A hitchhiker's guide to the multiverse

Different theories spin off very different kinds of multiverses. Our current standard theory of how the universe came to be, for example, predicts an infinite expanse of other universes, including an infinite number in which duplicates of you are reading this sentence and wondering if those other versions of you really exist. Meanwhile, string theory, which hoped to derive the particles, forces and constants of our universe from fundamental principles, instead discovered a wilderness of 10^500 universes fundamentally different from ours. Even quantum mechanics implies that our universe is a single snowflake in a blizzard of parallel universes.

-- from The Ultimate Guide to the Multiverse.