Universe

=== Multiverse hypotheses ===
{{Main|Multiverse|Many-worlds interpretation|Bubble universe theory}}
{{See also|Eternal inflation}}
Some speculative theories have proposed that our universe is but one of a [[set (mathematics)|set]] of disconnected universes, collectively denoted as the [[multiverse]], challenging or enhancing more limited definitions of the universe.<ref name="EllisKS032" /><ref>{{cite journal |author=Munitz |first=M. K. |date=1959 |title=One Universe or Many? |journal=Journal of the History of Ideas |volume=12 |issue=2 |pages=231–255 |doi=10.2307/2707516 |jstor=2707516}}</ref> Scientific multiverse models are distinct from concepts such as [[plane (esotericism)|alternate planes of consciousness]] and [[simulated reality]].

[[Max Tegmark]] developed a four-part [[Multiverse#Max Tegmark's four levels|classification scheme]] for the different types of multiverses that scientists have suggested in response to various problems in [[physics]]. An example of such multiverses is the one resulting from the [[bubble universe theory|chaotic inflation]] model of the early universe.<ref name="chaotic_inflation">{{cite journal |author=Linde |first=A. |author-link=Andrei Linde |date=1986 |title=Eternal chaotic inflation |url=https://cds.cern.ch/record/167897 |url-status=live |journal=Mod. Phys. Lett. A |volume=1 |issue=2 |pages=81–85 |bibcode=1986MPLA....1...81L |doi=10.1142/S0217732386000129 |s2cid=123472763 |archive-url=https://web.archive.org/web/20190417211031/https://cds.cern.ch/record/167897/ |archive-date=April 17, 2019 |access-date=August 6, 2017}}<br />{{cite journal |author=Linde |first=A. |author-link=Andrei Linde |date=1986 |title=Eternally existing self-reproducing chaotic inflationary Universe |url=http://www.stanford.edu/~alinde/Eternal86.pdf |url-status=live |journal=Phys. Lett. B |volume=175 |issue=4 |pages=395–400 |bibcode=1986PhLB..175..395L |doi=10.1016/0370-2693(86)90611-8 |archive-url=https://web.archive.org/web/20131127164909/http://www.stanford.edu/~alinde/Eternal86.pdf |archive-date=November 27, 2013 |access-date=March 17, 2011}}</ref> Another is the multiverse resulting from the [[many-worlds interpretation]] of quantum mechanics. In this interpretation, parallel worlds are generated in a manner similar to [[quantum superposition]] and [[decoherence]], with all states of the [[wave function]]s being realized in separate worlds. Effectively, in the many-worlds interpretation the multiverse evolves as a [[universal wavefunction]]. If the Big Bang that created our multiverse created an ensemble of multiverses, the wave function of the ensemble would be entangled in this sense.<ref name=everett1957>{{cite journal |last1=Everett |first1=Hugh |author-link=Hugh Everett |year=1957 |title=Relative State Formulation of Quantum Mechanics |journal=Reviews of Modern Physics |volume=29 |issue=3 |pages=454–462 |bibcode=1957RvMP...29..454E |doi=10.1103/RevModPhys.29.454 |s2cid=17178479 }}</ref> Whether scientifically meaningful probabilities can be extracted from this picture has been and continues to be a topic of much debate, and multiple versions of the many-worlds interpretation exist.<ref name="ball">{{Cite web |last=Ball |first=Philip |author-link=Philip Ball |date=February 17, 2015 |title=Too many worlds |url=https://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy |url-status=live |access-date=September 23, 2021 |website=[[Aeon.co]]|archive-url=https://web.archive.org/web/20210927130915/https://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy|archive-date=September 27, 2021}}</ref><ref>{{Cite book |last=Peres |first=Asher |title=[[Quantum Theory: Concepts and Methods]] |publisher=Kluwer Academic Publishers |year=1995 |isbn=0-7923-2549-4 |pages=374 |author-link=Asher Peres}}</ref><ref>{{Cite journal |last=Kent |first=Adrian |author-link=Adrian Kent |date=February 2015 |title=Does it Make Sense to Speak of Self-Locating Uncertainty in the Universal Wave Function? Remarks on Sebens and Carroll |journal=Foundations of Physics |language=en |volume=45 |issue=2 |pages=211–217 |arxiv=1408.1944 |bibcode=2015FoPh...45..211K |doi=10.1007/s10701-014-9862-5 |issn=0015-9018 |s2cid=118471198}}</ref> (The subject of the [[Interpretations of quantum mechanics|interpretation of quantum mechanics]] is in general marked by disagreement.)<ref>{{Cite journal |last1=Schlosshauer |first1=Maximilian |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |author-link3=Anton Zeilinger |date=August 1, 2013 |title=A snapshot of foundational attitudes toward quantum mechanics |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |volume=44 |issue=3 |pages=222–230 |arxiv=1301.1069 |bibcode=2013SHPMP..44..222S |doi=10.1016/j.shpsb.2013.04.004 |issn=1355-2198 |s2cid=55537196}}</ref><ref name=":22">{{Cite journal |last=Mermin |first=N. David |author-link=N. David Mermin |date=July 1, 2012 |title=Commentary: Quantum mechanics: Fixing the shifty split |journal=[[Physics Today]] |volume=65 |issue=7 |pages=8–10 |bibcode=2012PhT....65g...8M |doi=10.1063/PT.3.1618 |issn=0031-9228 |quote=New interpretations appear every year. None ever disappear. |doi-access=free}}</ref><ref name=":5">{{Cite book |last=Cabello |first=Adán |title=What is Quantum Information? |publisher=Cambridge University Press |year=2017 |isbn=9781107142114 |editor-last=Lombardi |editor-first=Olimpia |editor-link=Olimpia Lombardi |pages=138–143 |chapter=Interpretations of quantum theory: A map of madness |bibcode=2015arXiv150904711C |doi=10.1017/9781316494233.009 |editor2-last=Fortin |editor2-first=Sebastian |editor3-last=Holik |editor3-first=Federico |editor4-last=López |editor4-first=Cristian |arxiv=1509.04711 |s2cid=118419619}}</ref>

