View source for Cosmic microwave background

{{Short description|Trace radiation from the early universe}}
{{Redirect2|CMB|CMBR|the Republic of [[Cuba]] radio station|Radio Enciclopedia|other uses|CMB (disambiguation)}}
[[File:WMAP 2012.png|upright=1.5|thumb|Nine-year [[Wilkinson Microwave Anisotropy Probe]] heat map of temperature fluctuations in the cosmic microwave background]]
{{Cosmology|early}}

The '''cosmic microwave background''' ('''CMB''', '''CMBR'''), or '''relic radiation''', is [[microwave radiation]] that fills all space in the [[observable universe]]. With a standard [[optical telescope]], the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive [[radio telescope]] detects a faint background glow that is almost [[isotropic|uniform]] and is not associated with any star, galaxy, or other [[astronomical object|object]]. This glow is strongest in the [[microwave]] region of the electromagnetic spectrum. The accidental [[Discovery of cosmic microwave background radiation|discovery of the CMB]] in 1965 by American radio astronomers [[Arno Allan Penzias]] and [[Robert Woodrow Wilson]] was the culmination of work initiated in the 1940s.

The CMB is landmark evidence of the [[Big Bang]] [[scientific theory|theory]] for the origin of the universe. In the Big Bang [[cosmological model]]s, during the earliest periods, the universe was filled with an [[Opacity (optics)|opaque]] fog of dense, hot [[Plasma (physics)|plasma]] of [[sub-atomic particle]]s. As the universe expanded, this plasma cooled to the point where protons and electrons combined to form neutral atoms of mostly hydrogen. Unlike the plasma, these atoms could not scatter thermal radiation by [[Thomson scattering]], and so the universe became transparent. Known as the [[Recombination (cosmology)|recombination]] epoch, this [[Decoupling (cosmology)|decoupling]] event released [[photons]] to travel freely through space. However, the photons have grown less [[photon energy|energetic]] due to the [[cosmological redshift]] associated with the [[expansion of the universe]]. The ''surface of last scattering'' refers to a shell at the right distance in space so photons are now received that were originally emitted at the time of decoupling.

The CMB is not completely smooth and uniform, showing a faint [[anisotropy]] that can be mapped by sensitive detectors. Ground and space-based experiments such as [[Cosmic Background Explorer|COBE]], [[Wilkinson Microwave Anisotropy Probe|WMAP]] and [[Planck (spacecraft)|Planck]] have been used to measure these temperature inhomogeneities. The anisotropy structure is determined by various interactions of matter and photons up to the point of decoupling, which results in a characteristic lumpy pattern that varies with angular scale. The [[Distribution (mathematics)|distribution]] of the anisotropy across the sky has [[frequency]] components that can be represented by a [[power spectrum]] displaying a sequence of peaks and valleys. The peak values of this spectrum hold important information about the physical properties of the early universe: the first peak determines the overall [[curvature of the universe]], while the second and third peak detail the density of normal matter and so-called [[dark matter]], respectively. Extracting fine details from the CMB data can be challenging, since the emission has undergone modification by foreground features such as [[galaxy cluster]]s.

==Features==
[[File:Cmbr.svg|thumb|300px|Graph of cosmic microwave background spectrum around its peak in the [[microwave]] frequency range,<ref name="Komatsu2022Review" /> as measured by the FIRAS instrument on the [[Cosmic Background Explorer|COBE]].<ref>{{Cite web |title=LAMBDA - Cosmic Background Explorer |url=https://lambda.gsfc.nasa.gov/product/cobe/firas_monopole_spect.html |access-date=2024-05-17 |website=lambda.gsfc.nasa.gov}}</ref><ref>{{Cite journal |last1=Fixsen |first1=D. J. |last2=Mather |first2=J. C. |date=2002-12-20 |title=The Spectral Results of the Far-Infrared Absolute Spectrophotometer Instrument on COBE |url=https://iopscience.iop.org/article/10.1086/344402 |journal=The Astrophysical Journal |language=en |volume=581 |issue=2 |pages=817–822 |doi=10.1086/344402 |bibcode=2002ApJ...581..817F |issn=0004-637X}}</ref> While vastly exaggerated "[[standard error of estimation|error bars]]" were included here to show the measured data points, the true error bars are too small to be seen even in an enlarged image, and it is impossible to distinguish the observed data from the [[blackbody]] spectrum for 2.725&nbsp;[[Kelvin|K]].]]
The cosmic microwave background radiation is an emission of uniform [[black body]] [[thermal energy]] coming from all directions. Intensity of the CMB is expressed in [[kelvin]] (K), the [[System International|SI]] unit of temperature. The CMB has a thermal [[black body spectrum]] at a temperature of {{val|2.72548|0.00057|u=K}}.<ref name="apj707_2_916">{{Cite journal |last1=Fixsen |first1=D. J. |year=2009 |title=The Temperature of the Cosmic Microwave Background |journal=[[The Astrophysical Journal]] |volume=707 |issue=2 |pages=916–920 |arxiv=0911.1955 |bibcode=2009ApJ...707..916F |doi=10.1088/0004-637X/707/2/916 |s2cid=119217397}}</ref> Variations in intensity are expressed as variations in temperature. The blackbody temperature uniquely characterizes the intensity of the radiation at all wavelengths; a measured [[brightness temperature]] at any wavelength can be converted to a blackbody temperature.<ref name="WrightUCLASite" />

The radiation is remarkably uniform across the sky, very unlike the almost point-like structure of stars or clumps of stars in galaxies.<ref name="HuDodelsonReview">{{Cite journal |last1=Hu |first1=Wayne |last2=Dodelson |first2=Scott |date=September 2002 |title=Cosmic Microwave Background Anisotropies |url=https://www.annualreviews.org/doi/10.1146/annurev.astro.40.060401.093926 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=40 |issue=1 |pages=171–216 |doi=10.1146/annurev.astro.40.060401.093926 |issn=0066-4146|arxiv=astro-ph/0110414 |bibcode=2002ARA&A..40..171H }}</ref> The radiation is [[isotropic]] to roughly one part in 25,000: the [[root mean square]] variations are just over 100&nbsp;μK,<ref name="PlanckV">
{{citation | author=The Planck Collaboration | title= Planck 2018 results V. CMB power spectra and likelihoods | journal= Astronomy and Astrophysics |arxiv= 1907.12875 | year= 2020 | volume= 641 |  pages= A5 | doi= 10.1051/0004-6361/201936386 | bibcode= 2020A&A...641A...5P}}</ref> after subtracting a [[dipole]] anisotropy from the [[Doppler shift]] of the background radiation. The latter is caused by the [[peculiar velocity]] of the Sun relative to the [[Comoving distance#Comoving coordinates|comoving]] cosmic rest frame as it moves at 369.82&nbsp;±&nbsp;0.11&nbsp;km/s towards the constellation [[Crater (constellation)|Crater]] near its boundary with the constellation [[Leo (constellation)|Leo]]<ref name="PlanckI">{{citation | author=The Planck Collaboration | title= Planck 2018 results. I. Overview, and the cosmological legacy of Planck | journal= Astronomy and Astrophysics |arxiv=1807.06205| year= 2020 | volume= 641 | pages= A1 | doi= 10.1051/0004-6361/201833880 | bibcode= 2020A&A...641A...1P | s2cid= 119185252 }}</ref> The CMB dipole and [[aberration of light|aberration]] at higher multipoles have been measured, consistent with galactic motion.<ref name="PlanckXXVII">{{citation | author=The Planck Collaboration | title= Planck 2013 results. XXVII. Doppler boosting of the CMB: Eppur si muove |arxiv=1303.5087 |bibcode = 2014A&A...571A..27P |doi=10.1051/0004-6361/201321556 |volume=571 | issue= 27 |journal=Astronomy |pages=A27| year= 2014| s2cid= 5398329 }}</ref>
Despite the very small degree of anisotropy in the CMB, many aspects can be measured with high precision and such measurements are critical for cosmological theories.<ref name="HuDodelsonReview"/>

In addition to temperature anisotropy, the CMB should have an angular variation in [[polarization (physics)|polarization]]. The polarisation at each direction in the sky has an orientation described in terms of E-mode and B-mode polarization. The E-mode signal is a factor of 10 less strong than the temperature anisotropy; it supplements the temperature data as they are correlated. The B-mode signal is even weaker but may contain additional cosmological data.<ref name="HuDodelsonReview"/>

The anisotropy is related to physical origin of the polarisation. Excitation of an electron by linear polarised light generates polarized light at 90 degrees to the incident direction. If the incoming radiation is isotropic, different incoming directions create polarizations that cancel out. If the incoming radiation has quadrupole anisotropy, residual polarization will be seen.<ref>Hu, Wayne, and Martin White. "A CMB polarization primer." arXiv preprint astro-ph/9706147 (1997).</ref>

Other than the temperature and polarization anisotropy, the CMB frequency spectrum is expected to feature tiny departures from the black-body law known as [[Cosmic microwave background spectral distortions|spectral distortions]]. These are also at the focus of an active research effort with the hope of a first measurement within the forthcoming decades, as they contain a wealth of information about the primordial universe and the formation of structures at late time.<ref name="Voyage2050">{{cite journal|last=Chluba|first=J.|display-authors=etal|title=New Horizons in Cosmology with Spectral Distortions of the Cosmic Microwave Background|journal=Voyage 2050 Proposals|year=2021|volume=51|issue=3|pages=1515–1554|doi=10.1007/s10686-021-09729-5|arxiv=1909.01593|bibcode=2021ExA....51.1515C|s2cid=202539910|url=https://www.cosmos.esa.int/documents/1866264/3219248/ChlubaJ_Voyage-2050-SDWP-main.pdf/b91871ad-75c4-5b75-3300-049682255629?t=1565184628801}}</ref>

The CMB contains the vast majority of photons in the universe by a factor of 400 to 1;<ref name=HistoryOfAlternatives>{{Cite journal |last1=Ćirković |first1=Milan M. |last2=Perović |first2=Slobodan |date=2018-05-01 |title=Alternative explanations of the cosmic microwave background: A historical and an epistemological perspective |url=https://www.sciencedirect.com/science/article/pii/S1355219816302039 |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |volume=62 |pages=1–18 |doi=10.1016/j.shpsb.2017.04.005 |arxiv=1705.07721 |bibcode=2018SHPMP..62....1C |issn=1355-2198}}</ref>{{rp|5}} the number density of photons in the CMB is one billion times  (10<sup>9</sup>) the number density of matter in the universe. Without the expansion of the universe to cause the cooling of the CMB, the night sky would shine as brightly as the Sun.<ref>K.A. Olive and J.A. Peacock
(September 2017) [https://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf "21. Big-Bang Cosmology"]
in .S. Navas et al. (Particle Data Group), to be published in Phys. Rev. D 110, 030001 (2024)</ref> The energy density of the CMB is {{convert|0.260|eV/cm3|J/m3|abbr=on}}, about 411 photons/cm<sup>3</sup>.<ref>{{cite web| url = https://pdg.lbl.gov/2020/reviews/rpp2020-rev-cosmic-microwave-background.pdf| title = 29. Cosmic Microwave Background: Particle Data Group P.A. Zyla (LBL, Berkeley) et al.}}</ref>

==History==

===Early speculations===
In 1931, [[Georges Lemaître]] speculated that remnants of the early universe may be observable as radiation, but his candidate was [[cosmic rays]].<ref name=PeeblesPrinciples/>{{rp|140}} [[Richard C. Tolman]] showed in 1934 that expansion of the universe would cool blackbody radiation while maintaining a thermal spectrum.
The cosmic microwave background was first predicted in 1948 by [[Ralph Alpher]] and [[Robert Herman]], in a correction<ref>
{{cite journal|last1=Alpher|first1=R. A.|last2=Herman|first2=R. C.|s2cid=4113488|date=1948|title=Evolution of the Universe|journal=[[Nature (journal)|Nature]]|volume=162|issue=4124|pages=774–775|doi=10.1038/162774b0|bibcode=1948Natur.162..774A}}</ref> they prepared for a paper by Alpher's PhD advisor [[George Gamow]].<ref>
{{cite journal|last=Gamow|first=G.|s2cid=4793163|date=1948|title=The evolution of the universe|journal=[[Nature (journal)|Nature]]|volume=162|pages=680–682|doi=10.1038/162680a0|pmid=18893719|bibcode = 1948Natur.162..680G|issue=4122}}</ref> Alpher and Herman were able to estimate the temperature of the cosmic microwave background to be 5&nbsp;K.<ref>
{{cite journal|last1=Assis|first1=A. K. T.|last2=Neves|first2=M. C. D.|date=1995|title=History of the 2.7&nbsp;K Temperature Prior to Penzias and Wilson|url=http://www.ifi.unicamp.br/~assis/Apeiron-V2-p79-84(1995).pdf|issue=3|pages=79–87 |periodical=Apeiron }}</ref>

