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The multiverse (or meta-universe, metaverse) is the hypothetical set of multiple possible universes (including our universe) that together comprise  everything that physically exists: the entirety of space and time, all forms of matter, energy and momentum, and the physical laws and constants that govern them. The different 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, philosophy, transpersonal psychology and fiction, particularly in science fiction and fantasy. The term was coined in 1895 by psychologist 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> In these contexts, parallel universes are also called "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel worlds", "alternative realities", and "alternative timelines", among others.

== Multiverse hypotheses in physics ==
=== Tegmark's classification ===
Cosmologist Max Tegmark has provided a taxonomy of universes beyond the familiar observable universe. The levels according to Tegmark's classification are briefly described below.<ref>Template:Cite journal</ref><ref>{{cite book
 | first = Max
 | last = Tegmark
 | year = 2003
 | month = January 23
 | title = Parallel Universes
 | url =
 |format=PDF| accessdate=2006-02-07
}} (PDF)</ref>

==== Level I: Beyond our cosmological horizon ====
A generic prediction of cosmic inflation is an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.

An infinite universe should contain an infinite number of Hubble volumes. All will have the same physical laws and physical constants. However, almost all will be different from our Hubble volume regarding configurations such as how matter is distributed in the volume. But since there are an infinite number of such volumes, then some of these will be very similar or even identical to our own. Thus, far beyond our cosmological horizon, there will eventually be a Hubble volume identical to our own. Tegmark estimates that such an identical volume should be about 10<sup>118</sup> meters away.<ref name="TegmarkPUstaple">"Parallel universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations.", Tegmark M., Sci Am. 2003 May;288(5):40-51.</ref>

==== Level II: Universes with different physical constants ====
File:Multiverse - level II.svg

In the chaotic inflation theory, a variant of the cosmic inflation theory, the multiverse as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles, like gas pockets in a loaf of rising bread. There exists an infinite number of such bubbles which are embryonic level I universes of infinite size. Different bubbles may experience different spontaneous symmetry breaking resulting in different properties such as different physical constants.<ref name="TegmarkPUstaple"/>

This level also includes John Archibald Wheeler's oscillatory universe theory and Lee Smolin's fecund universes theory.

==== Level III: Many-worlds interpretation of quantum mechanics ====
Hugh Everett's many-worlds interpretation (MWI) is one of several mainstream interpretations of quantum mechanics. In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is a range of possible observations each with a different probability. According to the MWI, each of these possible observations correspond to a different universe. Suppose a die is thrown that contains 6 sides and that the result correspond to a quantum mechanics observable. All 6 possible ways the die can fall correspond to 6 different universes. (More correctly, in MWI there is only a single universe but after the "split" into "many worlds" these cannot in general interact.)<ref>
Tegmark, Max, The Interpretation of Quantum Mechanics: Many Worlds or Many Words?, 1998.
Deutsch, David, David Deutsch's Many Worlds, Frontiers, 1998.</ref>

Tegmark argues that a level III multiverse does not contain more possibilities in the Hubble volume than a level I-II multiverse. In effect, all the different "worlds" created by "splits" in a level III multiverse with the same physical constants can be found in some Hubble volume in a level I multiverse. Tegmark writes that "The only difference between Level I and Level III is where your doppelgängers reside. In Level I they live elsewhere in good old three-dimensional space. In Level III they live on another quantum branch in infinite-dimensional Hilbert space." Similarly, all level II bubble universes with different physical constants can in effect be found as "worlds" created by "splits" at the moment of spontaneous symmetry breaking in a level III multiverse.<ref name="TegmarkPUstaple"/>

Related to the many-worlds idea are Richard Feynman's multiple histories interpretation and H. Dieter Zeh's many-minds interpretation.

==== Level IV: Ultimate Ensemble ====
The Ultimate Ensemble hypothesis of Tegmark himself. This level considers equally real all universes that can be defined by mathematical structures. This also includes those having physical laws different from our observable universe. Tegmark writes that "abstract mathematics is so general that any TOE that is definable in purely formal terms (independent of vague human terminology) is also a mathematical structure. For instance, a TOE involving a set of different types of entities (denoted by words, say) and relations between them (denoted by additional words) is nothing but what mathematicians call a set-theoretical model, and one can generally find a formal system that it is a model of." He argues this "it implies that any conceivable parallel universe theory can be described at Level IV" and "it subsumes all other ensembles, therefore brings closure to the hierarchy of multiverses, and there cannot be say a Level V."<ref>{{cite book
 | first = Max
 | last = Tegmark
 | year = 2003
 | month = January 23
 | title = Parallel Universes
 | url =
 |format=PDF| accessdate=2006-02-07
}} (PDF).</ref>

