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The multiverse (or meta-universe, metaverse) is the hypothetical set of multiple possible universes (including the historical 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 term was coined in 1895 by the American philosopher and psychologist William James.[1] The various universes within the multiverse are sometimes called parallel universes.
The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered. Multiverses 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 "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "parallel worlds", "alternative realities", and "alternative timelines", among others.
Contents [hide]
1 Multiverse hypotheses in physics
1.1 Tegmark's classification
1.1.1 Level I: Beyond our cosmological horizon
1.1.2 Level II: Universes with different physical constants
1.1.3 Level III: Many-worlds interpretation of quantum mechanics
1.1.4 Level IV: Ultimate Ensemble
1.2 Cyclic theories
1.3 M-theory
1.4 Anthropic principle
1.5 WMAP cold spot
1.6 Criticisms
1.6.1 Non-scientific claims
1.6.2 Indirect Evidence
1.6.3 Occam's Razor
2 Multiverse hypotheses in philosophy and logic