View source for Multiverse

{{Short description|Hypothetical group of multiple universes}}
{{Distinguish|Metaverse}}
{{Redirect-distinguish|Multiverses|MultiVersus{{!}}''MultiVersus''}}
{{Other uses}}
{{Pp-pc|small=yes}}
{{Use dmy dates|date=April 2019}}
{{Cosmology}}

The '''multiverse''' is the [[Hypothesis|hypothetical]] set of all [[universe]]s.<ref>{{cite news |title=We are closer than ever to finally proving the multiverse exists |url=https://www.newscientist.com/article/mg26234971-300-we-are-closer-than-ever-to-finally-proving-the-multiverse-exists/ |access-date=18 July 2024 |work=New Scientist|url-access=subscription}}</ref>{{efn|In some models, such as those of [[brane cosmology]], many parallel structures may exist within the same universe.}} Together, these universes are presumed to comprise everything that exists: the entirety of [[space]], [[time]], [[matter]], [[energy]], [[information]], and the [[physical law]]s and [[Physical constant|constants]] that describe them. The different universes within the multiverse are called "parallel universes", "flat universes", "other universes", "alternate universes", "multiple universes", "plane universes", "parent and child universes", "many universes", or "many worlds". One common assumption is that the multiverse is a "patchwork quilt of separate universes all bound by the same laws of physics."<ref>{{cite book |last1=Swain |first1=Frank |title=The Universe Next Door: A Journey Through 55 Alternative Realities, Parallel Worlds and Possible Futures |date=2017 |publisher=New Scientist |location=London |isbn=9781473658677 |page=12}}</ref>

The concept of multiple universes, or a multiverse, has been discussed throughout history. It has evolved and has been debated in various fields, including cosmology, physics, and philosophy. Some physicists have argued that the multiverse is a philosophical notion rather than a scientific hypothesis, as it cannot be empirically falsified. In recent years, there have been proponents and skeptics of multiverse theories within the physics community. Although some scientists have analyzed data in search of evidence for other universes, no statistically significant evidence has been found. Critics argue that the multiverse concept lacks testability and falsifiability, which are essential for scientific inquiry, and that it raises unresolved metaphysical issues.

[[Max Tegmark]] and [[Brian Greene]] have proposed different classification schemes for multiverses and universes. Tegmark's four-level classification consists of Level I: an extension of our universe, Level II: universes with different physical constants, Level III: many-worlds interpretation of quantum mechanics, and Level IV: [[ultimate ensemble]]. Brian Greene's nine types of multiverses include quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate. The ideas explore various dimensions of space, physical laws, and mathematical structures to explain the existence and interactions of multiple universes. Some other multiverse concepts include twin-world models, cyclic theories, M-theory, and [[Black hole cosmology|black-hole cosmology]].

The [[anthropic principle]] suggests that the existence of a multitude of universes, each with different physical laws, could explain the asserted appearance of [[Fine-tuned universe|fine-tuning of our own universe]] for conscious life. The weak anthropic principle posits that we exist in one of the few universes that support life. Debates around [[Occam's razor]] and the simplicity of the multiverse versus a single universe arise, with proponents like Max Tegmark arguing that the multiverse is simpler and more elegant. The [[Many Worlds Interpretation of Quantum Mechanics|many-worlds interpretation of quantum mechanics]] and [[modal realism]], the belief that all possible worlds exist and are as real as our world, are also subjects of debate in the context of the anthropic principle.

== History of the concept ==
According to some, the idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher [[Anaximander]] in the sixth century BCE.<ref>{{Citation |last=Tarán |first=Leonardo |title=The Text of Simplicius' Commentary on Aristotle's Physics |work=Simplicius. Sa vie, son oeuvre, sa survie |year=1987 |url=http://dx.doi.org/10.1515/9783110862041.246 |access-date=2022-09-21 |place=Berlin, Germany; Boston, Massachusetts |publisher=DE GRUYTER |doi=10.1515/9783110862041.246 |isbn=9783110862041|url-access=subscription }}</ref> However, there is debate as to whether he believed in multiple worlds, and if he did, whether those worlds were co-existent or successive.<ref>
{{Cite journal |last=Kočandrle |first=Radim |title=Infinite Worlds in the Thought of Anaximander |date=December 2019 |url=https://www.cambridge.org/core/product/identifier/S000983882000004X/type/journal_article |journal=The Classical Quarterly |language=en |volume=69 |issue=2 |pages=483–500 |doi=10.1017/S000983882000004X |s2cid=216169543 |issn=0009-8388|url-access=subscription }}
</ref><ref>
{{cite book |last1=Gregory |first1=Andrew |title=Anaximander: A Re-assessment |date=25 February 2016 |publisher=Bloomsbury Publishing |isbn=978-1-4725-0625-2 |page=121 |url=https://books.google.com/books?id=7TE0CwAAQBAJ&pg=PA121}}
</ref><ref>
{{cite book |last1=Curd |first1=Patricia |last2=Graham |first2=Daniel W. |title=The Oxford Handbook of Presocratic Philosophy |date=27 October 2008 |publisher=Oxford University Press |isbn=978-0-19-972244-0 |pages=239–241 |url=https://books.google.com/books?id=lDvRCwAAQBAJ&pg=PA239}}
</ref><ref name="Hatleback2014">
{{cite thesis |last=Hatleback |first=Eric Nelson |date=2014 |title=Chimera of the Cosmos |type=PhD |chapter= |publisher=University of Pittsburgh |url=https://d-scholarship.pitt.edu/22668/1/Chimera_of_the_Cosmos_2.pdf |place=Pittsburgh, Pennsylvania}}</ref>

