Editing Big Bang (section)

=== Pre–Big Bang cosmology ===
The Big Bang explains the evolution of the universe from a starting density and temperature that is well beyond humanity's capability to replicate, so extrapolations to the most extreme conditions and earliest times are necessarily more speculative. Lemaître called this initial state the "''primeval atom''" while Gamow called the material "''[[ylem]]''". How the initial state of the universe originated is still an open question, but the Big Bang model does constrain some of its characteristics. For example, if specific [[scientific law|laws of nature]] were to come to existence in a random way, inflation models show, some combinations of these are far more probable,{{sfn|Hawking|1988|p=69}} partly explaining why our Universe is rather stable. Another possible explanation for the stability of the Universe could be a hypothetical multiverse, which assumes every possible universe to exist, and thinking species could only emerge in those stable enough.<ref>{{Cite web |last=Kuhn |first=Robert Lawrence |date=2015-12-23 |title=Confronting the Multiverse: What 'Infinite Universes' Would Mean |url=https://www.space.com/31465-is-our-universe-just-one-of-many-in-a-multiverse.html |access-date=2024-01-07 |website=Space.com |language=en}}</ref> A flat universe implies a balance between [[Gravitational energy|gravitational potential energy]] and other energy forms, requiring no additional energy to be created.<ref name=Filippenko2002/><ref name=Krauss2009/>

The Big Bang theory is built upon the equations of classical general relativity, which are not expected to be valid at the origin of cosmic time, as the temperature of the universe approaches the Planck scale. Correcting this will require the development of a correct treatment of quantum gravity.<ref name=Hawking_Ellis_1973 /> Certain quantum gravity treatments, such as the [[Wheeler–DeWitt equation]], imply that time itself could be an [[Emergence#Nonliving, physical systems|emergent property]].<ref>{{harvnb|Carroll|n.d.}}</ref> As such, physics may conclude that [[time]] did not exist before the Big Bang.<ref>{{cite magazine |last=Beckers |first=Mike |date=16 February 2015 |title=Quantentrick schafft Urknall-Singularität ab |trans-title=Quantum Trick Eliminates the Big Bang Singularity |url=https://www.spektrum.de/news/quantentrick-schafft-urknall-singularitaet-ab/1332377 |url-status=live |department=Cosmology |magazine=[[Spektrum der Wissenschaft]] |language=de |archive-url=https://web.archive.org/web/20170721041648/https://www.spektrum.de/news/quantentrick-schafft-urknall-singularitaet-ab/1332377 |archive-date=21 July 2017 |access-date=19 December 2019}} {{Google translation|en|de|www.spektrum.de/news/quantentrick-schafft-urknall-singularitaet-ab/1332377}}</ref><ref>{{cite journal |last1=Ali |first1=Ahmed Farag |author1-link=Ahmed Farag Ali |last2=Das |first2=Saurya |date=4 February 2015 |title=Cosmology from quantum potential |journal=[[Physics Letters|Physics Letters B]] |volume=741 |pages=276–279 |arxiv=1404.3093v3 |doi=10.1016/j.physletb.2014.12.057 |bibcode=2015PhLB..741..276F |s2cid=55463396}}</ref><ref>{{cite journal |last=Lashin |first=Elsayed I. |date=7 March 2016 |title=On the correctness of cosmology from quantum potential |journal=[[Modern Physics Letters A]] |volume=31 |issue=7 |pages=1650044 |arxiv=1505.03070 |bibcode=2016MPLA...3150044L |doi=10.1142/S0217732316500449 |s2cid=119220266}}</ref><ref>{{cite journal |last1=Das |first1=Saurya |last2=Rajat K. |first2=Bhaduri |date=21 May 2015 |title=Dark matter and dark energy from a Bose–Einstein condensate |journal=[[Classical and Quantum Gravity]] |volume=32 |issue=10 |pages=105003 |arxiv=1411.0753 |bibcode=2015CQGra..32j5003D |doi=10.1088/0264-9381/32/10/105003 |s2cid=119247745}}</ref><ref>{{cite web |url=http://www.hawking.org.uk/the-beginning-of-time.html |url-status=live |title=The Beginning of Time |last=Hawking |first=Stephen W. |author-link=Stephen Hawking |year=1996 |website=Stephen Hawking |publisher=The Stephen Hawking Foundation |location=London |type=Lecture |archive-url=https://web.archive.org/web/20191106162705/http://www.hawking.org.uk/the-beginning-of-time.html |archive-date=6 November 2019 |access-date=26 April 2017}}</ref>

