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{{Short description|Physical theory}}
{{About|the theory|the television series|The Big Bang Theory{{!}}''The Big Bang Theory''||Big Bang (disambiguation)|and|Big Bang Theory (disambiguation)}}
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{{Use dmy dates|date=March 2020}}
{{Use American English|date=May 2016}}
[[File:CMB Timeline300 no WMAP.jpg|upright=1.5|thumb|alt=A model of the expanding universe opening up from the viewer's left, facing the viewer in a 3/4 pose.|Timeline of the [[expansion of the universe]], where space, including hypothetical non-observable portions of the universe, is represented at each time by the circular sections. On the left, the dramatic expansion occurs in the [[inflationary epoch]]; and at the center, the expansion [[accelerating expansion of the universe|accelerates]] (artist's concept; neither time nor size are to scale).]]
{{Cosmology|cTopic=Key topics}}

The '''Big Bang''' is a [[physical theory]] that describes how the [[Expansion of the universe|universe expanded]] from an initial state of high [[Energy density|density]] and [[temperature]].<ref name="HTUW">{{cite serial|title=First Second of the Big Bang|url=https://www.sciencechannel.com/tv-shows/how-the-universe-works/full-episodes/first-second|series=[[How the Universe Works#Season 3 (2014)|How The Universe Works]]|last=Bridge|first=Mark (Director)|network=[[Science Channel]]|location=Silver Spring, Maryland|date=30 July 2014}}</ref> The notion of an expanding universe was first scientifically originated by physicist [[Alexander Friedmann]] in 1922 with the mathematical derivation of the [[Friedmann equations]].<ref name="m853">{{cite journal | last=Belenkiy | first=Ari | title=Alexander Friedmann and the origins of modern cosmology | journal=Physics Today | volume=65 | issue=10 | date=October 1, 2012 | issn=0031-9228 | doi=10.1063/PT.3.1750 | pages=38–43| bibcode=2012PhT....65j..38B }}</ref><ref name="z155">{{cite journal | last1=Nemiroff | first1=Robert J. | last2=Patla | first2=Bijunath | title=Adventures in Friedmann cosmology: A detailed expansion of the cosmological Friedmann equations | journal=American Journal of Physics | volume=76 | issue=3 | date=March 1, 2008 | issn=0002-9505 | doi=10.1119/1.2830536 | pages=265–276| arxiv=astro-ph/0703739 | bibcode=2008AmJPh..76..265N }}</ref><ref name="c686">{{cite journal | last1=Carroll | first1=Sean M. | last2=Kaplinghat | first2=Manoj | title=Testing the Friedmann equation: The expansion of the universe during big-bang nucleosynthesis | journal=Physical Review D | volume=65 | issue=6 | date=February 27, 2002 | issn=0556-2821 | doi=10.1103/PhysRevD.65.063507 | page=063507| arxiv=astro-ph/0108002 | bibcode=2002PhRvD..65f3507C }}</ref><ref name="c625">{{cite journal | last=Mörtsell | first=Edvard | title=Cosmological histories from the Friedmann equation: the Universe as a particle | journal=European Journal of Physics | volume=37 | issue=5 | date=September 1, 2016 | issn=0143-0807 | doi=10.1088/0143-0807/37/5/055603 | page=055603| arxiv=1606.09556 | bibcode=2016EJPh...37e5603M }}</ref>

[[List_of_multiple_discoveries#20th_century|Independent]] of Friedmann's work, the Big Bang was first proposed in 1931 by [[Roman Catholic priest]] and [[physicist]] [[Georges Lemaître]] when he suggested the universe emerged from a "primeval [[atom]]". Various [[Physical cosmology|cosmological models]] of the Big Bang explain the evolution of the [[observable universe]] from the [[Planck units#Cosmology|earliest known periods]] through its subsequent large-scale form.{{sfn|Silk|2009|p=208}}{{sfn|Singh|2004|p=560|ps=. Book limited to 532 pages. Correct source page requested.}}<ref>{{cite web |url=https://map.gsfc.nasa.gov/universe/ |title=Cosmology: The Study of the Universe |author=NASA/WMAP Science Team |date=6 June 2011 |work=Universe 101: Big Bang Theory |publisher=[[NASA]] |location=Washington, D.C. |access-date=18 December 2019 |archive-url=https://web.archive.org/web/20110629050256/https://map.gsfc.nasa.gov/universe/ |archive-date=29 June 2011 |url-status=live |quote=The second section discusses the classic tests of the Big Bang theory that make it so compelling as the most likely valid and accurate description of our universe.}}</ref> These models offer a comprehensive explanation for a broad range of observed phenomena, including the abundance of [[light element]]s, the [[cosmic microwave background]] (CMB) [[Electromagnetic radiation|radiation]], and [[Large-scale structure of the Universe|large-scale structure]]. The uniformity of the universe, known as the [[flatness problem]], is explained through [[Inflation (cosmology)|cosmic inflation]]: a sudden and very rapid expansion of space during the earliest moments.

Crucially, these models are compatible with the [[Hubble–Lemaître law]]—the observation that the farther away a [[galaxy]] is, the faster it is moving away from Earth. Extrapolating this cosmic expansion backward in time using the known [[Scientific law#Laws of physics|laws of physics]], the models describe an increasingly concentrated cosmos preceded by [[Initial singularity|a singularity]] in which [[Spacetime|space and time]] lose meaning (typically named "the Big Bang singularity").<ref name="books.google.com">{{harvnb|Chow|2008|p=[https://books.google.com/books?id=fp9wrkMYHvMC&pg=PA211 211]}}</ref> Physics lacks a widely accepted theory of [[quantum gravity]] that can model the earliest conditions of the Big Bang. In 1964 the CMB was discovered, which convinced many cosmologists that the competing [[steady-state model]] of cosmic evolution was [[Falsifiability|falsified]], since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in the distant past.<ref>{{harvnb|Partridge|1995|p=[https://books.google.com/books?id=5G3wdV1IPE4C&pg=PR17 xvii]}}</ref> A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted.<ref name="Kragh_1996">{{harvnb|Kragh|1996|p=[https://archive.org/details/cosmologycontrov00helg 319]}}: "At the same time that observations tipped the balance definitely in favor of the relativistic big-bang theory, ..."</ref> Detailed measurements of the expansion rate of the [[universe]] place the Big Bang singularity at an estimated {{val|13.787|0.020}}&nbsp;[[1,000,000,000 (number)|billion]] years ago, which is considered the [[age of the universe]].<ref name="esa">{{cite web | url=https://www.mpg.de/7044245/Planck_cmb_universe |title=Planck reveals an almost perfect universe | date=March 21, 2013 | publisher=Max-Planck-Gesellschaft | access-date=2020-11-17 }}</ref>

There remain aspects of the observed universe that are not yet adequately explained by the Big Bang models.  After its initial expansion, the universe cooled sufficiently to allow the formation of [[subatomic particle]]s, and later [[atom]]s. The unequal abundances of matter and [[antimatter]] that allowed this to occur is an unexplained effect known as [[baryon asymmetry]]. These primordial elements—mostly [[hydrogen]], with some [[helium]] and [[lithium]]—later coalesced through [[gravity]], forming early [[star]]s and galaxies. Astronomers observe the gravitational effects of an unknown [[dark matter]] surrounding galaxies. Most of the [[gravitational potential]] in the universe seems to be in this form, and the Big Bang models and various observations indicate that this excess gravitational potential is not created by [[baryonic matter]], such as normal atoms. Measurements of the redshifts of [[type Ia supernova|supernovae]] indicate that the [[Accelerating expansion of the universe|expansion of the universe is accelerating]], an observation attributed to an unexplained phenomenon known as [[dark energy]].<ref name="peebles">{{cite journal|last1=Peebles|first1=P. J. E.|last2=Ratra|first2=Bharat|author-link2=Bharat Ratra|date=22 April 2003|title=The cosmological constant and dark energy|journal=[[Reviews of Modern Physics]]|volume=75|issue=2|pages=559–606|arxiv=astro-ph/0207347|bibcode=2003RvMP...75..559P|doi=10.1103/RevModPhys.75.559|author-link1=Jim Peebles|s2cid=118961123|issn = 0034-6861}}</ref>

== Features of the models ==
The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the [[Chemical element|light element]]s, the [[cosmic microwave background|CMB]], [[Observable universe#Large-scale structure|large-scale structure]], and [[Hubble's law]].<ref name=Wright2009>{{cite web |url=http://www.astro.ucla.edu/~wright/cosmology_faq.html#BBevidence |url-status=live |title=Frequently Asked Questions in Cosmology: What is the evidence for the Big Bang? |last=Wright |first=Edward L. |author-link=Edward L. Wright |date=24 May 2013 |website=Ned Wright's Cosmology Tutorial |publisher=Division of Astronomy & Astrophysics, [[University of California, Los Angeles]] |location=Los Angeles |archive-url=https://web.archive.org/web/20130620105441/http://www.astro.ucla.edu/~wright/cosmology_faq.html |archive-date=20 June 2013 |access-date=25 November 2019}}</ref> The models depend on two major assumptions: the universality of physical laws and the [[cosmological principle]]. The universality of physical laws is one of the underlying principles of the [[theory of relativity]]. The cosmological principle states that on large scales the [[universe]] is [[Homogeneous space|homogeneous]] and [[isotropy|isotropic]]—appearing the same in all directions regardless of location.<ref name=Francis2018>{{cite book
 | title=Light after Dark I: Structures of the Sky
 | first=Charles | last=Francis
 | publisher=Troubador Publishing Ltd
 | date=2018 | isbn=9781785897122 | page=199
 | url=https://books.google.com/books?id=TVhiDAAAQBAJ&pg=PA199 }}</ref>

These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that the largest possible deviation of the [[fine-structure constant]] over much of the age of the universe is of order 10<sup>−5</sup>.<ref>{{cite journal |last1=Ivanchik |first1=Alexandre V. |last2=Potekhin |first2=Alexander Y. |last3=Varshalovich |first3=Dmitry A. |date=March 1999 |title=The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences |journal=[[Astronomy & Astrophysics]] |volume=343 |issue=2 |pages=439–445 |arxiv=astro-ph/9810166 |bibcode=1999A&A...343..439I}}</ref> Also, [[general relativity]] has passed stringent [[tests of general relativity|tests]] on the scale of the [[Solar System]] and [[binary star]]s.<ref>{{cite journal
 | title=Experimental Tests of General Relativity
 | last=Turyshev | first=Slava G.
 | journal=Annual Review of Nuclear and Particle Science
 | volume=58 | issue=1 | pages=207–248 | date=November 2008
 | arxiv=0806.1731 | bibcode=2008ARNPS..58..207T
 | doi=10.1146/annurev.nucl.58.020807.111839
| s2cid=119199160 }}</ref><ref>{{cite journal
 | title=Testing general relativity in cosmology
 | last=Ishak | first=Mustapha
 | journal=Living Reviews in Relativity
 | volume=22 | issue=1 | id=1 | pages=204 | date=December 2019
 | arxiv=1806.10122 | bibcode=2019LRR....22....1I
 | doi=10.1007/s41114-018-0017-4
| pmid=30613193 | pmc=6299071 }}</ref><ref group="notes">Further information of, and references for, tests of general relativity are given in the article [[tests of general relativity]].</ref>

The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler [[Copernican principle]], which states that there is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10<sup>−5</sup> via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound [[on the order of]] 10% inhomogeneity, as of 1995.<ref>{{cite journal |last=Goodman |first=Jeremy |date=15 August 1995 |title=Geocentrism reexamined |url=https://cds.cern.ch/record/283096/files/9506068.pdf |url-status=live |journal=[[Physical Review D]] |volume=52 |issue=4 |pages=1821–1827 |arxiv=astro-ph/9506068 |bibcode=1995PhRvD..52.1821G |doi=10.1103/PhysRevD.52.1821 |pmid=10019408 |s2cid=37979862 |archive-url=https://web.archive.org/web/20190502001358/https://cds.cern.ch/record/283096/files/9506068.pdf |archive-date=2 May 2019 |access-date=2 December 2019}}</ref>

===Horizons===
{{Main|Cosmological horizon}}
An important feature of the Big Bang spacetime is the presence of [[particle horizon]]s. Since the universe has a finite age, and [[light]] travels at a finite speed, there may be events in the past whose light has not yet had time to reach earth. This places a limit or a ''past horizon'' on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a ''future horizon'', which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the [[Friedmann–Lemaître–Robertson–Walker metric|Friedmann–Lemaître–Robertson–Walker (FLRW) metric]] that describes the expansion of the universe.<ref name="kolb_c3"/>

Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well.<ref name="kolb_c3">{{harvnb|Kolb|Turner|1988|loc=chpt. 3}}</ref>

===Thermalization===
Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate [[thermodynamic equilibrium]]. Others were fast enough to reach [[thermalization]]. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the [[Hubble parameter]]. The larger the ratio, the more time particles had to thermalize before they were too far away from each other.<ref>{{cite journal | last1=Enqvist | first1=K. | last2=Sirkka | first2=J. | date=September 1993 | title=Chemical equilibrium in QCD gas in the early universe | journal=Physics Letters B | volume=314 | issue=3–4 | pages=298–302 | doi=10.1016/0370-2693(93)91239-J | arxiv=hep-ph/9304273 | bibcode=1993PhLB..314..298E  | s2cid=119406262 }}</ref>

== Timeline ==
{{Main|Chronology of the universe}}
{{External Timeline|Graphical timeline of the Big Bang|Graphical timeline of the Big Bang}}

According to the Big Bang models, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling.