The first to whom we can definitively attribute the concept of innumerable worlds are the Ancient Greek [[Atomists]], beginning with [[Leucippus]] and [[Democritus]] in the 5th century BCE, followed by [[Epicurus]] (341–270 BCE) and [[Lucretius]] (1st century BCE).<ref name="Siegfried2019">
{{cite book |last1=Siegfried |first1=Tom |title=The Number of the Heavens: A History of the Multiverse and the Quest to Understand the Cosmos |date=17 September 2019 |publisher=Harvard University Press |isbn=978-0-674-97588-0 |pages=51–61 |url=https://books.google.com/books?id=L36mDwAAQBAJ&pg=PA51 |quote="In some worlds there is no sun and moon, in others they are larger than in our world, and in others more numerous. The intervals between the worlds are unequal; in some parts there are more worlds, in others fewer; some are increasing, some at their height, some decreasing; in some parts they are arising, in others falling. They are destroyed by collision one with another. There are some worlds devoid of living creatures or plants or any moisture." ... Only an infinite number of atoms could have created the complexity of the known world by their random motions... In this sense, the atomist-multiverse theory of antiquity presents a striking parallel to the situation in science today. The Greek atomists' theory of the ultimate nature of matter on the smallest scales implied the existence of multiple universes on cosmic scales. Modern science's most popular attempt to describe the fundamental nature of matter—superstring theory—also turns out (much to the theorists' surprise) to imply a vast multiplicity of vacuum states, essentially the same thing as predicting the existence of a multiverse.}}
</ref><ref>
{{cite book |last1=Dick |first1=Steven J. |title=Plurality of Words: The Extraterrestrial Life Debate from Democritus to Kant |date=29 June 1984 |publisher=Cambridge University Press |isbn=978-0-521-31985-0 |pages=6–10 |url=https://books.google.com/books?id=Uak5AAAAIAAJ&pg=PA6 |quote=Why should other worlds have become the subject of scientific discourse, when they were neither among the phenomena demanding explanation?... it derived from the cosmogonic assumption of ancient atomism: the belief that the constituent bodies of the cosmos are formed by the chance coalescence of moving atoms, the same type of indivisible particles of which matter on Earth was composed... Given the occurrence of these natural processes, and the obvious example of potential stability revealed in our own finite world, it was not unreasonable to suppose the existence of other stable conglomerations. The atomists further employed the principle that when causes were present, effects must occur.6 Atoms were the agents of causality and their number was infinite. The effect was innumerable worlds in formation, in collision, and in decay."}}
</ref><ref name="Hatleback2014"/><ref name="Rubenstein2014">
{{cite book |last1=Rubenstein |first1=Mary-Jane |title=Worlds Without End: The Many Lives of the Multiverse |date=11 February 2014 |publisher=Columbia University Press |isbn=978-0-231-15662-2 |pages=40–69 |chapter-url=https://books.google.com/books?id=fV-sAgAAQBAJ&pg=PA40 |chapter=Ancient Openings of Multiplicity}}
</ref><ref name="Sedacca2017">
{{cite web |last1=Sedacca |first1=Matthew |title=The Multiverse Is an Ancient Idea |url=https://nautil.us/the-multiverse-is-an-ancient-idea-236401/ |website=Nautilus |access-date=4 December 2022 |date=30 January 2017 |quote=The earliest hints of the multiverse are found in two ancient Greek schools of thought, the Atomists and the Stoics. The Atomists, whose philosophy dates to the fifth century B.C., argued that that the order and beauty of our world was the accidental product of atoms colliding in an infinite void. The atomic collisions also give rise to an endless number of other, parallel worlds less perfect than our own.}}
</ref><ref>
{{cite web |last1=Siegfried |first1=Tom |title=Long Live the Multiverse! |url=https://blogs.scientificamerican.com/observations/long-live-the-multiverse/ |website=Scientific American Blog Network |date=2019 |quote=Leucippus and Democritus believed that their atomic theory required an infinity of worlds... Their later follower, Epicurus of Samos, also professed the reality of multiple worlds. "There are infinite worlds both like and unlike this world of ours"...}}
</ref> In the third century BCE, the philosopher [[Chrysippus]] suggested that the world eternally expired and regenerated, effectively suggesting the existence of multiple universes across time.<ref name=Sedacca2017/> The concept of multiple universes became more defined in the [[Middle Ages]].<ref>{{Cite news |last=Laskow |first=Sarah |date=20 June 2017 |title=Medieval Scholars Believed in the Possibility of Parallel Universes |url=https://www.atlasobscura.com/articles/medieval-multiverse-theory |access-date=20 June 2025 |work=Atlas Obscura}}</ref>

The American philosopher and psychologist [[William James]] used the term "multiverse" in 1895, but in a different context.<ref>James, William, ''The Will to Believe'', 1895; and earlier in 1895, as cited in [[OED]]'s new 2003 entry for "multiverse": {{citation|url=https://books.google.com/books?id=HA0MAAAAIAAJ&pg=PA10|date=October 1895|last=James|first=William|journal=Int. J. Ethics|volume=6|page=10|title=Is Life Worth Living?|quote=Visible nature is all plasticity and indifference, a multiverse, as one might call it, and not a universe.|doi=10.1086/205378|issue=1}}</ref>

The concept first appeared in the modern scientific context in the course of the debate between [[Ludwig Boltzmann|Boltzmann]] and [[Ernst Zermelo|Zermelo]] in 1895.<ref>{{cite book |last1=Ćirković |first1=Milan M. |editor1-last=Kragh |editor1-first=Helge |editor2-last=Longair |editor2-first=Malcolm |title=The Oxford Handbook of the History of Modern Cosmology |date=6 March 2019 |publisher=Oxford University Press |isbn=978-0-19-254997-6 |chapter-url=https://books.google.com/books?id=OsKKDwAAQBAJ&pg=PA467 |chapter=Stranger things: multiverse, string cosmology, physical eschatology}}</ref>

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".<ref>{{Cite web |date=2012-04-05 |title=Erwin Schrödinger and the Quantum Revolution by John Gribbin: review |url=https://www.telegraph.co.uk/culture/books/bookreviews/9188438/Erwin-Schrodinger-and-the-Quantum-Revolution-by-John-Gribbin-review.html |access-date=2023-09-24 |website=The Telegraph |language=en}}</ref> This sort of duality is called "[[Quantum superposition|superposition]]".