While it is not known what could have preceded the hot dense state of the early universe or how and why it originated, or even whether such questions are sensible, speculation abounds on the subject of "[[cosmogony]]".

Some speculative proposals in this regard, each of which entails untested hypotheses, are:
* The simplest models, in which the Big Bang was caused by [[quantum fluctuation]]s. That scenario had very little chance of happening, but, according to the [[totalitarian principle]], even the most improbable event will eventually happen. It took place instantly, in our perspective, due to the absence of perceived time before the Big Bang.<ref>{{Cite web|url=https://www.space.com/16281-big-bang-god-intervention-science.html|title=The Big Bang Didn't Need God to Start Universe, Researchers Say|last=Wall|first=Mike|date=24 June 2012|website=Space.com}}</ref><ref>{{Cite news|last=Overbye|first=Dennis|url=https://www.nytimes.com/2001/05/22/science/before-the-big-bang-there-was-what.html |archive-url=https://web.archive.org/web/20130227035220/http://www.nytimes.com/2001/05/22/science/before-the-big-bang-there-was-what.html |archive-date=2013-02-27 |url-access=subscription |url-status=live|title=Before the Big Bang, There Was . . . What?|date=22 May 2001|work=The New York Times}}</ref><ref>{{Cite journal|last1=He|first1=Dongshan|last2=Gao|first2=Dongfeng|last3=Cai|first3=Qing-yu|date=3 April 2014|title=Spontaneous creation of the universe from nothing|journal=Physical Review D|volume=89|issue=8|page=083510|doi=10.1103/PhysRevD.89.083510|arxiv=1404.1207|bibcode=2014PhRvD..89h3510H|s2cid=118371273}}</ref><ref>{{Cite journal|last1=Lincoln|first1=Maya|last2=Wasser|first2=Avi|date=1 December 2013|title=Spontaneous creation of the Universe Ex Nihilo|journal=Physics of the Dark Universe|volume=2|issue=4|pages=195–199|doi=10.1016/j.dark.2013.11.004|issn=2212-6864|bibcode=2013PDU.....2..195L|doi-access=free}}</ref>
* [[Emergent Universe]] models, which feature a low-activity past-eternal era before the Big Bang, resembling ancient ideas of a [[cosmic egg]] and birth of the world out of [[chaos (cosmogony)|primordial chaos]].
* Models in which the whole of spacetime is finite, including the [[Hartle–Hawking state|Hartle–Hawking no-boundary condition]]. For these cases, the Big Bang does represent the limit of time but without a singularity.<ref>{{cite journal |last1=Hartle |first1=James H. |author1-link=James Hartle |last2=Hawking |first2=Stephen W. |author2-link=Stephen Hawking |date=15 December 1983 |title=Wave function of the Universe |journal=[[Physical Review D]] |volume=28 |issue=12 |pages=2960–2975 |bibcode=1983PhRvD..28.2960H |doi=10.1103/PhysRevD.28.2960|s2cid=121947045 }}</ref> In such a case, the universe is self-sufficient.{{sfn|Hawking|1988|p=71}}