=== Singularity ===
{{see also|Gravitational singularity|Initial singularity|Planck units#Cosmology}}
In the absence of a [[perfect cosmological principle]], extrapolation of the expansion of the universe backwards in time using general relativity yields an [[Infinity#Physics|infinite]] [[density]] and [[temperature]] at a finite time in the past.<ref name=Hawking_Ellis_1973>{{harvnb|Hawking|Ellis|1973}}</ref> This irregular behavior, known as the [[gravitational singularity]], indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone cannot fully extrapolate toward the singularity.<ref name="books.google.com"/> In some proposals, such as the [[emergent Universe]] models, the singularity is replaced by another cosmological epoch. A different approach identifies the [[initial singularity]] as a [[Gravitational singularity|singularity]] predicted by some models of the Big Bang theory to have existed before the Big Bang event.<ref name="handf-space">{{cite news|url=http://www.space.com/13347-big-bang-origins-universe-birth.html|title=The Big Bang: What Really Happened at Our Universe's Birth?|author=Wall, Mike|date=21 October 2011|work=The History & Future of the Cosmos|accessdate=September 7, 2023|agency=Space.com}}</ref>{{clarify|date=September 2023}}

This primordial singularity is itself sometimes called "the Big Bang",<ref>{{harvnb|Roos|2012|p=216}}: "This singularity is termed the Big Bang."</ref> but the term can also refer to a more generic early hot, dense phase<ref>{{harvnb|Drees|1990|pp=[https://archive.org/details/beyondbigbangqua0000dree/page/223 223–224]}}</ref>{{refn|There is no consensus about how long the Big Bang phase lasted. For some writers, this denotes only the initial singularity, for others the whole history of the universe. Usually, at least the first few minutes (during which helium is synthesized) are said to occur "during the Big Bang".|group="notes"}} of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a [[Equation of state (cosmology)|regime]] where the laws of physics as we understand them (specifically general relativity and the [[Standard Model]] of [[particle physics]]) work. Based on measurements of the expansion using [[Type Ia supernova]]e and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the "[[age of the universe]]"—is 13.8&nbsp;billion years.<ref name="Planck 2015">{{cite journal |author=Planck Collaboration |date=October 2016 |title=''Planck'' 2015 results. XIII. Cosmological parameters |journal=[[Astronomy & Astrophysics]] |volume=594 |page=Article A13 |arxiv=1502.01589 |bibcode=2016A&A...594A..13P |doi=10.1051/0004-6361/201525830 |s2cid=119262962 }} (See Table 4, Age/Gyr, last column.)</ref>

Despite being extremely dense at this time—far denser than is usually required to form a [[black hole]]—the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining [[gravitational collapse]] are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as the Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible [[density gradient]].<ref name=Musser2003>{{cite web | title=Why didn't all this matter immediately collapse into a black hole? | first=George | last=Musser | date=22 September 2003 | publisher=Scientific American | url=https://www.scientificamerican.com/article/according-to-the-big-bang/ | access-date=22 March 2020 }}</ref>

=== Inflation and baryogenesis ===
{{Main|Inflation (cosmology)|Baryogenesis}}
The earliest phases of the Big Bang are subject to much speculation, given the lack of available data. In the most common models the universe was filled homogeneously and isotropically with a very high [[energy density]] and huge temperatures and [[pressure]]s, and was very rapidly expanding and cooling. The period up to 10<sup>−43</sup> seconds into the expansion, the [[Planck epoch]], was a phase in which the four [[fundamental force]]s—the [[electromagnetic force]], the [[strong nuclear force]], the [[Weak-Nuclear Force|weak nuclear force]], and the [[gravitational force]], were unified as one.<ref name=":0">{{Cite book|editor1-last=Unruh |editor1-first=W.G. |editor2-last=Semenoff |editor2-first=G.W.|title=The early universe|date=1988|publisher=Reidel|isbn=90-277-2619-1|oclc=905464231}}</ref> In this stage, the [[Characteristic length|characteristic scale length]] of the universe was the [[Planck length]], {{val|1.6|e=-35|u=m}}, and consequently had a temperature of approximately 10<sup>32</sup> degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of [[quantum gravity]].<ref name=Hawley_Holcomb_2005>{{cite book | title=Foundations of Modern Cosmology | first1=John F. | last1=Hawley | first2=Katherine A. | last2=Holcomb | date=July 7, 2005 | publisher=OUP Oxford | isbn=9780198530961 | page=355 | url=https://books.google.com/books?id=s5MUDAAAQBAJ&pg=PA355 }}</ref><ref>{{Cite web|url=http://www.astro.ucla.edu/~wright/BBhistory.html|title=Brief History of the Universe|website=www.astro.ucla.edu|access-date=2020-04-28}}</ref> The Planck epoch was succeeded by the [[grand unification epoch]] beginning at 10<sup>−43</sup> seconds, where gravitation separated from the other forces as the universe's temperature fell.<ref name=":0" />

At approximately 10<sup>−37</sup> seconds into the expansion, a [[phase transition]] caused a [[Inflation (cosmology)|cosmic inflation]], during which the universe grew [[Exponential growth|exponentially]], unconstrained by the [[speed of light|light speed invariance]], and temperatures dropped by a factor of 100,000. This concept is motivated by the [[flatness problem]], where the [[density of the universe|density of matter and energy]] is very close to the critical density needed to produce a [[flat universe]]. That is, the [[shape of the universe]] has no overall [[Spacetime curvature|geometric curvature]] due to gravitational influence. Microscopic [[quantum fluctuation]]s that occurred because of [[uncertainty principle|Heisenberg's uncertainty principle]] were "frozen in" by inflation, becoming amplified into the seeds that would later form the large-scale structure of the universe.<ref name="Guth1998">{{harvnb|Guth|1998}}</ref> At a time around 10<sup>−36</sup> seconds, the [[electroweak epoch]] begins when the strong nuclear force separates from the other forces, with only the electromagnetic force and weak nuclear force remaining unified.<ref name=":1">{{Cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/planck.html|title=Big Bang models back to Planck time|website=hyperphysics.phy-astr.gsu.edu|access-date=2020-04-28}}</ref>

Inflation stopped locally at around 10<sup>−33</sup> to 10<sup>−32</sup> seconds, with the observable universe's volume having increased by a factor of at least 10<sup>78</sup>. Reheating followed as the [[inflaton field]] decayed, until the universe obtained the temperatures required for the [[Pair production|production]] of a [[quark–gluon plasma]] as well as all other [[elementary particle]]s.<ref>{{cite magazine |last1=Schewe |first1=Phillip F. |last2=Stein |first2=Ben P. |date=20 April 2005 |title=An Ocean of Quarks |url=http://www.aip.org/pnu/2005/split/728-1.html |url-status=dead |magazine=Physics News Update |volume=728 |issue=1 |archive-url=https://web.archive.org/web/20050423224100/http://www.aip.org/pnu/2005/split/728-1.html |archive-date=23 April 2005 |access-date=30 November 2019}}</ref><ref>{{Cite journal|last=Høg|first=Erik|date=2014|title=Astrosociology: Interviews about an infinite universe|journal=Asian Journal of Physics|arxiv=1408.4795|bibcode=2014arXiv1408.4795H}}</ref> Temperatures were so high that the random motions of particles were at [[Special relativity|relativistic]] [[Relativistic speed|speeds]], and [[Antimatter|particle–antiparticle pairs]] of all kinds were being continuously created and destroyed in collisions.<ref name="HTUW" /> At some point, an unknown reaction called [[baryogenesis]] violated the conservation of [[baryon number]], leading to a very small excess of [[quark]]s and [[lepton]]s over antiquarks and antileptons—of the order of one part in 30&nbsp;million. This resulted in the predominance of matter over antimatter in the present universe.<ref name="kolb_c6">{{harvnb|Kolb|Turner|1988|loc=chpt. 6}}</ref>

=== Cooling ===
{{Main|Big Bang nucleosynthesis|Cosmic microwave background}}
[[File:2MASS LSS chart-NEW Nasa.jpg|thumb|right|upright=2.5|Panoramic view of the entire [[Infrared#Regions within the infrared|near-infrared]] sky reveals the distribution of galaxies beyond the [[Milky Way]]. Galaxies are color-coded by [[redshift]].|alt=A map of the universe, with specks and strands of light of different colors.]]
The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. [[Explicit symmetry breaking|Symmetry-breaking]] phase transitions put the [[fundamental force]]s of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10<sup>−12</sup> seconds.<ref name=":1" /><ref name="kolb_c7">{{harvnb|Kolb|Turner|1988|loc=chpt. 7}}</ref>

After about 10<sup>−11</sup> seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in [[particle accelerator]]s. At about 10<sup>−6</sup> seconds, [[quark]]s and [[gluon]]s combined to form [[baryon]]s such as [[proton]]s and [[neutron]]s. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass [[annihilation]] immediately followed, leaving just one in 10<sup>8</sup> of the original matter particles and none of their [[antiparticle]]s.<ref>{{Cite web | last=Weenink | first=Jan | date=February 26, 2009 | title=Baryogenesis | url=https://webspace.science.uu.nl/~proko101/JanGWeenink_bg3.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://webspace.science.uu.nl/~proko101/JanGWeenink_bg3.pdf |archive-date=2022-10-09 |url-status=live | publisher=Tomislav Prokopec }}</ref> A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by [[photon]]s (with a minor contribution from [[neutrino]]s).

A few minutes into the expansion, when the temperature was about a billion [[kelvin]] and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's [[deuterium]] and [[helium]] [[atomic nucleus|nuclei]] in a process called [[Big Bang nucleosynthesis]] (BBN).<ref name="kolb_c4"/> Most protons remained uncombined as hydrogen nuclei.<ref name="peacock_c9"/>

As the universe cooled, the [[rest mass|rest energy]] density of matter came to gravitationally dominate that of the photon [[electromagnetic radiation|radiation]]. The [[Recombination (cosmology)|recombination]] epoch began after about 379,000 years, when the electrons and nuclei combined into [[atom]]s (mostly [[hydrogen]]), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background.<ref name="peacock_c9">{{harvnb|Peacock|1999|loc=chpt. 9}}</ref>

=== Structure formation ===
{{Main|Structure formation}}
[[File:Heic1401a-Abell2744-20140107.jpg|thumb|right|upright=1.5|[[Abell 2744]] [[galaxy cluster]] – [[Hubble Space Telescope#Frontier Fields program|Hubble Frontier Fields view]]<ref name="NASA-20140107">{{cite web |url=https://www.jpl.nasa.gov/news/news.php?release=2014-007 |url-status=live |title=NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies |last1=Clavin |first1=Whitney |last2=Jenkins |first2=Ann |last3=Villard |first3=Ray |date=7 January 2014 |website=[[Jet Propulsion Laboratory]] |publisher=[[NASA]] |location=Washington, D.C. |archive-url=https://web.archive.org/web/20190903225105/https://www.jpl.nasa.gov/news/news.php?release=2014-007 |archive-date=3 September 2019 |access-date=8 January 2014}}</ref>]]
After the recombination epoch, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today.<ref name="HTUW"/> The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as [[cold dark matter]] (CDM), [[warm dark matter]], [[hot dark matter]], and [[Baryon#Baryonic matter|baryonic matter]]. The best measurements available, from the [[Wilkinson Microwave Anisotropy Probe]] (WMAP), show that the data is well-fit by a [[Lambda-CDM model]] in which dark matter is assumed to be cold. (Warm dark matter is ruled out by early [[reionization]].)<ref name="WMAP2003Spergel">{{cite journal |last1=Spergel |first1=David N. |author1-link=David Spergel |last2=Verde |first2=Licia |author2-link=Licia Verde |last3=Peiris |first3=Hiranya V. |author3-link=Hiranya Peiris |last4=Komatsu |first4=E. |last5=Nolta |first5=M. R. |last6=Bennett |first6=C. L. |last7=Halpern |first7=M. |last8=Hinshaw |first8=G. |last9=Jarosik |first9=N. |last10=Kogut |first10=A. |last11=Limon |first11=M. |last12=Meyer |first12=S. S. |last13=Page |first13=L. |last14=Tucker |first14=G. S. |last15=Weiland |first15=J. L. |last16=Wollack |first16=E. |last17=Wright |first17=E. L. |display-authors=3 |date=September 2003 |title=First-Year ''Wilkinson Microwave Anisotropy Probe (WMAP)'' Observations: Determination of Cosmological Parameters |journal=[[The Astrophysical Journal|The Astrophysical Journal Supplement Series]] |volume=148 |issue=1 |pages=175–194 |arxiv=astro-ph/0302209 |bibcode=2003ApJS..148..175S |doi=10.1086/377226 |s2cid=10794058}}</ref> This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%.<ref name="wmap7year">{{cite journal |last1=Jarosik |first1=Norman |author1-link=Norman Jarosik |last2=Bennett |first2=Charles L. |author2-link=Charles L. Bennett |last3=Dunkley |first3=Jo |author3-link=Jo Dunkley |last4=Gold |first4=B. |last5=Greason |first5=M. R. |last6=Halpern |first6=M. |last7=Hill |first7=R. S. |last8=Hinshaw |first8=G. |last9=Kogut |first9=A. |last10=Komatsu |first10=E. |last11=Larson |first11=D. |last12=Limon |first12=M. |last13=Meyer |first13=S. S. |last14=Nolta |first14=M. R. |last15=Odegard |first15=N. |last16=Page |first16=L. |last17=Smith |first17=K. M. |last18=Spergel |first18=D. N. |last19=Tucker |first19=G. S. |last20=Weiland |first20=J. L. |last21=Wollack |first21=E. |last22=Wright |first22=E. L. |display-authors=3 |date=February 2011 |title=Seven-Year ''Wilkinson Microwave Anisotropy Probe (WMAP)'' Observations: Sky Maps, Systematic Errors, and Basic Results |url=https://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/basic_results/wmap_7yr_basic_results.pdf |url-status=live |journal=[[The Astrophysical Journal|The Astrophysical Journal Supplement Series]] |volume=192 |issue=2 |page=Article 14 |bibcode=2011ApJS..192...14J |arxiv=1001.4744 |doi=10.1088/0067-0049/192/2/14 |hdl=2152/43001 |s2cid=46171526 |archive-url=https://web.archive.org/web/20190914181522/https://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/basic_results/wmap_7yr_basic_results.pdf |archive-date=14 September 2019 |access-date=2 December 2019}} (See Table 8.)</ref>

In an "extended model" which includes hot dark matter in the form of neutrinos,<ref name="NYT-20200415">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Why The Big Bang Produced Something Rather Than Nothing – How did matter gain the edge over antimatter in the early universe? Maybe, just maybe, neutrinos. |url=https://www.nytimes.com/2020/04/15/science/physics-neutrino-antimatter-ichikawa-t2k.html |archive-url=https://web.archive.org/web/20200415151520/https://www.nytimes.com/2020/04/15/science/physics-neutrino-antimatter-ichikawa-t2k.html |archive-date=2020-04-15 |url-access=subscription |url-status=live |date=15 April 2020 |work=[[The New York Times]] |access-date=16 April 2020 }}</ref> then the "physical baryon density" <math>\Omega _\text{b} h^2</math> is estimated at 0.023. (This is different from the 'baryon density' <math>\Omega _\text{b}</math> expressed as a fraction of the total matter/energy density, which is about 0.046.) The corresponding cold dark matter density <math>\Omega _\text{c} h^2</math> is about 0.11, and the corresponding neutrino density <math>\Omega _\text{v} h^2</math> is estimated to be less than 0.0062.<ref name="wmap7year" />

=== Cosmic acceleration ===
{{Main|Accelerating expansion of the universe}}
Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as [[dark energy]], which appears to homogeneously permeate all of space. Observations suggest that 73% of the total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together, [[gravity]] predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate.<ref name="peebles" />

Dark energy in its simplest formulation is modeled by a [[cosmological constant]] term in [[Einstein field equations]] of general relativity, but its composition and mechanism are unknown. More generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theory.<ref name="peebles" />

All of this cosmic evolution after the [[inflationary epoch]] can be rigorously described and modeled by the lambda-CDM model of cosmology, which uses the independent frameworks of [[quantum mechanics]] and general relativity. There are no easily testable models that would describe the situation prior to approximately 10<sup>−15</sup> seconds.{{sfn|Manly|2011|loc=chpt. 7: "The Ultimate Free Lunch"{{page needed|date=January 2020}}}} Understanding this earliest of eras in the history of the universe is one of the greatest [[List of unsolved problems in physics|unsolved problems in physics]].