== Search for evidence ==
In the 1990s, after recent works of fiction about the concept gained popularity, scientific discussions about the multiverse and [[Scientific journal|journal]] articles about it gained prominence.<ref>{{Cite web |last=Romeo |first=Jess |date=2022-01-07 |title=The Real Science of the Multiverse |url=https://daily.jstor.org/the-real-science-of-the-multiverse/ |access-date=2023-07-15 |website=JSTOR Daily |publisher=[[JSTOR]] |language=en-US}}</ref>

Around 2010, scientists such as Stephen M. Feeney analyzed [[Wilkinson Microwave Anisotropy Probe]] (WMAP) data and claimed to find evidence suggesting that this universe collided with other (parallel) universes in the distant past.<ref>{{cite web|title=Astronomers Find First Evidence Of Other Universe|url=https://www.technologyreview.com/s/421999/astronomers-find-first-evidence-of-other-universes/|publisher=technologyreview.com|access-date=12 October 2013|date=13 December 2010}}</ref><ref>{{cite web |author=Tegmark |first1=Max |last2=Vilenkin |first2=Alexander |date=19 July 2011 |title=The Case for Parallel Universes |url=https://www.scientificamerican.com/article/multiverse-the-case-for-parallel-universe/ |access-date=12 October 2013 |website=[[Scientific American]]}}</ref><ref>{{cite web|title=Is Our Universe Inside a Bubble? First Observational Test of the 'Multiverse'|url=https://www.sciencedaily.com/releases/2011/08/110803102844.htm|work=Science Daily|publisher=sciencedaily.com|access-date=12 October 2013|date=3 August 2011}}</ref> However, a more thorough analysis of data from the WMAP and from the [[Planck (spacecraft)|Planck satellite]], which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a [[bubble universe]] collision.<ref name="Feeney">{{cite journal |title=First observational tests of eternal inflation: Analysis methods and WMAP 7-year results  |journal=Physical Review D |volume=84 |issue=4 |page=43507  |date=2011 |arxiv=1012.3667 |last1=Feeney|first1=Stephen M. |display-authors=1 |bibcode=2011PhRvD..84d3507F |last2=Johnson |first2=Matthew C.|last3=Mortlock |first3=Daniel J.  |last4=Peiris  |first4=Hiranya V.  |doi=10.1103/PhysRevD.84.043507  |s2cid=43793857 }}</ref><ref name="Feeney2">{{cite journal|title=First observational tests of eternal inflation |journal=Physical Review Letters |volume=107 |issue=7 |page=071301 |date=2011 |arxiv=1012.1995 |author=Feeney |display-authors=etal   |bibcode = 2011PhRvL.107g1301F |doi = 10.1103/PhysRevLett.107.071301 |pmid=21902380 |s2cid=23560957 }}. {{Cite journal|arxiv=1309.4060|last1=Bousso|first1=Raphael|title=Inflation after False Vacuum Decay: Observational Prospects after Planck|journal=Physical Review D|volume=91|issue=8|page=083527|last2=Harlow|first2=Daniel|last3=Senatore|first3=Leonardo|year=2015|doi=10.1103/PhysRevD.91.083527|bibcode = 2015PhRvD..91h3527B |s2cid=118488797}}</ref> In addition, there was no evidence of any gravitational pull of other universes on ours.<ref>{{Cite journal|arxiv=1303.5090 |title= Planck intermediate results. XIII. Constraints on peculiar velocities |journal=Astronomy & Astrophysics |volume=561 |pages=A97 |date=2013-03-20 |last1=Collaboration |first1=Planck |last2= Ade |first2=P. A. R. |last3= Aghanim |first3=N.|author3-link=Nabila Aghanim |last4= Arnaud |first4=M. |last5= Ashdown |first5=M. |last6= Aumont |first6=J. |last7= Baccigalupi |first7=C. |last8= Balbi |first8=A. |last9= Banday |first9=A. J. |last10= Barreiro |first10=R. B. |last11= Battaner |first11=E. |last12= Benabed |first12=K. |last13= Benoit-Levy |first13=A. |last14= Bernard |first14=J. -P. |last15= Bersanelli |first15=M. |last16= Bielewicz |first16=P. |last17= Bikmaev |first17=I. |last18= Bobin |first18=J. |last19= Bock |first19=J. J. |last20= Bonaldi |first20=A. |last21= Bond |first21=J. R. |last22= Borrill |first22=J. |last23= Bouchet |first23=F. R. |last24= Burigana |first24=C. |last25= Butler |first25=R. C. |last26= Cabella |first26=P. |last27= Cardoso |first27=J. -F. |last28= Catalano |first28=A. |last29= Chamballu |first29=A. |last30= -Y Chiang |first30=L. |display-authors=29 |doi=10.1051/0004-6361/201321299 |bibcode = 2014A&A...561A..97P |s2cid= 2745526 }}</ref><ref>{{cite news|title=Blow for 'dark flow' in Planck's new view of the cosmos|url=https://www.newscientist.com/article/dn23340-blow-for-dark-flow-in-plancks-new-view-of-the-cosmos|access-date=10 March 2014|newspaper=[[New Scientist]]|date=3 April 2013}}</ref>

In 2015, an astrophysicist may have found evidence of alternate or parallel universes by looking back in time to a time immediately after the [[Big Bang]], although it is still a matter of debate among physicists.<ref name="www.usatoday.com">{{cite web |title=Study may have found evidence of alternate, parallel universes |url=http://www.usatoday.com/story/tech/2015/11/03/alternate-universes-discovered/75102502/ |publisher=www.usatoday.com}}<!-- auto-translated by Module:CS1 translator --> por Doyle Rice, ''[[USA Today]]'' (2015).</ref> Dr. Ranga-Ram Chary, after analyzing the [[Cosmic microwave background|cosmic radiation spectrum]], found a signal 4,500 times brighter than it should have been, based on the number of [[Proton|protons]] and [[Electron|electrons]] scientists believe existed in the very early universe. This signal—an emission line that arose from the formation of atoms during the era of recombination—is more consistent with a universe whose ratio of matter particles to photons is about 65 times greater than our own. There is a 30% chance that this signal is noise, and not really a signal at all; however, it is also possible that it exists because a parallel universe dumped some of its matter particles into our universe. If additional protons and electrons had been added to our universe during recombination, more atoms would have formed, more photons would have been emitted during their formation, and the signature line that arose from all of these emissions would be greatly enhanced. Chary himself is skeptical:{{quotation|text=Many other regions beyond our observable universe would exist with each such region governed by a different set of physical parameters than the ones we have measured for our universe.<ref name="www.usatoday.com"/>|author=Ranga-Ram Chary|title=''USA Today''|source=}}