Jürgen Schmidhuber, however, says the "set of mathematical structures" is not even well-defined, and admits only universe representations describable by constructive mathematics, that is, computer programs. He explicitly includes universe representations describable by non-halting programs whose output bits converge after finite time, although the convergence time itself may not be predictable by a halting program, due to Kurt Gödel's limitations.<ref>J. Schmidhuber (1997): A Computer Scientist's View of Life, the Universe, and Everything. Lecture Notes in Computer Science, pp. 201-208, Springer: IDSIA - Dalle Molle Institute for Artificial Intelligence </ref><ref>J. Schmidhuber (2000): Algorithmic Theories of Everything e-Print archive</ref><ref>J. Schmidhuber (2002): Hierarchies of generalized Kolmogorov complexities and nonenumerable universal measures computable in the limit. International Journal of Foundations of Computer Science 13(4):587-612 IDSIA - Dalle Molle Institute for Artificial Intelligence</ref> He also explicitly discusses the more restricted ensemble of quickly computable universes.<ref>J. Schmidhuber (2002): The Speed Prior: A New Simplicity Measure Yielding Near-Optimal Computable Predictions. Proc. 15th Annual Conference on Computational Learning Theory (COLT 2002), Sydney, Australia, Lecture Notes in Artificial Intelligence, pp. 216-228. Springer: IDSIA - Dalle Molle Institute for Artificial Intelligence</ref>

=== Cyclic theories ===

Main article: Cyclic model
In several theories there is a series of infinite, self-sustaining cycles (for example: an eternity of Big Bang-Big crunches).

=== M-theory ===
Template:See also
A multiverse of a somewhat different kind has been envisaged within the multi-dimensional extension of string theory known as M-theory.<ref>Steven Weinberg(2005)"Living in the Multiverse"</ref> In M-theory our universe and others are created by collisions between p-branes in a space with 11 and 26 dimensions (the number of dimensions depends on the chirality of the observer)<ref>Richard J Szabo, An introduction to string theory and D-brane dynamics (2004)</ref><ref>Maurizio Gasperini, Elements of String Cosmology (2007)</ref>; each universe takes the form of a D-brane<ref>Richard J Szabo, An introduction to string theory and D-brane dynamics (2004)</ref><ref>Maurizio Gasperini, Elements of String Cosmology (2007)</ref>. Objects in each universe are essentially confined to the D-brane of their universe, but may be able to interact with other universes via gravity, a force which is not restricted to D-branes<ref>Paul Halpern, The Great Beyond, 2005</ref>. This is unlike the universes in the "quantum multiverse", but both concepts can operate at the same time.

=== Anthropic principle ===

Main article: Anthropic principle
The concept of other universes has been proposed to explain why our universe seems to be fine-tuned for conscious life as we experience it. If there were a large number (possibly infinite) of different physical laws (or fundamental constants) in as many universes, some of these would have laws that were suitable for stars, planets and life to exist. The anthropic principle could then be applied to conclude that we would only consciously exist in those universes which were finely-tuned for our conscious existence. Thus, while the probability might be extremely small that there is life in most of the universes, this scarcity of life-supporting universes does not imply intelligent design as the only explanation of our existence.

=== WMAP cold spot ===
Laura Mersini-Houghton claims that the WMAP cold spot may provide testable empirical evidence for a parallel universe within the multiverse.

=== Criticisms ===
==== Non-scientific claims ====
Critics claim that many of these theories lack empirical testability, and without hard physical evidence are unfalsifiable; outside the methodology of scientific investigation to confirm or disprove.

==== Occam's Razor  ====
Template:See alsoTegmark answers: "A skeptic worries about all the information necessary to specify all those unseen worlds. But an entire ensemble is often much simpler than one of its members. This principle can be stated more formally using the notion of algorithmic information content. The algorithmic information content in a number is, roughly speaking, the length of the shortest computer program that will produce that number as output. For example, consider the set of all integers. Which is simpler, the whole set or just one number? Naively, you might think that a single number is simpler, but the entire set can be generated by quite a trivial computer program, whereas a single number can be hugely long. Therefore, the whole set is actually simpler. Similarly, the set of all solutions to Einstein's field equations is simpler than a specific solution. The former is described by a few equations, whereas the latter requires the specification of vast amounts of initial data on some hypersurface. The lesson is that complexity increases when we restrict our attention to one particular element in an ensemble, thereby losing the symmetry and simplicity that were inherent in the totality of all the elements taken together. In this sense, the higher-level multiverses are simpler. Going from our universe to the Level I multiverse eliminates the need to specify initial conditions, upgrading to Level II eliminates the need to specify physical constants, and the Level IV multiverse eliminates the need to specify anything at all." He continues "A common feature of all four multiverse levels is that the simplest and arguably most elegant theory involves parallel universes by default. To deny the existence of those universes, one needs to complicate the theory by adding experimentally unsupported processes and ad hoc postulates: finite space, wave function collapse and ontological asymmetry. Our judgment therefore comes down to which we find more wasteful and inelegant: many worlds or many words. Perhaps we will gradually get used to the weird ways of our cosmos and find its strangeness to be part of its charm."<ref name="TegmarkPUstaple"/><ref>==Further reading==


Main article: Multiverse (religion)

=== Planes of existence ===

Main article: Plane (esotericism)
Certain religions and esoteric cosmologies propound the idea of a whole series of subtle emanated planes or worlds.

