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A '''multiverse''' (or '''meta-universe''') is the hypothetical set of multiple possible [[universe]]s (including our universe) that together comprise all of [[reality]]. The different universes within a 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.  

A '''multiverse''' (or '''meta-universe''') is the hypothetical set of multiple possible [[universe]]s (including our universe) that together comprise all of [[reality]]. The different universes within a 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]], [[philosophy]], [[theology]], and [[fiction]], particularly in [[science fiction]] and [[fantasy]]. The specific term "multiverse," which was coined by [[William James]], (James, William, ''The Will to Believe'', 1895; and earlier in 1895, as cited in [[OED]]'s new 2003 entry for "multiverse": "1895 W. JAMES in Internat. Jrnl. Ethics 6 10) Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe." was popularized by science fiction author [[Michael Moorcock]]. In these contexts, parallel universes are also called "alternative universes," "quantum universes," "parallel worlds," "alternate realities," "alternative timelines," etc.

Multiverses have been hypothesized in [[cosmology]], [[physics]], [[astronomy]], [[philosophy]], [[theology]], and [[fiction]], particularly in [[science fiction]] and [[fantasy]]. The specific term "multiverse," which was coined by [[William James]], (James, William, ''The Will to Believe'', 1895; and earlier in 1895, as cited in [[OED]]'s new 2003 entry for "multiverse": "1895 W. JAMES in Internat. Jrnl. Ethics 6 10) Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe." was popularized by science fiction author [[Michael Moorcock]].

The possibility of many universes raises various scientific and philosophical questions.

The possibility of many universes raises various scientific and philosophical questions.

Thus, according to Tegmark, paradoxically the multiverse scenario is more parsimonious than that of a single universe.

Thus, according to Tegmark, paradoxically the multiverse scenario is more parsimonious than that of a single universe.

David Lewis, however, draws a distinction between qualitative and quantitative excess. Postulating extra universe just like our own does not increase the number of kinds of things there are, and thus there is only qualitative invarience.{{Fact|date=January 2008}}

David Lewis, however, draws a distinction between qualitative and quantitative excess. Postulating extra universe just like our own does not increase the number of kinds of things there are, and thus there is only qualitative invarience.

===One unique universe===

===One unique universe===