==Concept history==
{{Main|History of the Big Bang theory}}
{{See also|Timeline of cosmological theories}}

===Etymology===
[[English people|English]] [[astronomer]] [[Fred Hoyle]] is credited with coining the term "Big Bang" during a talk for a March 1949 [[BBC Radio]] broadcast,<ref>{{cite news |author=<!--Staff writer(s); no by-line.--> |date=22 August 2001 |title='Big bang' astronomer dies |url=http://news.bbc.co.uk/2/hi/uk_news/1503721.stm |url-status=live |department=Sci/Tech |work=[[BBC News]] |location=London |publisher=[[BBC]] |access-date=2 December 2019 |archive-url=https://web.archive.org/web/20190903152416/http://news.bbc.co.uk/2/hi/uk_news/1503721.stm |archive-date=3 September 2019}}</ref> saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past."<ref>{{cite web |url=https://www.joh.cam.ac.uk/library/special_collections/hoyle/exhibition/radio/ |url-status=live |title=Hoyle on the Radio: Creating the 'Big Bang' |author=|website=Fred Hoyle: An Online Exhibition |location=Cambridge |publisher=[[St John's College, Cambridge|St John's College]] |archive-url=https://web.archive.org/web/20140526084945/https://www.joh.cam.ac.uk/library/special_collections/hoyle/exhibition/radio/ |archive-date=26 May 2014 |access-date=2 December 2019}}</ref><ref name="Kragh2013">{{cite journal |last=Kragh |first=Helge |author-link=Helge Kragh |date=April 2013 |title=Big Bang: the etymology of a name |journal=[[Astronomy & Geophysics]] |volume=54 |issue=2 |pages=2.28–2.30 |doi=10.1093/astrogeo/att035 |bibcode=2013A&G....54b2.28K|doi-access=free }}</ref> However, it did not catch on until the 1970s.<ref name="Kragh2013"/>

It is popularly reported that Hoyle, who favored an alternative "[[Steady-state model|steady-state]]" cosmological model, intended this to be pejorative,<ref>{{cite web |last=Mattson |first=Barbara (Project Leader) |date=8 December 2017 |title=Hoyle Scoffs at 'Big Bang' Universe Theory |url=https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/online_edition/1955/hoyle.html |url-status=live |archive-url=https://web.archive.org/web/20180310172435/https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/online_edition/1955/hoyle.html |archive-date=10 March 2018 |access-date=2 December 2019 |website=Cosmic Times (hosted by Imagine the Universe!) |publisher=[[NASA]]: [[Goddard Space Flight Center#High Energy Astrophysics Science Archive Research Center|High Energy Astrophysics Science Archive Research Center]] |oclc=227004453 |location=Greenbelt, Maryland}}
</ref><ref name="Mathew2013">
{{cite book |last1=Mathew |first1=Santhosh |title=Essays on the Frontiers of Modern Astrophysics and Cosmology |date=2013 |publisher=Springer Science & Business Media |isbn=978-3-319-01887-4 |page=13 |url=https://books.google.com/books?id=1--3BAAAQBAJ&pg=PA13}}
</ref>{{refn|name=Ferris Quote}} but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.<ref name="Croswell1995_chpt9">{{harvnb|Croswell|1995|loc=chapter 9|p=113}}</ref><ref name="Mitton2011_p129">{{harvnb|Mitton|2011|p=[https://books.google.com/books?id=MWKQhXo2eaIC&pg=PA129 129]}}: "To create a picture in the mind of the listener, Hoyle had likened the explosive theory of the universe's origin to a 'big bang'."</ref>{{refn|Hoyle stated:
"I was constantly striving over the radio – where I had no visual aids, nothing except the spoken word – for visual images. And that seemed to be one way of distinguishing between the steady-state and the explosive big bang. And so that was the language I used."<ref name="Kragh2013"/><ref>{{cite book |last1=Kragh |first1=Helge |title=Masters of the Universe: Conversations with Cosmologists of the Past |date=2014 |publisher=Oxford University Press |isbn=978-0-19-103442-8 |page=210n30 |url=https://books.google.com/books?id=ZripBAAAQBAJ&pg=PT210}}</ref>}} [[Helge Kragh]] writes that the evidence for the claim that it was meant as a pejorative is "unconvincing", and mentions a number of indications that it was not a pejorative.<ref name="Kragh2013"/>

The term itself has been argued to be a misnomer because it evokes an explosion.<ref name="Kragh2013"/><ref>{{cite book |last1=Kaler |first1=James B. |title=The Little Book of Stars |date=2013 |publisher=Springer Science & Business Media |isbn=978-0-387-21621-8 |page=3 |url=https://books.google.com/books?id=l8S9BwAAQBAJ&pg=PA4}}</ref> The argument is that whereas an explosion suggests expansion into a surrounding space, the Big Bang only describes the intrinsic expansion of the contents of the universe.<ref>
{{cite book |last1=Emam |first1=Moataz |url=https://books.google.com/books?id=wX4fEAAAQBAJ&pg=PA208 |title=Covariant Physics: From Classical Mechanics to General Relativity and Beyond |date=2021 |publisher=Oxford University Press |isbn=978-0-19-886489-9 |pages=208–246 |quote=The term "Big Bang" is an unfortunate misnomer. It implies an "explosion," and explosions are events that happen ''in'' space. This is incorrect; the term describes the first instant in the expansion ''of'' space itself. Some would even interpret it as the very beginning of the universe, evolving from "nothing." It is hard to imagine exactly what it was, but an explosion it most definitely wasn't.}}
</ref><ref>
{{cite web |last1=Moskowitz |first1=Clara |title=Was the Big Bang Really an Explosion? |url=https://www.livescience.com/32278-was-the-big-bang-really-an-explosion.html |website=Live Science |date=2010}}
</ref> Another issue pointed out by Santhosh Mathew is that bang implies sound, which is not an important feature of the model.<ref name="Mathew2013"/> An attempt to find a more suitable alternative was not successful.<ref name="Kragh2013"/>{{refn|name=Ferris Quote| [[Timothy Ferris]] writes:
"The term "big bang" was coined with derisive intent by Fred Hoyle, and its endurance testifies to Sir Fred's creativity and wit. Indeed, the term survived an international competition in which three judges — the television science reporter Hugh Downs, the astronomer Carl Sagan, and myself — sifted through 13,099 entries from 41 countries and concluded that none was apt enough to replace it. No winner was declared, and like it or not, we are stuck with "big bang.""<ref name="Ferris">
* {{cite book |last1=Ferris |first1=Timothy |title=The Whole Shebang: A State of the Universe Report |date=1998 |publisher=Simon and Schuster |isbn=978-0-684-83861-8 |page=323n10 |url=https://books.google.com/books?id=qjYbQ7EBAKwC&pg=PA323}}
* {{cite book |last1=Gaither |first1=Carl C. |last2=Cavazos-Gaither |first2=Alma E. |title=Gaither's Dictionary of Scientific Quotations |date=2012 |publisher=Springer Science & Business Media |isbn=978-1-4614-1114-7 |edition=2nd |url=https://books.google.com/books?id=zQaCSlEM-OEC&pg=PA272}} (quoting Ferris)</ref>}}

===Development===
{{Multiple image |direction=vertical |align=right |width=400|image1=XDF-scale.jpg|image2=The Hubble eXtreme Deep Field.jpg |image3=XDF-separated.jpg|caption1=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|XDF]]'' size compared to the size of the [[Moon]] (''XDF'' is the small box to the left of, and nearly below, the Moon)  – several thousand galaxies, each consisting of billions of stars, are in this small view. |caption2=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|XDF]]'' (2012) view – each light speck is a galaxy – some of these are as old as 13.2 billion years<ref name="Space-20120925">{{cite web |url=https://www.space.com/17755-farthest-universe-view-hubble-space-telescope.html |url-status=live |last=Moskowitz |first=Clara |date=25 September 2012 |title=Hubble Telescope Reveals Farthest View Into Universe Ever |website=[[Space.com]] |location=New York |publisher=[[Future plc]] |archive-url=https://web.archive.org/web/20191012164808/https://www.space.com/17755-farthest-universe-view-hubble-space-telescope.html |archive-date=12 October 2019 |access-date=3 December 2019}}</ref> – the universe is estimated to contain 200 billion galaxies. |caption3=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|XDF]]'' image shows fully mature galaxies in the foreground plane – nearly mature galaxies from 5 to 9 billion years ago – [[Protogalaxy|protogalaxies]], blazing with [[young star]]s, beyond 9 billion years. |header=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|Hubble eXtreme Deep Field (XDF)]]''}}
The Big Bang models developed from observations of the structure of the universe and from theoretical considerations. In 1912, [[Vesto Slipher]] measured the first [[Doppler effect|Doppler shift]] of a "[[Spiral galaxy#Spiral nebula|spiral nebula]]" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was [[Great Debate (astronomy)|highly controversial]] whether or not these nebulae were "island universes" outside our [[Milky Way]].<ref>{{cite journal |last=Slipher |first=Vesto M. |author-link=Vesto Slipher |year=1913 |title=The Radial Velocity of the Andromeda Nebula |journal=[[Lowell Observatory Bulletin]] |volume=1 |issue=8 |pages=56–57 |bibcode=1913LowOB...2...56S}}</ref><ref>{{cite journal |last=Slipher |first=Vesto M. |author-link=Vesto Slipher |date=January 1915 |title=Spectrographic Observations of Nebulae |journal=[[Popular Astronomy (US magazine)|Popular Astronomy]] |volume=23 |pages=21–24 |bibcode=1915PA.....23...21S}}</ref> Ten years later, [[Alexander Friedmann]], a [[Russia]]n [[physical cosmology|cosmologist]] and [[mathematician]], derived the [[Friedmann equations]] from the Einstein field equations, showing that the universe might be expanding in contrast to the [[static universe]] model advocated by [[Albert Einstein]] at that time.<ref name=af1922>{{cite journal |last=Friedman |first=Alexander |author-link=Alexander Friedmann |date=December 1922 |title=Über die Krümmung des Raumes |journal=[[Zeitschrift für Physik]] |language=de |volume=10 |issue=1 |pages=377–386 |bibcode=1922ZPhy...10..377F |doi=10.1007/BF01332580 |s2cid=125190902 }}
* Translated in: {{cite journal |last=Friedmann |first=Alexander |author-link=Alexander Friedmann |date=December 1999 |title=On the Curvature of Space |journal=[[General Relativity and Gravitation]] |volume=31 |issue=12 |pages=1991–2000 |bibcode=1999GReGr..31.1991F |doi=10.1023/A:1026751225741 |s2cid=122950995 }}</ref>

In 1924, [[Americans|American]] astronomer [[Edwin Hubble]]'s measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed a series of distance indicators, the forerunner of the [[cosmic distance ladder]], using the {{convert|100|in|m|adj=on}} [[100-inch Hooker telescope|Hooker telescope]] at [[Mount Wilson Observatory]]. This allowed him to estimate distances to galaxies whose [[redshift]]s had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and [[recessional velocity]]—now known as Hubble's law.<ref name="hubble">{{cite journal|last=Hubble|first=Edwin|author-link=Edwin Hubble|date=15 March 1929|title=A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae|url=https://apod.nasa.gov/debate/1996/hub_1929.html|url-status=live|journal=[[Proceedings of the National Academy of Sciences]]|volume=15|issue=3|pages=168–173|bibcode=1929PNAS...15..168H|doi=10.1073/pnas.15.3.168|pmc=522427|pmid=16577160|archive-url=https://web.archive.org/web/20061001060258/https://apod.nasa.gov/debate/1996/hub_1929.html|archive-date=1 October 2006|access-date=28 November 2019|doi-access=free}}</ref><ref name="christianson">{{harvnb|Christianson|1995}}</ref>

Independently deriving Friedmann's equations in 1927, [[Georges Lemaître]], a [[Belgium|Belgian]] [[physicist]] and [[Roman Catholic priest]], proposed that the recession of the nebulae was due to the expansion of the universe.<ref name="gl1927">{{cite journal |last=Lemaître |first=Georges |author-link=Georges Lemaître |date=April 1927 |title=Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques |url=https://archive.org/details/B-001-004-204 |journal=Annales de la Société scientifique de Bruxelles |language=fr |volume=47 |pages=49–59 |bibcode=1927ASSB...47...49L }}
* Translated in: {{cite journal |last=Lemaître |first=Georges |author-link=Georges Lemaître |date=March 1931 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=91 |issue=5 |pages=483–490 |title=A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-galactic Nebulæ |bibcode=1931MNRAS..91..483L |doi=10.1093/mnras/91.5.483 |doi-access= free}}</ref> He inferred the relation that Hubble would later observe, given the cosmological principle.<ref name="peebles" /> In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence.<ref>{{cite journal |last=Lemaître |first=Abbé Georges |author-link=Georges Lemaître |date=24 October 1931 |title=Contributions to a British Association Discussion on the Evolution of the Universe |journal=[[Nature (journal)|Nature]] |volume=128 |issue=3234 |pages=704–706 |bibcode=1931Natur.128..704L |doi=10.1038/128704a0 |s2cid=4028196}}</ref>

In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady-state theory.<ref>{{harvnb|Kragh|1996}}</ref> This perception was enhanced by the fact that the originator of the Big Bang concept, Lemaître, was a Roman Catholic priest.<ref name="WGBH1927">{{cite web |author=<!--Staff writer(s); no by-line.--> |year=1998 |title=Big bang theory is introduced – 1927 |url=https://www.pbs.org/wgbh/aso/databank/entries/dp27bi.html |url-status=live |archive-url=https://web.archive.org/web/19990423033457/https://www.pbs.org/wgbh/aso/databank/entries/dp27bi.html |archive-date=23 April 1999 |access-date=31 July 2014 |website=A Science Odyssey |publisher=[[WGBH-TV|WGBH Boston]] |location=Boston, Massachusetts}}</ref> [[Arthur Eddington]] agreed with [[Aristotle]] that the universe did not have a beginning in time, ''viz''., that [[Eternity of the world|matter is eternal]]. A beginning in time was "repugnant" to him.<ref>{{cite journal |last=Eddington |first=Arthur S. |author-link=Arthur Eddington |date=21 March 1931 |title=The End of the World: from the Standpoint of Mathematical Physics |journal=[[Nature (journal)|Nature]] |volume=127 |issue=3203 |pages=447–453 |bibcode=1931Natur.127..447E |doi=10.1038/127447a0 |s2cid=4140648}}</ref><ref>{{cite journal |last=Appolloni |first=Simon |date=17 June 2011 |title='Repugnant', 'Not Repugnant at All': How the Respective Epistemic Attitudes of Georges Lemaitre and Sir Arthur Eddington Influenced How Each Approached the Idea of a Beginning of the Universe |url=https://journal.ibsu.edu.ge/index.php/ibsusj/article/view/180 |journal=IBSU Scientific Journal |volume=5 |issue=1 |pages=19–44}}</ref> Lemaître, however, disagreed:
{{blockquote|text=If the world has begun with a single [[quantum]], the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.<ref>{{cite journal |last=Lemaître |author-link=Georges Lemaître |first=Georges |date=9 May 1931 |title=The Beginning of the World from the Point of View of Quantum Theory |journal=[[Nature (journal)|Nature]] |volume=127 |issue=3210 |page=706 |bibcode=1931Natur.127..706L |doi=10.1038/127706b0 |s2cid=4089233 |issn=0028-0836|doi-access=free }}</ref>}}

