Editing Big Bang (section)

== Implications ==
Given current understanding, scientific extrapolations about the future of the universe are only possible for finite durations, albeit for much longer periods than the current age of the universe. Anything beyond that becomes increasingly speculative. Likewise, at present, a proper understanding of the origin of the universe can only be subject to conjecture.<ref name=Starobinsky_2000>{{cite book | chapter=Future and Origin of Our Universe: Modern View | last=Starobinsky | first=Alexei | title=The Future of the Universe and the Future of Our Civilization | series= Proceedings of a symposium held in Budapest-Debrecen, Hungary, 2–6 July 1999 |chapter-url=https://books.google.com/books?id=V7zhUBg1qesC&pg=PA71 | editor1-first=V. | editor1-last=Burdyuzha | editor2-first=G. | editor2-last=Khozin | publication-place=Singapore | publisher=World Scientific Publishing | isbn=9810242646 | page=71 | year=2000 | doi=10.1142/9789812793324_0008 | bibcode=2000fufc.conf...71S | s2cid=37813302 }}</ref>

=== 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]].

=== Ultimate fate of the universe ===
{{Main|Ultimate fate of the universe}}
Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state similar to that in which it started—a [[Big Crunch]].<ref name="kolb_c3"/>

Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down but never stop. Star formation would cease with the consumption of interstellar gas in each galaxy; stars would burn out, leaving [[white dwarf]]s, [[neutron star]]s, and black holes. Collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would very gradually asymptotically approach [[absolute zero]]—a [[Future of an expanding universe|Big Freeze]].<ref>{{cite web |url=https://map.gsfc.nasa.gov/universe/uni_fate.html |title=What is the Ultimate Fate of the Universe? |author=NASA/WMAP Science Team |date=29 June 2015 |work=Universe 101: Big Bang Theory |publisher=[[NASA]] |location=Washington, D.C. |url-status=live |archive-url=https://web.archive.org/web/20191015052245/https://map.gsfc.nasa.gov/universe/uni_fate.html |archive-date=15 October 2019 |access-date=18 December 2019}}</ref> Moreover, if protons are [[proton decay|unstable]], then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting [[Hawking radiation]]. The [[entropy]] of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.<ref name=dying>{{cite journal |last1=Adams |first1=Fred C. |author1-link=Fred Adams |last2=Laughlin |first2=Gregory |author2-link=Gregory P. Laughlin |date=April 1997 |title=A dying universe: the long-term fate and evolution of astrophysical objects |journal=[[Reviews of Modern Physics]] |volume=69 |issue=2 |pages=337–372 |arxiv=astro-ph/9701131 |bibcode=1997RvMP...69..337A |doi=10.1103/RevModPhys.69.337 |s2cid=12173790 }}.</ref>

Modern observations of accelerating expansion imply that more and more of the currently visible universe will pass beyond our [[event horizon]] and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, will remain together, and they too will be subject to heat death as the universe expands and cools. Other explanations of dark energy, called [[phantom energy]] theories, suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei, and matter itself will be torn apart by the ever-increasing expansion in a so-called [[Big Rip]].<ref>{{cite journal |last1=Caldwell |first1=Robert R. |author1-link=Robert R. Caldwell |last2=Kamionkowski |first2=Marc |author2-link=Marc Kamionkowski |last3=Weinberg |first3=Nevin N. |date=15 August 2003 |title=Phantom Energy: Dark Energy with w<−1 Causes a Cosmic Doomsday |journal=[[Physical Review Letters]] |volume=91 |issue=7 |page=071301 |arxiv=astro-ph/0302506 |bibcode=2003PhRvL..91g1301C |doi=10.1103/PhysRevLett.91.071301 |pmid=12935004|s2cid=119498512 }}</ref>

===Religious and philosophical interpretations===
{{Main|Religious interpretations of the Big Bang theory}}
<!--
Please do not add quotes from religious texts in this article,
the main article treating religious views on the Big Bang is
[[Religious interpretations of the Big Bang theory]].
-->
As a description of the origin of the universe, the Big Bang has significant bearing on religion and philosophy.<ref name="Harris2002">{{harvnb|Harris|2002|p=[https://books.google.com/books?id=Rx2Qf9ieFKYC&pg=PA128 128]}}</ref><ref name="Frame2009">{{harvnb|Frame|2009|pp=[https://books.google.com/books?id=1mb-h1lom9IC&pg=PA137 137–141]}}</ref> As a result, it has become one of the liveliest areas in the discourse between [[Relationship between religion and science|science and religion]].<ref name="Harrison2010">{{harvnb|Harrison|2010|p=[https://books.google.com/books?id=0mSCHC0QMUgC&pg=PA9 9]}}</ref> Some believe the Big Bang implies a creator,<ref>{{harvnb|Block|Puerari|Stockton|Ferreira|2000|pp=723–740}}{{doi|10.1007/978-94-011-4114-7_85}}</ref><ref>{{harvnb|Harris|2002|p=[https://books.google.com/books?id=Rx2Qf9ieFKYC&pg=PA129 129]}}</ref><ref>{{cite journal |last=Craig |first=William Lane |author-link=William Lane Craig |date=December 1999 |title=The Ultimate Question of Origins: God and the Beginning of the Universe |journal=[[Astrophysics and Space Science]] |type=Lecture |volume=269–270 |issue=1–4 |pages=721–738 |doi=10.1023/A:1017083700096 |bibcode=1999Ap&SS.269..721C |s2cid=117794135}}</ref><ref>{{cite web |url=https://www.reasonablefaith.org/writings/scholarly-writings/the-existence-of-god/the-ultimate-question-of-origins-god-and-the-beginning-of-the-universe |url-status=live |title=The Ultimate Question of Origins: God and the Beginning of the Universe |last=Craig |first=William Lane |author-link=William Lane Craig |department=Scholarly Writings: The Existence of God |website=Reasonable Faith |location=Dallas, TX |archive-url=https://web.archive.org/web/20191229042029/https://www.reasonablefaith.org/writings/scholarly-writings/the-existence-of-god/the-ultimate-question-of-origins-god-and-the-beginning-of-the-universe |archive-date=29 December 2019 |access-date=21 December 2019 }}</ref> while others argue that Big Bang cosmology makes the notion of a creator superfluous.<ref name="Frame2009"/>{{sfn|Hawking|1988|loc=Introduction: "...&nbsp;a universe with no edge in space, no beginning or end in time, and nothing for a Creator to do." — [[Carl Sagan]]}}