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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]],<ref>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."</ref> 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]]. In these contexts, parallel universes are also called "alternative universes," "quantum universes," "parallel worlds," "alternate realities," "alternative timelines," etc.

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

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

==Multiverse hypotheses in physics==

==Multiverse hypotheses in physics==

[[Laura Mersini|Laura Mersini-Houghton]] claims that the [[WMAP cold spot#Parallel universe|WMAP cold spot]] may provide testable empirical evidence for a parallel universe within the multiverse. According to [[Max Tegmark]],<ref>{{cite journal| first=Max| last=Tegmark| journal=Scientific American| year=2003| month=May| title=Parallel Universes}}</ref> the existence of other universes is a direct implication of cosmological observations. Tegmark describes the set of related concepts which share the notion that there are universes beyond the familiar observable one, and goes on to provide a taxonomy of parallel universes organized by levels. (Parallel Universes [http://www.wintersteel.com/files/ShanaArticles/multiverse.pdf])

[[Laura Mersini|Laura Mersini-Houghton]] claims that the [[WMAP cold spot#Parallel universe|WMAP cold spot]] may provide testable empirical evidence for a parallel universe within the multiverse. According to [[Max Tegmark]], (Scientific American, 2003, May, Parallel Universes) the existence of other universes is a direct implication of cosmological observations. Tegmark describes the set of related concepts which share the notion that there are universes beyond the familiar observable one, and goes on to provide a taxonomy of parallel universes organized by levels. (Parallel Universes [http://www.wintersteel.com/files/ShanaArticles/multiverse.pdf])