===Discovery===
{{See also|Discovery of cosmic microwave background radiation}}
[[File:Horn Antenna-in Holmdel, New Jersey - restoration1.jpg|thumb|left|The [[Holmdel Horn Antenna]] on which Penzias and Wilson discovered the cosmic microwave background.<ref name="NYT-20230905" />]]

The first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by [[Soviet Union|Soviet]] astrophysicists [[A. G. Doroshkevich]] and [[Igor Dmitriyevich Novikov|Igor Novikov]], in the spring of 1964.<ref name="Penzias-Nobel-1979">{{Cite journal |last=Penzias |first=Arno A. |date=1979-07-01 |title=The origin of the elements |url=https://link.aps.org/doi/10.1103/RevModPhys.51.425 |journal=Reviews of Modern Physics |archive-url=https://web.archive.org/web/20060925205437/http://nobelprize.org/nobel_prizes/physics/laureates/1978/penzias-lecture.pdf|archive-date=2006-09-25 | language=en |volume=51 |issue=3 |pages=425–431 |doi=10.1103/RevModPhys.51.425 |issn=0034-6861|url-access=subscription }}</ref> In 1964, [[David Todd Wilkinson]] and Peter Roll, [[Robert H. Dicke]]'s colleagues at [[Princeton University]], began constructing a [[Dicke radiometer]] to measure the cosmic microwave background.<ref>
{{cite journal|last=Dicke|first=R. H.|date=1946|title=The Measurement of Thermal Radiation at Microwave Frequencies|journal=[[Review of Scientific Instruments]]|volume=17|pages=268–275|doi=10.1063/1.1770483|pmid=20991753|bibcode = 1946RScI...17..268D|issue=7 |s2cid=26658623 |doi-access=free}} This basic design for a radiometer has been used in most subsequent cosmic microwave background experiments.</ref> In 1964, [[Arno Penzias]] and [[Robert Woodrow Wilson]] at the [[Crawford Hill]] location of [[Bell Telephone Laboratories]] in nearby [[Holmdel Township, New Jersey]] had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. The antenna was constructed in 1959 to support [[Project Echo]]—the National Aeronautics and Space Administration's passive communications satellites, which used large Earth orbiting aluminized plastic balloons as reflectors to bounce radio signals from one point on the Earth to another.<ref name="NYT-20230905">{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye |title=Back to New Jersey, Where the Universe Began - A half-century ago, a radio telescope in Holmdel, N.J., sent two astronomers 13.8 billion years back in time — and opened a cosmic window that scientists have been peering through ever since.|url=https://www.nytimes.com/2023/09/04/science/astronomy-holmdel-antenna-microwaves.html |date=5 September 2023 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20230905113310/https://www.nytimes.com/2023/09/04/science/astronomy-holmdel-antenna-microwaves.html |archivedate=5 September 2023 |accessdate=5 September 2023 }}</ref> On 20&nbsp;May 1964 they made their first measurement clearly showing the presence of the microwave background,<ref>{{cite web| url = https://www.nobelprize.org/uploads/2018/06/wilson-lecture-1.pdf| title = The Cosmic Microwave Background Radiation (Nobel Lecture) by Robert Wilson 8 Dec 1978, p.&nbsp;474}}</ref> with their instrument having an excess 4.2K [[noise temperature|antenna temperature]] which they could not account for. After receiving a telephone call from Crawford Hill, Dicke said "Boys, we've been scooped."<ref name="Penzias&Wilson">{{cite journal |last1=Penzias |first1=A. A. |last2=Wilson|first2=R. W. |date=1965 |title=A Measurement of Excess Antenna Temperature at 4080 Mc/s |journal=[[The Astrophysical Journal]] |volume=142 |issue=1 |pages=419–421 |bibcode=1965ApJ...142..419P |doi=10.1086/148307|doi-access=free }}</ref><ref>{{cite web |author=Smoot Group |date=28 March 1996 |title=The Cosmic Microwave Background Radiation. |url=http://aether.lbl.gov/www/science/cmb.html |publisher=[[Lawrence Berkeley Lab]] |access-date=2008-12-11}}</ref><ref>
{{cite journal|last=Dicke|first=R. H.|date=1965|title=Cosmic Black-Body Radiation|journal=[[Astrophysical Journal]]|volume=142|pages=414–419|doi=10.1086/148306|bibcode=1965ApJ...142..414D|display-authors=etal}}</ref><ref name=PeeblesPrinciples>{{cite book|last=Peebles|first=P. J. E|date=1993|title=Principles of Physical Cosmology|pages=[https://archive.org/details/principlesofphys00pjep/page/139 139–148]|publisher=[[Princeton University Press]]|isbn=978-0-691-01933-8|url=https://archive.org/details/principlesofphys00pjep/page/139}}</ref>{{rp|140}} A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 [[Nobel Prize in Physics]] for their discovery.<ref name="PenziasWilsonNobelSummary">{{cite web|date=1978|title=The Nobel Prize in Physics 1978|url=http://nobelprize.org/nobel_prizes/physics/laureates/1978/|publisher=[[Nobel Foundation]]|access-date=2009-01-08}}</ref>

===Cosmic origin===
The interpretation of the cosmic microwave background was a controversial issue in the late 1960s. Alternative explanations included energy from within the [[Solar System]], from galaxies, from intergalactic plasma and from multiple extragalactic radio sources. Two requirements would show that the microwave radiation was truly "cosmic". First, the intensity vs frequency or spectrum needed to be shown to match a thermal or blackbody source. This was accomplished by 1968 in a series of measurements of the radiation temperature at higher and lower wavelengths. Second, the radiation needed be shown to be isotropic, the same from all directions. This was also accomplished by 1970, demonstrating that this radiation was truly cosmic in origin.<ref name=PartridgeReview>{{Cite book |last=Partridge |first=R. Bruce |url=https://academic.oup.com/edited-volume/34295/chapter/290745058 |title=The Oxford Handbook of the History of Modern Cosmology |date=2019-04-04 |publisher=Oxford University Press |isbn=978-0-19-881766-6 |editor-last=Kragh |editor-first=Helge |edition=1 |pages=292–345 |language=en |chapter=The cosmic microwave background: from discovery to precision cosmology |doi=10.1093/oxfordhb/9780198817666.013.8 |editor-last2=Longair |editor-first2=Malcolm S.}}</ref>

=== Progress on theory ===
In the 1970s numerous studies showed that tiny deviations from isotropy in the CMB could result from events in the early universe.<ref name=PartridgeReview/>{{rp|8.5.1}}
Harrison,<ref>
{{cite journal|last=Harrison|first=E. R.|date=1970|title=Fluctuations at the threshold of classical cosmology|journal=[[Physical Review D]]|volume=1|pages=2726–2730|doi=10.1103/PhysRevD.1.2726|bibcode = 1970PhRvD...1.2726H|issue=10 }}</ref> Peebles and Yu,<ref>{{cite journal|last1=Peebles|first1=P. J. E.|last2=Yu|first2=J. T.|date=1970|title=Primeval Adiabatic Perturbation in an Expanding Universe|journal=[[Astrophysical Journal]]|volume=162|pages=815–836|doi=10.1086/150713|bibcode=1970ApJ...162..815P}}</ref> and Zel'dovich<ref>
{{cite journal|last=Zeldovich|first=Y. B.|date=1972|title=A hypothesis, unifying the structure and the entropy of the Universe|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=160|pages=1P–4P|doi=10.1093/mnras/160.1.1P |doi-access=free |bibcode=1972MNRAS.160P...1Z}}</ref> realized that the early universe would require quantum inhomogeneities that would result in temperature anisotropy at the level of 10<sup>−4</sup> or 10<sup>−5</sup>.<ref name=PartridgeReview/>{{rp|8.5.3.2}} [[Rashid Sunyaev]], using the alternative name ''relic radiation'',  calculated the observable imprint that these inhomogeneities would have on the cosmic microwave background.<ref>{{cite journal|author=Sunyaev RA|author2=Zel'dovich YB|title=Small-scale fluctuations of relic radiation|journal=Astrophys. Space Sci.|volume=7|issue=1|pages=3–19|date=1970|bibcode=1970Ap&SS...7....3S|doi=10.1007/BF00653471|s2cid=117050217|url=https://link.springer.com/article/10.1007/BF00653471|url-access=subscription}}</ref>

===COBE===
After a lull in the 1970s caused in part by the many experimental difficulties in measuring CMB at high precision,<ref name=PartridgeReview/>{{rp|8.5.1}}
increasingly stringent limits on the anisotropy of the cosmic microwave background were set by ground-based experiments during the 1980s. [[RELIKT-1]], a Soviet cosmic microwave background anisotropy experiment on board the Prognoz 9 satellite (launched 1 July 1983), gave the first upper limits on the large-scale anisotropy.<ref name=PartridgeReview/>{{rp|8.5.3.2}}

The other key event in the 1980s was the proposal by [[Alan Guth]] for [[cosmic inflation]]. This theory of rapid spatial expansion gave an explanation for large-scale isotropy by allowing causal connection just before the epoch of last scattering.<ref name=PartridgeReview/>{{rp|8.5.4}} With this and similar theories, detailed prediction encouraged larger and more ambitious experiments.

The [[NASA]] Cosmic Background Explorer ([[Cosmic Background Explorer|COBE]]) satellite orbited Earth in 1989–1996 detected and quantified the large-scale anisotropies at the limit of its detection capabilities.
The [[NASA]] [[Cosmic Background Explorer|COBE]] mission clearly confirmed the primary anisotropy with the Differential Microwave Radiometer instrument, publishing their findings in 1992.<ref>
{{cite journal|last=Smoot|first=G. F.|date=1992|title=Structure in the COBE differential microwave radiometer first-year maps|journal=[[Astrophysical Journal Letters]]|volume=396|issue=1|pages=L1–L5|doi=10.1086/186504|bibcode=1992ApJ...396L...1S| s2cid=120701913 |display-authors=etal|doi-access=free}}</ref><ref>
{{cite journal|last=Bennett|first=C.L.|year=1996|title=Four-Year COBE DMR Cosmic Microwave Background Observations: Maps and Basic Results|journal=[[Astrophysical Journal Letters]]|volume=464|pages=L1–L4|doi=10.1086/310075|bibcode=1996ApJ...464L...1B|arxiv = astro-ph/9601067 |s2cid=18144842|display-authors=etal}}</ref> The team received the [[Nobel Prize]] in physics for 2006 for this discovery.

