The multiverse hypothesis postulates that multiple universes exist, and it 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. The multiverse hypothesis attempts to avoid the significant body of evidence supporting the fine-tuning of the universe (see: Anthropic principle).
In his 2003 NY Times opinion piece, A Brief History of the Multiverse, the cosmologist Paul Davies offers a variety of arguments that multiverse hypotheses are non-scientific:
|“||For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there are an infinite number of universes, credibility reaches a limit. As one slips down that slope, more and more must be accepted on faith, and less and less is open to scientific verification. Extreme multiverse explanations are therefore reminiscent of theological discussions. Indeed, invoking an infinity of unseen universes to explain the unusual features of the one we do see is just as ad hoc as invoking an unseen Creator. The multiverse theory may be dressed up in scientific language, but in essence it requires the same leap of faith.||”|
In most formulations of the hypothesis, 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.
Related ideas are Richard Feynman's multiple histories interpretation, Dieter Zeh's many-minds interpretation, and David Kellogg Lewis's modal realism theory of possible worlds.
The multiverse is suggested as a possible explanation for the "bumps and wiggles" in temperature observed for cosmic background radiation, and for "dark flow" (the streaming of galaxy clusters across space).
- Universe or Multiverse. p. 19. ISBN 9780521848411. "Some physicists would prefer to believe that string theory, or M-theory, will answer these questions and uniquely predict the features of the Universe. Others adopt the view that the initial state of the Universe is prescribed by an outside agency, code-named God, or that there are many universes, with ours being picked out by the anthropic principle. Hawking argues that string theory is unlikely to predict the distinctive features of the Universe. But neither is he is an advocate of God. He therefore opts for the last approach, favouring the type of multiverse which arises naturally within the context of his own work in quantum cosmology."
- Davies, Paul. "New York Times, April 12, 2003 A Brief History of the Multiverse".