Editing Multiverse (section)

== Types ==
[[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 ===
{{anchor|Tegmark's classification}}[[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.<ref>{{cite journal | first=Max| last=Tegmark| journal=Scientific American|date=May 2003| title=Parallel Universes| doi=10.1038/scientificamerican0503-40| volume=288| issue=5| pages=40–51| pmid=12701329| url=https://cds.cern.ch/record/604580|arxiv = astro-ph/0302131 |bibcode = 2003SciAm.288e..40T }}</ref><ref>{{cite book
 | first = Max | last = Tegmark | date = 23 January 2003
 | title = Parallel Universes
 | url = http://space.mit.edu/home/tegmark/multiverse.pdf | access-date=7 February 2006}}</ref>

==== Level I: An extension of our universe ====
A prediction of [[Inflation (cosmology)|cosmic inflation]] is the existence of an infinite [[ergodic hypothesis|ergodic]] universe, which, being infinite, must contain [[Hubble volume]]s realizing all initial conditions.

Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having the same [[physical law]]s and [[physical constant]]s. 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|10<sup>10<sup>115</sup></sup>]] meters away from us.<ref name="X0302131"/>

Given infinite space, there would be an infinite number of Hubble volumes identical to ours in the universe.<ref name="TegmarkPUstaple">"Parallel universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations.", Tegmark, Max, Scientific American. May 2003; 288 (5): 40–51.</ref> 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 ====

In the [[eternal 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,<ref>{{cite serial |title=[[How the Universe Works#Season 3|How The Universe Works 3]]|episode=First Second of the Big Bang |network=[[Science (TV network)|Discovery Science]] |date=2014}}</ref> 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 constant]]s.<ref name="TegmarkPUstaple"/>

Level II also includes [[John Archibald Wheeler]]'s [[oscillatory universe]] theory and [[Lee Smolin]]'s [[Cosmological natural selection|fecund universes theory]].

==== Level III: Many-worlds interpretation of quantum mechanics ====
[[File:Schroedingers cat film.svg|thumb|[[Schrödinger's cat]] in the many-worlds interpretation, where a branching of the universe occurs through a superposition of two quantum mechanical states]]
[[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 "world" within the [[Universal wavefunction]], with each world as real as ours. Suppose a six-sided dice is thrown and that the result of the throw corresponds to [[observable]] quantum mechanics. All six possible ways the dice can fall correspond to six different worlds. In the case of the Schrödinger's cat thought experiment, [[Schrödinger's cat#Many-worlds interpretation and consistent histories|both outcomes would be "real" in at least one "world"]].

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 [[doppelgänger]]s 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.<ref name="TegmarkPUstaple"/> According to [[Yasunori Nomura]],<ref name="Nomura"/> [[Raphael Bousso]], and [[Leonard Susskind]],<ref name="BoussoSusskind"/> this is because global spacetime appearing in the (eternally) inflating multiverse is a redundant concept. This implies that the multiverses of Levels I, II, and III are, in fact, the same thing. This hypothesis is referred to as "Multiverse = Quantum Many Worlds". According to [[Yasunori Nomura]], this quantum multiverse is static, and time is a simple illusion.<ref>{{Cite journal |arxiv = 1205.5550|doi = 10.1103/PhysRevD.86.083505|title = Static quantum multiverse|journal = Physical Review D|volume = 86|issue = 8|pages = 083505|year = 2012|last1 = Nomura|first1 = Yasunori|last2 = Johnson|first2 = Matthew C.|last3 = Mortlock|first3 = Daniel J.|last4 = Peiris|first4 = Hiranya V.|bibcode = 2012PhRvD..86h3505N|s2cid = 119207079}}</ref>

Another version of the many-worlds idea is [[H. Dieter Zeh]]'s [[many-minds interpretation|''many-minds'' interpretation]].

==== Level IV: Ultimate ensemble ====
The ultimate [[mathematical universe hypothesis]] is Tegmark's own hypothesis.<ref name="Tegmark2014">{{cite book |first=Max |last=Tegmark |date=2014 |title=Our Mathematical Universe: My Quest for the Ultimate Nature of Reality |publisher=Knopf Doubleday Publishing Group |isbn=9780307599803|title-link=Our Mathematical Universe: My Quest for the Ultimate Nature of Reality }}</ref>

This level considers all universes to be equally real which can be described by different mathematical structures.