=== Precision cosmology ===
Inspired by the COBE results, a series of ground and balloon-based experiments measured cosmic microwave background anisotropies on smaller angular scales over the{{Which|date=June 2024}} two decades. The sensitivity of the new experiments improved dramatically, with a reduction in internal noise by three orders of magnitude.<ref name="Komatsu2022Review">{{Cite journal |last=Komatsu |first=Eiichiro |date=2022-05-18 |title=New physics from the polarized light of the cosmic microwave background |url=https://www.nature.com/articles/s42254-022-00452-4 |journal=Nature Reviews Physics |language=en |volume=4 |issue=7 |pages=452–469 |doi=10.1038/s42254-022-00452-4 |issn=2522-5820|arxiv=2202.13919 |bibcode=2022NatRP...4..452K }}</ref> The primary goal of these experiments was to measure the scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve. This peak corresponds to large scale density variations in the early universe that are created by gravitational instabilities, resulting in acoustical oscillations in the plasma.<ref>
{{cite book|last=Grupen|first=C. |date=2005|title=Astroparticle Physics|pages=240–241|publisher=[[Springer Science+Business Media|Springer]]|isbn=978-3-540-25312-9|display-authors=etal}}</ref> The first peak in the anisotropy was tentatively detected by the [[Mobile Anisotropy Telescope|MAT/TOCO]] experiment<ref>
{{cite journal|last=Miller|first=A. D.|date=1999|title=A Measurement of the Angular Power Spectrum of the Microwave Background Made from the High Chilean Andes|journal=[[Astrophysical Journal]]|volume=521|issue=2|pages=L79–L82|doi=10.1086/312197|bibcode=1999ApJ...521L..79T|arxiv = astro-ph/9905100 |s2cid=16534514|display-authors=etal}}</ref> and the result was confirmed by the [[BOOMERanG experiment|BOOMERanG]]<ref>
{{cite journal|last=Melchiorri|first=A.|date=2000|title=A Measurement of Ω from the North American Test Flight of Boomerang|journal=[[The Astrophysical Journal Letters]]|volume=536|issue=2|pages=L63–L66|doi=10.1086/312744|pmid=10859119|bibcode=2000ApJ...536L..63M|arxiv = astro-ph/9911445 |s2cid=27518923|display-authors=etal}}</ref> and [[Millimeter Anisotropy eXperiment IMaging Array|MAXIMA]] experiments.<ref>
{{cite journal|last=Hanany|first=S.|date=2000|title=MAXIMA-1: A Measurement of the Cosmic Microwave Background Anisotropy on Angular Scales of 10'–5°|journal=[[Astrophysical Journal]]|volume=545|issue=1|pages=L5–L9|doi=10.1086/317322|bibcode=2000ApJ...545L...5H|arxiv = astro-ph/0005123 |s2cid=119495132|display-authors=etal}}</ref> These measurements demonstrated that the [[Shape of the universe|geometry of the universe]] is approximately flat, rather than [[curved space|curved]].<ref>
{{cite journal|last=de Bernardis|first=P.|year=2000|title=A flat Universe from high-resolution maps of the cosmic microwave background radiation|journal=[[Nature (journal)|Nature]]|volume=404|pmid=10801117|issue=6781|pages=955–959|bibcode=2000Natur.404..955D|doi=10.1038/35010035|arxiv = astro-ph/0004404 |display-authors=etal|hdl=10044/1/60851|s2cid=4412370}}</ref> They ruled out [[cosmic string]]s as a major component of cosmic structure formation and suggested [[cosmic inflation]] was the right theory of structure formation.<ref>
{{cite journal|last=Pogosian|first=L.|author-link1=Levon Pogosian|year=2003|title=Observational constraints on cosmic string production during brane inflation|journal=[[Physical Review D]]|volume=68|issue=2|pages=023506|doi=10.1103/PhysRevD.68.023506|arxiv = hep-th/0304188 |bibcode = 2003PhRvD..68b3506P |display-authors=etal}}</ref>

===Observations after COBE===
{{Main|List of cosmic microwave background experiments}}
[[File:PIA16874-CobeWmapPlanckComparison-20130321.jpg|thumb|240px|Comparison of [[CMB]] results from [[Cosmic Background Explorer|COBE]], [[WMAP]] and [[Planck (spacecraft)|Planck]]<br />(March 21, 2013)]]
Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out [[cosmic string]]s as the leading theory of cosmic structure formation, and suggested [[cosmic inflation]] was the right theory.

During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the [[BOOMERanG experiment]] reported that the highest power fluctuations occur at scales of approximately one angular degree. Together with other cosmological data, these results implied that the geometry of the universe is [[flat spacetime|flat]]. A number of ground-based [[interferometer]]s provided measurements of the fluctuations with higher accuracy over the next three years, including the [[Very Small Array]], [[Degree Angular Scale Interferometer]] (DASI), and the [[Cosmic Background Imager]] (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum.

===Wilkinson Microwave Anisotropy Probe===
{{main |Wilkinson Microwave Anisotropy Probe}}
In June 2001, [[NASA]] launched a second CMB space mission, [[WMAP]], to make much more precise measurements of the large-scale anisotropies over the full sky. [[WMAP]] used symmetric, rapid-multi-modulated scanning, rapid switching radiometers at five frequencies to minimize non-sky signal noise.<ref name="FirstWMAP"/> The data from the mission was released in five installments, the last being the nine-year summary.
The results are broadly consistent [[Lambda CDM]] models based on 6 free parameters and fitting in to Big Bang cosmology with [[cosmic inflation]].<ref name=WMAP9Map>{{Cite journal |last1=Bennett |first1=C. L. |last2=Larson |first2=D. |last3=Weiland |first3=J. L. |last4=Jarosik |first4=N. |last5=Hinshaw |first5=G. |last6=Odegard |first6=N. |last7=Smith |first7=K. M. |last8=Hill |first8=R. S. |last9=Gold |first9=B. |last10=Halpern |first10=M. |last11=Komatsu |first11=E. |last12=Nolta |first12=M. R. |last13=Page |first13=L. |last14=Spergel |first14=D. N. |last15=Wollack |first15=E. |date=2013-09-20 |title=NINE-YEAR ''WILKINSON MICROWAVE ANISOTROPY PROBE'' ( ''WMAP'' ) OBSERVATIONS: FINAL MAPS AND RESULTS |url=https://iopscience.iop.org/article/10.1088/0067-0049/208/2/20 |journal=The Astrophysical Journal Supplement Series |volume=208 |issue=2 |pages=20 |doi=10.1088/0067-0049/208/2/20 |issn=0067-0049|arxiv=1212.5225 |bibcode=2013ApJS..208...20B }}</ref>

===Degree Angular Scale Interferometer===
{{excerpt|article=Degree Angular Scale Interferometer|paragraph=1}}

===Atacama Cosmology Telescope===
{{excerpt|article=Atacama Cosmology Telescope|paragraph=1}}

===Planck Surveyor===
{{main | Planck Surveyor}}
A third space mission, the [[European Space Agency|ESA]] (European Space Agency) [[Planck Surveyor]], was launched in May 2009 and performed an even more detailed investigation until it was shut down in October 2013. Planck employed both [[HEMT]] radiometers and [[bolometer]] technology and measured the CMB at a smaller scale than WMAP. Its detectors were trialled in the Antarctic [[Viper telescope]] as ACBAR ([[Arcminute Cosmology Bolometer Array Receiver]]) experiment—which has produced the most precise measurements at small angular scales to date—and in the [[Archeops]] balloon telescope.

On 21 March 2013, the European-led research team behind the [[Planck (spacecraft)|''Planck'' cosmology probe]] released the mission's all-sky map ([https://web.archive.org/web/20131202233029/http://esacmt.esac.esa.int/science-e-media/img/61/51553_Planck_CMB_Mollweide_565.jpg 565x318 jpeg], [https://web.archive.org/web/20170215024745/https://www.nasa.gov/images/content/735683main_pia16873-full_full.jpg 3600x1800 jpeg]) of the cosmic microwave background.<ref name="NASA-20130321">{{cite web|last1=Clavin |first1=Whitney |last2=Harrington |first2=J.D. |title=Planck Mission Brings Universe Into Sharp Focus |url=http://www.jpl.nasa.gov/news/news.php?release=2013-109&rn=news.xml&rst=3739 |date=21 March 2013|website=[[NASA]] |access-date=21 March 2013 }}</ref><ref name="NYT-20130321g">{{cite web |author=Staff |title=Mapping the Early Universe |url=https://www.nytimes.com/interactive/2013/03/21/science/space/0321-universe.html |date=21 March 2013 |website=[[The New York Times]] |access-date=23 March 2013 }}</ref> The map suggests the universe is slightly older than researchers expected. According to the map, subtle fluctuations in temperature were imprinted on the deep sky when the cosmos was about {{val|370000}} years old. The imprint reflects ripples that arose as early, in the existence of the universe, as the first [[Nonillion#Standard dictionary numbers|nonillionth]] (10<sup>−30</sup>) of a second. Apparently, these ripples gave rise to the present vast [[Cosmic Web#Large-scale structure|cosmic web]] of [[galaxy cluster]]s and [[dark matter]]. Based on the 2013 data, the universe contains 4.9% [[matter|ordinary matter]], 26.8% [[dark matter]] and 68.3% [[dark energy]]. On 5 February 2015, new data was released by the ''Planck'' mission, according to which the age of the universe is {{val|13.799|0.021}} [[1,000,000,000 (number)|billion]] years old and the [[Hubble constant]] was measured to be {{val|67.74|0.46|u=(km/s)/Mpc}}.<ref name="Planck 2015">{{cite journal
 |author=Planck Collaboration
 |year=2016
 |title=Planck 2015 results. XIII. Cosmological parameters (See Table 4 on page 31 of pfd)
 |arxiv=1502.01589
|bibcode = 2016A&A...594A..13P
 |doi=10.1051/0004-6361/201525830
 |volume=594
 |issue=13
 |journal=Astronomy & Astrophysics
 |pages=A13|s2cid=119262962
 }}</ref>

=== South Pole Telescope ===
{{excerpt|article= South Pole Telescope| paragraph=1}}

==Theoretical models==
{{For|details about the reasoning that the radiation is evidence for the Big Bang|Big Bang#Cosmic microwave background radiation}}
The cosmic microwave background radiation and the [[Hubble's law|cosmological redshift]]-distance relation are together regarded as the best available evidence for the [[Big Bang]] event. Measurements of the CMB have made the inflationary Big Bang model the [[Standard Cosmological Model]].<ref>{{cite journal|last=Scott|first=D.|date=2005|title=The Standard Cosmological Model|arxiv=astro-ph/0510731|doi=10.1139/P06-066|volume=84|issue=6–7|journal=Canadian Journal of Physics|pages=419–435|bibcode = 2006CaJPh..84..419S |citeseerx=10.1.1.317.2954|s2cid=15606491}}</ref> The discovery of the CMB in the mid-1960s curtailed interest in [[non-standard cosmology|alternatives]] such as the [[steady state theory]].<ref>{{cite book|author=Durham, Frank|author2=Purrington, Robert D.|title=Frame of the universe: a history of physical cosmology|url=https://archive.org/details/frameofuniverseh0000durh|url-access=registration|publisher=Columbia University Press|date=1983|isbn=978-0-231-05393-8|pages=[https://archive.org/details/frameofuniverseh0000durh/page/193 193–209]}}</ref>

In the [[Big Bang]] model for the formation of the [[universe]], [[inflation (cosmology)|inflationary cosmology]] predicts that after about 10<sup>−37</sup> seconds<ref>{{cite book|last=Guth|first=A. H.|date=1998|title=The Inflationary Universe: The Quest for a New Theory of Cosmic Origins|page=[https://archive.org/details/inflationaryuniv0000guth/page/186 186]|publisher=[[Basic Books]]|isbn=978-0201328400|oclc=35701222|url=https://archive.org/details/inflationaryuniv0000guth/page/186}}</ref> the nascent universe underwent [[exponential growth]] that smoothed out nearly all irregularities. The remaining irregularities were caused by quantum fluctuations in the [[inflaton]] field that caused the inflation event.<ref>{{cite journal |last1=Cirigliano |first1=D. |last2=de Vega |first2=H.J. |last3=Sanchez |first3=N. G. |author-link3=Norma Sanchez |date=2005 |title=Clarifying inflation models: The precise inflationary potential from effective field theory and the WMAP data |url=https://cds.cern.ch/record/812888 |journal=[[Physical Review D]] |type=Submitted manuscript |volume=71 |issue=10 |pages=77–115 |arxiv=astro-ph/0412634 |bibcode=2005PhRvD..71j3518C |doi=10.1103/PhysRevD.71.103518 |s2cid=36572996}}</ref> Long before the formation of stars and planets, the early universe was more compact, much hotter and, starting 10<sup>−6</sup> seconds after the Big Bang, filled with a uniform glow from its white-hot fog of interacting [[Plasma (physics)|plasma]] of [[photon]]s, [[electron]]s, and [[baryon]]s.

