Multiverse Totally Explained

Everything about The Multiverse totally explained

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 or dimensions. 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, 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.

Multiverse hypotheses in physics

Laura Mersini-Houghton claims that the WMAP cold spot may provide testable empirical evidence for a parallel universe within the multiverse. According to Max Tegmark, 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.

Classification

In order to clarify terminology, George Ellis, U. Kirchner and W. R. Stoeger recommend using the term "the Universe" for the theoretical model of the whole of the causally connected spacetime in which we live, universe domain for the observable universe or a similar part of the same space-time, "universe" for a general space-time, either our own "Universe" or another one disconnected from our own, multiverse for a set of disconnected space-times, and multi-domain universe to refer to a model of the whole of a single connected space-time in the sense of chaotic inflation models.
The levels according to Tegmark's classification and using Ellis, Kirchner and Stoeger's terminology are briefly described below.

Multi-domain universes (Ellis, Kirchner and Stoeger sense):

Level I: (Open multiverse) A generic prediction of cosmic inflation is an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.

Universes with different physical constants

Level II: (Andrei Linde's bubble theory) In chaotic inflation, other thermalized regions may have different effective physical constants, dimensionality and particle content. (Surprisingly, this level includes Wheeler's oscillating universe theory as well.)

Multiverses (Ellis, Kirchner and Stoeger sense)

Level III: (Hugh Everett III's many-worlds interpretation) An interpretation of quantum mechanics that proposes the existence of multiple universes, all of which are "identical", but exist in possibly different states. It is widely believed that Everett's interpretation (considered as a formal theory) is a conservative extension of standard quantum mechanics – that is, as far as results expressible in the language of ordinary quantum mechanics are concerned, it leads to no new results. This, according to Tegmark, "is ironic given that this level has historically been the most controversial". In September 2007 David Deutsch presented what is considered a proof of the many-worlds interpretation.

Ultimate ensemble

Level IV: (The ultimate "Ensemble theory" of Tegmark) Other mathematical structures give different fundamental equations of physics. This level considers "real" any hypothetical universe based on one of these structures. Since this subsumes all other possible ensembles, it brings closure to the hierarchy of multiverses: there can't be a Level V. Jürgen Schmidhuber, however, says the "set of mathematical structures" isn't 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 .

Bubble theory

Bubble theory posits an infinite number of open multiverses, each with different physical constants. (The set of bubble universes is thus a Level II multiverse.) Counter-intuitively, these universes are farther away than even the farthest universe in our open multiverse.
The formation of our universe from a "bubble" of a multiverse was proposed by Andre Linde. This Bubble universe theory fits well with the widely accepted theory of cosmic inflation. The bubble universe concept involves creation of universes from the elementary particles in one universe. if the theory of relativity and quantum physics are true then in a infinite amount of time any particle combanation is possible and may form another universe. If the energy fluctuation isn't very large, a tiny bubble universe may form, experience some expansion like an inflating balloon, and then contract and disappear from existence. However, if the energy fluctuation is greater than a particular critical value, a tiny bubble universe forms from the parent universe, experiences long-term expansion, and allows matter and large-scale galactic structures to form.

Many worlds interpretation of quantum physics

Hugh Everett's many-worlds interpretation (MWI) is one of several mainstream interpretations of quantum mechanics. Other interpretations include the Copenhagen and the consistent histories interpretations. The multiverse proposed by MWI has a shared time parameter. In most formulations, all the constituent universes are structurally identical to each other and though they've the same physical laws and values for the fundamental constants, they may exist in different states. The constituent universes are furthermore non-communicating, in the sense that no information can pass between them, although in Everett's formulation they may potentially affect each other through quantum interference. The state of the entire multiverse is related to the states of the constituent universes by quantum superposition, and is described by a single universal wavefunction. Related are Richard Feynman's multiple histories interpretation and H. Dieter Zeh's many-minds interpretation.
Many worlds interpretation can't explain the apparently Fine-tuned universe. The physical constants of all the "many worlds" are the same. Many worlds interpretation can, however explain the apparent improbability of a planet like Earth existing. See Rare Earth hypothesis. If the Many worlds interpretation is true there are so many copies of our universe that the existence of at least one planet like Earth isn't surprising.

