The multiverse (or meta-universe) is the hypothetical set of infinite or finite possible universes (including the Universe we consistently experience) that together comprise everything that exists: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The various universes within the multiverse are sometimes called parallel universes or "alternate universes".
The structure of the multiverse, the nature of each universe within
it and the relationships among the various constituent universes, depend
on the specific multiverse hypothesis considered. Multiple universes
have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology, and fiction, particularly in science fiction and fantasy.
In these contexts, parallel universes are also called "alternate
universes", "quantum universes", "interpenetrating dimensions",
"parallel dimensions", "parallel worlds", "alternate realities",
"alternate timelines", and "dimensional planes," among others. The term
'multiverse' was coined in 1895 by the American philosopher and
psychologist William James in a different context.
The multiverse hypothesis is a source of debate within the physics
community. Physicists disagree about whether the multiverse exists, and
whether the multiverse is a proper subject of scientific inquiry. Supporters of one of the multiverse hypotheses include Stephen Hawking, Steven Weinberg, Brian Greene, Max Tegmark, Alan Guth, Andrei Linde, Michio Kaku, David Deutsch, Leonard Susskind, Raj Pathria, Sean Carroll, Alex Vilenkin, Laura Mersini-Houghton, and Neil deGrasse Tyson. In contrast, critics such as Jim Baggott, David Gross, Paul Steinhardt, George Ellis and Paul Davies
have argued that the multiverse question is philosophical rather than
scientific, that the multiverse cannot be a scientific question because
it lacks falsifiability, or even that the multiverse hypothesis is harmful or pseudoscientific.