Chary also noted:<ref name="phys.org">{{cite web |title=Cosmologist thinks a strange signal may be evidence of a parallel universe |url=http://phys.org/news/2015-11-cosmologist-strange-evidence-parallel-universe.html#jCp |publisher=phys.org}}<!-- auto-translated by Module:CS1 translator --> por Vanessa Janek, "Universe Today" (2015).</ref>{{quotation|text=Unusual claims like evidence for alternate universes require a very high burden of proof.<ref name="phys.org"/>|author=Ranga-Ram Chary|title="Universe Today"|source=}}

The signature that Chary has isolated may be a consequence of incoming light from distant [[Galaxy|galaxies]], or even from clouds of dust surrounding our own galaxy.<ref name="phys.org"/>

== Proponents and skeptics ==
Modern proponents of one or more of the multiverse hypotheses include <!-- To add anyone to this list we MUST have a citation that shows that they belong on the list. -->[[Lee Smolin]],<ref>Smolin, Lee. The Life of the Cosmos. Oxford University Press. ISBN 978-0195126648.</ref> [[Don Page (physicist)|Don Page]],<ref>{{Cite web |last=Page |first=Don |date=8 March 2018 |title=Does God exist in the multiverse? |work=Grandin Media |url=https://grandinmedia.ca/god-exist-multiverse/}}</ref> [[Brian Greene]],<ref name="C4WDefault-4326764">{{cite interview |url= https://www.npr.org/2011/01/24/132932268/a-physicist-explains-why-parallel-universes-may-exist |title= A Physicist Explains Why Parallel Universes May Exist |date= 24 January 2011 |access-date= 12 September 2014 |work= npr.org |last= Greene |first= Brian |interviewer= Terry Gross |archive-url= https://web.archive.org/web/20140913065530/http://www.npr.org/2011/01/24/132932268/a-physicist-explains-why-parallel-universes-may-exist |archive-date= 13 September 2014 |url-status= live |df= dmy-all }}</ref><ref name="C4WDefault-379179">{{cite interview |url=https://www.npr.org/templates/transcript/transcript.php?storyId=132932268 |title=Transcript:A Physicist Explains Why Parallel Universes May Exist |date=24 January 2011 |access-date=12 September 2014 |work=npr.org |last=Greene |first=Brian |interviewer=Terry Gross |archive-url=https://web.archive.org/web/20140913065827/http://www.npr.org/templates/transcript/transcript.php?storyId=132932268 |archive-date=13 September 2014 |url-status=live  }}</ref> [[Max Tegmark]],<ref name="X0302131" /> [[Alan Guth]],<ref>{{cite web |last=Guth |first=Alan |title=Inflationary Cosmology: Is Our Universe Part of a Multiverse? |url=https://www.youtube.com/watch?v=d1bImPhPw9c |url-status=live |archive-url=https://ghostarchive.org/varchive/youtube/20211211/d1bImPhPw9c |archive-date=2021-12-11 |access-date=6 October 2014 |work=YouTube|date=May 2014 }}{{cbignore}}</ref> [[Andrei Linde]],<ref name="C4WDefault-6178546">{{cite web |url=http://www.ctc.cam.ac.uk/stephen70/talks/swh70_linde.pdf |title=Inflation in Supergravity and String Theory: Brief History of the Multiverse |date=27 January 2012 <!-- No date available; last modification date used. --> |access-date=13 September 2014 |work=ctc.cam.ac.uk |last=Linde |first=Andrei |archive-url=https://web.archive.org/web/20140714173722/http://www.ctc.cam.ac.uk/stephen70/talks/swh70_linde.pdf |archive-date=14 July 2014 |url-status= live}}</ref> [[Michio Kaku]],<ref>{{Cite web |last=Kaku |first=Michio |title=e-reading.ws |url=http://www.e-reading.ws/bookreader.php/136469/Parallel_Worlds:_A_Journey_Through_Creation,_Higher_Dimensions,_and_the_Future_of_the_Cosmos.pdf |website=www.e-reading.ws}}</ref> [[David Deutsch]],<ref>David Deutsch (1997). "The Ends of the Universe". The Fabric of Reality: The Science of Parallel Universes—and Its Implications. London, England: Penguin Press. {{ISBN|0-7139-9061-9}}.</ref> [[Leonard Susskind]],<ref name="BoussoSusskind">{{Cite journal |last1=Bousso |first1=Raphael |last2=Susskind |first2=Leonard |year=2012 |title=Multiverse interpretation of quantum mechanics |journal=Physical Review D |volume=85 |issue=4 |pages=045007 |arxiv=1105.3796 |bibcode=2012PhRvD..85d5007B |doi=10.1103/PhysRevD.85.045007 |s2cid=118507872}}</ref> [[Alexander Vilenkin]],<ref>{{Cite book|url= https://books.google.com/books?id=9nRGwQnvGx0C|isbn= 9780374707149|date= 2007|title= Many Worlds in One: The Search for Other Universes|first= Alex |last= Vilenkin|publisher= Farrar, Straus and Giroux}}</ref> [[Yasunori Nomura]],<ref name="Nomura">{{Cite journal |last1=Nomura |first1=Yasunori |year=2011 |title=Physical theories, eternal inflation, and the quantum universe |journal=Journal of High Energy Physics |volume=2011 |issue=11 |pages=63 |arxiv=1104.2324 |bibcode=2011JHEP...11..063N |doi=10.1007/JHEP11(2011)063 |s2cid=119283262}}</ref> [[Raj Pathria]],<ref>{{Cite journal | doi = 10.1038/240298a0| title = The Universe as a Black Hole| journal = Nature| volume = 240| issue = 5379| pages = 298–299| year = 1972| last1 = Pathria | first1 = R. K.| bibcode=1972Natur.240..298P| s2cid = 4282253}}</ref> [[Laura Mersini-Houghton]],<ref name="TG-20220827">{{cite news |last=Fox |first=Killian |date=27 August 2022 |title=Cosmologist Laura Mersini-Houghton: 'Our universe is one tiny grain of dust in a beautiful cosmos' – Interview |url=https://www.theguardian.com/science/2022/aug/27/cosmologist-laura-mersini-houghton-before-the-big-bang-interview |accessdate=28 August 2022 |work=[[The Guardian]]}}</ref> [[Neil deGrasse Tyson]],<ref name="C4WDefault-2084173">{{cite news |url=http://www.huffingtonpost.com/2014/03/04/neil-degrasse-tyson-cosmos-god-alien-life-multiverses-interview_n_4790408.html |title=Why Revive 'Cosmos?' Neil DeGrasse Tyson Says Just About Everything We Know Has Changed |date=4 March 2014 |access-date=12 September 2014 |work=huffingtonpost.com |last=Freeman |first=David |archive-url=https://web.archive.org/web/20140913063109/http://www.huffingtonpost.com/2014/03/04/neil-degrasse-tyson-cosmos-god-alien-life-multiverses-interview_n_4790408.html |archive-date=13 September 2014 |url-status=live  }}</ref> [[Sean M. Carroll|Sean Carroll]]<ref>{{cite magazine |author=Carroll |first=Sean |date=18 October 2011 |title=Welcome to the Multiverse |url=http://discovermagazine.com/2011/oct/18-out-there-welcome-to-the-multiverse |magazine=[[Discover (magazine)|Discover]] |access-date=5 May 2015}}</ref> and [[Stephen Hawking]].<ref>{{cite book|title= Universe or Multiverse|isbn= 9780521848411|page= 19|quote= Some physicists would prefer to believe that string theory, or M-theory, will answer these questions and uniquely predict the features of the Universe. Others adopt the view that the initial state of the Universe is prescribed by an outside agency, code-named God, or that there are many universes, with ours being picked out by the anthropic principle. Hawking argued that string theory is unlikely to predict the distinctive features of the Universe. But neither is he is an advocate of God. He therefore opts for the last approach, favoring the type of multiverse which arises naturally within the context of his own work in quantum cosmology.|last1= Carr|first1= Bernard|date= 2007-06-21|publisher= Cambridge University Press}}</ref>