Tegmark writes: {{quotation|text=[[Abstract mathematics]] is so general that any [[theory of everything|Theory Of Everything (TOE)]] which 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 theory|set-theoretical]] model, and one can generally find a [[formal system]] that it is a model of.}}

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."<ref name="X0302131">{{Cite journal |arxiv=astro-ph/0302131 |bibcode = 2003SciAm.288e..40T |doi = 10.1038/scientificamerican0503-40 |pmid=12701329 |title = Parallel Universes |year = 2003 |last1 = Tegmark |first1 = Max |journal = Scientific American |volume = 288 |issue = 5 |pages = 40–51 }}</ref>

[[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 a 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]].<ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (1997): A Computer Scientist's View of Life, the Universe, and Everything. Lecture Notes in Computer Science, pp. 201–208, Springer: [http://www.idsia.ch/~juergen/everything/ IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref><ref>{{Cite arXiv|eprint=quant-ph/0011122|last1=Schmidhuber|first1=Juergen|title=Algorithmic Theories of Everything|year=2000}}</ref><ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (2002): Hierarchies of generalized Kolmogorov complexities and nonenumerable universal measures computable in the limit. ''International Journal of Foundations of Computer Science'' 13 (4): 587–612. [http://www.idsia.ch/~juergen/kolmogorov.html IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref> He also explicitly discusses the more restricted ensemble of quickly computable universes.<ref>[[Jürgen Schmidhuber|J. Schmidhuber]] (2002): The Speed Prior: A New Simplicity Measure Yielding Near-Optimal Computable Predictions. Proc. 15th Annual Conference on Computational Learning Theory (COLT 2002), Sydney, Australia, Lecture Notes in Artificial Intelligence, pp. 216–228. Springer: [http://www.idsia.ch/~juergen/speedprior.html IDSIA – Dalle Molle Institute for Artificial Intelligence].</ref>

=== Brian Greene's nine types ===
{{anchor|Brian Greene's nine types of multiverses}}The American [[theoretical physicist]] and [[Super-string theory|string theorist]] [[Brian Greene]] discussed nine types of multiverses:<ref>Greene, Brian. ''[[The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos]]'', 2011.</ref>

;Quilted
: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.

;Inflationary
:The [[Eternal inflation|inflationary multiverse]] is composed of various pockets in which inflation fields collapse and form new universes.
[[File:Brane theory.ogg|thumb|upright=1.6|Animation showing the multiple brane universes in the bulk]]
;Brane
:The [[Brane cosmology|brane multiverse]] version postulates that our entire universe exists on a membrane ([[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]]. 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.
[[File:Cosmos-animation Friedmann-closed.gif|thumb|upright=0.8|Cosmos animation of a cyclic universe]]
;Cyclic
:The [[cyclic model|cyclic multiverse]] has multiple [[brane]]s that have collided, causing [[Big Bang]]s. 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.

;Landscape
: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.

;Quantum
:The [[Many-worlds interpretation|quantum multiverse]] creates a new universe when a diversion in events occurs, as in the real-worlds variant of the [[many-worlds interpretation]] of quantum mechanics.

;Holographic
: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.

;Simulated
:The [[Simulated reality|simulated multiverse]] exists on complex computer systems that simulate entire universes. A related hypothesis, as put forward as a possibility by astronomer [[Avi Loeb]], is that universes may be creatable in laboratories of advanced technological civilizations who have a [[theory of everything]].<ref>{{cite web |last1=Loeb |first1=Avi |title=Was Our Universe Created in a Laboratory? |url=https://www.scientificamerican.com/article/was-our-universe-created-in-a-laboratory/ |website=Scientific American |date=December 2021 |access-date=12 July 2022 |language=en}}</ref> Other related hypotheses include [[brain in a vat]]<ref>{{cite web |title=What if we're living in a computer simulation? |url=https://www.theguardian.com/technology/2017/apr/22/what-if-were-living-in-a-computer-simulation-the-matrix-elon-musk |work=The Guardian |access-date=12 July 2022 |language=en |date=22 April 2017}}</ref>-type scenarios where the perceived universe is either simulated in a low-resource way or not perceived directly by the virtual/simulated inhabitant species.{{additional citation needed|date=July 2022}}

;Ultimate
:The [[Mathematical universe hypothesis|ultimate multiverse]] contains every mathematically possible universe under different laws of physics.

