This article is about physical cosmology. For the article on how publishing this Wiki has potentially created a different series of future events, see Multiverse (Influence of this Wiki).
The multiverse, also known as an omniverse or meta-universe, is a Hypothesis|hypothetical group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, and the physical laws and Physical constant|constants that describe them.
History of the concept
In Dublin in 1952, Erwin Schrödinger gave a lecture in which he jocularly warned his audience that what he was about to say might "seem lunatic". He said that when his equations seemed to describe several different histories, these were "not alternatives, but all really happen simultaneously".
The American philosopher and psychologist William James used the term "multiverse" in 1895, but in a different context. The term was first used in fiction and in its current Physics context by Michael Moorcock in his 1963 SF Adventures novella The Sundered Worlds.
Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology, and literature, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternate universes", "quantum universes", "interpenetrating dimensions", "parallel universes", "parallel dimensions", "parallel worlds", "parallel realities", "quantum realities", "alternate realities", "alternate timelines", "alternate dimensions" and "dimensional planes".
The physics community has debated the various multiverse theories over time. Prominent physicists are divided about whether any other universes exist outside of our own.
Some physicists say the multiverse is not a legitimate topic of scientific inquiry.
In 2007, Nobel laureate Steven Weinberg suggested that if the multiverse existed, "the hope of finding a rational explanation for the precise values of quark masses and other constants of the standard model that we observe in our Big Bang is doomed, for their values would be an accident of the particular part of the multiverse in which we live."
Search for evidence
Around 2010, scientists such as Stephen M. Feeney analyzed Wilkinson Microwave Anisotropy Probe (WMAP) data and claimed to find evidence suggesting that our universe collided with other (parallel) universes in the distant past.
Proponents and skeptics
Proponents of one or more of the multiverse hypotheses include Hugh Everett,
Scientists who are generally skeptical of the multiverse hypothesis include: David Gross, Carlo Rovelli, Marcelo Gleiser, and Paul Davies.
Arguments against Multiverse Theories
In his 2003 New York Times opinion piece, "A Brief History of the Multiverse", author and cosmologist Paul Davies offered a variety of arguments that multiverse theories are non-scientific:
George F. R. Ellis|George Ellis, writing in August 2011, provided a criticism of the multiverse, and pointed out that it is not a traditional scientific theory. He accepts that the multiverse is thought to exist far beyond the List of cosmological horizons|cosmological horizon. He emphasized that it is theorized to be so far away that it is unlikely any evidence will ever be found. Ellis also explained that some theorists do not believe the lack of Empiricism|empirical testability falsifiability is a major concern, but he is opposed to that line of thinking:
Ellis says that scientists have proposed the idea of the multiverse as a way of explaining the nature of existence. He points out that it ultimately leaves those questions unresolved because it is a metaphysical issue that cannot be resolved by empirical science. He argues that observational testing is at the core of science and should not be abandoned:
Max Tegmark and Brian Greene have devised classification schemes for the various theoretical types of multiverses and universes that they might comprise.
Max Tegmark's four levels
Cosmology|Cosmologist Max Tegmark has provided a taxonomy (general)|taxonomy of universes beyond the familiar observable universe. The four levels of Tegmark's classification are arranged such that subsequent levels can be understood to encompass and expand upon previous levels. They are briefly described below.
Level I: An extension of our Universe
A prediction of chaotic inflation is the existence of an infinite ergodic hypothesis|ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.
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 Double exponential function|1010115 meters away from us.
Given infinite space, there would, in fact, be an infinite number of Hubble volumes identical to ours in the universe. This follows directly from the cosmological principle, wherein it is assumed that our Hubble volume is not special or unique.
Level II: Universes with different physical constants
File:Multiverse - level II.svg|thumb|Eternal inflation|Bubble universes – every disk represents a bubble universe. Our universe is represented by one of the disks.
Universe 1 to Universe 6 represent bubble universes. Five of them have different physical constants than our universe has. In the Eternal inflation|chaotic inflation theory, which is a variant of the Inflation (cosmology)|cosmic inflation theory, the multiverse or space 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.
Different bubbles may experience different spontaneous symmetry breaking, which results in different properties, such as different physical constants.
Level II 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 III'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 six-sided die is thrown and that the result of the throw corresponds to a quantum mechanics observable. All six possible ways the die can fall correspond to six different universes.
Tegmark argues that a Level III multiverse does not contain more possibilities in the Hubble volume than a Level I or Level 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.
Related to the many-worlds idea are Richard Feynman's multiple histories interpretation and H. Dieter Zeh's many-minds interpretation|many-minds interpretation.
Level IV: Ultimate ensemble
The ultimate mathematical universe hypothesis is Tegmark's own hypothesis.