Scientists who are generally skeptical of the concept of a multiverse or popular multiverse hypotheses include <!-- To add anyone to this list we MUST have a citation that shows that they belong on the list. -->[[Sabine Hossenfelder]],<ref name="Sabine">{{cite magazine |last1=Geek's Guide to the Galaxy |title=Have Some Scientists Gotten Too Excited About the Multiverse? |url=https://www.wired.com/2022/09/geeks-guide-sabine-hossenfelder/ |access-date=16 February 2024 |magazine=Wired |publisher=Wired |date=9 September 2022}}</ref> [[David Gross]],<ref name="Davies2008">{{cite book |first=Paul |last=Davies |date=2008 |title=The Goldilocks Enigma: Why Is the Universe Just Right for Life? |chapter=Many Scientists Hate the Multiverse Idea |publisher=Houghton Mifflin Harcourt |isbn=9780547348469 |chapter-url=https://books.google.com/books?id=ktkvIoOanJsC&pg=PT207 |page=207}}</ref> [[Paul Steinhardt]],<ref name="edge-steinhardt-2014retire">{{cite web |url=http://www.edge.org/response-detail/25405 |title=Theories of Anything |work=edge.org |date=9 March 2014 <!-- No date available; last modification date used. --> |access-date=9 March 2014 |first=Paul |last=Steinhardt |author-link=Paul Steinhardt |archive-url=https://web.archive.org/web/20140310063831/http://www.edge.org/response-detail/25405 |archive-date=10 March 2014 |url-status=live |at=2014 : WHAT SCIENTIFIC IDEA IS READY FOR RETIREMENT? |quote=Theories of Anything<br />A pervasive idea in fundamental physics and cosmology that should be retired: the notion that we live in a multiverse in which the laws of physics and the properties of the cosmos vary randomly from one patch of space to another }}</ref><ref name="Multimess"/> Anna Ijjas,<ref name="Multimess">{{citation |last1=Ijjas |first1=Anna |title=Cosmic Inflation Theory Faces Challenges |date=February 2017 |journal=Scientific American |volume=316 |issue=2 |pages=32–39 |doi=10.1038/scientificamerican0217-32 |pmid=28118351 |last2=Loeb |first2=Abraham |last3=Steinhardt |first3=Paul}}.</ref> [[Abraham Loeb]],<ref name="Multimess"/> [[David Spergel]],<ref>{{Cite news| url = https://www.youtube.com/watch?v=hgCaBxAIcGE| title = Is Nature Simple? 2018 Breakthrough Prize Symposium Panel | work = YouTube | access-date = 14 January 2018}}</ref> [[Neil Turok]],<ref>{{cite journal |last1=Gibbons |first1=G. W. |last2=Turok |first2=Neil |year=2008 |title=The Measure Problem in Cosmology |journal=Physical Review D |volume=77 |issue=6 |page=063516 |arxiv=hep-th/0609095 |bibcode=2008PhRvD..77f3516G |doi=10.1103/PhysRevD.77.063516 |s2cid=16394385}}</ref> [[Viatcheslav Mukhanov]],<ref>{{cite journal | last1 = Mukhanov | first1 = Viatcheslav| year =  2014 | title = Inflation without Selfreproduction | journal = Fortschritte der Physik | volume = 63 | issue = 1  | pages = 36–41 | doi = 10.1002/prop.201400074  | bibcode =  2015ForPh..63...36M|arxiv = 1409.2335 | s2cid = 117514254}}</ref> [[Michael S. Turner]],<ref>{{cite web|url=http://www.math.columbia.edu/~woit/wordpress/?p=7793 | title= A Crisis at the (Western) Edge of Physics | last1=Woit|first1= Peter| work= [[Not Even Wrong]] |date= 9 June 2015}}</ref> [[Roger Penrose]],<ref>{{cite web|url= http://www.math.columbia.edu/~woit/wordpress/?p=7812 | title= CMB @ 50 | last1=Woit|first1= Peter| work= [[Not Even Wrong]] |date= 14 June 2015}}</ref> [[George Francis Rayner Ellis|George Ellis]],<ref name="SciAmer-731548">{{cite journal |url= http://www.scientificamerican.com/article/does-the-multiverse-really-exist/ |title= Does the Multiverse Really Exist? |last=Ellis |first=George F. R. |date=1 August 2011 |journal= [[Scientific American]] |volume=305 |issue=2 |pages=38–43 |access-date= 12 September 2014 |doi= 10.1038/scientificamerican0811-38 |pmid= 21827123 |url-access=subscription|bibcode = 2011SciAm.305a..38E }}</ref><ref name="C4WDefault-1835434">{{cite web |url=http://www.aei.mpg.de/~axkl/HermannFestProceedings/Ellis.pdf |title=The Multiverse: Conjecture, Proof, and Science |date=2012 |access-date=12 September 2014 |work=Slides for a talk at Nicolai Fest Golm 2012 |last=Ellis |first=George |archive-url=https://web.archive.org/web/20140913064557/http://www.aei.mpg.de/~axkl/HermannFestProceedings/Ellis.pdf |archive-date=13 September 2014 |url-status=dead }}</ref> [[Joe Silk]],<ref>{{citation |title= Scientific Method: Defend the Integrity of Physics |last1= Ellis  |first1= George |last2=Silk |first2=Joe |date=16 December 2014   |journal= Nature |doi=10.1038/516321a |volume=516 |issue= 7531 |pages=321–323|bibcode = 2014Natur.516..321E |pmid=25519115|doi-access= free }}</ref> [[Carlo Rovelli]],<ref>{{citation |last1=Scoles |first1=Sarah |title=Can Physics Ever Prove the Multiverse is Real |date=19 April 2016 |work=Smithsonian.com |url=http://www.smithsonianmag.com/science-nature/can-physicists-ever-prove-multiverse-real-180958813/?no-ist }}.</ref> [[Adam Frank]],<ref name="Crisis" >{{cite news |url= https://www.nytimes.com/2015/06/07/opinion/a-crisis-at-the-edge-of-physics.html?_r=0 |title= A Crisis at the Edge of Physics |last1= Frank|first1= Adam |last2=Gleiser |first2=Marcelo |work = [[The New York Times]] |date= 5 June 2015}}</ref> [[Marcelo Gleiser]],<ref name = "Crisis" /> [[Jim Baggott]]<ref name="Amazon-1605984728">{{cite book|title= Farewell to Reality: How Modern Physics Has Betrayed the Search for Scientific Truth|last= Baggott|first= Jim|author-link= Jim Baggott|date= 1 August 2013|publisher= Pegasus|isbn= 978-1-60598-472-8|url-access= registration|url= https://archive.org/details/isbn_9781605984728}}
</ref><!-- The person who added the original ref did not specify which version of the book they used. --> and [[Paul Davies]].<ref>{{cite news |url= https://www.nytimes.com/2003/04/12/opinion/a-brief-history-of-the-multiverse.html |title= A Brief History of the Multiverse |last1= Davies |first1= Paul | work = [[The New York Times]] |date= 12 April 2003}}</ref>