During the 1930s, other ideas were proposed as [[Non-standard cosmology|non-standard cosmologies]] to explain Hubble's observations, including the [[Milne model]],<ref>{{harvnb|Milne|1935}}</ref> the [[Cyclic model|oscillatory universe]] (originally suggested by Friedmann, but advocated by Albert Einstein and [[Richard C. Tolman]])<ref>{{harvnb|Tolman|1934}}</ref> and [[Fritz Zwicky]]'s [[tired light]] hypothesis.<ref>{{cite journal |last=Zwicky |first=Fritz |author-link=Fritz Zwicky |date=15 October 1929 |title=On the Red Shift of Spectral Lines through Interstellar Space |journal=Proceedings of the National Academy of Sciences |volume=15 |issue=10 |pages=773–779 |bibcode=1929PNAS...15..773Z |doi=10.1073/pnas.15.10.773 |pmc=522555 |pmid=16577237|doi-access=free }}</ref>

After [[World War II]], two distinct possibilities emerged. One was Fred Hoyle's steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time.<ref>{{cite journal |last=Hoyle |first=Fred |author-link=Fred Hoyle |date=October 1948 |title=A New Model for the Expanding Universe |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=108 |issue=5 |pages=372–382 |bibcode=1948MNRAS.108..372H |doi=10.1093/mnras/108.5.372|doi-access=free }}</ref> The other was Lemaître's Big Bang theory, advocated and developed by [[George Gamow]], who introduced BBN<ref>{{cite journal |last1=Alpher |first1=Ralph A. |author1-link=Ralph Asher Alpher |last2=Bethe |first2=Hans |author2-link=Hans Bethe |last3=Gamow |first3=George |author3-link=George Gamow |date=1 April 1948 |title=The Origin of Chemical Elements |journal=[[Physical Review]] |volume=73 |issue=7 |pages=803–804 |bibcode=1948PhRv...73..803A |doi=10.1103/PhysRev.73.803 |pmid=18877094|doi-access=free }}</ref> and whose associates, [[Ralph Alpher]] and [[Robert Herman]], predicted the CMB.<ref>{{cite journal |last1=Alpher |first1=Ralph A. |author1-link=Ralph Asher Alpher |last2=Herman |first2=Robert |author2-link=Robert Herman |date=13 November 1948 |title=Evolution of the Universe |journal=[[Nature (journal)|Nature]]  |volume=162 |issue=4124 |pages=774–775 |bibcode=1948Natur.162..774A |doi=10.1038/162774b0 |s2cid=4113488}}</ref> Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this ''big bang'' idea" during a BBC Radio broadcast in March 1949.<ref name="Mitton2011_p129" /><ref name="Kragh2013" />{{refn|It is commonly reported that Hoyle intended this to be pejorative. However, Hoyle later denied that, saying that it was just a striking image meant to emphasize the difference between the two theories for radio listeners.<ref name="Croswell1995_chpt9" />|group="notes"}} For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio [[source counts]], began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe.<ref name="penzias">{{cite journal |last1=Penzias |first1=Arno A. |author1-link=Arno Allan Penzias |last2=Wilson |first2=R. W. |author2-link=Robert Woodrow Wilson |date=July 1965 |title=A Measurement of Excess Antenna Temperature at 4080&nbsp;Mc/s |url=https://fermatslibrary.com/s/a-measurement-of-excess-antenna-temperature-at-4080-mc-s |url-status=live |journal=[[The Astrophysical Journal]] |volume=142 |pages=419–421 |bibcode=1965ApJ...142..419P |doi=10.1086/148307 |archive-url=https://web.archive.org/web/20191014185903/https://fermatslibrary.com/s/a-measurement-of-excess-antenna-temperature-at-4080-mc-s |archive-date=14 October 2019 |access-date=5 December 2019|doi-access=free }}</ref>

In 1968 and 1970, [[Roger Penrose]], [[Stephen Hawking]], and [[George F. R. Ellis]] published papers where they showed that [[Singularity (mathematics)|mathematical singularities]] were an inevitable initial condition of relativistic models of the Big Bang.<ref>{{cite journal |last1=Hawking |first1=Stephen W. |author1-link=Stephen Hawking |last2=Ellis |first2=George F. R. |author2-link=George F. R. Ellis |date=April 1968 |title=The Cosmic Black-Body Radiation and the Existence of Singularities in our Universe |journal=[[The Astrophysical Journal]] |volume=152 |page=25 |bibcode=1968ApJ...152...25H |doi=10.1086/149520}}</ref><ref>{{cite journal |last1=Hawking |first1=Stephen W. |author1-link=Stephen Hawking |last2=Penrose |first2=Roger |author2-link=Roger Penrose |date=27 January 1970 |title=The Singularities of Gravitational Collapse and Cosmology |volume=314 |issue=1519 |pages=529–548 |journal=[[Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences]] |bibcode=1970RSPSA.314..529H |doi=10.1098/rspa.1970.0021|s2cid=120208756 |doi-access= }}</ref> Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, [[Alan Guth]] made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang models with the introduction of an epoch of rapid expansion in the early universe he called "inflation".<ref>{{cite journal |last=Guth |first=Alan |author-link=Alan Guth |date=15 January 1981 |title=Inflationary universe: A possible solution to the horizon and flatness problems |journal=[[Physical Review D]] |volume=23 |issue=2 |pages=347–356 |bibcode=1981PhRvD..23..347G |doi=10.1103/PhysRevD.23.347 |doi-access=free }}</ref> Meanwhile, during these decades, two questions in [[observational cosmology]] that generated much discussion and disagreement were over the precise values of the Hubble Constant<ref>{{cite journal |url=https://www.cfa.harvard.edu/~dfabricant/huchra/hubble/ |title=The Hubble Constant |last1=Huchra |first1=John P. |volume=256 |issue=5055 |pages=321–5 |author-link=John Huchra |year=2008 |journal=Science |archive-url=https://web.archive.org/web/20190930124013/https://www.cfa.harvard.edu/~dfabricant/huchra/hubble/ |archive-date=30 September 2019 |access-date=5 December 2019|pmid=17743107 |doi=10.1126/science.256.5055.321 |s2cid=206574821 }}</ref> and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual [[Ultimate fate of the universe|fate of the universe]]).<ref>{{harvnb|Livio|2000|p=160}}</ref>

In the mid-1990s, observations of certain [[globular cluster]]s appeared to indicate that they were about 15&nbsp;billion years old, which [[cosmic age problem|conflicted]] with most then-current estimates of the age of the universe (and indeed with the age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to [[stellar wind]]s, indicated a much younger age for globular clusters.<ref>{{cite journal |last1=Navabi |first1=Ali Akbar |last2=Riazi |first2=Nematollah |date=March 2003 |title=Is the Age Problem Resolved? |journal=[[Journal of Astrophysics and Astronomy]] |volume=24 |issue=1–2 |pages=3–10 |bibcode=2003JApA...24....3N |doi=10.1007/BF03012187 |s2cid=123471347}}</ref>

Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in [[telescope]] technology as well as the analysis of data from satellites such as the [[Cosmic Background Explorer]] (COBE),<ref name=cobe>{{cite journal |last1=Boggess |first1=Nancy W. |author2-last=Mather |author2-first=John C. |author2-link=John C. Mather |author3-last=Weiss |author3-first=Rainer |author3-link=Rainer Weiss |last4=Bennett |first4=C. L. |last5=Cheng |first5=E. S. |last6=Dwek |first6=E. |last7=Gulkis |first7=S. |last8=Hauser |first8=M. G. |last9=Janssen |first9=M. A. |last10=Kelsall |first10=T. |last11=Meyer |first11=S. S. |last12=Moseley |first12=S. H. |last13=Murdock |first13=T. L. |last14=Shafer |first14=R. A. |last15=Silverberg |first15=R. F. |last16=Smoot |first16=G. F. |last17=Wilkinson |first17=D. T. |last18=Wright |first18=E. L. |display-authors=3 |date=1 October 1992 |title=The COBE Mission: Its Design and Performance Two Years after the launch |journal=[[The Astrophysical Journal]] |volume=397 |pages=420–429 |bibcode=1992ApJ...397..420B |doi=10.1086/171797|doi-access=free }}</ref> the [[Hubble Space Telescope]] and WMAP.<ref name="wmap1year">{{cite journal |last1=Spergel |first1=David N. |author1-link=David Spergel |last2=Bean |first2=Rachel |author2-link=Rachel Bean |last3=Doré |first3=Olivier |author-link3=Olivier Doré |display-authors=etal |date=June 2007 |title=Three-Year ''Wilkinson Microwave Anisotropy Probe (WMAP)'' Observations: Implications for Cosmology |journal=[[The Astrophysical Journal Supplement Series]] |volume=170 |issue=2 |pages=377–408 |arxiv=astro-ph/0603449 |bibcode=2007ApJS..170..377S |doi=10.1086/513700 |s2cid=1386346}}</ref> Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.<ref>{{cite journal |last1=Reiss |first1=Adam G. |last2=Filippenko |first2=Alexei V. |last3=Challis |first3=Peter |last4=Clocchiatti |first4=Alejandro |last5=Diercks |first5=Alan |last6=Garnavich |first6=Peter M. |last7=Gilliland |first7=Ron L. |last8=Hogan |first8=Craig J. |last9=Jha |first9=Saurabh |last10=Kirshner |first10=Robert P. |last11=Leibundgut |first11=B. |last12=Phillips |first12=M. M. |last13=Reiss |first13=David |last14=Schmidt |first14=Brian P. |last15=Schommer |first15=Robert A. |last16=Smith |first16=R. Chris |last17=Spyromilio |first17=J. |last18=Stubbs |first18=Christopher |last19=Suntzeff |first19=Nicholas B. |last20=Tonry |first20=John |title=Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant |date=1998 |journal=[[The Astronomical Journal]] |volume=116 |issue=3 |pages=1009–1038 |doi=10.1086/300499 |arxiv=astro-ph/9805201|bibcode=1998AJ....116.1009R |s2cid=15640044 }}</ref><ref>{{cite journal |last1=Perlmutter |first1=S. |last2=Aldering |first2=G. |last3=Goldhaber |first3=G. |last4=Knop |first4=R.A. |last5=Nugent |first5=P. |last6=Castro |first6=P.G. |last7=Deustua |first7=S. |last8=Fabbro |first8=S. |last9=Goobar |first9=A. |last10=Groom |first10=D.E. |last11=Hook |first11=I.M. |last12=Kim |first12=A.G. |last13=Kim |first13=M.Y. |last14=Lee |first14=J.C. |last15=Nunes |first15=N.J. |last16=Pain |first16=R. |last17=Pennypacker |first17=C.R. |last18=Quimby |first18=R. |last19=Lidman |first19=C. |last20=Ellis |first20=R.S. |last21=Irwin |first21=M. |last22=McMahon |first22=R.G.  |last23=Ruiz-Lapuente |first23=P.  |last24=Walton  |first24=N.  |last25=Schaefer  |first25=B. |last26=Boyle |first26=B.J. |last27=Filippenko |first27=A.V. |last28=Matheson |first28=T. |last29=Fruchter |first29=A.S. |last30=Panagia |first30=N. |last31=Newberg |first31=H.J.M. |last32=Couch |first32=W.J.|date=1999 |title=Measurements of Omega and Lambda from 42 High-Redshift Supernovae |volume=517 |issue=2 |journal=[[The Astrophysical Journal]] |pages=565–586 |arxiv=astro-ph/9812133 |doi=10.1086/307221|bibcode=1999ApJ...517..565P |s2cid=118910636 }}</ref>

==Observational evidence==
{{Quote box
 |quote="[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its general features."
 |source=— [[Lawrence Krauss]]<ref>{{harvnb|Krauss|2012|p=[https://archive.org/details/universefromnoth0000krau/page/118 118]}}</ref>
 |width=27%
 |align=right
 |style=padding:8px;
}}

The earliest and most direct observational evidence of the validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by [[Big Bang nucleosynthesis]] (BBN). More recent evidence includes observations of [[galaxy formation and evolution]], and the distribution of [[large-scale structure of the cosmos|large-scale cosmic structures]].<ref>{{cite journal |last1=Gladders |first1=Michael D. |last2=Yee |first2=H. K. C. |last3=Majumdar |first3=Subhabrata |last4=Barrientos |first4=L. Felipe |last5=Hoekstra |first5=Henk |last6=Hall |first6=Patrick B. |last7=Infante |first7=Leopoldo |display-authors=3 |date=20 January 2007 |title=Cosmological Constraints from the Red-Sequence Cluster Survey |journal=[[The Astrophysical Journal]] |volume=655 |issue=1 |pages=128–134 |arxiv=astro-ph/0603588 |bibcode=2007ApJ...655..128G |doi=10.1086/509909 |s2cid=10855653}}</ref> These are sometimes called the "four pillars" of the Big Bang models.<ref>{{cite web |url=http://www.ctc.cam.ac.uk/outreach/origins/big_bang_four.php |url-status=live |title=The Four Pillars of the Standard Cosmology |editor-last=Shellard |editor-first=Paul |display-editors=etal |year=2012 |website=Outreach |publisher=[[Centre for Theoretical Cosmology]]; [[University of Cambridge]] |location=Cambridge, UK |archive-url=https://web.archive.org/web/20131102133646/http://www.ctc.cam.ac.uk/outreach/origins/big_bang_four.php |archive-date=2 November 2013 |access-date=6 December 2019}}
* From retired website: {{cite web |url=http://www.damtp.cam.ac.uk/user/gr/public/bb_pillars.html |url-status=dead |title=The Four Pillars of the Standard Cosmology |editor-last=Shellard |editor-first=Paul |display-editors=etal |year=2006 |website=Cambridge Relativity and Cosmology |publisher=University of Cambridge |location=Cambridge, UK |archive-url=https://web.archive.org/web/19980128054235/http://www.damtp.cam.ac.uk/user/gr/public/bb_pillars.html |archive-date=28 January 1998 |access-date=6 December 2019}}</ref>

Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, [[dark matter]] is currently the subject of most active laboratory investigations.<ref>{{cite web |url=https://www8.nationalacademies.org/astro2010/DetailFileDisplay.aspx?id=225 |url-status=live |title=Direct Searches for Dark Matter |last=Sadoulet |first=Bernard |author-link=Bernard Sadoulet |display-authors=etal |work=[[Astronomy and Astrophysics Decadal Survey|Astro2010: The Astronomy and Astrophysics Decadal Survey]] |publisher=[[National Academies Press]] on behalf of the [[National Academies of Sciences, Engineering, and Medicine#Program units|National Research Council]] of the [[National Academy of Sciences]] |location=Washington, D.C. |type=white paper |format=PDF |oclc=850950122 |archive-url=https://web.archive.org/web/20090413141208/https://www8.nationalacademies.org/astro2010/DetailFileDisplay.aspx?id=225 |archive-date=13 April 2009 |access-date=8 December 2019}}</ref> Remaining issues include the [[cuspy halo problem]]<ref name="Diemand2005" /> and the [[dwarf galaxy problem]]<ref name="Martínez-Delgado" /> of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible.<ref>{{cite journal |url=https://www8.nationalacademies.org/astro2010/DetailFileDisplay.aspx?id=243 |url-status=live |title=Whitepaper: For a Comprehensive Space-Based Dark Energy Mission |last=Cahn |first=Robert N. |volume=2010 |pages=35 |display-authors=etal |year=2009 |journal=[[Astronomy and Astrophysics Decadal Survey|Astro2010: The Astronomy and Astrophysics Decadal Survey, Science White Papers, no. 35]] |publisher=[[National Academies Press]] on behalf of the [[National Academies of Sciences, Engineering, and Medicine#Program units|National Research Council]] of the [[National Academy of Sciences]] |location=Washington, D.C. |type=white paper |format=PDF |oclc=850950122 |archive-url=https://web.archive.org/web/20110807103919/http://www8.nationalacademies.org/astro2010/DetailFileDisplay.aspx?id=243 |archive-date=7 August 2011 |access-date=8 December 2019|bibcode=2009astro2010S..35B}}</ref> Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are unsolved problems in physics.