As the universe [[metric expansion of space|expanded]], [[Adiabatic process|adiabatic]] cooling caused the energy density of the plasma to decrease until it became favorable for [[electron]]s to combine with [[proton]]s, forming [[hydrogen]] atoms. This [[recombination (cosmology)|recombination]] event happened when the temperature was around 3000&nbsp;K or when the universe was approximately 379,000&nbsp;years old.<ref>{{cite web|last=Abbott |first=B. |date=2007 |title=Microwave (WMAP) All-Sky Survey |url=http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php |publisher=[[Hayden Planetarium]] |access-date=2008-01-13 |url-status=dead |archive-url=https://web.archive.org/web/20130213023246/http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php |archive-date=2013-02-13 }}</ref> As photons did not interact with these electrically neutral atoms, the former began to travel [[free streaming|freely]] through space, resulting in the [[Decoupling (cosmology)|decoupling]] of matter and radiation.<ref>{{cite journal|last1=Gawiser|first1=E.|last2=Silk|first2=J.|date=2000|title=The cosmic microwave background radiation|journal=[[Physics Reports]]|volume=333–334|issue=2000|pages=245–267|doi=10.1016/S0370-1573(00)00025-9|arxiv=astro-ph/0002044|bibcode = 2000PhR...333..245G |citeseerx=10.1.1.588.3349|s2cid=15398837}}</ref>

The [[color temperature]] of the ensemble of decoupled photons has continued to diminish ever since; now down to {{val|2.7260|0.0013|u=K}},<ref name=apj707_2_916/> it will continue to drop as the universe expands. The intensity of the radiation corresponds to black-body radiation at 2.726&nbsp;K because red-shifted black-body radiation is just like black-body radiation at a lower temperature. According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called ''the surface of last scattering''. This represents the set of locations in space at which the decoupling event is estimated to have occurred<ref>
{{cite web|last=Smoot|first=G. F.|date=2006|title=Cosmic Microwave Background Radiation Anisotropies: Their Discovery and Utilization|url=http://nobelprize.org/nobel_prizes/physics/laureates/2006/smoot-lecture.html|website=Nobel Lecture|publisher=[[Nobel Foundation]]|access-date=2008-12-22}}</ref><ref>{{cite web|url=https://map.gsfc.nasa.gov/media/990053/index.html|title=NASA's "CMB Surface of Last Scatter"|access-date=2023-07-05}}</ref> and at a point in time such that the photons from that distance have just reached observers. Most of the radiation energy in the universe is in the cosmic microwave background,<ref>{{cite book|last1=Hobson|first1=M.P.|last2=Efstathiou|first2=G.|last3=Lasenby|first3=A.N.|date=2006|title=General Relativity: An Introduction for Physicists|url=https://archive.org/details/generalrelativit00hobs_383|url-access=limited|pages=[https://archive.org/details/generalrelativit00hobs_383/page/n407 388]|publisher=[[Cambridge University Press]]|isbn=978-0-521-82951-9}}</ref> making up a fraction of roughly {{val|6|e=-5}} of the total density of the universe.<ref>{{cite book|last1=Unsöld|first1=A.|last2=Bodo|first2=B.|date=2002|title=The New Cosmos, An Introduction to Astronomy and Astrophysics|edition=5th|page=485|publisher=[[Springer-Verlag]]|isbn=978-3-540-67877-9|bibcode=2001ncia.book.....U}}</ref>

Two of the greatest successes of the Big Bang theory are its prediction of the almost perfect black body spectrum and its detailed prediction of the anisotropies in the cosmic microwave background. The CMB spectrum has become the most precisely measured black body spectrum in nature.<ref name="dpf99">
{{cite conference|last=White|first=M.|date=1999|title=Anisotropies in the CMB|book-title=Proceedings of the Los Angeles Meeting, DPF 99|publisher=[[UCLA]]|arxiv=astro-ph/9903232 |bibcode= 1999dpf..conf.....W }}</ref>

=== Predictions based on the Big Bang model ===

In the late 1940s Alpher and Herman reasoned that if there was a Big Bang, the expansion of the universe would have stretched the high-energy radiation of the very early universe into the microwave region of the [[electromagnetic spectrum]], and down to a temperature of about 5&nbsp;K. They were slightly off with their estimate, but they had the right idea. They predicted the CMB. It took another 15 years for Penzias and Wilson to discover that the microwave background was actually there.<ref name="Apeiron2_3_79"/>

According to standard cosmology, the CMB gives a snapshot of the hot early [[universe]] at the point in time when the temperature dropped enough to allow [[electron]]s and [[proton]]s to form [[hydrogen]] atoms. This event made the universe nearly transparent to radiation because light was no longer being [[Thomson scattering|scattered]] off free electrons.<ref>{{cite episode |last=Kaku |first=M. |author-link=Michio Kaku |date=2014 |title=First Second of the Big Bang |series=[[How the Universe Works]] |season=3 |number=4 |network=[[Science Channel|Discovery Science]]}}</ref> When this occurred some 380,000 years after the Big Bang, the temperature of the universe was about 3,000&nbsp;K. This corresponds to an ambient energy of about {{val|0.26|ul=eV}}, which is much less than the {{val|13.6|u=eV}} ionization energy of hydrogen.<ref>{{cite arXiv |eprint=astro-ph/9508159|last1=Fixsen|first1=D. J.|title=Formation of Structure in the Universe|year=1995}}</ref> This epoch is generally known as the "time of last scattering" or the period of [[recombination (cosmology)|recombination]] or [[Decoupling (cosmology)|decoupling]].<ref>{{cite web | url=https://physics.nist.gov/cgi-bin/cuu/Convert?exp=3&num=3&From=k&To=ev&Action=Convert+value+and+show+factor | title=Converted number: Conversion from K to eV}}</ref>

Since decoupling, the color temperature of the background radiation has dropped by an average factor of 1,089<ref name="FirstWMAP"/> due to the expansion of the universe. As the universe expands, the CMB photons are [[redshift]]ed, causing them to decrease in energy. The color temperature of this radiation stays [[inversely proportional]] to a parameter that describes the relative expansion of the universe over time, known as the [[scale factor (universe)|scale length]]. The color temperature ''T''<sub>r</sub> of the CMB as a function of redshift, ''z'', can be shown to be proportional to the color temperature of the CMB as observed in the present day (2.725&nbsp;K or 0.2348&nbsp;meV):<ref>{{cite journal | author=Noterdaeme, P. | author2=Petitjean, P. | author3=Srianand, R. | author4=Ledoux, C. | author5=López, S. | title=The evolution of the cosmic microwave background temperature. Measurements of T<sub>CMB</sub> at high redshift from carbon monoxide excitation | journal=Astronomy and Astrophysics | volume=526 |date=February 2011 | doi=10.1051/0004-6361/201016140 | bibcode=2011A&A...526L...7N | arxiv=1012.3164 | pages=L7 | s2cid=118485014 }}</ref>
:''T''<sub>r</sub> = 2.725&nbsp;K × (1 + ''z'')

The high degree of uniformity throughout the [[observable universe]] and its faint but measured anisotropy lend strong support for the Big Bang model in general and the [[Lambda-CDM model|ΛCDM ("Lambda Cold Dark Matter") model]] in particular. Moreover, the fluctuations are [[coherence (physics)|coherent]] on angular scales that are larger than the apparent [[cosmological horizon]] at recombination. Either such coherence is acausally [[fine-tuning (physics)|fine-tuned]], or [[cosmic inflation]] occurred.<ref name="hep-ph/0309057">{{cite journal |last=Dodelson |first=S. |year=2003 |title=Coherent Phase Argument for Inflation |journal=[[AIP Conference Proceedings]] |volume=689 |pages=184–196 |arxiv=hep-ph/0309057 |bibcode=2003AIPC..689..184D |doi=10.1063/1.1627736|citeseerx=10.1.1.344.3524 |s2cid=18570203 }}</ref><ref>{{Cite web |last=Baumann |first=D. |date=2011 |title=The Physics of Inflation |url=http://www.damtp.cam.ac.uk/user/db275/TEACHING/INFLATION/Lectures.pdf |publisher=[[University of Cambridge]] |access-date=2015-05-09 |archive-url=https://web.archive.org/web/20180921195002/http://www.damtp.cam.ac.uk/user/db275/TEACHING/INFLATION/Lectures.pdf |archive-date=2018-09-21 |url-status=dead }}</ref>

====Primary anisotropy====
[[File:PowerSpectrumExt.svg|thumb|right|300px|The power spectrum of the cosmic microwave background radiation temperature anisotropy in terms of the angular scale (or [[multipole moment]]). The data shown comes from the [[WMAP]] (2006), [[Arcminute Cosmology Bolometer Array Receiver|Acbar]] (2004) [[BOOMERanG experiment|Boomerang]] (2005), [[Cosmic Background Imager|CBI]] (2004), and [[Very Small Array|VSA]] (2004) instruments. Also shown is a theoretical model (solid line).]]
The [[anisotropy]], or directional dependency, of the cosmic microwave background is divided into two types: primary anisotropy, due to effects that occur at the surface of last scattering and before; and secondary anisotropy, due to effects such as interactions of the background radiation with intervening hot gas or gravitational potentials, which occur between the last scattering surface and the observer.

The structure of the cosmic microwave background anisotropies is principally determined by two effects: acoustic oscillations and [[diffusion damping]] (also called collisionless damping or [[Joseph Silk|Silk]] damping). The acoustic oscillations arise because of a conflict in the [[photon]]–[[baryon]] plasma in the early universe. The pressure of the photons tends to erase anisotropies, whereas the gravitational attraction of the baryons, moving at speeds much slower than light, makes them tend to collapse to form overdensities. These two effects compete to create acoustic oscillations, which give the microwave background its characteristic peak structure. The peaks correspond, roughly, to resonances in which the photons decouple when a particular mode is at its peak amplitude.

The peaks contain interesting physical signatures. The angular scale of the first peak determines the [[shape of the universe|curvature of the universe]] (but not the [[topology]] of the universe). The next peak—ratio of the odd peaks to the even peaks—determines the reduced baryon density.<ref>{{cite web |url=http://background.uchicago.edu/~whu/intermediate/baryons.html |title=Baryons and Inertia |author=Wayne Hu}}</ref> The third peak can be used to get information about the dark-matter density.<ref>{{cite web |url=http://background.uchicago.edu/~whu/intermediate/driving.html |title=Radiation Driving Force |author=Wayne Hu}}</ref>

The locations of the peaks give important information about the nature of the primordial density perturbations. There are two fundamental types of density perturbations called ''adiabatic'' and ''isocurvature''. A general density perturbation is a mixture of both, and different theories that purport to explain the primordial density perturbation spectrum predict different mixtures.
; Adiabatic density perturbations:In an adiabatic density perturbation, the fractional additional number density of each type of particle (baryons, [[photon]]s, etc.) is the same. That is, if at one place there is a 1% higher number density of baryons than average, then at that place there is a 1% higher number density of photons (and a 1% higher number density in neutrinos) than average. [[Cosmic inflation]] predicts that the primordial perturbations are adiabatic.
; Isocurvature density perturbations:In an isocurvature density perturbation, the sum (over different types of particle) of the fractional additional densities is zero. That is, a perturbation where at some spot there is 1% more energy in baryons than average, 1% more energy in photons than average, and 2% {{em|less}} energy in neutrinos than average, would be a pure isocurvature perturbation. Hypothetical [[cosmic string]]s would produce mostly isocurvature primordial perturbations.

The CMB spectrum can distinguish between these two because these two types of perturbations produce different peak locations. Isocurvature density perturbations produce a series of peaks whose angular scales (''ℓ'' values of the peaks) are roughly in the ratio 1&nbsp;:&nbsp;3&nbsp;:&nbsp;5&nbsp;:&nbsp;..., while adiabatic density perturbations produce peaks whose locations are in the ratio 1&nbsp;:&nbsp;2&nbsp;:&nbsp;3&nbsp;:&nbsp;...<ref name="hu_white_1996">{{cite journal|last1=Hu |first1=W.|last2=White|first2=M.|year=1996|title=Acoustic Signatures in the Cosmic Microwave Background|journal=[[Astrophysical Journal]]|volume=471|pages=30–51|doi=10.1086/177951|bibcode=1996ApJ...471...30H|arxiv = astro-ph/9602019 |s2cid=8791666}}</ref> Observations are consistent with the primordial density perturbations being entirely adiabatic, providing key support for inflation, and ruling out many models of structure formation involving, for example, cosmic strings.

Collisionless damping is caused by two effects, when the treatment of the primordial plasma as [[fluid]] begins to break down:
* the increasing [[mean free path]] of the photons as the primordial plasma becomes increasingly rarefied in an expanding universe,
* the finite depth of the last scattering surface (LSS), which causes the mean free path to increase rapidly during decoupling, even while some Compton scattering is still occurring.
These effects contribute about equally to the suppression of anisotropies at small scales and give rise to the characteristic exponential damping tail seen in the very small angular scale anisotropies.

The depth of the LSS refers to the fact that the decoupling of the photons and baryons does not happen instantaneously, but instead requires an appreciable fraction of the age of the universe up to that era. One method of quantifying how long this process took uses the ''photon visibility function'' (PVF). This function is defined so that, denoting the PVF by ''P''(''t''), the probability that a CMB photon last scattered between time ''t'' and {{nowrap|''t'' + ''dt''}} is given by ''P''(''t''){{thin space}}''dt''.

