Many physicists and astronomers have predicted that our universe is part of a larger (some say inifinite) set of universes called a multiverse; University of Pennsylvania physicist Max Tegmark classifies these multiverse theories into four different types:
Type 1 Multiverse. If the space in our universe is infinite (if our universe stretches on forever, in other words), then every event which has ever occurred in the observeable universe is being repeated somewhere else about 10 to the 10 to the 28th light years-away. There is a exact copy of you reading this essay about 10 to the 10 to the 28th light years-away from wherever you are now, for example. This idea appears to be related to Nietzsche's Eternal Return.
Type 2. Multiverse. Inflationary cosmologists like Andrei Linde predict that our universe is one in an infinite sea of universes, each with different laws of phsyics, expanding like bubbles from within "space as a whole" via the process of "chaotic inflation." Not all of these universes support life, but many do. Some of these universes may be inhabited by living beings so alien as to defy human imagination.
Type 3. Multiverse. The Many Worlds Interpretation of quantum physics holds that an infinite number of universes exist - every time a possibility manifests itself on the quantum level, a universe splits in two, and thus a new universe is created.
In the MWI view, very single thing which ever could have happened, has happened, and there exist an infinite number of parallel universes: universes where the Confederacy won the Civil War, universes where Elvis married Marilyn Monroe and even universes where Jesus Christ had a third arm sprouting from His back.
Type 3 Multiverses are deterministic in the extreme; if a monkey in one universe is pounding on a keyboard, then we must find another monkey in a parallel universe somewhere pounding out the complete works of Shakespeare.
Type 4 Multiverse. The simplest and most logical prediction is that every sort of universe we can imagine mathematically actually exists in real physical space; Tegmark calls this idea the Ultimate Ensemble Theory.
Some of the universes in this "ultimate ensemble" obey our laws of physics, while others do not. Some universes are too simple and short-lived to be inhabited or observed; others again suffer from overpopulation. Some are inhabited by primitive forms of life while others again are technologically advanced beyond our wildest dreams.
THE MULTIVERSE AND SIMULATION
Australian physicist Paul Davies notes that Type 4 Multiverses strongly support the "Simulation Argument" advanced by Nick Bostrom of Oxford University mathematician.
An infinite multiverse, Bostrom argues, is bound to produce to produce at least one civilization sophisticated enough to program virtual universes of its own. This opens the door to the possibility that our own world might just be some sort of sophisticated simulation created in a computer lab, perhaps by our own descendants in the far future!
Assuming that virtual worlds are easier to create than "real" ones, then once a civilization learned how to program them then it would have no reason not to produce numerous simulated worlds populated by artifically intelligent lifeforms. Simulated worlds would soon greatly outnumber "real" worlds, increasing the chances that any randomly selected inhabitant from the multiverse as a whole would be virtual instead of real. Complains Davies:
"...if simulated systems are every bit as good at simulating their own conscious sub-systems, sub-sub-systems, and so on ad infinitum, gods and worlds, creators and creatures in an infinite regress, embedded within each other... then there would seem to be little point in pursuing scientific inquiry at all..."
There will of course be those who object that simulated universes inhabited by artificially intelligent, virtual people aren't (and will never be possible), but Bostrom argues convincingly that human civilization - assuming that it continues to develop increasingly sophisticated computer programming - can only hope to follow one of three possible courses:
1. the human species is very likely to go extinct before reaching a “posthuman” [i.e., capable of creating virtual universes] stage
2. any posthuman civilization is extremely unlikely to run a significant number of simulations of their evolutionary history (or variations thereof);
3. we are almost certainly living in a computer simulation.
Alternately, our descendants may discover that real universes are easier to create than virtual ones; if so, then we should expect to see physical (as opposed to virtual) universes proliferate instead.