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For other uses, see Multiverse (disambiguation).

The multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of reality. 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 specific term "multiverse" was coined in 1895 by psychologist William James.[1] In these contexts, parallel universes are also called "alternative universes", "quantum universes", "interpenetrating dimensions", "parallel worlds", "alternative realities", "alternative timelines", etc.

Multiverse hypotheses in physics

[edit] 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.[2][3]

[edit] 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 10118 meters away.[4]

[edit] Level II: Universes with different physical constants

"Bubble universes", every disk is a bubble universe (Universe 1 to Universe 6 are different bubbles, they have physical constants that are different from our universe), our universe is just one of the bubbles.
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.[4]
This level also include John Archibald Wheeler's oscillatory universe theory and Lee Smolin's fecund universes theory.

[edit] Level III: Many-worlds interpretation of quantum mechanics
Hugh Everett's many-worlds interpretation (MWI) is one of several mainstream interpretation of quantum mechanics. In brief, one aspect of quantum mechanics is that certain observations cannot be predicted absolutely. Instead, there is 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.)[5]
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.[4]

[edit] 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 including 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."[6]
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.[7][8][9] He also explicitly discusses the more restricted ensemble of quickly computable universes.[10]

[edit] Cyclic theories
Main articles: Cyclic model and Oscillatory universe
In several theories there is a series of infinite, self-sustaining cycles (for example: an eternity of Big Bang-Big crunches).

[edit] String theory
See also: Brane cosmology and String theory landscape
A multiverse has been envisaged within some versions of the 11-dimensional extension of string theory known as M-theory. In one version of M-theory our universe and others are created by collisions between membranes in an 11-dimensional space.

[edit] 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.

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

[edit] Criticisms

[edit] 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.

[edit] Specified complexity
William Dembski, a proponent of intelligent design, criticizes multiverse theories using arguments based on specified complexity.[11]

[edit] Occam's Razor
See also: Kolmogorov Complexity
Critics argue that to postulate usually unobservable universes just to explain our universe seems contrary to Occam's razor.[12][13]
Tegmark 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. "[4]

[edit] Multiverse hypotheses in philosophy and logic

[edit] Modal realism
Possible worlds are a way of explaining probability, hypothetical statements and the like, and some philosophers such as David Lewis believe that all possible worlds exist, and are just as real as the actual world (a position known as modal realism).[14]

[edit] Trans-world identity
A metaphysical issue that crops up in multiverse schema that posit infinite identical copies of any given universe is that of the notion that there can be identical objects in different possible worlds. According to the counterpart theory of David Lewis, the objects should be regarded as similar rather than identical.[15][16]

[edit] Fictional realism
The view that because fictions exist, fictional characters exist as well. There are fictional entities, in the same sense as that in which, setting aside philosophical disputes, there are people, Mondays, numbers and planets.[17][18]

[edit] Multiverse hypotheses in religion and spirituality
Main article: Multiverse (religion)

[edit] Planes of existence
Main article: Plane (metaphysics)
Certain religions and esoteric cosmologies propound the idea of a whole series of subtle emanated planes or worlds.

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

[edit] Eschatology
See also: End time
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.[19]

[edit] Multiverse Hypothesis in fiction
See also: Parallel universe (fiction) and Fictional universe
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 (as it had previously been used both by William James in 1895 and by J.C.Powys in his 1955 novel The Brazen Head p.279).

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