The maximum of the PVF (the time when it is most likely that a given CMB photon last scattered) is known quite precisely. The first-year [[Wilkinson Microwave Anisotropy Probe|WMAP]] results put the time at which ''P''(''t'') has a maximum as 372,000 years.<ref name="WMAP_1_cosmo_params">{{cite journal|author=WMAP Collaboration|year=2003|title=First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters|journal=[[Astrophysical Journal Supplement Series]]|volume=148|last2=Verde|issue=1|pages=175–194|first2=L.|doi=10.1086/377226|last3=Peiris|first3=H. V.|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.|arxiv=astro-ph/0302209|bibcode=2003ApJS..148..175S|s2cid=10794058| display-authors = 8}}</ref> This is often taken as the "time" at which the CMB formed. However, to figure out how {{em|long}} it took the photons and baryons to decouple, we need a measure of the width of the PVF. The WMAP team finds that the PVF is greater than half of its maximal value (the "full width at half maximum", or FWHM) over an interval of 115,000 years.<ref name="WMAP_1_cosmo_params"/>{{rp|179}} By this measure, decoupling took place over roughly 115,000 years, and thus when it was complete, the universe was roughly 487,000 years old.

====Late time anisotropy====
Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy. When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of the volume of the [[intergalactic medium]] (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of [[reionization]] during which some of the material of the universe was broken into hydrogen ions.

The CMB photons are scattered by free charges such as electrons that are not bound in atoms. In an ionized universe, such charged particles have been liberated from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are at sufficiently low density in most of the volume of the universe that they do not measurably affect the CMB. However, if the IGM was ionized at very early times when the universe was still denser, then there are two main effects on the CMB:
# Small scale anisotropies are erased. (Just as when looking at an object through fog, details of the object appear fuzzy.)
# The physics of how photons are scattered by free electrons ([[Thomson scattering]]) induces polarization anisotropies on large angular scales. This broad angle polarization is correlated with the broad angle temperature perturbation.

Both of these effects have been observed by the WMAP spacecraft, providing evidence that the universe was ionized at very early times, at a [[redshift]] around 10.<ref name="WMAP9Cosmo"/> The detailed provenance of this early ionizing radiation is still a matter of scientific debate. It may have included starlight from the very first population of stars ([[population III]] stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes.

The time following the emission of the cosmic microwave background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as the [[Timeline of the Big Bang#Dark Ages|Dark Age]], and is a period which is under intense study by astronomers (see [[21 centimeter radiation]]).

Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, are the [[Sunyaev–Zeldovich effect]], where a cloud of high-energy electrons scatters the radiation, transferring some of its energy to the CMB photons, and the [[Sachs–Wolfe effect]], which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields.

===Alternative theories===
{{main| Non-standard cosmology }}
The standard cosmology that includes the Big Bang "enjoys considerable popularity among the practicing cosmologists"<ref name=NarlikarPadmanabhan>{{Cite journal |last1=Narlikar |first1=Jayant V. |last2=Padmanabhan |first2=T. |date=September 2001 |title=Standard Cosmology and Alternatives: A Critical Appraisal |url=https://www.annualreviews.org/doi/10.1146/annurev.astro.39.1.211 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=39 |issue=1 |pages=211–248 |doi=10.1146/annurev.astro.39.1.211 |bibcode=2001ARA&A..39..211N |issn=0066-4146|url-access=subscription }}</ref>{{rp|211}}
However, there are challenges to the standard big bang framework for explaining CMB data. In particular standard cosmology requires [[Fine-tuning (physics)|fine-tuning]] of some free parameters, with different values supported by different experimental data.<ref name=NarlikarPadmanabhan/>{{rp|245}}
As an example of the fine-tuning issue, standard cosmology cannot predict the present temperature of the relic radiation, <math>T_0</math>.<ref name=NarlikarPadmanabhan/>{{rp|229}} This value of <math>T_0</math> is one of the best results of experimental cosmology and the [[steady state model]] can predict it.<ref name="Apeiron2_3_79"/>
However, alternative models have their own set of problems and they have only made post-facto explanations of existing observations.<ref name=NarlikarPadmanabhan/>{{rp|239}} Nevertheless, these alternatives have played an important historic role in providing ideas for and challenges to the standard explanation.<ref name=HistoryOfAlternatives/>

==Polarization==
[[File:CMB_power_spectra_-_TT,_EE,_BB.pdf|thumb|400px|right|Temperature power spectrum and E-mode and B-mode polarization power spectra of the cosmic microwave background]]
<!-- This section is linked from [[Cosmic inflation]] -->
The cosmic microwave background is [[polarization (waves)|polarized]] at the level of a few microkelvin. There are two types of polarization, called E-mode (or gradient-mode) and B-mode (or curl mode).<ref name=Trippe2014/> This is in analogy to [[electrostatics]], in which the electric field (''E''-field) has a vanishing [[curl (mathematics)|curl]] and the magnetic field (''B''-field) has a vanishing [[divergence]].

=== E-modes ===
The E-modes arise from [[Thomson scattering]] in a heterogeneous plasma.<ref name=Trippe2014>{{Cite journal |last=Trippe |first=Sascha |date=2014 |title=Polarization and Polarimetry: A Review |url=http://koreascience.or.kr/article/JAKO201408739562367.page |journal=Journal of the Korean Astronomical Society |volume=47 |issue=1 |pages=15–39 |doi=10.5303/JKAS.2014.47.1.15 |issn=1225-4614|arxiv=1401.1911 |bibcode=2014JKAS...47...15T }}</ref>
E-modes were first seen in 2002 by the [[Degree Angular Scale Interferometer]] (DASI).<ref>{{Cite journal |last1=Kovac |first1=J. M. |last2=Leitch |first2=E. M. |last3=Pryke |first3=C. |last4=Carlstrom |first4=J. E. |last5=Halverson |first5=N. W. |last6=Holzapfel |first6=W. L. |date=December 2002 |title=Detection of polarization in the cosmic microwave background using DASI |url=https://www.nature.com/articles/nature01269 |journal=Nature |language=en |volume=420 |issue=6917 |pages=772–787 |doi=10.1038/nature01269 |pmid=12490941 |issn=0028-0836|arxiv=astro-ph/0209478 |bibcode=2002Natur.420..772K }}</ref><ref>{{Cite journal |last1=Ade |first1=P. A. R. |last2=Aikin |first2=R. W. |last3=Barkats |first3=D. |last4=Benton |first4=S. J. |last5=Bischoff |first5=C. A. |last6=Bock |first6=J. J. |last7=Brevik |first7=J. A. |last8=Buder |first8=I. |last9=Bullock |first9=E. |last10=Dowell |first10=C. D. |last11=Duband |first11=L. |last12=Filippini |first12=J. P. |last13=Fliescher |first13=S. |last14=Golwala |first14=S. R. |last15=Halpern |first15=M. |date=2014-06-19 |title=Detection of B -Mode Polarization at Degree Angular Scales by BICEP2 |url=https://link.aps.org/doi/10.1103/PhysRevLett.112.241101 |journal=Physical Review Letters |language=en |volume=112 |issue=24 |page=241101 |doi=10.1103/PhysRevLett.112.241101 |pmid=24996078 |issn=0031-9007|arxiv=1403.3985 |bibcode=2014PhRvL.112x1101B }}</ref>

=== B-modes ===
B-modes are expected to be an order of magnitude weaker than the E-modes. The former are not produced by standard scalar type perturbations, but are generated by [[gravitational wave]]s during [[inflation (cosmology)|cosmic inflation]] shortly after the big bang.<ref name=SeljakMeasuring>{{cite journal|first=U.|last=Seljak|title=Measuring Polarization in the Cosmic Microwave Background|journal=Astrophysical Journal|date=June 1997|volume=482|issue=1|pages=6–16|doi=10.1086/304123|arxiv = astro-ph/9608131 |bibcode = 1997ApJ...482....6S |s2cid=16825580}}</ref><ref name=SeljakSignature>{{cite journal|first=U.|last=Seljak|author2=Zaldarriaga M.|title=Signature of Gravity Waves in the Polarization of the Microwave Background|journal=Phys. Rev. Lett.|date=March 17, 1997|volume=78|issue=11|doi=10.1103/PhysRevLett.78.2054|arxiv = astro-ph/9609169 |bibcode = 1997PhRvL..78.2054S|pages=2054–2057|s2cid=30795875}}</ref><ref>{{cite journal|first=M.|last=Kamionkowski|author2= Kosowsky A.|author3= Stebbins A.|name-list-style= amp|title=A Probe of Primordial Gravity Waves and Vorticity|journal=Phys. Rev. Lett.|year=1997|volume=78|issue=11|doi=10.1103/PhysRevLett.78.2058|arxiv = astro-ph/9609132 |bibcode = 1997PhRvL..78.2058K|pages=2058–2061|s2cid=17330375}}</ref>
However, gravitational lensing of the stronger E-modes can also produce B-mode polarization.<ref name=SeljakGraviational>{{cite journal|first=M.|last=Zaldarriaga|author2=Seljak U.|title=Gravitational lensing effect on cosmic microwave background polarization|journal=Physical Review D|date=July 15, 1998|volume=58|issue=2|pages=023003|series=2|doi=10.1103/PhysRevD.58.023003|arxiv = astro-ph/9803150 |bibcode = 1998PhRvD..58b3003Z |s2cid=119512504}}</ref><ref>{{cite journal|last1=Lewis|first1=A.|last2=Challinor|first2=A.|date=2006|title=Weak gravitational lensing of the CMB|journal=[[Physics Reports]]|volume=429|issue=1|pages=1–65|doi = 10.1016/j.physrep.2006.03.002|arxiv=astro-ph/0601594|bibcode = 2006PhR...429....1L |s2cid=1731891}}</ref> Detecting the original B-modes signal requires analysis of the contamination caused by lensing of the relatively strong E-mode signal.<ref>{{cite journal|last=Hanson|first=D.|year=2013|title=Detection of B-mode polarization in the Cosmic Microwave Background with data from the South Pole Telescope|journal=[[Physical Review Letters]]|volume=111|issue=14|pages=141301|doi = 10.1103/PhysRevLett.111.141301|pmid=24138230|arxiv=1307.5830|url = http://www.nature.com/news/polarization-detected-in-big-bang-s-echo-1.13441 |bibcode = 2013PhRvL.111n1301H |s2cid=9437637|display-authors=etal}}</ref>

==== Primordial gravitational waves ====
Models of "slow-roll" [[cosmic inflation]] in the [[early universe]] predicts primordial [[gravitational waves]] that would impact the polarisation of the cosmic microwave background, creating a specific pattern of B-mode polarization. Detection of this pattern would support the theory of inflation and their strength can confirm and exclude different models of inflation.<ref name=SeljakSignature/><ref name="KamionkowskiReview">{{Cite journal |last1=Kamionkowski |first1=Marc |last2=Kovetz |first2=Ely D. |date=2016-09-19 |title=The Quest for B Modes from Inflationary Gravitational Waves |url=https://www.annualreviews.org/doi/10.1146/annurev-astro-081915-023433 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=54 |issue=1 |pages=227–269 |doi=10.1146/annurev-astro-081915-023433 |issn=0066-4146|arxiv=1510.06042 |bibcode=2016ARA&A..54..227K }}</ref>
Claims that this characteristic pattern of B-mode polarization had been measured by [[BICEP and Keck Array|BICEP2]] instrument<ref name="NYT-20140922"/> were later attributed to [[cosmic dust]] due to new results of the [[Planck (spacecraft)|Planck experiment]].<ref name="AXV-20140919">{{Cite journal |author=Planck Collaboration Team |title=Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes |date=9 February 2016 |arxiv=1409.5738 | journal = Astronomy & Astrophysics | volume = 586 |issue=133 | doi = 10.1051/0004-6361/201425034 | pages=A133 | bibcode=2016A&A...586A.133P|s2cid=9857299 }}<!--|access-date=22 September 2014 --></ref><ref name="KamionkowskiReview"/>{{rp|253}}

