Editing Big Bang (section)

==Observational evidence==
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 |source=— [[Lawrence Krauss]]<ref>{{harvnb|Krauss|2012|p=[https://archive.org/details/universefromnoth0000krau/page/118 118]}}</ref>
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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]], [[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 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=et al |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}}</ref><ref>{{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=et al |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|ul=kyr}},<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> 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">{{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> 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>{{cite journal | last1=Yang | first1=R. J. | last2=Zhang | first2=S. N. | year=2010| title=The age problem in the ΛCDM model | journal=Monthly Notices of the Royal Astronomical Society | volume=407 | issue=3 | pages=1835–1841 | doi=10.1111/j.1365-2966.2010.17020.x | doi-access=free | arxiv=0905.2683 | bibcode=2010MNRAS.407.1835Y }}</ref><ref>{{cite journal | last1=Yu | first1=H. | last2=Wang | first2=F. Y. | year=2014 | title=Reconciling the cosmic age problem in the ''R''<sub>h</sub> = ''ct'' universe | journal=The European Physical Journal C | volume=74 | issue=10 | at=id. 3090 | doi=10.1140/epjc/s10052-014-3090-1 | arxiv=1402.6433 | bibcode=2014EPJC...74.3090Y }}</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|Belusevic|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>