in decimal would take more paper than could fit in the OU.)
If all that is not enough (and it *is* serious, mathematically rigorous science), there could also be more than one multiverse, and some bubbles could be spawned within other bubbles. Truly mind-bending.… (plus d'informations)