==== Gravitational lensing ====
[[Image:Gravitational lens-full.jpg|thumb|right|Artist impression of the gravitational lensing effect of massive cosmic structures]]
The second type of B-modes was discovered in 2013 using the [[South Pole Telescope]] with help from the [[Herschel Space Observatory]].<ref>{{Cite journal|url=http://www.nature.com/news/polarization-detected-in-big-bang-s-echo-1.13441|title=Polarization detected in Big Bang's echo|journal=Nature|doi=10.1038/nature.2013.13441|year=2013|last1=Samuel Reich|first1=Eugenie|s2cid=211730550|url-access=subscription}}</ref> In October 2014, a measurement of the B-mode polarization at 150&nbsp;GHz was published by the [[POLARBEAR]] experiment.<ref name="pc1" /> Compared to BICEP2, POLARBEAR focuses on a smaller patch of the sky and is less susceptible to dust effects. The team reported that POLARBEAR's measured B-mode polarization was of cosmological origin (and not just due to dust) at a 97.2% confidence level.<ref name="cs" />

==Multipole analysis==
[[File:WMAP 2008 TT spectra.png|thumb|Example Multipole Power Spectrum. WMAP Data are represented as points, curves correspond to the best-fit LCDM model<ref name="WMAP9Cosmo">{{Cite journal |last1=Hinshaw |first1=G. |last2=Larson |first2=D. |last3=Komatsu |first3=E. |last4=Spergel |first4=D. N. |last5=Bennett |first5=C. L. |last6=Dunkley |first6=J. |last7=Nolta |first7=M. R. |last8=Halpern |first8=M. |last9=Hill |first9=R. S. |last10=Odegard |first10=N. |last11=Page |first11=L. |last12=Smith |first12=K. M. |last13=Weiland |first13=J. L. |last14=Gold |first14=B. |last15=Jarosik |first15=N. |date=2013-09-20 |title=NINE-YEAR ''WILKINSON MICROWAVE ANISOTROPY PROBE'' ( ''WMAP'' ) OBSERVATIONS: COSMOLOGICAL PARAMETER RESULTS |url=https://iopscience.iop.org/article/10.1088/0067-0049/208/2/19 |journal=The Astrophysical Journal Supplement Series |volume=208 |issue=2 |pages=19 |doi=10.1088/0067-0049/208/2/19 |issn=0067-0049|arxiv=1212.5226 |bibcode=2013ApJS..208...19H }}</ref>]]
The CMB angular anisotropies are usually presented in terms of power per multipole.<ref name="cmbreview">{{cite journal |author1=P.A. Zyla et al. (Particle Data Group) |title=Review of Particle Physics |journal=Progress of Theoretical and Experimental Physics |date=2020 |volume=2020 |issue=8 |page=083C01 |doi=10.1093/ptep/ptaa104 |url=https://pdg.lbl.gov/2020/reviews/rpp2020-rev-cosmic-microwave-background.pdf|doi-access=free }} Cosmic Microwave Background review by Scott and Smoot.</ref>
The map of temperature across the sky, <math>T(\theta,\varphi),</math> is written as coefficients of [[spherical harmonics]],
<math display="block">T(\theta,\varphi) = \sum_{\ell m} a_{\ell m} Y_{\ell m}(\theta,\varphi)</math>
where the <math>a_{\ell m}</math> term measures the strength of the angular oscillation in <math>Y_{\ell m}(\theta,\varphi)</math>, and ''ℓ'' is the multipole number while ''m'' is the azimuthal number.
The azimuthal variation is not significant and is removed by applying the [[angular correlation function]], giving power spectrum term <math>C_{\ell}\equiv \langle |a_{\ell m}|^2 \rangle.</math> Increasing values of ''ℓ'' correspond to higher multipole moments of CMB, meaning more rapid variation with angle.

===CMBR monopole term (''ℓ'' = 0)===
The monopole term, {{nowrap|1=''ℓ'' = 0}}, is the constant isotropic mean temperature of the CMB, {{nowrap|1=''T''<sub>''γ''</sub> = {{val|2.7255|0.0006|u=K}}}}<ref name="cmbreview"/> with one standard deviation confidence. This term must be measured with absolute temperature devices, such as the FIRAS instrument on the [[Cosmic Background Explorer|COBE]] satellite.<ref name="cmbreview"/>{{rp|499}}

===CMBR dipole anisotropy (''ℓ'' = 1)===
CMB dipole represents the largest anisotropy, which is in the first spherical harmonic ({{nowrap|1=''ℓ'' = 1}}), a cosine function. The amplitude of CMB dipole is around {{val|3.3621|0.0010|u=mK}}.<ref name="cmbreview"/> The CMB dipole moment is interpreted as the peculiar motion of the Earth relative to the CMB. Its amplitude depends on the time due to the Earth's orbit about the barycenter of the solar system. This enables us to add a time-dependent term to the dipole expression. The modulation of this term is 1 year,<ref name="cmbreview"/><ref name="cobe">{{Cite web|title=COBE Differential Microwave Radiometers: Calibration Techniques.| url=http://articles.adsabs.harvard.edu/pdf/1992ApJ...391..466B| last= Bennett| first=C.}}</ref> which fits the observation done by COBE FIRAS.<ref name="cobe"/><ref>{{Cite journal|title=Dipole Modulation of Cosmic Microwave Background Temperature and Polarization.| last=Shosh| first=S.| journal=Journal of Cosmology and Astroparticle Physics| year=2016| volume =2016 |issue=1|page=046| doi=10.1088/1475-7516/2016/01/046 | arxiv=1507.04078 | bibcode=2016JCAP...01..046G | s2cid=118553819}}</ref> The dipole moment does not encode any primordial information.

From the CMB data, it is seen that the Sun appears to be moving at {{val|369.82|0.11|u=km/s}} relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB). The [[Local Group]] — the galaxy group that includes our own Milky Way galaxy — appears to be moving at {{val|620|15|u=km/s}} in the direction of [[galactic longitude]] {{nowrap|1=''ℓ'' = {{val|271.9|2|u=°}}}}, {{nowrap|1=''b'' = {{val|30|3|u=°}}}}.<ref name="cmbreview"/> The dipole is now used to calibrate mapping studies.

===Multipole (''ℓ'' ≥ 2)===
The temperature variation in the CMB temperature maps at higher multipoles, or {{nowrap|''ℓ'' ≥ 2}}, is considered to be the result of perturbations of the density in the early Universe, before the recombination epoch at a redshift of around&nbsp;{{nowrap|1=''z'' ⋍ 1100}}. Before recombination, the Universe consisted of a hot, dense plasma of electrons and baryons. In such a hot dense environment, electrons and protons could not form any neutral atoms. The baryons in such early Universe remained highly ionized and so were tightly coupled with photons through the effect of Thompson scattering. These phenomena caused the pressure and gravitational effects to act against each other, and triggered fluctuations in the photon-baryon plasma. Quickly after the recombination epoch, the rapid expansion of the universe caused the plasma to cool down and these fluctuations are "frozen into" the CMB maps we observe today.<ref name="cmbreview"/>

===Data analysis challenges===
Raw CMBR data, even from space vehicles such as WMAP or Planck, contain foreground effects that completely obscure the fine-scale structure of the cosmic microwave background. The fine-scale structure is superimposed on the raw CMBR data but is too small to be seen at the scale of the raw data. The most prominent of the foreground effects is the dipole anisotropy caused by the Sun's motion relative to the CMBR background. The dipole anisotropy and others due to Earth's annual motion relative to the Sun and numerous microwave sources in the galactic plane and elsewhere must be subtracted out to reveal the extremely tiny variations characterizing the fine-scale structure of the CMBR background.
The detailed analysis of CMBR data to produce maps, an angular power spectrum, and ultimately cosmological parameters is a complicated, computationally difficult problem.

In practice it is hard to take the effects of noise and foreground sources into account. In particular, these foregrounds are dominated by galactic emissions such as [[bremsstrahlung]], [[Synchrotron radiation#Synchrotron radiation in astronomy|synchrotron]], and [[dust#Dust in other contexts|dust]] that emit in the microwave band; in practice, the galaxy has to be removed, resulting in a CMB map that is not a full-sky map. In addition, point sources like galaxies and clusters represent foreground sources which must be removed so as not to distort the short scale structure of the CMB power spectrum.

Constraints on many cosmological parameters can be obtained from their effects on the power spectrum, and results are often calculated using [[Markov chain Monte Carlo]] sampling techniques.

===Anomalies===
{{See also|Cosmological principle|Axis of evil (cosmology)|CMB cold spot}}

With the increasingly precise data provided by WMAP, there have been a number of claims that the CMB exhibits anomalies, such as very large scale anisotropies, anomalous alignments, and non-Gaussian distributions.<ref name="arXiv:0905.2854v2">{{cite journal| last1=Rossmanith |first1=G. |year=2009 |title=Non-Gaussian Signatures in the five-year WMAP data as identified with isotropic scaling indices |doi=10.1111/j.1365-2966.2009.15421.x| journal=Monthly Notices of the Royal Astronomical Society|volume=399|issue=4| pages=1921–1933|arxiv=0905.2854|bibcode = 2009MNRAS.399.1921R |last2=Räth |first2=C. |last3=Banday|first3=A. J.|last4=Morfill| first4=G. |doi-access=free |s2cid=11586058 }}</ref><ref name="arXiv:astro-ph/0511666">{{cite journal| last1=Bernui| first1=A. |year=2007 |title=Mapping the large-scale anisotropy in the WMAP data| doi=10.1051/0004-6361:20065585 |journal=Astronomy and Astrophysics| volume=464 |issue=2 |pages=479–485 |arxiv=astro-ph/0511666 |bibcode = 2007A&A...464..479B |last2=Mota |first2=B. |last3=Rebouças|first3=M. J.| last4=Tavakol|first4=R.| s2cid=16138962 }}</ref><ref name="arXiv:astro-ph/0503213">{{cite journal|last1=Jaffe|first1=T.R.| year=2005|title=Evidence of vorticity and shear at large angular scales in the WMAP data: a violation of cosmological isotropy?| doi=10.1086/444454|journal=The Astrophysical Journal|volume=629 | issue=1| pages=L1–L4|arxiv=astro-ph/0503213|bibcode = 2005ApJ...629L...1J |last2=Banday |first2=A. J. | last3=Eriksen|first3=H. K. | last4=Górski|first4=K. M.| last5=Hansen|first5=F. K.|s2cid=15521559}}</ref> The most longstanding of these is the low-''ℓ'' multipole controversy. Even in the COBE map, it was observed that the [[quadrupole]] ({{nowrap|1=''ℓ'' = 2}}, spherical harmonic) has a low amplitude compared to the predictions of the Big Bang. In particular, the quadrupole and octupole ({{nowrap|1=''ℓ'' = 3}}) modes appear to have an unexplained alignment with each other and with both the [[plane of the ecliptic|ecliptic plane]] and [[equinox]]es.<ref>{{cite journal |last1=de Oliveira-Costa |first1=A. |year=2004 |title=The significance of the largest scale CMB fluctuations in WMAP|journal=[[Physical Review D]]|volume=69 |pages=063516 | doi=10.1103/PhysRevD.69.063516 |arxiv=astro-ph/0307282 |bibcode = 2004PhRvD..69f3516D |issue=6 |last2=Tegmark |first2=Max |last3=Zaldarriaga |first3=Matias |last4=Hamilton |first4=Andrew| s2cid=119463060 |url=https://cds.cern.ch/record/628847 |type=Submitted manuscript }}</ref><ref>{{cite journal|last1=Schwarz|first1=D. J.|date=2004|title=Is the low-''ℓ'' microwave background cosmic?| journal=[[Physical Review Letters]]| volume=93| pages=221301| doi=10.1103/PhysRevLett.93.221301| pmid=15601079| arxiv=astro-ph/0403353| bibcode=2004PhRvL..93v1301S |issue=22 | last2=Starkman |first2=Glenn D. |last3=Huterer|first3=Dragan|last4=Copi|first4=Craig|s2cid=12554281|display-authors=2|url=https://cds.cern.ch/record/725179|type=Submitted manuscript}}</ref><ref>{{cite journal|last1=Bielewicz| first1=P.|last2=Gorski|first2=K. M.|last3=Banday|first3=A. J.|date=2004 |title=Low-order multipole maps of CMB anisotropy derived from WMAP|journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=355|pages=1283–1302 |doi=10.1111/j.1365-2966.2004.08405.x | arxiv=astro-ph/0405007 |bibcode=2004MNRAS.355.1283B |issue=4 | doi-access=free|s2cid=5564564}}</ref> A number of groups have suggested that this could be the signature of quantum corrections or new physics at the greatest observable scales; other groups suspect systematic errors in the data.<ref>{{Cite journal |last=Cao |first=F. J. |last2=de Vega |first2=H. J. |last3=Sánchez |first3=N. G. |date=2004-10-22 |title=Quantum inflaton, primordial perturbations, and CMB fluctuations |url=https://journals.aps.org/prd/abstract/10.1103/PhysRevD.70.083528 |journal=Physical Review D |volume=70 |issue=8 |pages=083528 |doi=10.1103/PhysRevD.70.083528|url-access=subscription }}</ref><ref name="arXiv:0907.2731v3">{{cite arXiv |last1=Liu|first1=Hao|last2=Li|first2=Ti-Pei|date=2009|title=Improved CMB Map from WMAP Data|class=astro-ph |eprint=0907.2731v3}}</ref><ref name="arXiv:1006.1270v1">{{cite arXiv|last1=Sawangwit|first1=Utane|last2=Shanks|first2=Tom|date=2010|title=Lambda-CDM and the WMAP Power Spectrum Beam Profile Sensitivity|class=astro-ph |eprint=1006.1270v1}}</ref><ref name="arXiv:1009.2701v1">{{cite journal |last=Liu|first=Hao|date=2010|title=Diagnosing Timing Error in WMAP Data |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=413 |issue=1|pages=L96–L100|arxiv=1009.2701v1 |display-authors=etal |bibcode=2011MNRAS.413L..96L| doi=10.1111/j.1745-3933.2011.01041.x |doi-access=free |s2cid=118739762}}</ref>