This level considers all universes to be equally real which can be described by different mathematical structures.
He argues that 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 that the set of mathematical structures is not even well-defined and that it admits only universe representations describable by constructive mathematics—that is, computer programs.
Schmidhuber 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 the Undecidable problem|undecidability of the halting problem.
Brian Greene's nine types
The American theoretical physicist and Super-string theory|string theorist Brian Greene discussed nine types of multiverses:
The quilted multiverse works only in an infinity|infinite universe. With an infinite amount of space, every possible event will occur an infinite number of times. However, the speed of light prevents us from being aware of these other identical areas.
The Eternal inflation|inflationary multiverse is composed of various pockets in which inflation fields collapse and form new universes.
The Brane cosmology|brane multiverse version postulates that our entire universe exists on a membrane (Brane multiverse|brane) which floats in a higher dimension or "bulk". In this bulk, there are other membranes with their own universes. These universes can interact with one another, and when they collide, the violence and energy produced is more than enough to give rise to a Big Bang|big bang. The branes float or drift near each other in the bulk, and every few trillion years, attracted by gravity or some other force we do not understand, collide and bang into each other. This repeated contact gives rise to multiple or "cyclic" Big Bang|big bangs. This particular hypothesis falls under the string theory umbrella as it requires extra spatial dimensions.
The cyclic model|cyclic multiverse has multiple branes that have collided, causing Big Bangs. The universes bounce back and pass through time until they are pulled back together and again collide, destroying the old contents and creating them anew.
The String theory landscape|landscape multiverse relies on string theory's Calabi–Yau manifold|Calabi–Yau spaces. Quantum fluctuations drop the shapes to a lower energy level, creating a pocket with a set of laws different from that of the surrounding space.
The Many-worlds interpretation|quantum multiverse creates a new universe when a diversion in events occurs, as in the many-worlds interpretation of quantum mechanics.
The Holographic principle|holographic multiverse is derived from the theory that the surface area of a space can encode the contents of the volume of the region.
The Simulated reality|simulated multiverse exists on complex computer systems that simulate entire universes.
The Mathematical universe hypothesis|ultimate multiverse contains every mathematically possible universe under different laws of physics.
In several theories, there is a series of infinite, self-sustaining cycles (for example, an eternity of Big Bangs, Big Crunches, and/or Big Freezes).
A multiverse of a somewhat different kind has been envisaged within string theory and its higher-dimensional extension, Introduction to M-theory|M-theory.
These theories require the presence of 10 or 11 spacetime dimensions respectively. The extra 6 or 7 dimensions may either be compactified on a very small scale, or our universe may simply be localized on a dynamical (3+1)-dimensional object, a D3-brane. This opens up the possibility that there are other branes which could support other universes.
Black-hole Physical cosmology|cosmology is a cosmological model in which the observable universe is the interior of a black hole existing as one of possibly many universes inside a larger universe. This includes the theory of white holes, which are on the opposite side of space-time.
The concept of other universes has been proposed to explain how our Observable universe|own universe appears to be Fine-tuned Universe|fine-tuned for Consciousness|conscious life as we experience it.
If there were a large (possibly infinite) number of universes, each with possibly different physical laws (or different dimensionless physical constant|fundamental physical constants), then some of these universes (even if very few) would have the combination of laws and fundamental parameters that are suitable for the development of matter, astronomical structures, elemental diversity, stars, and planets that can exist long enough for life to emerge and evolve.
The weak anthropic principle could then be applied to conclude that we (as conscious beings) would only exist in one of those few universes that Selection bias|happened to be finely tuned, permitting the existence of life with developed consciousness. Thus, while the probability might be extremely small that any particular universe would have the requisite conditions for life (Carbon chauvinism|as we understand life), those conditions do not require intelligent design as an explanation for the conditions in the Universe that promote our existence in it.
An early form of this reasoning is evident in Arthur Schopenhauer's 1844 work "Von der Nichtigkeit und dem Leiden des Lebens", where he argues that our world must be the worst of all possible worlds, because if it were significantly worse in any respect it could not continue to exist.
Proponents and critics disagree about how to apply Occam's razor. Critics argue that to postulate an almost infinite number of unobservable universes, just to explain our own universe, is contrary to Occam's razor. However, proponents argue that in terms of Kolmogorov complexity the proposed multiverse is simpler than a single idiosyncratic universe.
For example, multiverse proponent Max Tegmark argues:
Possible worlds are a way of explaining probability and hypothetical statements. Some philosophers, such as David Lewis (philosopher)|David Lewis, believe that all possible worlds exist and that they are just as real as the world we live in (a position known as modal realism).