== Arguments against multiverse hypotheses ==
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 hypotheses are non-scientific:<ref>{{cite news |title=''A Brief History of the Multiverse'' |first=Paul |last=Davies |url=https://www.nytimes.com/2003/04/12/opinion/a-brief-history-of-the-multiverse.html?pagewanted=all |newspaper=New York Times |date=12 April 2003 |access-date=16 August 2011}}</ref> {{quotation|text=For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there is an infinite number of universes, credibility reaches a limit. As one slips down that slope, more and more must be accepted on faith, and less and less is open to scientific verification. Extreme multiverse explanations are therefore reminiscent of theological discussions. Indeed, invoking an infinity of unseen universes to explain the unusual features of the one we do see is just as ad hoc as invoking an unseen Creator. The multiverse theory may be dressed up in scientific language, but in essence, it requires the same leap of faith.|author=Paul Davies|title="A Brief History of the Multiverse"|source=''[[The New York Times]]''}}

[[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 [[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]] and [[falsifiability]] is a major concern, but he is opposed to that line of thinking: {{quotation|text=Many physicists who talk about the multiverse, especially advocates of the [[string landscape]], do not care much about parallel universes ''per se''. For them, objections to the multiverse as a concept are unimportant. Their theories live or die based on internal consistency and, one hopes, eventual laboratory testing.}}

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:<ref name="SciAmer-9723382">{{cite magazine |first=George F. R. |last=Ellis |date=1 August 2011 |title=Does the Multiverse Really Exist? |magazine=[[Scientific American]] |volume=305 |issue=2 |pages=38–43 |url=http://www.scientificamerican.com/article/does-the-multiverse-really-exist/ |access-date=16 August 2011 |doi=10.1038/scientificamerican0811-38 |url-access=subscription |bibcode = 2011SciAm.305a..38E }}</ref> {{quotation|text=As skeptical as I am, I think the contemplation of the multiverse is an excellent opportunity to reflect on the nature of science and on the ultimate nature of existence: why we are here. {{omission}} In looking at this concept, we need an open mind, though not too open. It is a delicate path to tread. Parallel universes may or may not exist; the case is unproved. We are going to have to live with that uncertainty. Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is.|author=George Ellis|title="Does the Multiverse Really Exist?"|source=''[[Scientific American]]''}}

Philosopher [[Philip Goff (philosopher)|Philip Goff]] argues that the inference of a multiverse to explain the apparent fine-tuning of the universe is an example of [[Inverse Gambler's Fallacy]].<ref>{{Cite web|url=https://www.scientificamerican.com/article/our-improbable-existence-is-no-evidence-for-a-multiverse/|title=Our Improbable Existence Is No Evidence for a Multiverse|first=Philip|last=Goff|author-link=Philip Goff (philosopher)|website=Scientific American}}</ref>

Stoeger, Ellis, and Kircher<ref>{{cite arXiv |eprint=astro-ph/0407329 |first1=W. R. |last1=Stoeger |first2=G. F. R. |last2=Ellis |title=Multiverses and Cosmology: Philosophical Issues |date=2006-01-19 |last3=Kirchner |first3=U.}}</ref>{{Rp|at=sec. 7}} note that in a true multiverse theory, "the universes are then completely disjoint and nothing that happens in any one of them is causally linked to what happens in any other one. This lack of any causal connection in such multiverses really places them beyond any scientific support".