* [[Brane cosmology]] models, in which inflation is due to the movement of [[brane]]s in [[string theory]]; the pre-Big Bang model; the [[Ekpyrotic universe|ekpyrotic]] model, in which the Big Bang is the result of a collision between branes; and the [[cyclic model]], a variant of the ekpyrotic model in which collisions occur periodically. In the latter model the Big Bang was preceded by a Big Crunch and the universe cycles from one process to the other.<ref>{{cite journal |last=Langlois |first=David |year=2003 |title=Brane Cosmology |journal=[[Progress of Theoretical Physics Supplement]] |volume=148 |pages=181–212 |arxiv=hep-th/0209261 |bibcode=2002PThPS.148..181L |doi=10.1143/PTPS.148.181 |s2cid=9751130}}</ref><ref>{{harvnb|Gibbons|Shellard|Rankin|2003|pp=801–838|loc=chpt. 43: "Inflationary theory versus the ekpyrotic/cyclic scenario" by [[Andrei Linde]].}} {{Bibcode|2003ftpc.book..801L}}</ref><ref name="rebirth">{{cite web |url=https://www.space.com/2372-recycled-universe-theory-solve-cosmic-mystery.html |url-status=live |title=Recycled Universe: Theory Could Solve Cosmic Mystery |last=Than |first=Ker |date=8 May 2006 |website=[[Space.com]] |location=New York |publisher=[[Future plc]] |archive-url=https://web.archive.org/web/20190906000057/https://www.space.com/2372-recycled-universe-theory-solve-cosmic-mystery.html |archive-date=6 September 2019 |access-date=19 December 2019}}</ref><ref name="rebirth2">{{cite web |url=https://science.psu.edu/news-and-events/2007-news/Bojowald6-2007.htm |url-status=live |last=Kennedy |first=Barbara K. |title=What Happened Before the Big Bang? |date=1 July 2007 |website=News and Events |publisher=[[Eberly College of Science]], [[Pennsylvania State University]] |location=University Park, PA |access-date=19 December 2019 |archive-url=https://web.archive.org/web/20191215041942/http://science.psu.edu/news-and-events/2007-news/Bojowald6-2007.htm/ |archive-date=15 December 2019}}</ref><ref>{{cite journal |last=Bojowald |first=Martin |author-link=Martin Bojowald |date=August 2007 |title=What happened before the Big Bang? |journal=[[Nature Physics]] |volume=3 |issue=8 |pages=523–525 |doi=10.1038/nphys654 |bibcode=2007NatPh...3..523B|url=https://zenodo.org/record/896670 |doi-access=free }}</ref>
* [[Eternal inflation]], in which universal inflation ends locally here and there in a random fashion, each end-point leading to a ''bubble universe'', expanding from its own big bang.<ref>{{cite journal |last=Linde |first=Andrei D. |author-link=Andrei Linde |date=May 1986 |title=Eternal Chaotic Inflation |url=https://cds.cern.ch/record/167897 |url-status=live |journal=[[Modern Physics Letters 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=17 April 2019}}</ref><ref>{{cite journal |last=Linde |first=Andrei D. |author-link=Andrei Linde |date=14 August 1986 |title=Eternally Existing Self-Reproducing Chaotic Inflationary Universe |journal=[[Physics Letters|Physics Letters B]] |volume=175 |issue=4 |pages=395–400 |bibcode=1986PhLB..175..395L |doi=10.1016/0370-2693(86)90611-8}}</ref> This is sometimes referred to as pre-big bang inflation.<ref>{{cite journal | title=Primordial black holes from pre-big bang inflation | last1=Conzinu | first1=P. | last2=Gasperini | first2=M. | last3=Marozzi | first3=G. | journal=Journal of Cosmology and Astroparticle Physics | issue=8 | at=id. 031 | date=August 2020 | doi=10.1088/1475-7516/2020/08/031 | arxiv=2004.08111 | bibcode=2020JCAP...08..031C }}</ref>

Proposals in the last two categories see the Big Bang as an event in either a much larger and [[Roger Penrose#An earlier universe|older universe]] or in a [[multiverse]].