=== Afterlife ===

Many religions include an afterlife existence in realms, such as heavens and hells, which may be very different from the observable universe.

=== Eschatology ===
Template:See also
Eschatological scenarios may include a new different world after the end time of the current one. For example, Hindu cosmology include the idea of an infinite cycle of births and deaths and an infinite number of universes with each cycle lasting 8.4 billion years.<ref>Carl Sagan, Placido P D'Souza (1980s). Hindu cosmology's time-scale for the universe is in consonance with modern science.; Dick Teresi (2002). Lost Discoveries: The Ancient Roots of Modern Science – from the Babylonians to the Maya.</ref>
Similar eschatological scenarios appear in other religions, in the form of belief in there being a new and different world after the end time of the current one.

== Multiverse hypothesis in fiction ==
Template:See also
Fiction by definition does not claim to be a completely accurate description of our observable universe. All fiction could thus be seen as describing different universes. Some genres, such as crime fiction and historical fiction, may describe universes similar to the observable one, while others, such as fantasy, science fiction, and alternate history, may describe ones more different.

Parallel universes in fiction may interact. For example, in science fiction a common plot device is hyperspace which is temporarily entered and used for faster than light travel.

The term 'Multiverse' was used in 1962 by science fiction author Michael Moorcock, though not coined by him, having previously been used by both William James in 1895 and John Cowper Powys in his 1955 novel The Brazen Head (p. 279). The author and editor Paul le Page Barnett, best known by the pseudonym John Grant, later used the term 'polycosmos' for a similar concept binding together a number of his works.<ref>"John Grant" interviewed by Lou Anders, accessed 24 October 2009</ref>

The television show Sliders was based entirely on the possibility of parallel universes. In each episode the stars experienced an alternate universe through the use of a device that would create a portal through which to travel to those alternate realities.

In the card game Magic: The Gathering the different worlds that the characters of the game inhabit, are located throughout what is described as the Multiverse. Certain characters called "Planeswalkers" have the ability to travel to different planes within the Multiverse.

In the season 8 premiere of the comedy television show Family Guy, Stewie and Brian use an enhanced device to travel to different universes.

== See also ==
* The Fabric of Reality
* Philosophy of physics
* Philosophy of space and time
* Reductionism
* Simulated reality
* Omniverse
* Modal realism
* Impossible world
* As Contrary concepts
** Determinism
** Fatalism
** Predestination

== Notes ==

== References ==
* {{cite book
 | first = David
 | last = Deutsch
 | authorlink =
 | coauthors =
 | year = 1985
 | month = 45841
 | title = Quantum theory, the Church-Turing principle and the universal quantum computer
 | chapter =
 | editor = Splash
 | others =
 | edition = Proceedings of the Royal Society of London A 400
 | pages = 97–117
 | publisher =
New wormhole

 | location =
 | id =
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== External links ==
* Physics in the Multiverse, published by Aurélien Barrau in the CERN Courier
* Preprint of David Deutsch's paper The Structure of the Multiverse
* BBC Horizon -Parallel Universes
* Michael Price's Everett FAQ
* Mulitverse Cosmological Models by P.C.W. Davies
* The ensemble of universes describable by constructive mathematics by Jürgen Schmidhuber
*Interview with Tufts cosmologist Alex Vilenkin on his new book, "Many Worlds in One: The Search for Other Universes" on the podcast and public radio interview program ThoughtCast.
* Joseph Pine II about Multiverse, Presentation at Mobile Monday Amsterdam, 2008
* Multiple Universes ?, in French by A. Barrau
* Parallel universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations by Max Tegmark
* {{cite journal
 | last =Ellis
 | first = George F.R.
 | authorlink = George Ellis
 | coauthors = U. Kirchner, W.R. Stoeger
 | title =Multiverses and physical cosmology
 | journal =Monthly Notices of the Royal Astronomical Society
 | volume =347
 | issue =
 | pages =921–936
 | publisher =
 | url =
 | doi =10.1111/j.1365-2966.2004.07261.x
 | id =
 | accessdate = 2007-01-09}}

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