The multiverse, a theory in which multiple universes exist, is a consequence of one of the interpretations of quantum mechanics. Its origins lie in a 1957 thesis by Hugh Everett, who offered a 'many worlds' interpretation (MWI) of some of the phenomena observed in quantum mechanical situations, such as the double-slit experiment.
In most formulations of the theory, the constituent "universes" are identical, in that they have the same physical laws and the same values for the fundamental constants but exist in different states, and are arranged so that no information can pass between them. The state of the entire multiverse is constituted by a quantum superposition of states of the constituent universes and is described by a single universal wave function. In a recent work, The Fabric of Reality (1997), David Deutsch argues that the universe as we experience it consists of an infinity of physically possible worlds all co-existing in all possible times, and that what we regard as reality is simply an individual mind's rendering of his or her journey through a sequence of individual panes of the multiverse.