Books, television shows, and films have different theories about the existence of parallel universes. It's a fascinating notion that many believe is good for science fiction. However, it seems that their existence is now becoming a respectable possibility among physicists.
By definition, people cannot visit "alternate" universes. Meaning, even if they are true, there is no way to prove that they can exist. BBC noted that the most common notion is that in alternate universes, people have doppelgangers who live similar lives. This is the also the theory that many seem to like and have been popular among many fictional works such as "The Man in the High Castle" and "Worlds Without End."
There have been different theories about parallel universes. The patchwork universe theory says that the multiverse is the result of the infinite size of our universe and is then divided into a patchwork of regions so that we can't see one another.
Another is the inflationary multiverse, which says that when the Big Bang happened, there is a patch of ordinary space that contained no matter but has energy within it that is a sort of "false vacuum." This "false vacuum" should also experience an inflation, and when it expands, other bubble universe can appear within it in a scenario called "eternal inflation."
The most popular theory remains to be the quantum multiverse theory, where particles are treated like waves and allow for quantum particles to exist in several states at once. However, objects do not switch from multiple state to a single state when they are observed, so all possibilities encoded in the wave are equally real. This is what is known to be as the "many worlds interpretation."
Without real evidence, however, the existence of parallel or multiple universes remains a theory and lies between physics and metaphysics -- reality or science fiction. At any rate, it 's still odd that the possibility of multiverses keep cropping up.
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