=== Twin-world models ===
[[File:Universe Antiuniverse model.png|thumb|upright=1.4|<!--{{better image needed}}-->Concept of a twin universe, with the [[Cosmogony|beginning of time]] in the middle]]
There are models of two related universes that e.g. attempt to explain the [[baryon asymmetry]] – why there was more matter than [[antimatter]] at the beginning – [[Baryon asymmetry#Mirror anti-universe|with a mirror anti-universe]].<ref>{{cite news |title=Our universe has antimatter partner on the other side of the Big Bang, say physicists |url=https://physicsworld.com/a/our-universe-has-antimatter-partner-on-the-other-side-of-the-big-bang-say-physicists/ |access-date=22 June 2022 |work=Physics World |date=3 January 2019}}</ref><ref>{{cite news |last1=Letzter |first1=Rafi |title=Why some physicists really think there's a 'mirror universe' hiding in space-time |url=https://www.space.com/truth-behind-nasa-mirror-parallel-universe.html |access-date=22 June 2022 |work=Space.com |date=23 June 2020 |language=en}}</ref><ref>{{cite journal |last1=Boyle |first1=Latham |last2=Finn |first2=Kieran |last3=Turok |first3=Neil |title=CPT-Symmetric Universe |journal=Physical Review Letters |date=20 December 2018 |volume=121 |issue=25 |pages=251301 |doi=10.1103/PhysRevLett.121.251301|pmid=30608856 |arxiv=1803.08928 |bibcode=2018PhRvL.121y1301B |s2cid=58638592 }}</ref> One two-universe cosmological model<!-- – already extensively studied to find out why gravity appears much weaker than other known forces –--> could explain the [[Hubble's law#Hubble tension|Hubble constant (H<sub>0</sub>) tension]] via interactions between the two worlds. The "mirror world" would contain copies of all existing fundamental particles.<ref>{{cite news |title=Mirror world of dark particles could explain cosmic anomaly |url=https://physicsworld.com/a/mirror-world-of-dark-particles-could-explain-cosmic-anomaly/ |access-date=22 June 2022 |work=Physics World |date=31 May 2022}}</ref><ref>{{cite journal |last1=Cyr-Racine |first1=Francis-Yan |last2=Ge |first2=Fei |last3=Knox |first3=Lloyd |title=Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant |journal=Physical Review Letters |date=18 May 2022 |volume=128 |issue=20 |pages=201301 |doi=10.1103/PhysRevLett.128.201301|pmid=35657861 |arxiv=2107.13000 |bibcode=2022PhRvL.128t1301C |s2cid=248904936 }}</ref> Another twin/pair-world or "bi-world" cosmology is shown to theoretically be able to solve the [[cosmological constant problem|cosmological constant (Λ) problem]], closely related to [[dark energy]]<!--while describing other important parts of physics-->: two interacting worlds with a large Λ each could result in a small shared effective Λ.<ref>{{cite news |last1=Bedford |first1=Bailey |title=Bilayer graphene inspires two-universe cosmological model |url=https://phys.org/news/2022-05-bilayer-graphene-two-universe-cosmological.html |access-date=22 June 2022 |work=Joint Quantum Institute |language=en}}</ref><ref>{{cite journal |last1=Parhizkar |first1=Alireza |last2=Galitski |first2=Victor |title=Strained bilayer graphene, emergent energy scales, and moir\'e gravity |journal=Physical Review Research |date=2 May 2022 |volume=4 |issue=2 |pages=L022027 |doi=10.1103/PhysRevResearch.4.L022027|arxiv=2108.04252 |bibcode=2022PhRvR...4b2027P |s2cid=236965490 }}</ref><ref>{{cite arXiv |last1=Parhizkar |first1=Alireza |last2=Galitski |first2=Victor |date=2022 |title=Moiré Gravity and Cosmology |class=hep-th |eprint=2204.06574}}</ref>

=== Cyclic theories ===
{{main|Cyclic model}}
In several theories, there is a series of, in some cases [[eternity|infinite]], self-sustaining cycles – typically a series of [[Big Crunch]]es (or [[Big Bounce]]s). However, the respective universes do not exist at once but are forming or following in a logical order or sequence, with key natural constituents potentially varying between universes (see [[#Anthropic principle|§ Anthropic principle]]).