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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 and can exist: 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. 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", "alternative timelines", and "dimensional planes," among others.
Multiverse hypotheses in physics
Cosmologist Max Tegmark has provided a taxonomy of universes beyond the familiar observable universe. The levels according to Tegmark's classification are arranged such that subsequent levels can be understood to encompass and expand upon previous levels, and they are briefly described below. 
Level I: Beyond our cosmological horizon
Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having the same physical laws and physical constants. In regard to configurations such as the distribution of matter, almost all will differ from our Hubble volume. However, because there are infinitely many, far beyond the cosmological horizon, there will eventually be Hubble volumes with similar, and even identical, configurations. Tegmark estimates that an identical volume to ours should be about 1010115 meters away from us (a number larger than a googolplex). This estimate implies use of the cosmological principle, wherein one assumes our Hubble volume is not special or unique. By extension of the same reasoning, there would, in fact, be an infinite number of Hubble volumes identical to ours in the universe.
Level II: Universes with different physical constants
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. Such bubbles are embryonic level I multiverses. Linde and Vanchurin calculated the number of these universes to be on the scale of 101010000000.
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 corresponds to a different universe. Suppose a die is thrown that contains six sides and that the result corresponds to a quantum mechanics observable. All six possible ways the die can fall correspond to six different universes. (More correctly, in MWI there is only a single universe but after the "split" into "many worlds" these cannot in general interact.)
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.
Level IV: Ultimate Ensemble
The Ultimate Ensemble hypothesis of Tegmark himself. This level considers equally real all universes that can be described by different mathematical structures. This does not include different low-energy physical laws not of our observable universe. Tegmark writes that "abstract mathematics is so general that any Theory Of Everything (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 "implies that any conceivable parallel universe theory can be described at Level IV" and "subsumes all other ensembles, therefore brings closure to the hierarchy of multiverses, and there cannot be say a Level V."
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. He also explicitly discusses the more restricted ensemble of quickly computable universes.
A multiverse of a somewhat different kind has been envisaged within the multi-dimensional extension of string theory known as M-theory, also known as Membrane Theory. 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); each universe takes the form of a D-brane. 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. This is unlike the universes in the "quantum multiverse", but both concepts can operate at the same time.
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 weak 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.
People have criticized the name "multiverse". In the July 5, 2010 episode of the Colbert Report (Episode 6088) Steven confronted his guest, Michio Kaku, on this issue. His logic was that the phase "multiverse" is self-contradictory due to the definition of universe in the original "Greek". Colbert was slightly wrong on this: universe is a Latin term, but he was right overall as it means "entirety"  and it is impossible to have more than one entirety.
Some cosmologists, such as Paul Davies and George Ellis, argue that many Multiverse theories lack empirical testability and are unfalsifiable; they are thus outside the methodology of scientific investigation to confirm or disprove. Reasons why such claims lack empirical evidence or testability according to most Multiverse theories is that other universes are in a different spacetime framework, so in principle they cannot be observed.
The logical foundation of modern science is hypothetico-deductive logic which permits a theory to propose unobservable entities if these help explain observable outcomes, either by theory-based predictions (of future observations) or retroductionism (of already known observations).
Max 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."
"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."
Multiverse hypotheses in philosophy and logic
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).
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.
The view that because fictions exist, fictional characters exist as well. There are fictional entities, in the same sense in which, setting aside philosophical disputes, there are people, Mondays, numbers and planets.
Multiverse hypotheses in religion and spirituality
Cosmology in medieval Islam
Fakhr al-Din al-Razi (1149–1209), in dealing with his conception of physics and the physical world in his Matalib al-'Aliya, criticizes the idea of the Earth's centrality within the universe and "explores the notion of the existence of a multiverse in the context of his commentary" on the Qur'anic verse, "All praise belongs to God, Lord of the Worlds." He raises the question of whether the term "worlds" in this verse refers to "multiple worlds within this single universe or cosmos, or to many other universes or a multiverse beyond this known universe."
The concept of multiple universes is mentioned many times in Hindu Puranic literature, such as in the Bhagavata Purana:
Because You are unlimited, neither the lords of heaven nor even You Yourself can ever reach the end of Your glories. The countless universes, each enveloped in its shell, are compelled by the wheel of time to wander within You, like particles of dust blowing about in the sky. The śrutis, following their method of eliminating everything separate from the Supreme, become successful by revealing You as their final conclusion (Bhagavata Purana 10.87.41)
"Our mind follows the development of possibilities always in one direction only. But in fact every moment contains a very large number of possibilities. And all of them are actualised, only we do not see it and do not know it. We always see only one of the actualisations, and in this lie the poverty and limitation of the human mind. But if we try to imagine the actualisation of all the possibilities of the present moment, then of the next moment, and so on, we shall feel the world growing infinitely, incessantly multiplying by itself and becoming immeasurably rich and utterly unlike the flat and limited world we have pictured to ourselves up to this moment." 
Planes of existence
Certain religions and esoteric cosmologies propound the idea of a whole series of subtle emanated planes or worlds.
Eschatological scenarios may include a new different world after the end time of the current one. For example, Hindu cosmology includes the idea of an infinite cycle of births and deaths and an infinite number of universes with each cycle lasting 8.4 billion years.
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.
- Impossible world
- Modal realism
- Roger Penrose
- Parallel universe (fiction)
- Philosophy of physics
- Philosophy of space and time
- Simulated reality
- The Fabric of Reality
- As contrary concepts
- ^ 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."
- ^ Tegmark, Max (May 2003). "Parallel Universes". Scientific American.
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- ^ Craig Rusbult, Ph.D
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- Max Tegmark (2003) "Parallel Universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations."
- David Deutsch (2001) "The Structure of the Multiverse."
- BBC Horizon -Parallel Universes.
- Michael Price's Everett FAQ.
- Jürgen Schmidhuber, "The ensemble of universes describable by constructive mathematics."
- 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
- Multiverse - Radio-discussion on BBC Four with Melvyn Bragg
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