{{anchor|Hubble's law expansion}}<!-- previous header name, so as not to disturb hashlinks if any -->

===Hubble's law and the expansion of the universe===
{{Main|Hubble's law|Expansion of the universe}}
{{See also|Distance measures (cosmology)|Scale factor (cosmology)}}
[[File:Redshifted.png|right|thumb|Redshift of absorption lines due to recessional velocity]]
Observations of distant galaxies and [[quasar]]s show that these objects are redshifted: the light emitted from them has been shifted to longer wavelengths. This can be seen by taking a [[Spectral density|frequency spectrum]] of an object and matching the [[spectroscopy|spectroscopic]] pattern of [[Spectral line|emission or absorption line]]s corresponding to atoms of the chemical elements interacting with the light. These redshifts are [[Homogeneity (physics)|uniformly]] isotropic, distributed evenly among the observed objects in all directions. If the redshift is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the [[cosmic distance ladder]]. When the recessional velocities are plotted against these distances, a linear relationship known as [[Hubble's law]] is observed:<ref name="hubble" />
<math>v = H_0D</math>
where
* <math>v</math> is the recessional velocity of the galaxy or other distant object,
* <math>D</math> is the [[proper length|proper distance]] to the object, and
* <math>H_0</math> is [[Hubble's law|Hubble's constant]], measured to be {{val|70.4|+1.3|-1.4}} [[kilometres|km]]/[[second|s]]/[[Megaparsec|Mpc]] by the WMAP.<ref name="wmap7year" />

Hubble's law implies that the universe is uniformly expanding everywhere. This cosmic expansion was predicted from general relativity by Friedmann in 1922<ref name=af1922 /> and Lemaître in 1927,<ref name=gl1927 /> well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker.

The theory requires the relation <math>v = HD</math> to hold at all times, where <math>D</math> is the proper distance, <math>v</math> is the recessional velocity, and <math>v</math>, <math>H</math>, and <math>D</math> vary as the universe expands (hence we write <math>H_0</math> to denote the present-day Hubble "constant"). For distances much smaller than the size of the [[observable universe]], the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity <math>v</math>. For distances comparable to the size of the observable universe, the attribution of the cosmological redshift becomes more ambiguous, although its interpretation as a kinematic Doppler shift remains the most natural one.<ref name="Hogg">{{cite journal |author=Bunn |first1=E. F. |last2=Hogg |first2=D. W. |year=2009 |title=The kinematic origin of the cosmological redshift |journal=American Journal of Physics |volume=77 |issue=8 |pages=688–694 |arxiv=0808.1081 |bibcode=2009AmJPh..77..688B |doi=10.1119/1.3129103 |s2cid=1365918}}</ref>

An unexplained discrepancy with the determination of the Hubble constant is known as [[Hubble tension]]. Techniques based on observation of the CMB suggest a lower value of this constant compared to the quantity derived from measurements based on the cosmic distance ladder.<ref name="di Valentino 2021 153001">{{cite journal | last1=Di Valentino | first1=Eleonora | last2=Mena | first2=Olga | last3=Pan | first3=Supriya | last4=Visinelli | first4=Luca | last5=Yang | first5=Weiqiang | last6=Melchiorri | first6=Alessandro | last7=Mota | first7=David F. | last8=Riess | first8=Adam G. | last9=Silk | first9=Joseph | year=2021 | title=In the realm of the Hubble tension—a review of solutions | journal=Classical and Quantum Gravity | volume=38 | issue=15 | page=153001 | doi=10.1088/1361-6382/ac086d | arxiv=2103.01183|bibcode=2021CQGra..38o3001D | s2cid=232092525 }}</ref>

===Cosmic microwave background radiation===
{{Main|Cosmic microwave background}}
[[File:Cmbr.svg|thumb|left|The [[cosmic microwave background]] spectrum measured by the FIRAS instrument on the [[Cosmic Background Explorer|COBE]] satellite is the most-precisely measured [[Black body|blackbody]] spectrum in nature.<ref name="dpf99">{{cite conference |url=http://www.dpf99.library.ucla.edu/session9/white0910.pdf |url-status=live |title=Anisotropies in the CMB |last=White |first=Martin |year=1999 |conference=Division of Particles and Fields Conference 1999 (DPF '99) |conference-url=http://home.physics.ucla.edu/calendar/conferences/dpf99/ |editor1-last=Arisaka |editor1-first=Katsushi |editor2-last=Bern |editor2-first=Zvi |editor2-link=Zvi Bern |book-title=DPF 99: Proceedings of the Los Angeles Meeting |publisher=[[University of California, Los Angeles]] on behalf of the [[American Physical Society]] |archive-url=https://web.archive.org/web/20170204083018/http://www.dpf99.library.ucla.edu/session9/white0910.pdf |archive-date=4 February 2017 |location=Los Angeles |id=Talk #9–10: The Cosmic Microwave Background |arxiv=astro-ph/9903232 |bibcode=1999dpf..conf.....W |oclc=43669022 |access-date=9 December 2019}}</ref> The [[data point]]s and [[standard error of estimation|error bars]] on this graph are obscured by the theoretical curve.]]

In 1964, [[Arno Allan Penzias|Arno Penzias]] and [[Robert Woodrow Wilson|Robert Wilson]] serendipitously discovered the cosmic background radiation, an omnidirectional signal in the [[microwave]] band.<ref name="penzias" /> Their discovery provided substantial confirmation of the big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a [[Black body|blackbody]] spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725&nbsp;K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded the 1978 [[Nobel Prize in Physics]].

The ''surface of last scattering'' corresponding to emission of the CMB occurs shortly after ''[[Recombination (cosmology)|recombination]]'', the epoch when neutral hydrogen becomes stable. Prior to this, the universe comprised a hot dense photon-baryon plasma sea where photons were quickly [[Thomson scattering|scattered]] from free charged particles. Peaking at around {{val|372|14|u=kyr}},<ref name="WMAP2003Spergel" /> the mean free path for a photon becomes long enough to reach the present day and the universe becomes transparent.

[[File:WMAP 2012.png|thumb|upright=1.25|right|9 year WMAP image of the cosmic microwave background radiation (2012).<ref name="arXiv-20121220">{{cite journal |last1=Bennett |first1=Charles L. |author1-link=Charles L. Bennett |last2=Larson |first2=Davin |last3=Weiland |first3=Janet L. |date=October 2013 |title=Nine-Year ''Wilkinson Microwave Anisotropy Probe (WMAP)'' Observations: Final Maps and Results |arxiv=1212.5225 |display-authors=etal |doi=10.1088/0067-0049/208/2/20 |volume=208 |issue=2 |page=Article 20 |journal=[[The Astrophysical Journal|The Astrophysical Journal Supplement Series]] |bibcode=2013ApJS..208...20B |s2cid=119271232}}</ref><ref name="Space-20121221">{{cite web |url=https://www.space.com/19027-universe-baby-picture-wmap.html |url-status=live |title=New 'Baby Picture' of Universe Unveiled |last=Gannon |first=Megan |date=21 December 2012 |website=[[Space.com]] |location=New York |publisher=[[Future plc]] |archive-url=https://web.archive.org/web/20191029114309/https://www.space.com/19027-universe-baby-picture-wmap.html |archive-date=29 October 2019 |access-date=9 December 2019}}</ref> The radiation is [[Isotropy|isotropic]] to roughly one part in 100,000.<ref>{{harvnb|Wright|2004|p=291}}</ref>]]

In 1989, [[NASA]] launched COBE, which made two major advances: in 1990, high-precision spectrum measurements showed that the CMB frequency spectrum is an almost perfect blackbody with no deviations at a level of 1 part in 10<sup>4</sup>, and measured a residual temperature of 2.726&nbsp;K (more recent measurements have revised this figure down slightly to 2.7255&nbsp;K); then in 1992, further COBE measurements discovered tiny fluctuations ([[Anisotropy|anisotrop]]ies) in the CMB temperature across the sky, at a level of about one part in 10<sup>5</sup>.<ref name="cobe" /> [[John C. Mather]] and [[George Smoot]] were awarded the 2006 Nobel Prize in Physics for their leadership in these results.

During the following decade, CMB anisotropies were further investigated by a large number of ground-based and balloon experiments. In 2000–2001, several experiments, most notably [[BOOMERanG experiment|BOOMERanG]], found the [[shape of the universe]] to be spatially almost flat by measuring the typical angular size (the size on the sky) of the anisotropies.<ref>{{cite journal |last1=Melchiorri |first1=Alessandro |last2=Ade |first2=Peter A.R. |last3=de Bernardis |first3=Paolo |display-authors=etal |date=20 June 2000 |title=A Measurement of Ω from the North American Test Flight of Boomerang |journal=[[The Astrophysical Journal|The Astrophysical Journal Letters]] |volume=536 |issue=2 |pages=L63–L66 |arxiv=astro-ph/9911445 |bibcode=2000ApJ...536L..63M |doi=10.1086/312744 |pmid=10859119|s2cid=27518923 }}</ref><ref>{{cite journal |last1=de Bernardis |first1=Paolo |last2=Ade |first2=Peter A.R. |last3=Bock |first3=James J. |display-authors=etal |date=27 April 2000 |title=A Flat Universe from High-Resolution Maps of the Cosmic Microwave Background Radiation |url=https://spiral.imperial.ac.uk/bitstream/10044/1/60851/2/0004404v1.pdf |url-status=live |journal=[[Nature (journal)|Nature]] |volume=404 |issue=6781 |pages=955–959 |arxiv=astro-ph/0004404 |bibcode=2000Natur.404..955D |doi=10.1038/35010035 |pmid=10801117 |hdl=10044/1/60851 |s2cid=4412370 |archive-url=https://web.archive.org/web/20190502001358/https://spiral.imperial.ac.uk/bitstream/10044/1/60851/2/0004404v1.pdf |archive-date=2 May 2019 |access-date=10 December 2019}}</ref><ref>{{cite journal |last1=Miller |first1=Andre D. |last2=Caldwell |first2=Robert H. |last3=Devlin |first3=Mark Joseph |display-authors=etal |date=10 October 1999 |title=A Measurement of the Angular Power Spectrum of the Cosmic Microwave Background from l = 100 to 400 |journal=[[The Astrophysical Journal|The Astrophysical Journal Letters]] |volume=524 |issue=1 |pages=L1–L4 |arxiv=astro-ph/9906421 |bibcode=1999ApJ...524L...1M |doi=10.1086/312293 |s2cid=1924091 }}</ref>

In early 2003, the first results of the Wilkinson Microwave Anisotropy Probe were released, yielding what were at the time the most accurate values for some of the cosmological parameters. The results disproved several specific cosmic inflation models, but are consistent with the inflation theory in general.<ref name="wmap1year" /> The ''[[Planck (spacecraft)|Planck]]'' space probe was launched in May 2009. Other ground and balloon-based [[List of cosmic microwave background experiments|cosmic microwave background experiments]] are ongoing.

===Abundance of primordial elements===
{{Main|Big Bang nucleosynthesis}}
[[File:Universe-09-00183-g004.png|right|thumb|upright=1.6|Time evolution of light element abundances during Big Bang nucleosynthesis]]
Using Big Bang models, it is possible to calculate the expected concentration of the isotopes [[helium-4]] (<sup>4</sup>He), [[helium-3]] (<sup>3</sup>He), deuterium (<sup>2</sup>H), and [[Isotopes of lithium#Lithium-7|lithium-7]] (<sup>7</sup>Li) in the universe as ratios to the amount of ordinary hydrogen.<ref name="kolb_c4"/> The relative abundances depend on a single parameter, the ratio of photons to baryons. This value can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by abundance) are about 0.25 for <sup>4</sup>He:H, about 10<sup>−3</sup> for <sup>2</sup>H:H, about 10<sup>−4</sup> for <sup>3</sup>He:H, and about 10<sup>−9</sup> for <sup>7</sup>Li:H.<ref name="kolb_c4">{{harvnb|Kolb|Turner|1988|loc=chpt. 4}}</ref>

The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for <sup>4</sup>He, and off by a factor of two for <sup>7</sup>Li (this anomaly is known as the [[cosmological lithium problem]]); in the latter two cases, there are substantial [[Observational error#Random errors versus systematic errors|systematic uncertainties]]. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium.<ref>{{cite journal |last=Steigman |first=Gary |author-link=Gary Steigman |date=February 2006 |title=Primordial Nucleosynthesis: Successes And Challenges |journal=[[International Journal of Modern Physics E]] |volume=15 |issue=1 |pages=1–36 |arxiv=astro-ph/0511534 |bibcode=2006IJMPE..15....1S |doi=10.1142/S0218301306004028 |citeseerx=10.1.1.337.542 |s2cid=12188807}}</ref> Indeed, there is no obvious reason outside of the Big Bang that, for example, the young universe before [[star formation]], as determined by studying matter supposedly free of [[stellar nucleosynthesis]] products, should have more helium than deuterium or more deuterium than <sup>3</sup>He, and in constant ratios, too.<ref name="Ryden2003"/>{{rp|182–185}}