In May 2020, astrophysicist Ethan Siegel expressed criticism in a [[Forbes]] blog post that parallel universes would have to remain a science fiction dream for the time being, based on the scientific evidence available to us.<ref>{{cite web |author=Siegel |first=Ethan |date=22 May 2020 |title=Ask Ethan: Have We Finally Found Evidence For A Parallel Universe? |url=https://www.forbes.com/sites/startswithabang/2020/05/22/ask-ethan-have-we-finally-found-evidence-for-a-parallel-universe/ |access-date=18 September 2020 |website=[[Forbes]] |language=en-us}}<!-- auto-translated by Module:CS1 translator --></ref>

''[[Scientific American]]'' contributor [[John Horgan (journalist)|John Horgan]] also argues against the idea of a multiverse, claiming that they are "bad for science."<ref>{{cite web | url=https://www.scientificamerican.com/blog/cross-check/multiverse-theories-are-bad-for-science/ | title=Multiverse Theories Are Bad for Science | website=[[Scientific American]] }}</ref>

== 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 journal
 | first = Max | last = Tegmark | date = 23 January 2003
 | title = Parallel Universes
 | journal = Scientific American | volume = 288 | issue = 5 | page = 40 | doi = 10.1038/scientificamerican0503-40 | arxiv = astro-ph/0302131 | bibcode = 2003SciAm.288e..40T | 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]]).

== M-theory ==
{{see also|Introduction to M-theory|M-theory|Brane cosmology|String theory landscape}}
A multiverse of a somewhat different kind has been envisaged within [[string theory]] and its higher-dimensional extension, [[Introduction to M-theory|M-theory.]]<ref>{{cite arXiv|eprint=hep-th/0511037v1|last1=Weinberg|first1=Steven|title=Living in the Multiverse|date=2005}}</ref>

These theories require the presence of 10 or 11 spacetime dimensions respectively. The extra six or seven 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 [[brane]]s which could support other universes.<ref name="Richard J Szabo 2004">Richard J. Szabo, ''An introduction to string theory and D-brane dynamics'' (2004).</ref><ref name="Maurizio Gasperini 2007">Maurizio Gasperini, ''Elements of String Cosmology'' (2007).</ref>

== Black-hole cosmology ==
{{main|Black-hole cosmology}}
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.<ref>{{Cite journal|last=Pathria|first=R. K.|date=1972-12-01|title=The Universe as a Black Hole|bibcode=1972Natur.240..298P|journal=Nature|volume=240|issue=5379|pages=298–299|doi=10.1038/240298a0|s2cid=4282253|issn=0028-0836}}</ref> This includes the theory of [[white hole]]s, which are on the opposite side of [[space-time]].

== Anthropic principle ==
{{main|Anthropic principle}}
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.<ref>Arthur Schopenhauer, "Die Welt als Wille und Vorstellung" (in German), supplement to the 4th book "Von der Nichtigkeit und dem Leiden des Lebens" (in German). see also R. B. Haldane and J. Kemp's translation [[wikisource:The_World_as_Will_and_Representation/Supplements_to_the_Fourth_Book|"On the Vanity and Suffering of Life"]], pp. 395–396.</ref>

== Occam's razor ==
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.<ref>{{cite book |last=Trinh |first=Xuan Thuan |title=Science & the Search for Meaning: Perspectives from International Scientists |date=2006 |publisher=[[Templeton Foundation]] |isbn=978-1-59947-102-0 |editor=Staune |editor-first=Jean |location=West Conshohocken, Pennsylvania |page=186}}</ref> However, proponents argue that in terms of [[Kolmogorov complexity]] the proposed multiverse is simpler than a single idiosyncratic universe.<ref name="TegmarkPUstaple"/>

For example, multiverse proponent [[Max Tegmark]] argues: {{quotation|text=[A]n entire [[Statistical ensemble (mathematical physics)|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 (mathematics)|set]] of all [[integer]]s. 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 higher-level multiverses are simpler. Going from our universe to the Level I multiverse eliminates the need to specify [[initial condition]]s, upgrading to Level II eliminates the need to specify [[physical constants]], and the Level IV multiverse eliminates the need to specify anything at all... 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]], [[Wavefunction collapse|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.<ref name="TegmarkPUstaple"/><ref>{{cite journal | date = May 2003 | title = Parallel universes. Not just a staple of science fiction, other universes are a direct implication of cosmological observations | url = https://cds.cern.ch/record/604580| journal = Scientific American | volume = 288 | issue = 5| pages = 40–51 | doi=10.1038/scientificamerican0503-40 | pmid=12701329|arxiv = astro-ph/0302131 |bibcode = 2003SciAm.288e..40T | last1 = Tegmark | first1 = M. }}</ref>|author=Max Tegmark}}