M-theory

A multiverse of a somewhat different kind has been envisaged within the 11-dimensional extension of string theory known as M-theory. In M-theory our universe and others are created by collisions between membranes in an 11-dimensional space. This is unlike the universes in the "quantum multiverse".

String landscape

The string landscape theory asserts that a different universe exists for each of the very large ensemble of solutions generated when ten dimensional string theory is reduced to the four-dimensional low-energy world we see.

Criticisms of multiverse theories

Non-scientific claims

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

Bad science

Some have argued that the job of a scientist is to provide fundamental explanations for observed phenomena, without making reference to observers. Resorting to anthropic principles constitutes a "lazy way out" of accounting for features such as the apparent fine-tuning of parameters in relation to the existence of life.
Leonard Susskind claims, however, that some form of multiverse is unavoidable, given the current state of physics, and that observer effects are inevitable and have to be taken into account in other sciences.

Occam's Razor

   To postulate an infinity of unseen and unseeable universes just to explain the one we do see seems superficially contrary to Occam's Razor.
   Tegmark answers: "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." Thus, according to Tegmark, paradoxically the multiverse scenario is more parsimonious than that of a single universe. This reflects an old insight from algorithmic information theory: the information conveyed by a set may be far less than the information conveyed by its individual elements. According to Jürgen Schmidhuber, the appropriate mathematical theory of Occam's razor already exists, namely, Ray Solomonoff's theory of optimal inductive inference and its extensions .
   David Lewis, however, draws a distinction between qualitative and quantitative excess. Postulating extra universes just like our own doesn't increase the number of kinds of things there are, and thus there's only qualitative invarience.

One unique universe

It is sometimes argued that the observed universe is the unique possible universe, so that talk of "other" universes is ipso facto meaningless. Einstein raised this possibility when he wondered whether the universe could have been otherwise, or non-existent altogether. This possibility is also expressed in theories such as determinism and chaos theory. The hope is sometimes expressed that once a grand unified theory of everything is achieved, it'll turn out to have a unique "solution" corresponding to the observed universe.

Other objections

The entire range of multiverse hypotheses, with specific emphasis on Hugh Everett's many-worlds interpretation, have been criticised by proponents of intelligent design. William Dembski in particular, derides it as inflating explanatory resources without evidence or warrant, and terms such concepts "inflatons".

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'd only consciously exist in those universes which were finely-tuned for our conscious existence. Thus, while the probability might be extremely small that there's life in most of the multiverses, this scarcity of life-supporting universes doesn't imply intelligent design as the only explanation of our existence.
Critics of this argument (Steven Jay Gould, Richard Dawkins and many others) point out that the cause and the effect have been reversed by those who claim that the universe seems to be fine-tuned for our benefit. Dr. Gould compared it to claiming that sausages were originally made long and narrow so that they'd fit modern hotdog buns, or that humans evolved fingernails so that fingernail polish would be invented. Critics cite the vast store of evolutionary evidence which shows that life is perfectly and naturally tuned to the universe it arose in. Fossil, genetic and other biological evidence abundantly supports the observation that life adapts to physics, not the other way around.
The paleophysicist Caroline Miller writes: "The Anthropic Principle is based on the underlying belief that the universe was created for our benefit. Unfortunately for its adherents, all of the reality-based evidence at our disposal contradicts this belief. In a non-anthropocentric universe, there's no need for multiple universes or supernatural entities to explain life as we know it."

Modal realism

Additionally, 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).

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.

Virtual realities as a multiverse

artificial intelligence advocates even claim we're not far from producing conscious computers. It is then but a small step to the point where the engineered conscious beings inhabit a simulated reality. For such beings, their "fake" universe will appear indistinguishable from reality.

Multiverse hypotheses in religions around the globe

Hindu universes

The earliest known records describing the concept of a multiverse are found in ancient Hindu cosmology, in texts such as the Puranas. They expressed the idea of an infinite number of universes, each with its own gods, inhabitants and planets, and an infinite cycle of births, deaths, and rebirths of a universe, with each cycle lasting 8.4 billion years. The belief is too that the number of universes is infinite.

Fictional multiverses

The concept of the multiverse has been used in fiction and information on this can be found in the following article: Parallel universe (fiction).

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