===Galactic evolution and distribution===
{{Main|Galaxy formation and evolution|Structure formation}}
Detailed observations of the [[Galaxy morphological classification|morphology]] and distribution of galaxies and [[quasar]]s are in agreement with the current Big Bang models. A combination of observations and theory suggest that the first quasars and galaxies formed within a billion years after the Big Bang,<ref>{{cite web | title=Astronomers Grapple with JWST's Discovery of Early Galaxies | first=Jonathan | last=O'Callaghan | date=December 6, 2022 | publisher=Scientific American | url=https://www.scientificamerican.com/article/astronomers-grapple-with-jwsts-discovery-of-early-galaxies1/ | access-date=2023-02-13 }}</ref> and since then, larger structures have been forming, such as [[galaxy cluster]]s and [[supercluster]]s.<ref name="Bertschinger"/>

Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model. Observations of star formation, galaxy and quasar distributions and larger structures, agree well with Big Bang simulations of the formation of structure in the universe, and are helping to complete details of the theory.<ref name="Bertschinger">{{cite arXiv |last=Bertschinger |first=Edmund |author-link=Edmund Bertschinger |title=Cosmological Perturbation Theory and Structure Formation |eprint=astro-ph/0101009|date=2000}}</ref><ref>{{cite journal |last=Bertschinger |first=Edmund |author-link=Edmund Bertschinger |date=September 1998 |title=Simulations of Structure Formation in the Universe |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=36 |issue=1 |pages=599–654 |bibcode=1998ARA&A..36..599B |doi=10.1146/annurev.astro.36.1.599 |s2cid=29015610|url=http://pdfs.semanticscholar.org/ffc4/1045e433c10454ba32e811d25eafd3ac324f.pdf |archive-url=https://web.archive.org/web/20190309060807/http://pdfs.semanticscholar.org/ffc4/1045e433c10454ba32e811d25eafd3ac324f.pdf |url-status=dead |archive-date=2019-03-09 }}</ref>

=== Primordial gas clouds ===
[[File:PIA17993-DetectorsForInfantUniverseStudies-20140317.jpg|thumb|right|[[Focal plane]] of [[BICEP and Keck Array|BICEP2 telescope]] under a microscope – used to search for polarization in the CMB<ref name="BICEP2-2014">{{cite web |author=<!--Not stated--> |date=16 December 2014 |orig-date=Results originally released on 17 March 2014 |title=BICEP2 March 2014 Results and Data Products |url=http://bicepkeck.org/bicep2_2014_release.html |url-status=live |archive-url=https://web.archive.org/web/20140318190423/http://bicepkeck.org/ |archive-date=18 March 2014 |access-date=10 December 2019 |website=The BICEP and Keck Array CMB Experiments |publisher=[[Harvard Faculty of Arts and Sciences|FAS Research Computing]], [[Harvard University]] |location=Cambridge, Massachusetts}}</ref><ref name="NASA-20140317">{{cite web |url=https://www.jpl.nasa.gov/news/news.php?release=2014-082 |url-status=live |title=NASA Technology Views Birth of the Universe |last=Clavin |first=Whitney |date=17 March 2014 |website=[[Jet Propulsion Laboratory]] |publisher=[[NASA]] |location=Washington, D.C. |archive-url=https://web.archive.org/web/20191010183450/https://www.jpl.nasa.gov/news/news.php?release=2014-082 |archive-date=10 October 2019 |access-date=10 December 2019}}</ref><ref name="NYT-20140317">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=17 March 2014 |title=Space Ripples Reveal Big Bang's Smoking Gun |url=https://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-space-buttresses-landmark-theory-of-big-bang.html |url-status=live |url-access=registration |department=Space & Cosmos |newspaper=[[The New York Times]] |location=New York |issn=0362-4331 |archive-url=https://web.archive.org/web/20140317154023/https://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-space-buttresses-landmark-theory-of-big-bang.html |archive-date=17 March 2014 |access-date=11 December 2019}} "A version of this article appears in print on March 18, 2014, Section A, Page 1 of the New York edition with the headline: Space Ripples Reveal Big Bang's Smoking Gun." The online version of this article was originally titled "Detection of Waves in Space Buttresses Landmark Theory of Big Bang".</ref><ref name="NYT-20140324">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=24 March 2014 |title=Ripples From the Big Bang |url=https://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html |url-status=live |url-access=registration |department=Out There |newspaper=[[The New York Times]] |location=New York |issn=0362-4331 |archive-url=https://web.archive.org/web/20140325015901/https://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html |archive-date=25 March 2014 |access-date=24 March 2014}} "A version of this article appears in print on March 25, 2014, Section D, Page 1 of the New York edition with the headline: Ripples From the Big Bang."</ref>]]
In 2011, astronomers found what they believe to be pristine clouds of primordial gas by analyzing absorption lines in the spectra of distant quasars. Before this discovery, all other astronomical objects have been observed to contain heavy elements that are formed in stars. Despite being sensitive to carbon, oxygen, and silicon, these three elements were not detected in these two clouds.<ref>{{cite journal |last1=Fumagalli |first1=Michele |last2=O'Meara |first2=John M. |last3=Prochaska |first3=J. Xavier |date=2 December 2011 |title=Detection of Pristine Gas Two Billion Years After the Big Bang |journal=[[Science (journal)|Science]] |volume=334 |issue=6060 |pages=1245–1249 |arxiv=1111.2334 |bibcode=2011Sci...334.1245F |doi=10.1126/science.1213581 |pmid=22075722 |s2cid=2434386}}</ref><ref>{{cite news |url=https://news.ucsc.edu/2011/11/pristine-gas.html |title=Astronomers find clouds of primordial gas from the early universe |last=Stephens |first=Tim |date=10 November 2011 |newspaper=Uc Santa Cruz News |publisher=[[University of California, Santa Cruz]] |location=Santa Cruz, CA |archive-url=https://web.archive.org/web/20111114140012/https://news.ucsc.edu/2011/11/pristine-gas.html |archive-date=14 November 2011 |access-date=11 December 2019}}</ref> Since the clouds of gas have no detectable levels of heavy elements, they likely formed in the first few minutes after the Big Bang, during BBN.

===Other lines of evidence===
The age of the universe as estimated from the Hubble expansion and the CMB is now in agreement with other estimates using the ages of the oldest stars, both as measured by applying the theory of [[stellar evolution]] to globular clusters and through [[radiometric dating]] of individual [[Stellar population#Population II stars|Population II]] stars.<ref>{{cite web |last=Perley |first=Daniel |date=21 February 2005 |title=Determination of the Universe's Age, t<sub>o</sub> |url=https://astro.berkeley.edu/~dperley/univage/univage.html |url-status=dead |archive-url=https://web.archive.org/web/20060911000604/https://astro.berkeley.edu/~dperley/univage/univage.html |archive-date=11 September 2006 |access-date=11 December 2019 |publisher=Department of Astronomy, [[University of California, Berkeley]] |language=en-us |location=Berkeley, California}}</ref> It is also in agreement with age estimates based on measurements of the expansion using [[Type Ia supernova]]e and measurements of temperature fluctuations in the cosmic microwave background.<ref name="Planck 2015"/> The agreement of independent measurements of this age supports the [[Lambda-CDM model|Lambda-CDM]] (ΛCDM) model, since the model is used to relate some of the measurements to an age estimate, and all estimates turn agree. Still, some observations of objects from the relatively early universe (in particular quasar [[APM 08279+5255]]) raise concern as to whether these objects had enough time to form so early in the ΛCDM model.<ref>Yang, R. J., & Zhang, S. N. (2010). The age problem in the ΛCDM model. Monthly Notices of the Royal Astronomical Society, 407(3), pp. 1835–1841.</ref><ref>Yu, H., & Wang, F. Y. (2014). Reconciling the cosmic age problem in the $$ R_\mathrm {h}= ct $$ universe. The European Physical Journal C, 74(10), 3090.</ref>

The prediction that the CMB temperature was higher in the past has been experimentally supported by observations of very low temperature absorption lines in gas clouds at high redshift.<ref>{{cite journal |last1=Srianand |first1=Raghunathan |author1-link=Raghunathan Srianand |last2=Noterdaeme |first2=Pasquier |last3=Ledoux |first3=Cédric |last4=Petitjean |first4=Patrick |display-authors=3 |date=May 2008 |title=First detection of CO in a high-redshift damped Lyman-α system |journal=[[Astronomy & Astrophysics]] |volume=482 |issue=3 |pages=L39–L42 |bibcode=2008A&A...482L..39S |doi=10.1051/0004-6361:200809727|arxiv=0804.0116 |doi-access=free }}</ref> This prediction also implies that the amplitude of the [[Sunyaev–Zeldovich effect|Sunyaev–Zel'dovich effect]] in clusters of galaxies does not depend directly on redshift. Observations have found this to be roughly true, but this effect depends on cluster properties that do change with cosmic time, making precise measurements difficult.<ref>{{cite journal |last1=Avgoustidis |first1=Anastasios |last2=Luzzi |first2=Gemma |last3=Martins |first3=Carlos J.A.P. |last4=Monteiro |first4=Ana M.R.V.L. |display-authors=3 |date=14 February 2012 |title=Constraints on the CMB temperature-redshift dependence from SZ and distance measurements |arxiv=1112.1862|doi=10.1088/1475-7516/2012/02/013 |volume=2012 |issue=2 |page=Article 013 |journal=[[Journal of Cosmology and Astroparticle Physics]] |bibcode=2012JCAP...02..013A |citeseerx=10.1.1.758.6956 |s2cid=119261969}}</ref><ref>{{harvnb|Belušević|2008|p=[https://media.wiley.com/product_data/excerpt/42/35274076/3527407642.pdf 16]}}</ref>

===Future observations===
Future [[Gravitational-wave observatory|gravitational-wave observatories]] might be able to detect primordial [[gravitational wave]]s, relics of the early universe, up to less than a second after the Big Bang.<ref name="Ghosh">{{cite news |last=Ghosh |first=Pallab |author-link=Pallab Ghosh |date=11 February 2016 |title=Einstein's gravitational waves 'seen' from black holes|url=https://www.bbc.com/news/science-environment-35524440 |url-status=live |department=Science & Environment |work=[[BBC News]] |location=London |publisher=[[BBC]] |archive-url=https://web.archive.org/web/20160211235836/https://www.bbc.com/news/science-environment-35524440 |archive-date=11 February 2016 |access-date=13 April 2017}}</ref><ref name="Billings">{{cite magazine |last=Billings |first=Lee |date=12 February 2016 |title=The Future of Gravitational Wave Astronomy |url=https://www.scientificamerican.com/article/the-future-of-gravitational-wave-astronomy/ |url-status=live |magazine=[[Scientific American]] |archive-url=https://web.archive.org/web/20160213012852/https://www.scientificamerican.com/article/the-future-of-gravitational-wave-astronomy/ |archive-date=13 February 2016 |access-date=13 April 2017}}</ref>

=={{anchor|Problems}}Problems and related issues in physics==
{{See also|List of unsolved problems in physics}}
As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang models. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the [[horizon problem]], the [[Inflation (cosmology)#Magnetic-monopole problem|magnetic monopole problem]], and the [[flatness problem]] are most commonly resolved with inflation theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven.<ref>{{cite journal |last1=Earman |first1=John |author1-link=John Earman |last2=Mosterín |first2=Jesús |author2-link=Jesús Mosterín |date=March 1999 |title=A Critical Look at Inflationary Cosmology |journal=[[Philosophy of Science (journal)|Philosophy of Science]] |volume=66 |issue=1 |pages=1–49 |doi=10.1086/392675 |jstor=188736|s2cid=120393154 }}</ref><ref>{{harvnb|Hawking|Israel|2010|pp=581–638|loc=chpt. 12: "Singularities and time-asymmetry" by [[Roger Penrose]].}}</ref><ref>{{harvnb|Penrose|1989}}</ref><ref>{{cite magazine |last=Steinhardt |first=Paul J. |author-link=Paul Steinhardt |date=April 2011 |title=The Inflation Debate: Is the theory at the heart of modern cosmology deeply flawed? |url=https://physics.princeton.edu/~steinh/0411036.pdf |url-status=live |magazine=[[Scientific American]] |doi=10.1038/scientificamerican0411-36 |volume=304 |issue=4 |pages=36–43 |archive-url=https://web.archive.org/web/20191101165817/https://physics.princeton.edu/~steinh/0411036.pdf |archive-date=1 November 2019 |access-date=23 December 2019}}</ref> What follows are a list of the mysterious aspects of the Big Bang concept still under intense investigation by cosmologists and [[Astrophysics|astrophysicists]].

===Baryon asymmetry===
{{Main|Baryon asymmetry}}
It is not yet understood why the universe has more matter than antimatter.<ref name="kolb_c6"/> It is generally assumed that when the universe was young and very hot it was in [[Statistical physics|statistical equilibrium]] and contained equal numbers of baryons and antibaryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of normal matter, rather than antimatter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the [[Sakharov conditions]] must be satisfied. These require that baryon number is not conserved, that [[C-symmetry]] and [[CP violation|CP-symmetry]] are violated and that the universe depart from [[thermodynamic equilibrium]].<ref name="sakharov">{{cite journal |last=Sakharov |first=Andrei D. |author-link=Andrei Sakharov |date=10 January 1967 |title=Нарушение ''СР''-инвариантности, ''С''-асимметрия и барионная асимметрия Вселенной |trans-title=Violation of ''CP''-invariance, ''C''-asymmetry and baryon asymmetry of the Universe |url=http://www.jetpletters.ac.ru/ps/808/article_12459.pdf |url-status=live |journal=[[Journal of Experimental and Theoretical Physics|Pis'ma v ZhETF]] |language=ru |volume=5 |issue=1 |pages=32–35 |archive-url=https://web.archive.org/web/20180728190714/http://www.jetpletters.ac.ru/ps/808/article_12459.pdf |archive-date=28 July 2018}}
* Translated in: {{cite journal |last=Sakharov |first=Andrei D. |author-link=Andrei Sakharov |date=10 January 1967 |title=Violation of CP Invariance, С Asymmetry, and Baryon Asymmetry of the Universe |url=http://www.jetpletters.ac.ru/ps/1643/article_25089.pdf |url-status=live |journal=[[Journal of Experimental and Theoretical Physics|JETP Letters]] |volume=5 |issue=1 |pages=24–27 |archive-url=https://web.archive.org/web/20191109163819/http://www.jetpletters.ac.ru/ps/1643/article_25089.pdf |archive-date=9 November 2019 |access-date=13 December 2019}}
** Reprinted in: {{harvnb|Kolb|Turner|1988|pp=371–373}}</ref> All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.