Ultimately, due to the foregrounds and the [[cosmic variance]] problem, the greatest modes will never be as well measured as the small angular scale modes. The analyses were performed on two maps that have had the foregrounds removed as far as possible: the "internal linear combination" map of the WMAP collaboration and a similar map prepared by [[Max Tegmark]] and others.<ref name="hinshaw07">{{cite journal|last=Hinshaw|first=G.|author2= (WMAP collaboration)|year=2007|title=Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: temperature analysis|journal=[[Astrophysical Journal Supplement Series]]|volume=170|issue=2|pages=288–334|doi=10.1086/513698|arxiv=astro-ph/0603451|bibcode=2007ApJS..170..288H|last3=Bennett|first3=C. L.|last4=Bean|first4=R.|author-link4=Rachel Bean|last5=Doré|first5=O.|last6=Greason|first6=M. R.|last7=Halpern|first7=M.|last8=Hill|first8=R. S.| last9=Jarosik| first9=N.| last10=Kogut| first10=A.| last11=Komatsu|first11=E.|last12=Limon|first12=M.|last13=Odegard|first13=N.|last14=Meyer|first14=S. S.| last15=Page |first15=L. |last16=Peiris |first16=H. V.|last17=Spergel|first17=D. N.|last18=Tucker|first18=G. S.| last19=Verde| first19=L.| last20 =Weiland|first20=J. L.|last21=Wollack|first21=E.|last22=Wright|first22=E. L.|display-authors=etal| citeseerx=10.1.1.471.7186|s2cid=15554608}}</ref><ref name="FirstWMAP">
{{cite journal|last=Bennett|first=C. L.|author2= (WMAP collaboration)|year=2003|title=First-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: preliminary maps and basic results|journal=[[Astrophysical Journal Supplement Series]]|volume=148|issue=1|pages=1–27|doi=10.1086/377253|arxiv=astro-ph/0302207| bibcode=2003ApJS..148....1B| last3=Hinshaw|first3=G.| last4=Jarosik| first4=N.|last5=Kogut| first5=A.| last6=Limon|first6=M.|last7=Meyer|first7=S. S.|last8=Page|first8=L.|last9=Spergel|first9=D. N.|last10=Tucker|first10=G. S.| last11=Wollack |first11=E.| last12=Wright|first12=E. L.|last13=Barnes|first13=C.|last14=Greason|first14=M. R.|last15=Hill|first15=R. S.| last16=Komatsu |first16=E.| last17=Nolta| first17=M. R.|last18=Odegard|first18=N.|last19=Peiris|first19=H. V.|last20=Verde|first20=L.|last21=Weiland|first21=J. L.|s2cid=115601|display-authors=etal}} This paper warns that "the statistics of this internal linear combination map are complex and inappropriate for most CMB analyses."</ref><ref>
{{cite journal|last1=Tegmark|first1=M.|last2=de Oliveira-Costa|first2=A.|last3=Hamilton|first3=A.|year=2003|title=A high resolution foreground cleaned CMB map from WMAP|journal=[[Physical Review D]]|volume=68|pages=123523|doi=10.1103/PhysRevD.68.123523|arxiv=astro-ph/0302496|bibcode = 2003PhRvD..68l3523T|issue=12 |s2cid=17981329}} This paper states, "Not surprisingly, the two most contaminated multipoles are [the quadrupole and octupole], which most closely trace the galactic plane morphology."</ref> Later analyses have pointed out that these are the modes most susceptible to foreground contamination from synchrotron, dust, and bremsstrahlung emission, and from experimental uncertainty in the monopole and dipole.

A full [[Bayesian analysis]] of the WMAP power spectrum demonstrates that the quadrupole prediction of [[Lambda-CDM model|Lambda-CDM cosmology]] is consistent with the data at the 10% level and that the observed octupole is not remarkable.<ref>{{cite journal|last1=O'Dwyer|first1=I.|date=2004|title=Bayesian Power Spectrum Analysis of the First-Year Wilkinson Microwave Anisotropy Probe Data|journal=[[Astrophysical Journal Letters]]|volume=617|pages=L99–L102|doi=10.1086/427386|arxiv=astro-ph/0407027|bibcode=2004ApJ...617L..99O|issue=2|last2=Eriksen |first2=H. K. |last3=Wandelt|first3=B. D.|last4=Jewell|first4=J. B.|last5=Larson|first5=D. L.|last6=Górski|first6=K. M.|last7=Banday|first7=A. J.|last8=Levin|first8=S.|last9=Lilje|first9=P. B. |s2cid=118150531 }}</ref> Carefully accounting for the procedure used to remove the foregrounds from the full sky map further reduces the significance of the alignment by ~5%.<ref>{{cite journal|last1=Slosar|first1=A.|last2=Seljak|first2=U.|date=2004|title=Assessing the effects of foregrounds and sky removal in WMAP|journal=[[Physical Review D]]|volume=70|pages=083002|doi=10.1103/PhysRevD.70.083002|arxiv=astro-ph/0404567|bibcode = 2004PhRvD..70h3002S|issue=8|s2cid=119443655|url=https://cds.cern.ch/record/732816|type=Submitted manuscript}}</ref><ref>{{cite journal|last1=Bielewicz|first1=P.|year=2005|title=Multipole vector anomalies in the first-year WMAP data: a cut-sky analysis|journal=[[Astrophysical Journal]]|volume=635|pages=750–60|doi=10.1086/497263|arxiv=astro-ph/0507186|bibcode=2005ApJ...635..750B|issue=2|last2=Eriksen |first2=H. K. |last3=Banday|first3=A. J.|last4=Górski|first4=K. M.|last5=Lilje|first5=P. B. |s2cid=1103733}}</ref><ref>{{cite journal|last1=Copi |first1=C.J. |year=2006|title=On the large-angle anomalies of the microwave sky|journal=[[Monthly Notices of the Royal Astronomical Society]]|volume=367|issue=1 |pages=79–102|doi=10.1111/j.1365-2966.2005.09980.x|arxiv=astro-ph/0508047 |bibcode=2006MNRAS.367...79C|last2=Huterer |first2=Dragan |last3=Schwarz |first3=D. J. |last4=Starkman |first4=G. D. |doi-access=free |citeseerx=10.1.1.490.6391 |s2cid=6184966 }}</ref><ref>{{cite journal|last1=de Oliveira-Costa|first1=A.| last2=Tegmark| first2=M.|year=2006|title=CMB multipole measurements in the presence of foregrounds|journal=[[Physical Review D]]|volume=74| pages=023005|doi=10.1103/PhysRevD.74.023005|arxiv=astro-ph/0603369|bibcode = 2006PhRvD..74b3005D| issue=2|s2cid=5238226|url=https://cds.cern.ch/record/934594|type=Submitted manuscript}}</ref>
Recent observations with the [[Planck (spacecraft)|Planck telescope]], which is very much more sensitive than WMAP and has a larger angular resolution, record the same anomaly, and so instrumental error (but not foreground contamination) appears to be ruled out.<ref>{{cite web| url = https://www.newscientist.com/article/dn23301-planck-shows-almost-perfect-cosmos--plus-axis-of-evil.html| title = Planck shows almost perfect cosmos – plus axis of evil}}</ref> Coincidence is a possible explanation, chief scientist from [[WMAP]], [[Charles L. Bennett]] suggested coincidence and human psychology were involved, "I do think there is a bit of a psychological effect; people want to find unusual things."<ref>{{cite web| url = https://www.newscientist.com/article/dn18489-found-hawkings-initials-written-into-the-universe.html| title = Found: Hawking's initials written into the universe}}</ref>

Measurements of the density of quasars based on [[Wide-field Infrared Survey Explorer]] data finds a dipole significantly different from the one extracted from the CMB anisotropy.<ref name="SuburSarkar">{{cite journal |title=A Test of the Cosmological Principle with Quasars|journal=The Astrophysical Journal Letters|year=2021|doi=10.3847/2041-8213/abdd40|last1=Secrest|first1=Nathan J.|last2=Hausegger|first2=Sebastian von|last3=Rameez|first3=Mohamed|last4=Mohayaee|first4=Roya|last5=Sarkar|first5=Subir|last6=Colin|first6=Jacques|volume=908|issue=2|pages=L51|arxiv=2009.14826|bibcode=2021ApJ...908L..51S|s2cid=222066749 |doi-access=free }}</ref> This difference is conflict with the [[cosmological principle]].<ref>{{Cite journal |last1=Perivolaropoulos |first1=L. |last2=Skara |first2=F. |date=2022-12-01 |title=Challenges for ΛCDM: An update |url=https://www.sciencedirect.com/science/article/pii/S1387647322000185 |journal=New Astronomy Reviews |volume=95 |pages=101659 |doi=10.1016/j.newar.2022.101659 |issn=1387-6473|arxiv=2105.05208 |bibcode=2022NewAR..9501659P }}</ref>

==Future evolution==
Assuming the universe keeps expanding and it does not suffer a [[Big Crunch]], a [[Big Rip]], or another similar fate, the cosmic microwave background will continue redshifting until it will no longer be detectable,<ref name="endcosmology">
{{cite journal
 |bibcode=2007GReGr..39.1545K
 |doi=10.1007/s10714-007-0472-9
 |title=The return of a static universe and the end of cosmology
 |year=2007
 |last1=Krauss |first1=Lawrence M.
 |last2=Scherrer |first2=Robert J.
 |journal=General Relativity and Gravitation
 |volume=39 |issue=10 |pages=1545–1550
|arxiv = 0704.0221 |s2cid=123442313
 }}</ref> and will be superseded first by the one produced by [[starlight]], and perhaps, later by the background radiation fields of processes that may take place in the far future of the universe such as [[proton decay]], [[Hawking radiation|evaporation of black holes]], and [[positronium]] decay.<ref name="fate">
{{cite journal
 |bibcode=1997RvMP...69..337A
 |arxiv= astro-ph/9701131
 |doi= 10.1103/RevModPhys.69.337
 |title=A dying universe: The long-term fate and evolution of astrophysical objects
 |year=1997
 |last1=Adams |first1=Fred C.
 |last2=Laughlin |first2=Gregory
 |journal=[[Reviews of Modern Physics]]
 |volume=69 |issue=2 |pages=337–372
|s2cid= 12173790
 }}</ref>

== Timeline of prediction, discovery and interpretation ==
{{see also|Timeline of cosmological theories}}

