PARALLEL+UNIVERSE+(MULTIVERSE)

=Parallel Universe (Multiverse)=

__[|Age of the Universe]__ __[|Timeline of the Big Bang]__ __[|Ultimate fate of the Universe]__||~ Early Universe || __[|GWB]__ · __[|Neutrino Background]__ __[|Cosmic microwave background]__ ||
 * **__[|Physical cosmology]__** ||
 * [[image:http://upload.wikimedia.org/wikipedia/commons/thumb/2/28/WMAP_2008.png/180px-WMAP_2008.png width="180" height="90" caption="WMAP 2008.png" link="http://en.wikipedia.org/wiki/File:WMAP_2008.png"]] ||
 * __[|Universe]__ · __[|Big Bang]__
 * __[|Inflation]__ · __[|Nucleosynthesis]__

__[|Metric expansion of space]__ __[|Friedmann equations]__ __[|FLRW metric]__ ||
 * ~ Expanding Universe ||
 * __[|Redshift]__ · __[|Hubble's law]__

__[|Structure formation]__ __[|Reionization]__ __[|Galaxy formation]__ __[|Large-scale structure]__ __[|Galaxy filaments]__ ||
 * ~ Structure Formation ||
 * __[|Shape of the Universe]__

__[|Dark energy]__ · __[|Dark matter]__ ||
 * ~ Components ||
 * __[|Lambda-CDM model]__

__[|Timeline of the Big Bang]__ __[|Future of an expanding universe]__ ||
 * ~ Timeline ||
 * __[|Timeline of cosmological theories]__

__[|2dF]__ · __[|SDSS]__ __[|COBE]__ · __[|BOOMERanG]__ · __[|WMAP]__ ||
 * ~ Experiments ||
 * __[|Observational cosmology]__

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 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. 
 * ~ Scientists ||
 * __[|Isaac Newton]__ · __[|Einstein]__ · __[|Hawking]__ · __[|Friedman]__ ·__[|Lemaître]__ · __[|Hubble]__ · __[|Penzias]__ · __[|Wilson]__ · __[|Gamow]__ ·__[|Dicke]__ · __[|Zel'dovich]__ · __[|Mather]__ · __[|Rubin]__ · __[|Smoot]__·__[|//others//]__ || ||

** 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]]__

** 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]]__

** 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 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 __[|interpretation 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.)__[|[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]]__ 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."__[|[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]]__

** Cyclic theories **
The Cyclic Universe //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]__).

** M-theory **
//See also: __[|Brane cosmology]__ and __[|String theory landscape]__// A multiverse of a somewhat different kind has been envisaged within the multi-dimensional extension of __[|string theory]__ known as __[|M-theory]__.__[|[11]]__ 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)__[|[12]]____[|[13]]__; each //universe// takes the form of a __[|D-brane]____[|[14]]____[|[15]]__. 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__[|[16]]__. 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.

** 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 **
//See also: __[|Kolmogorov Complexity]__// 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]]____[|[17]]____[|[18]]__

** 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]__).__[|[19]]__

** 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.__[|[20]]____[|[21]]__

** 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.__[|[22]]____[|[23]]__

** Multiverse hypotheses in religion and spirituality **
//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 **
//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.__[|[24]]__ 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 **
//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). 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.