Editing Big Bang (section)

===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.