===Dark energy===
{{Main|Dark energy}}
Measurements of the redshift–[[apparent magnitude|magnitude]] relation for [[type Ia supernova]]e indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy".<ref name="peebles" />

Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the [[Friedmann equations#Density parameter|critical density]] of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density.<ref name="peebles" /> Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of [[gravitational lens]]es,<ref>{{cite journal | title=Constraining dark energy from the abundance of weak gravitational lenses  | first1=Nevin N. | last1=Weinberg | first2=Marc | last2=Kamionkowski | journal=Monthly Notices of the Royal Astronomical Society | volume=341 | issue=1 | date=May 2003 | pages=251–262 | bibcode=2003MNRAS.341..251W | arxiv=astro-ph/0210134 | doi=10.1046/j.1365-8711.2003.06421.x | doi-access=free | s2cid=1193946 }}</ref> and the other using the characteristic pattern of the large-scale structure--[[baryon acoustic oscillations]]--as a cosmic ruler.<ref>{{cite web |last1=White |first1=Martin |title=Baryon acoustic oscillations and dark energy |url=https://w.astro.berkeley.edu/~mwhite/bao/}}</ref><ref>
{{cite journal | title=Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological implications from two decades of spectroscopic surveys at the Apache Point Observatory | first1=Shadab | last1=Alam | display-authors=etal | journal=Physical Review D | volume=103 | issue=8 | date=April 2021 | page=083533 | bibcode=2021PhRvD.103h3533A | arxiv=2007.08991 | doi=10.1103/PhysRevD.103.083533}}</ref>

Negative pressure is believed to be a property of [[vacuum energy]], but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.<ref name="wmap7year" /> According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the [[far future]] as dark energy becomes even more dominant.{{citation needed|date=February 2023}}

The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein's cosmological constant but also extending to more exotic forms of [[Quintessence (physics)|quintessence]] or other modified gravity schemes.<ref>{{harvnb|Tanabashi, M.|2018|pp=[http://pdg.lbl.gov/2018/reviews/rpp2018-rev-dark-energy.pdf 406–413]|loc=chpt. 27: "Dark Energy" (Revised September 2017) by David H. Weinberg and Martin White.}}

* {{harvnb|Olive|2014|pp=[http://pdg.lbl.gov/2014/reviews/rpp2014-rev-dark-energy.pdf 361–368]|loc=chpt. 26: "Dark Energy" (November 2013) by Michael J. Mortonson, David H. Weinberg, and Martin White.}} {{bibcode|2014arXiv1401.0046M}}</ref> A [[cosmological constant problem]], sometimes called the "most embarrassing problem in physics", results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from [[Planck units]].<ref>{{cite journal |last1=Rugh |first1=Svend E. |last2=Zinkernagel |first2=Henrik |title=The quantum vacuum and the cosmological constant problem |pages=663–705 |volume=33 |issue=4 |date=December 2002 |journal=[[Studies in History and Philosophy of Science Part B]] |arxiv=hep-th/0012253 |bibcode=2002SHPMP..33..663R |doi=10.1016/S1355-2198(02)00033-3 |s2cid=9007190 }}</ref>

===Dark matter===
{{Main|Dark matter}}
[[File:Cosmological Composition – Pie Chart.svg|thumb|right|upright=1.6|[[pie chart|Chart]] shows the proportion of different components of the universe {{spaced ndash}} about 95% is [[dark matter]] and [[dark energy]].]]
During the 1970s and the 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, [[galaxy cluster]] [[velocity dispersion]]s, large-scale structure distributions, [[gravitational lens]]ing studies, and [[X-ray astronomy|X-ray measurements]] of galaxy clusters.<ref>{{cite web |url=http://pages.astronomy.ua.edu/keel/galaxies/darkmatter.html |url-status=live |last=Keel |first=William C. |date=October 2009 |orig-date=Last changes: February 2015 |title=Dark Matter |website=Bill Keel's Lecture Notes – Galaxies and the Universe |archive-url=https://web.archive.org/web/20190503112916/http://pages.astronomy.ua.edu/keel/galaxies/darkmatter.html |archive-date=3 May 2019 |access-date=15 December 2019}}</ref>

Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.<ref name="pdg">{{harvnb|Tanabashi, M.|2018|pp=[http://pdg.lbl.gov/2018/reviews/rpp2018-rev-dark-matter.pdf 396–405]|loc=chpt. 26: "Dark Matter" (Revised September 2017) by Manuel Drees and Gilles Gerbier.}}
* {{harvnb|Yao, W.-M.|2006|pp=[http://pdg.lbl.gov/2006/reviews/darkmatrpp.pdf 233–237]|loc=chpt. 22: "Dark Matter" (September 2003) by Manuel Drees and Gilles Gerbier.}}</ref>

Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the [[dwarf galaxy problem]]<ref name="Martínez-Delgado">{{Cite book |arxiv= 1009.4505|last1 = Bullock|first1 = James S.|title = Local Group Cosmology|chapter= Notes on the Missing Satellites Problem |pages = 95–122|year = 2010 |doi=10.1017/CBO9781139152303.004|isbn = 9781139152303|s2cid = 119270708|editor1-last = Martinez-Delgado|editor1-first = David|editor2-last = Mediavilla|editor2-first = Evencio}}</ref> and the [[cuspy halo problem]].<ref name="Diemand2005">{{cite journal |last1=Diemand |first1=Jürg |last2=Zemp |first2=Marcel |last3=Moore |first3=Ben |last4=Stadel |first4=Joachim |last5=Carollo |first5=C. Marcella |author-link5=C. Marcella Carollo |date=December 2005 |title=Cusps in cold dark matter haloes |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=364 |issue=2 |pages=665–673 |arxiv=astro-ph/0504215 |bibcode=2005MNRAS.364..665D |doi=10.1111/j.1365-2966.2005.09601.x |doi-access=free |s2cid=117769706 }}</ref> Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein; yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations.<ref>{{cite journal |last1=Dodelson |first1=Scott |date=31 December 2011 |title=The Real Problem with MOND |journal=[[International Journal of Modern Physics D]] |arxiv=1112.1320 |doi=10.1142/S0218271811020561 |volume=20 |issue=14 |pages=2749–2753 |bibcode=2011IJMPD..20.2749D |s2cid=119194106 }}</ref>

===Horizon problem===
{{Main|Horizon problem}}
The horizon problem results from the premise that information cannot travel [[Faster-than-light|faster than light]]. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in [[causality (physics)|causal]] contact.<ref name="kolb_c8">{{harvnb|Kolb|Turner|1988|loc=chpt. 8}}</ref> The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.<ref name="Ryden2003">{{harvnb|Ryden|2003}}</ref>{{rp|191–202}}

A resolution to this apparent inconsistency is offered by inflation theory in which a homogeneous and isotropic [[Scalar field|scalar energy field]] dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes [[Exponential growth|exponential]] expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.<ref name="Guth1998" />{{rp|180–186}}

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be [[primordial fluctuations|quantum thermal fluctuations]], which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe.<ref name="Ryden2003"/>{{rp|207}} Inflation predicts that the primordial fluctuations are nearly [[Scale invariance|scale invariant]] and [[Normal distribution|Gaussian]], which has been confirmed by measurements of the CMB.<ref name="wmap1year" />{{rp|sec 6}}

A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before [[Higgs mechanism|electroweak symmetry breaking]] occurs, so inflation should not be able to prevent large-scale discontinuities in the [[False vacuum|electroweak vacuum]] since distant parts of the observable universe were causally separate when the [[electroweak epoch]] ended.<ref>{{harvnb|Penrose|2007}}</ref>

===Magnetic monopoles===
The magnetic monopole objection was raised in the late 1970s. [[Grand Unified Theory|Grand unified theories]] (GUTs) predicted [[topological defect]]s in space that would manifest as [[magnetic monopole]]s. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that no monopoles have been found. This problem is resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.<ref name="kolb_c8"/>

===Flatness problem===
[[File:End of universe.jpg|thumb|upright=1.5|The overall [[Shape of the universe|geometry of the universe]] is determined by whether the [[Friedmann equations#Density parameter|Omega cosmological parameter]] is less than, equal to or greater than 1. Shown from top to bottom are a [[Shape of the universe#Universe with positive curvature|closed universe]] with positive curvature, a [[Shape of the universe#Universe with negative curvature|hyperbolic universe]] with negative curvature and a [[Shape of the universe#Universe with zero curvature|flat universe]] with zero curvature.]]

The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW.<ref name="kolb_c8"/> The universe may have positive, negative, or zero spatial [[curvature]] depending on its total energy density. Curvature is negative if its density is less than the critical density; positive if greater; and zero at the critical density, in which case space is said to be ''flat''. Observations indicate the universe is consistent with being flat.<ref name=Filippenko2002>{{cite magazine |last1=Filippenko |first1=Alexei V. |author1-link=Alex Filippenko |last2=Pasachoff |first2=Jay M. |author2-link=Jay Pasachoff |date=March–April 2002 |title=A Universe from Nothing |url=http://www.astrosociety.org/pubs/mercury/31_02/nothing.html |url-status=dead |magazine=[[Mercury (magazine)|Mercury]] |volume=31 |issue=2 |page=15 |bibcode=2002Mercu..31b..15F |access-date=10 March 2010 |archive-url=https://web.archive.org/web/20131022135932/http://www.astrosociety.org/pubs/mercury/31_02/nothing.html |archive-date=22 October 2013}}</ref><ref name="Krauss2009">{{cite AV media |url=https://www.youtube.com/watch?v=7ImvlS8PLIo |title='A Universe From Nothing' by Lawrence Krauss, AAI 2009 |date=21 October 2009 |medium=Video |language=en-us |publisher=[[Richard Dawkins Foundation for Reason and Science]] |location=Washington, D.C. |access-date=17 October 2011 |archive-url=https://ghostarchive.org/varchive/youtube/20211123/7ImvlS8PLIo |archive-date=2021-11-23 |url-status=live |people=[[Lawrence M. Krauss]] (Speaker); R. Elisabeth Cornwell (Producer)}}{{cbignore}}</ref>

The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.<ref group="notes">Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; however, our universe remained close to flat for several billion years before the dark energy density became significant.</ref> Given that a natural timescale for departure from flatness might be the [[Planck time]], 10<sup>−43</sup> seconds,<ref name="HTUW"/> the fact that the universe has reached neither a [[heat death of the universe|heat death]] nor a [[Big Crunch]] after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 10<sup>14</sup> of its critical value, or it would not exist as it does today.<ref>{{harvnb|Hawking|Israel|2010|pp=504–517|loc=chpt. 9: "The big bang cosmology — enigmas and nostrums" by [[Robert H. Dicke]] and [[Jim Peebles|Phillip J.E. Peebles]].}}</ref>

== Misconceptions ==
One of the common misconceptions about the Big Bang model is that it fully explains the [[Cosmogony|origin of the universe]]. However, the Big Bang model does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state.<ref>{{cite web |author=<!--Not stated--> |title=Brief Answers to Cosmic Questions |url=https://lweb.cfa.harvard.edu/seuforum/faq.htm#m12 |url-status=live |archive-url=https://web.archive.org/web/20160413195349/https://www.cfa.harvard.edu/seuforum/faq.htm |archive-date=13 April 2016 |access-date=18 December 2019 |website=Universe Forum |publisher=[[Harvard–Smithsonian Center for Astrophysics]] |location=Cambridge, Massachusetts}} Archival site: "The Universe Forum's role as part of NASA's Education Support Network concluded in September, 2009."</ref> It is misleading to visualize the Big Bang by comparing its size to everyday objects. When the size of the universe at Big Bang is described, it refers to the size of the observable universe, and not the entire universe.<ref name="Davis_Lineweaver2004">{{cite journal |last1=Davis |first1=Tamara M. |author1-link=Tamara Davis |last2=Lineweaver |first2=Charles H. |date=31 March 2004 |title=Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe |journal=[[Publications of the Astronomical Society of Australia]] |volume=21 |issue=1 |pages=97–109 |arxiv=astro-ph/0310808 |bibcode=2004PASA...21...97D |doi=10.1071/as03040 |s2cid=13068122 }}</ref>

Another common misconception is that the Big Bang must be understood as the expansion of space and not in terms of the contents of space exploding apart. In fact, either description can be accurate. The expansion of space (implied by the FLRW metric) is only a mathematical convention, corresponding to a choice of [[coordinate conditions|coordinates]] on spacetime. There is no [[general covariance|generally covariant]] sense in which space expands.<ref name="Peacock">{{cite arXiv |eprint=0809.4573 |class=astro-ph |first=J. A. |last=Peacock |title=A diatribe on expanding space |date=2008}}</ref>

The recession speeds associated with Hubble's law are not velocities in a relativistic sense (for example, they are not related to the spatial components of [[4-velocity|4-velocities]]). Therefore, it is not remarkable that according to Hubble's law, galaxies farther than the Hubble distance recede faster than the speed of light. Such recession speeds do not correspond to [[faster-than-light]] travel.

Many popular accounts attribute the cosmological redshift to the expansion of space. This can be misleading because the expansion of space is only a coordinate choice. The most natural interpretation of the cosmological redshift is that it is a [[Doppler shift]].<ref name="Hogg"/>

== 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, built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time, and such an infinite energy density may be a physical impossibility. However, the physical theories of general relativity and quantum mechanics as currently realized are not applicable before the Planck epoch, and 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}}
* {{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}}
** {{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}}
* {{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}}
* {{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>

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}}
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[[Religious interpretations of the Big Bang theory]].
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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|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}}
* {{harvnb|Block|Puerari|Stockton|Ferreira|2000|pp=723–740}} {{doi|10.1007/978-94-011-4114-7_85}}
* {{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]]}}

==See also==
* {{annotated link|Anthropic principle}}
* {{annotated link|Big Bounce}}
* {{annotated link|Big Crunch}}
* {{annotated link|Cold Big Bang}}
* {{annotated link|Cosmic Calendar}}
* {{annotated link|Cosmogony}}
* {{annotated link|Eureka: A Prose Poem|''Eureka: A Prose Poem''}}, a Big Bang speculation
* {{annotated link|Future of an expanding universe}}
* {{annotated link|Heat death of the universe}}. Also known as the Big Chill and the Big Freeze
* {{annotated link|Non-standard cosmology}}
* {{annotated link|Shape of the universe}}
* {{annotated link|Steady-state model}}, a discredited theory that denied the Big Bang and posited that the universe always existed