== Possible worlds and real worlds ==
In any given set of possible universes – e.g. in terms of histories or variables of nature – not all may be ever realized, and some may be realized many times.<ref>{{cite journal |last1=Ellis |first1=G. F. R. |last2=Kirchner |first2=U. |last3=Stoeger |first3=W. R. |title=Multiverses and physical cosmology |journal=Monthly Notices of the Royal Astronomical Society |date=21 January 2004 |volume=347 |issue=3 |pages=921–936 |doi=10.1111/j.1365-2966.2004.07261.x|doi-access=free |arxiv=astro-ph/0305292 |bibcode=2004MNRAS.347..921E |s2cid=119028830 }}</ref> For example, over infinite time there could, in some potential theories, be infinite universes, but only a small or relatively small real number of universes where humanity could exist and only one where it ever does exist (with a unique history).{{citation needed|date=July 2022}} It has been suggested that a universe that "contains life, in the form it has on Earth, is in a certain sense radically non-[[Ergodicity|ergodic]], in that the vast majority of possible organisms will never be realized".<ref>{{cite arXiv |last1=Cortês |first1=Marina |last2=Kauffman |first2=Stuart A. |last3=Liddle |first3=Andrew R. |last4=Smolin |first4=Lee |authorlink4=Lee Smolin |title=Biocosmology: Biology from a cosmological perspective |eprint=2204.09379 |date=28 April 2022|class=physics.hist-ph }}</ref> On the other hand, some scientists, theories and popular works conceive of a multiverse in which the universes are so similar that humanity exists in many equally real separate universes but with varying histories.<ref>{{cite web |title=What is the multiverse—and is there any evidence it really exists? |url=https://www.nationalgeographic.com/science/article/what-is-the-multiverse |archive-url=https://web.archive.org/web/20220504150431/https://www.nationalgeographic.com/science/article/what-is-the-multiverse |url-status=dead |archive-date=4 May 2022 |website=Science |access-date=12 July 2022 |language=en |date=4 May 2022}}</ref>

There is a [[Many-worlds interpretation#Debate whether the other worlds are real|debate about whether the other worlds are real]] in the many-worlds interpretation (MWI) of quantum [[mechanics]]. In [[Quantum Darwinism]] one does not need to adopt a MWI in which all of the branches are equally real.<ref>{{cite journal |authorlink1=Wojciech H. Zurek |last1=Zurek |first1=Wojciech Hubert |title=Quantum theory of the classical: quantum jumps, Born's Rule and objective classical reality via quantum Darwinism |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |date=13 July 2018 |volume=376 |issue=2123 |pages=20180107 |doi=10.1098/rsta.2018.0107|pmid=29807905 |pmc=5990654 |arxiv=1807.02092 |bibcode=2018RSPTA.37680107Z }}</ref>

=== Modal realism ===
[[Possible world]]s are a way of explaining probability and hypothetical statements. Some philosophers, such as [[David Lewis (philosopher)|David Lewis]], posit that all possible worlds exist and that they are just as real as the world we live in. This position is known as [[modal realism]].<ref>{{cite book|first=David|last=Lewis|date=1986|title=On the Plurality of Worlds|publisher= Basil Blackwell|isbn=978-0-631-22426-6|title-link=On the Plurality of Worlds}}</ref>

== See also ==

* {{Annotated link|Black hole thermodynamics#Beyond black holes|Beyond black holes}}
* {{Annotated link|Cosmogony}}
* {{Annotated link|Eternity}}
* {{Annotated link|Impossible world}}
* {{Annotated link|Measure problem (cosmology)}}
* {{Annotated link|Modal realism}}
* {{Annotated link|Parallel universes in fiction}}
* {{Annotated link|Philosophy of physics}}
* {{Annotated link|Philosophy of space and time}}
* {{Annotated link|Simulated reality}}
* {{Annotated link|Twin Earth thought experiment}}
* {{Annotated link|Ultimate fate of the universe}}

== References ==
'''Footnotes'''
{{notelist}}

'''Citations'''
{{reflist|30em}}

== Further reading ==
{{refbegin}}
* {{cite book |last=Carr |first=Bernard |edition=2007 |author-link=Bernard Carr |title=Universe or Multiverse? |publisher=Cambridge University Press}}
* {{cite journal |first=David |last=Deutsch |author-link=David Deutsch |date=1985 |title=Quantum theory, the Church–Turing principle and the universal quantum computer |journal=Proceedings of the Royal Society of London A |issue=1818 |pages=97–117 |url=http://www.cs.berkeley.edu/~christos/classics/Deutsch_quantum_theory.pdf |doi=10.1098/rspa.1985.0070 |volume=400 |bibcode=1985RSPSA.400...97D |citeseerx=10.1.1.41.2382 |s2cid=1438116 |access-date=15 September 2014 |archive-url=https://web.archive.org/web/20160309202304/http://www.cs.berkeley.edu/~christos/classics/Deutsch_quantum_theory.pdf |archive-date=9 March 2016 |url-status=dead}}
* {{cite journal |last1=Ellis |first1=George F.R. |author-link=George Francis Rayner Ellis |last2=Stoeger |title=Multiverses and physical cosmology |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=347 |issue=3 |pages=921–936 |date=2004 |arxiv=astro-ph/0305292 |doi=10.1111/j.1365-2966.2004.07261.x |bibcode=2004MNRAS.347..921E |first2=William R. |doi-access=free |author-link2=William R. Stoeger |s2cid=119028830}}
* Andrei Linde, [https://web.stanford.edu/~alinde/1032226.pdf The Self-Reproducing Inflationary Universe], [[Scientific American]], November 1994 – Touches on multiverse concepts at the end of the article.
{{refend}}

== External links ==
{{wiktionary}}
{{wikiquote}}
{{commons category|Multiverse}}
* [http://thoughtcast.org/casts/the-end-of-our-universe-among-other-timely-topics Interview with Tufts cosmologist Alex Vilenkin] on his book, "Many Worlds in One: The Search for Other Universes" on the podcast and public radio interview program [http://www.thoughtcast.org/ ThoughtCast.] {{Webarchive|url=https://web.archive.org/web/20200818134713/https://thoughtcast.org/ |date=18 August 2020 }}.
* [http://www.bbc.co.uk/programmes/b008z744 Multiverse] – an episode of the series [[In Our Time (radio series)|In Our Time]] with [[Melvyn Bragg]], on [[BBC Radio 4]].
* [http://www.bbc.com/earth/story/20160318-why-there-might-be-many-more-universes-besides-our-own Why There Might be Many More Universes Besides Our Own], by Phillip Ball, March 21, 2016, bbc.com.

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