The least controversial, but still highly disputed, category of multiverse in Tegmark's scheme is [[Multiverse#Level I: An extension of our universe|Level I]]. The multiverses of this level are composed by distant spacetime events "in our own universe". Tegmark and others<ref>{{cite journal |first1=Jaume |last1=Garriga |first2=Alexander |last2=Vilenkin |date=2007 |title=Many Worlds in One |journal=Physical Review D |volume=64 |issue=4 |page=043511 |doi=10.1103/PhysRevD.64.043511 |arxiv=gr-qc/0102010v2|s2cid=119000743 }}</ref> have argued that, if space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire [[Hubble volume]] occur every so often, simply by chance. Tegmark calculated that our nearest so-called [[doppelgänger]] is 10<sup>10<sup>115</sup></sup> metres away from us (a [[double exponential function]] larger than a [[googolplex]]).<ref name="TegmarkPUstaple">{{cite journal |author=Tegmark |first=Max |date=2003 |title=Parallel universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations |journal=Scientific American |volume=288 |issue=5 |pages=40–51 |arxiv=astro-ph/0302131 |bibcode=2003SciAm.288e..40T |doi=10.1038/scientificamerican0503-40 |pmid=12701329}}</ref><ref>{{cite journal |author=Tegmark, Max |journal=Scientific American |title=Parallel Universes |date=2003 |arxiv=astro-ph/0302131|bibcode=2003SciAm.288e..40T |pages=40–51|doi=10.1038/scientificamerican0503-40 |pmid=12701329 |volume=288|issue=5 }}</ref> However, the arguments used are of speculative nature.<ref>{{cite journal |last1=Gil |first1=Francisco José Soler |last2=Alfonseca |first2=Manuel |date=2013 |title=About the Infinite Repetition of Histories in Space |journal=Theoria: An International Journal for Theory, History and Foundations of Science |volume=29 |issue=3 |page=361 |arxiv=1301.5295 |doi=10.1387/theoria.9951 |s2cid=52996408 |hdl-access=free |hdl=10486/664735}}</ref> Additionally, it would be impossible to scientifically verify the existence of an identical Hubble volume.

It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another.<ref name="TegmarkPUstaple" /><ref name="EllisScA">{{cite journal |author=Ellis |first=G. F. |date=2011 |title=Does the Multiverse Really Exist? |journal=Scientific American |volume=305 |issue=2 |pages=38–43 |bibcode=2011SciAm.305a..38E |doi=10.1038/scientificamerican0811-38 |pmid=21827123}}</ref> An easily visualized metaphor of this concept is a group of separate [[soap bubble]]s, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle.<ref>{{cite web |url=http://www.livescience.com/15530-multiverse-universe-eternal-inflation-test.html |title=Weird! Our Universe May Be a 'Multiverse,' Scientists Say |first=Clara |last=Moskowitz|author-link= Clara Moskowitz |date=August 12, 2011 |work=livescience |access-date=May 4, 2015 |archive-date=May 5, 2015 |archive-url=https://web.archive.org/web/20150505003038/http://www.livescience.com/15530-multiverse-universe-eternal-inflation-test.html |url-status=live }}</ref> According to one common terminology, each "soap bubble" of spacetime is denoted as a ''universe'', whereas humans' particular spacetime is denoted as ''the universe'',<ref name="EllisKS032" /> just as humans call Earth's moon ''the [[Moon]]''. The entire collection of these separate spacetimes is denoted as the multiverse.<ref name="EllisKS032">{{cite journal |last1=Ellis |first1=George F. R. |author-link=George Francis Rayner Ellis |last2=Kirchner |first2=U. |last3=Stoeger |first3=W. R. |date=2004 |title=Multiverses and physical cosmology |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=347 |issue=3 |pages=921–936 |arxiv=astro-ph/0305292 |bibcode=2004MNRAS.347..921E |doi=10.1111/j.1365-2966.2004.07261.x |s2cid=119028830}}</ref>

With this terminology, different ''universes'' are not [[causality|causally connected]] to each other.<ref name="EllisKS032" /> In principle, the other unconnected ''universes'' may have different [[dimension]]alities and [[Topology|topologies]] of spacetime, different forms of [[matter]] and [[energy]], and different [[physical law]]s and [[physical constant]]s, although such possibilities are purely speculative.<ref name="EllisKS032" /> Others consider each of several bubbles created as part of [[chaotic inflation]] to be separate ''universes'', though in this model these universes all share a causal origin.<ref name="EllisKS032" />
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