==Notes==
{{reflist|group="notes"}}

==References==
{{reflist}}

===Bibliography===
{{Refbegin|30em}}
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* {{cite book |editor1-last=Block |editor1-first=David L. |editor2-last=Puerari |editor2-first=Ivânio |editor3-last=Stockton |editor3-first=Alan |editor4-last=Ferreira |editor4-first=DeWet |display-editors=3 |year=2000 |title=Toward a New Millennium in Galaxy Morphology: Proceedings of an International Conference 'Toward a New Millennium in Galaxy Morphology: from z=0 to the Lyman Break, held at the Eskom Conference Centre, Midrand, South Africa, September 13–18, 1999 |location=Dordrecht |publisher=[[Springer Science+Business Media|Kluwer Academic Publishers]] |doi=10.1007/978-94-011-4114-7 |isbn=978-94-010-5801-8 |lccn=00042415 |oclc=851369444}} "Reprinted from ''[[Astrophysics and Space Science]]'' Volumes 269–270, Nos. 1–4, 1999".
* {{cite book |last=Block |first=David L. |year=2012 |chapter=Georges Lemaître and Stigler's Law of Eponymy |editor1-last=Holder |editor1-first=Rodney D. |editor2-last=Mitton |editor2-first=Simon |editor2-link=Simon Mitton |title=A Hubble Eclipse: Lemaitre and Censorship |series=Astrophysics and Space Science Library |volume=395 |pages=89–96 |location=Heidelberg; New York |publisher=[[Springer Science+Business Media|Springer]] |bibcode=2012ASSL..395...89B |doi=10.1007/978-3-642-32254-9_8 |arxiv=1106.3928v2 |isbn=978-3-642-32253-2 |s2cid=119205665 |lccn=2012956159 |oclc=839779611}}
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* {{cite book |last=Hawking |first=Stephen W. |author-link=Stephen Hawking |year=1988 |title=A Brief History of Time: From the Big Bang to Black Holes |title-link=A Brief History of Time |location=New York |others=Introduction by [[Carl Sagan]]; illustrations by Ron Miller |publisher=[[Bantam Dell Publishing Group]] |isbn=978-0-553-10953-5 |lccn=87033333 |oclc=39256652}}
* {{cite book |editor1-last=Hawking |editor1-first=Stephen W. |editor1-link=Stephen Hawking |editor2-last=Israel |editor2-first=Werner |editor2-link=Werner Israel |year=2010 |orig-date=Originally published 1979 |title=General Relativity: An Einstein Centenary Survey |location=Cambridge, UK |publisher=[[Cambridge University Press]] |isbn=978-0-521-13798-0 |lccn=78062112 |oclc=759923541}}
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* {{cite book |last=Krauss |first=Lawrence M. |author-link=Lawrence M. Krauss |year=2012 |title=A Universe From Nothing: Why there is Something Rather than Nothing |url=https://archive.org/details/universefromnoth0000krau |url-access=registration |others=Afterword by [[Richard Dawkins]] |edition=1st Free Press hardcover |location=New York |publisher=[[Free Press (publisher)|Free Press]] |isbn=978-1-4516-2445-8 |lccn=2011032519 |oclc=709673181}}
* {{cite book |last=Livio |first=Mario |author-link=Mario Livio |year=2000 |title=The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos |url=https://archive.org/details/Mario_Livio-The_Accelerating_Universe-AUDiOBOOK-WEB-2018-PROLOG |type=Audio book performance by Tom Parks, [[Brilliance Audio]] |others=Foreword by [[Allan Sandage]] |location=New York |publisher=[[Wiley (publisher)|John Wiley & Sons]] |isbn=978-0-471-32969-5 |lccn=99022278 |oclc=226086793}}
* {{cite book |last=Manly |first=Steven L. |year=2011 |title=Visions of the Multiverse |editor-last=Brandon |editor-first=Jodi |location=[[Pequannock Township, New Jersey|Pompton Plains, NJ]] |publisher=[[Red Wheel/Weiser/Conari|New Page Books]] |isbn=978-1-60163-720-8 |lccn=2010052741 |oclc=609531953}}
* {{cite book |editor-last=Martínez-Delgado |editor-first=David |year=2013 |title=Local Group Cosmology |location=Cambridge, UK |publisher=[[Cambridge University Press]] |isbn=978-1-107-02380-2 |lccn=2013012345 |oclc=875920635}} "Lectures presented at the XX Canary Islands Winter School of Astrophysics, held in Tenerife, Spain, November 17–18, 2008."
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* {{cite book |last=Partridge |first=R. Bruce |year=1995 |title=3K: The Cosmic Microwave Background Radiation |edition=Illustrated |series=Cambridge Astrophysics Series |volume=25 |location=Cambridge, UK |publisher=[[Cambridge University Press]] |isbn=978-0-521-35808-8 |lccn=94014980 |oclc=1123849709}}
* {{cite book |last=Peacock |first=John A. |author-link=John A. Peacock |year=1999 |title=Cosmological Physics |url=https://archive.org/details/cosmologicalphys0000peac |url-access=registration |series=Cambridge Astrophysics Series |location=Cambridge, UK; New York |publisher=[[Cambridge University Press]] |isbn=978-0-521-42270-3 |lccn=98029460 |oclc=60157380}}
* {{cite book |last=Penrose |first=Roger |author-link=Roger Penrose |year=1989 |chapter=Difficulties with Inflationary Cosmology |editor-last=Fenyves |editor-first=Ervin J. |title=Fourteenth Texas Symposium on Relativistic Astrophysics |title-link=Ivor Robinson (physicist)#Symposium series |series=[[Annals of the New York Academy of Sciences]] |location=New York |publisher=[[New York Academy of Sciences]] |volume=571 |pages=249–264 |bibcode=1989NYASA.571..249P |doi=10.1111/j.1749-6632.1989.tb50513.x |isbn=978-0-89766-526-1 |s2cid=122383812 |lccn=89014030 |oclc=318253659 |issn=0077-8923}} "Symposium held in Dallas, Tex., Dec. 11-16, 1988."
* {{cite book |last=Penrose |first=Roger |author-link=Roger Penrose |year=2007 |orig-date=Originally published: London: [[Jonathan Cape]], 2004 |title=The Road to Reality |title-link=The Road to Reality |edition=1st Vintage Books |location=New York |publisher=[[Vintage Books]] |isbn=978-0-679-77631-4 |lccn=2008274126 |oclc=920157277}} The 2004 edition of the book is available from the [https://archive.org/details/RoadToRealityRobertPenrose Internet Archive]. Retrieved 20 December 2019.
* {{cite book |last=Roos |first=Matts |year=2012 |orig-date=Chapter originally published 2008 |chapter=Expansion of the Universe – Standard Big Bang Model |editor1-last=Engvold |editor1-first=Oddbjørn |editor1-link=Oddbjørn Engvold |editor2-last=Stabell |editor2-first=Rolf |editor3-last=Czerny |editor3-first=Bozena |editor4-last=Lattanzio |editor4-first=John |title=Astronomy and Astrophysics |chapter-url=http://www.eolss.net/ebooklib/bookinfo/astronomy-astrophysics.aspx |series=[[Encyclopedia of Life Support Systems]] |volume=II |location=Ramsey, Isle of Man |publisher=[[UNESCO]] in partnership with Eolss Publishers Co. Ltd. |isbn=978-1-84826-823-4 |arxiv=0802.2005 |bibcode=2008arXiv0802.2005R |oclc=691095693}}
* {{cite book |last=Ryden |first=Barbara Sue |year=2003 |title=Introduction to Cosmology |location=San Francisco |publisher=[[Addison-Wesley]] |isbn=978-0-8053-8912-8 |lccn=2002013176 |oclc=1087978842}}
* {{cite book |last=Silk |first=Joseph |author-link=Joseph Silk |year=2009 |title=Horizons of Cosmology: Exploring Worlds Seen and Unseen |series=Templeton Science and Religion Series |location=Conshohocken, PA |publisher=[[Templeton Press]] |isbn=978-1-59947-341-3 |lccn=2009010014 |oclc=818734366}}
* {{cite book |last=Singh |first=Simon |author-link=Simon Singh |year=2004 |title=Big Bang: The Origin of the Universe |url=https://archive.org/details/bigbang00simo_0 |url-access=registration |edition=1st U.S. |location=New York |publisher=[[HarperCollins#Imprints|Fourth Estate]] |isbn=978-0-00-716220-8 |lccn=2004056306 |oclc=475508230 |bibcode=2004biba.book.....S}}
* {{cite journal |author=Tanabashi, M. |collaboration=[[Particle Data Group]] |year=2018 |title=Review of Particle Physics |journal=[[Physical Review D]] |volume=98 |issue=3 |pages=1–708 |doi=10.1103/PhysRevD.98.030001 |pmid=10020536 |bibcode=2018PhRvD..98c0001T|doi-access=free |hdl=10044/1/68623 |hdl-access=free }}
* {{cite book |last=Tolman |first=Richard C. |author-link=Richard C. Tolman |year=1934 |title=Relativity, Thermodynamics and Cosmology |url=https://archive.org/details/in.ernet.dli.2015.177229/page/n1 |series=The International Series of Monographs on Physics |location=Oxford, UK; London |publisher=[[Clarendon Press]]; [[Oxford University Press]] |isbn=978-0-486-65383-9 |lccn=34032023 |oclc=919976}}
* {{cite book |last=Woolfson |first=Michael |author-link=Michael Woolfson |year=2013 |title=Time, Space, Stars & Man: The Story of Big Bang |edition=2nd |location=London |publisher=[[Imperial College Press]] |isbn=978-1-84816-933-3 |lccn=2013371163 |oclc=835115510}}
* {{cite book |last=Wright |first=Edward L. |author-link=Edward L. Wright |year=2004 |chapter=Theoretical Overview of Cosmic Microwave Background Anisotropy |editor-last=Freedman |editor-first=Wendy L. |editor-link=Wendy Freedman |title=Measuring and Modeling the Universe |series=Carnegie Observatories Astrophysics Series |volume=2 |pages=291 |location=Cambridge, UK |publisher=[[Cambridge University Press]] |arxiv=astro-ph/0305591 |bibcode=2004mmu..symp..291W |isbn=978-0-521-75576-4 |lccn=2005277053 |oclc=937330165}}
* {{cite journal |author=Yao, W.-M. |collaboration=[[Particle Data Group]] |year=2006 |title=Review of Particle Physics |url=http://pdg.lbl.gov/2006/download/rpp-2006-book.pdf |url-status=live |journal=[[Journal of Physics G|Journal of Physics G: Nuclear and Particle Physics]] |volume=33 |issue=1 |pages=1–1232 |bibcode=2006JPhG...33....1Y |doi=10.1088/0954-3899/33/1/001 |s2cid=117958297 |archive-url=https://web.archive.org/web/20170212055045/http://pdg.lbl.gov/2006/download/rpp-2006-book.pdf |archive-date=12 February 2017 |access-date=16 December 2019}}
{{Refend}}

==Further reading==
{{for|an annotated list of textbooks and monographs|Physical cosmology#Textbooks}}
* {{cite journal |last1=Alpher |first1=Ralph A. |author-link=Ralph Asher Alpher |last2=Herman |first2=Robert |author-link2=Robert Herman |date=August 1988 |title=Reflections on Early Work on 'Big Bang' Cosmology |journal=[[Physics Today]] |volume=41 |issue=8 |pages=24–34 |bibcode=1988PhT....41h..24A |doi=10.1063/1.881126}}
* {{cite book |last=Barrow |first=John D. |author-link=John D. Barrow |year=1994 |title=The Origin of the Universe |series=Science Masters |url=https://archive.org/details/originofuniverse00barr |url-access=registration |location=London |publisher=[[Weidenfeld & Nicolson]] |isbn=978-0-297-81497-9 |lccn=94006343 |oclc=490957073}}
* {{cite book |last=Davies |first=Paul |author-link=Paul Davies |year=1992 |title=The Mind of God: The Scientific Basis for a Rational World |title-link=The Mind of God |location=New York |publisher=[[Simon & Schuster]] |isbn=978-0-671-71069-9 |lccn=91028606 |oclc=59940452}}
* {{cite magazine |last1=Lineweaver |first1=Charles H. |last2=Davis |first2=Tamara M. |author2-link=Tamara Davis |date=March 2005 |title=Misconceptions about the Big Bang |url=https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf |url-status=live |magazine=[[Scientific American]] |volume=292 |issue=3 |pages=36–45 |archive-url=https://web.archive.org/web/20191009072713/https://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf |archive-date=9 October 2019 |access-date=23 December 2019}}
* {{cite book |last1=Mather |first1=John C. |author1-link=John C. Mather |last2=Boslough |first2=John |year=1996 |title=The Very First Light: The True Inside Story of the Scientific Journey Back to the Dawn of the Universe |url=https://archive.org/details/veryfirstlight00john |url-access=registration |edition=1st |location=New York |publisher=[[Basic Books]] |isbn=978-0-465-01575-7 |lccn=96010781 |oclc=34357391}}
* {{cite magazine|author1-link=Michael Riordan (physicist) |last1=Riordan |first1=Michael |last2=Zajc |first2=William A. |author2-link=William Allen Zajc |date=May 2006 |title=The First Few Microseconds |url=https://rhig.physics.yale.edu/Summer2014/sc%20am%20-%20zjac%20and%20riordan.pdf |url-status=live |magazine=[[Scientific American]] |volume=294 |issue=5 |pages=34–41 |bibcode=2006SciAm.294e..34R |doi=10.1038/scientificamerican0506-34a |archive-url=https://web.archive.org/web/20141130184142/http://rhig.physics.yale.edu/M_article_11_2005.pdf |archive-date=30 November 2014}}
* {{Cite book |last=Singh |first=Simon |title=Big Bang: The Origin of the Universe |title-link=Big Bang (Singh book) |publisher=[[Harper Perennial]] |year=2005 |isbn=978-0007162215 |edition=Harper Perennial; illustrated |location=New York, New York |language=en |author-link=Simon Singh |orig-date=First U.S. edition published 2004}}
* {{cite book |last=Weinberg |first=Steven |author-link=Steven Weinberg |year=1993 |orig-date=Originally published 1977 |title=The First Three Minutes: A Modern View of the Origin of the Universe |title-link=The First Three Minutes |edition=Updated |location=New York |publisher=[[Basic Books]] |isbn=978-0-465-02437-7 |lccn=93232406 |oclc=488469247}} 1st edition is available from the [https://archive.org/details/TheFirstThreeMinutesAModernViewOfTheOriginOfTheUniverseS.Weinberg Internet Archive]. Retrieved 23 December 2019.

==External links==
{{Spoken Wikipedia|en-BigBang.ogg|date=2011-11-12}}
* [https://onceuponauniverse.com/about/in-the-beginning/ Once Upon a Universe] {{Webarchive|url=https://web.archive.org/web/20200622230141/https://onceuponauniverse.com/about/in-the-beginning/ |date=22 June 2020 }} – [[Science and Technology Facilities Council|STFC]] funded project explaining the history of the universe in easy-to-understand language
* [https://map.gsfc.nasa.gov/universe/bb_theory.html "Big Bang Cosmology"] – [[Wilkinson Microwave Anisotropy Probe|NASA/WMAP]] Science Team
* [https://science.nasa.gov/astrophysics/focus-areas/what-powered-the-big-bang/ "The Big Bang"] – [[Science Mission Directorate|NASA Science]]
* [https://www.science20.com/hammock_physicist/big_bang_big_bewilderment "Big Bang, Big Bewilderment"] – Big bang model with animated graphics by Johannes Koelman
* [https://nautil.us/the-trouble-with-the-big-bang-238547/ "The Trouble With "The Big Bang""] – A rash of recent articles illustrates a longstanding confusion over the famous term. by Sabine Hossenfelde
{{Big Bang timeline}}
{{Cosmology topics}}
{{Big History}}
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[[Category:Big Bang| ]]
[[Category:Physical cosmology]]
[[Category:Concepts in astronomy]]
[[Category:Astronomical events]]
[[Category:Scientific models]]
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