===Thermal (non-microwave background) temperature predictions===
* 1896 – [[Charles Édouard Guillaume]] estimates the "radiation of the stars" to be 5–6&nbsp;[[Kelvin|K]].<ref name="Apeiron2_3_79"/><ref name="G 1896">Guillaume, C.-É., 1896, ''La Nature'' 24, series 2, p. 234</ref>
* 1926 – Sir [[Arthur Eddington]] estimates the non-thermal radiation of [[star#Radiation|starlight]] in the galaxy "... by the formula {{nowrap|1=''E'' = ''σT''<sup>4</sup>}} the effective temperature corresponding to this density is 3.18° absolute ... black body".<ref name="Apeiron2_3_79"/><ref>{{Cite book |date=1979-12-31 |editor-last=Lang |editor-first=Kenneth R. |editor2-last=Gingerich |editor2-first=Owen |chapter=45. The Internal Constitution of the Stars |chapter-url=https://www.degruyter.com/document/doi/10.4159/harvard.9780674366688.c50/html |publisher=Harvard University Press |pages=281–290 |doi=10.4159/harvard.9780674366688.c50 |isbn=978-0-674-36668-8 |title=A Source Book in Astronomy and Astrophysics, 1900–1975 }}</ref>
* 1930s – [[Cosmologist]] [[Erich Regener]] calculates that the non-thermal spectrum of cosmic rays in the galaxy has an effective temperature of 2.8&nbsp;K.<ref name="Apeiron2_3_79"/>
* 1931 – Term ''microwave'' first used in print: "When trials with wavelengths as low as 18 cm. were made known, there was undisguised surprise+that the problem of the micro-wave had been solved so soon." ''Telegraph & Telephone Journal'' XVII. 179/1
* 1934 – [[Richard Tolman]] shows that [[black-body]] radiation in an expanding universe cools but remains thermal.
* 1946 – [[Robert Dicke]] predicts "... radiation from cosmic matter" at <&nbsp;20&nbsp;K, but did not refer to background radiation.<ref name=Kragh>
{{cite book|first=H.|last=Kragh|date=1999|title=Cosmology and Controversy: The Historical Development of Two Theories of the Universe|publisher=Princeton University Press|url=https://archive.org/details/cosmologycontrov00helg|url-access=registration|page=[https://archive.org/details/cosmologycontrov00helg/page/135 135]|isbn=978-0-691-00546-1}} "In 1946, Robert Dicke and coworkers at MIT tested equipment that could test a cosmic microwave background of intensity corresponding to about 20K in the microwave region. However, they did not refer to such a background, but only to 'radiation from cosmic matter'. Also, this work was unrelated to cosmology and is only mentioned because it suggests that by 1950, detection of the background radiation might have been technically possible, and also because of Dicke's later role in the discovery". See also {{cite journal|last=Dicke|first=R. H.|date=1946|title=Atmospheric Absorption Measurements with a Microwave Radiometer|journal=[[Physical Review]]|volume=70|issue=5–6|pages=340–348|doi=10.1103/PhysRev.70.340|bibcode = 1946PhRv...70..340D |display-authors=etal}}</ref>
* 1946 – [[George Gamow]] calculates a temperature of 50&nbsp;K (assuming a 3-billion year old universe),<ref name="The Creation Of The Universe">George Gamow, ''[https://books.google.com/books?id=5awirwgmvAoC&pg=PA40 The Creation Of The Universe]'' p.50 (Dover reprint of revised 1961 edition) {{ISBN|0-486-43868-6}}</ref> commenting it "... is in reasonable agreement with the actual temperature of interstellar space", but does not mention background radiation.<ref>{{cite book|last=Gamow|first=G.|author-link=George Gamow|date=2004|orig-year=1961|title=Cosmology and Controversy: The Historical Development of Two Theories of the Universe|page=40|publisher=[[Courier Dover Publications]]|url=https://books.google.com/books?id=5awirwgmvAoC&pg=PA40|isbn=978-0-486-43868-9}}</ref>
* 1953 – [[Erwin Finlay-Freundlich]] in support of his [[tired light]] theory, derives a blackbody temperature for intergalactic space of 2.3&nbsp;K and in the following year values of 1.9K and 6.0K.<ref>Erwin Finlay-Freundlich, "[http://adsabs.harvard.edu/abs/1953CoStA...4...96F Ueber die Rotverschiebung der Spektrallinien]" (1953) ''Contributions from the Observatory, University of St. Andrews''; no. 4, p. 96–102. Finlay-Freundlich gave two extreme values of 1.9K and 6.0K in Finlay-Freundlich, E.: 1954, "Red shifts in the spectra of celestial bodies", Phil. Mag., Vol. 45, pp. 303–319.</ref>

===Microwave background radiation predictions and measurements===
* 1941 – [[Andrew McKellar]] detected a "rotational" temperature of 2.3&nbsp;[[Kelvin|K]] for the interstellar medium by comparing the population of CN doublet lines measured by W. S. Adams in a B star.<ref name="dao7">
{{cite journal|last=McKellar|first=A.|title=Molecular Lines from the Lowest States of Diatomic Molecules Composed of Atoms Probably Present in Interstellar Space|journal=Publications of the Dominion Astrophysical Observatory|place=Vancouver, B.C., Canada| year=1941|volume=7|pages=251–272|issue=6|bibcode = 1941PDAO....7..251M }}</ref><ref>{{Cite book |last=Weinberg |first=Steven |author-link=Steven Weinberg |url=https://archive.org/details/gravitationcosmo00stev_0/page/514 |title=Gravitation and cosmology: principles and applications of the general theory of relativity |date=1972 |publisher=Wiley |isbn=978-0-471-92567-5 |location=New York |pages=[https://archive.org/details/gravitationcosmo00stev_0/page/514 514]}}</ref>
* 1948 – [[Ralph Alpher]] and [[Robert Herman]] estimate "the temperature in the universe" at 5&nbsp;K. Although they do not specifically mention microwave background radiation, it may be inferred.<ref>[[Helge Kragh]], Cosmology and Controversy: [https://archive.org/details/cosmologycontrov00helg/page/132 The Historical Development of Two Theories of the Universe] (1999) {{ISBN|0-691-00546-X}}. "Alpher and Herman first calculated the present temperature of the decoupled primordial radiation in 1948, when they reported a value of 5&nbsp;K. Although it was not mentioned either then or in later publications that the radiation is in the microwave region, this follows immediately from the temperature ... Alpher and Herman made it clear that what they had called "the temperature in the universe" the previous year referred to a blackbody distributed background radiation quite different from the starlight."</ref>
* 1953 – [[George Gamow]] estimates 7&nbsp;K based on a model that does not rely on a free parameter<ref name=Kragh /><ref>{{Cite journal |last1=Alpher |first1=Ralph A. |last2=Gamow |first2=George |last3=Herman |first3=Robert |date=December 1967 |title=Thermal Cosmic Radiation and the Formation of Protogalaxies |journal=Proceedings of the National Academy of Sciences |language=en |volume=58 |issue=6 |pages=2179–2186 |doi=10.1073/pnas.58.6.2179 |doi-access=free |issn=0027-8424 |pmc=223817 |pmid=16591578|bibcode=1967PNAS...58.2179A }}</ref>{{rp|2181}}
* 1955 – Émile Le Roux of the [[Nançay Radio Observatory]], in a sky survey at ''λ'' = 33&nbsp;cm, initially reported a near-isotropic background radiation of 3 kelvins, plus or minus 2; he did not recognize the cosmological significance<ref name=Kragh /> {{rp|343}}<ref name=PartridgeReview/>{{rp|location=8.3.1}} and later revised the error bars to 20K.<ref>Delannoy, J., Denisse, J. F., Le Roux, E., & Morlet, B. (1957). Mesures absolues de faibles densités de flux de rayonnement à 900 MHz. Annales d'Astrophysique, Vol. 20, p. 222, 20, 222.</ref><ref name=WrightUCLASite>{{Cite web |last=Wright |first=Edward |title=Cosmic Microwave Background |url=https://astro.ucla.edu/~wright/CMB.html |access-date=2024-05-28 |website=astro.ucla.edu}}</ref> 
* 1957 – Tigran Shmaonov reports that "the absolute effective temperature of the radioemission background ... is 4±3&nbsp;K".<ref>{{cite journal|last=Shmaonov|first=T. A.|date=1957|title=Commentary|language=ru|journal=[[Pribory I Tekhnika Experimenta]]|volume=1|pages=83|doi=10.1016/S0890-5096(06)60772-3}}</ref> with radiation intensity was independent of either time or direction of observation. Although Shamonov did not recognize it at the time, it is now clear that Shmaonov did observe the cosmic microwave background at a wavelength of 3.2&nbsp;cm<ref>{{cite book|last1=Naselsky|first1=P. D.|last2=Novikov|first2=D.I.|last3=Novikov|first3=I. D.|date=2006|title=The Physics of the Cosmic Microwave Background|publisher=Cambridge University Press |url=https://books.google.com/books?id=J2KCisZsWZ0C&pg=RA1-PA1|isbn=978-0-521-85550-1}}</ref>
* 1964 – [[A. G. Doroshkevich]] and [[Igor Dmitrievich Novikov]] publish a brief paper suggesting microwave searches for the black-body radiation predicted by Gamow, Alpher, and Herman, where they name the CMB radiation phenomenon as detectable.<ref>{{cite journal|last1=Doroshkevich|first1=A. G.|last2=Novikov|first2=I.D.|s2cid=96773397|date=1964|title=Mean Density of Radiation in the Metagalaxy and Certain Problems in Relativistic Cosmology|journal=[[Soviet Physics Doklady]]|volume=9|pages=4292–4298|doi=10.1021/es990537g|issue=23|bibcode = 1999EnST...33.4292W }}</ref>
* 1964–65 – [[Arno Penzias]] and [[Robert Woodrow Wilson]] measure the temperature to be approximately 3&nbsp;K. [[Robert Dicke]], [[Philip James Edwin Peebles|James Peebles]], P. G. Roll, and [[David Todd Wilkinson|D. T. Wilkinson]] interpret this radiation as a signature of the Big Bang.
* 1966 – [[Rainer K. Sachs]] and [[Arthur M. Wolfe]] theoretically predict microwave background fluctuation amplitudes created by [[gravitational potential]] variations between observers and the last scattering surface (see ''[[Sachs–Wolfe effect]]'').
* 1968 – [[Martin Rees]] and [[Dennis Sciama]] theoretically predict microwave background fluctuation amplitudes created by photons traversing time-dependent wells of potential.
* 1969 – [[R. A. Sunyaev]] and [[Yakov Zel'dovich]] study the inverse [[Compton scattering]] of microwave background photons by hot electrons (see ''[[Sunyaev–Zel'dovich effect]]'').
* 1983 – Researchers from the [[Cavendish Astrophysics Group|Cambridge Radio Astronomy Group]] and the [[Owens Valley Radio Observatory]] first detect the [[Sunyaev–Zel'dovich effect]] from [[galaxy cluster|clusters of galaxies]].
* 1983 – [[RELIKT-1]] Soviet CMB anisotropy experiment was launched.
* 1990 – FIRAS on the [[Cosmic Background Explorer]] (COBE) satellite measures the black body form of the CMB spectrum with exquisite precision, and shows that the microwave background has a nearly perfect black-body spectrum with T = 2.73 K and thereby strongly constrains the density of the [[intergalactic medium]].
* January 1992 – Scientists that analysed data from the [[RELIKT-1]] report the discovery of [[anisotropy]] in the cosmic microwave background at the Moscow astrophysical seminar.<ref>''Nobel Prize In Physics: Russia's Missed Opportunities'', [[RIA Novosti]], Nov 21, 2006</ref>
* 1992 – Scientists that analysed data from [[Cosmic Background Explorer|COBE]] DMR report the discovery of [[anisotropy]] in the cosmic microwave background.<ref>
{{cite news|last=Sanders|first=R.|author2=Kahn, J.|date=13 October 2006|title=UC Berkeley, LBNL cosmologist George F. Smoot awarded 2006 Nobel Prize in Physics|url=http://www.berkeley.edu/news/media/releases/2006/10/03_nobelph.shtml|publisher=[[UC Berkeley|UC Berkeley News]]|access-date=2008-12-11}}</ref>
* 1995 – The [[Cosmic Anisotropy Telescope]] performs the first high resolution observations of the cosmic microwave background.
* 1999 – First measurements of acoustic oscillations in the CMB anisotropy angular power spectrum from the [[Mobile Anisotropy Telescope|MAT/TOCO]], BOOMERANG, and Maxima Experiments. The [[BOOMERanG experiment]] makes higher quality maps at intermediate resolution, and confirms that the universe is "flat".
* 2002 – Polarization discovered by [[Degree Angular Scale Interferometer|DASI]].<ref>{{cite journal|last=Kovac|first=J.M.|year=2002|title=Detection of polarization in the cosmic microwave background using DASI|journal=[[Nature (journal)|Nature]]|pmid=12490941|volume=420|issue=6917|pages=772–787|doi=10.1038/nature01269|arxiv=astro-ph/0209478 |bibcode = 2002Natur.420..772K |s2cid=4359884|display-authors=etal|url=https://cds.cern.ch/record/582473|type=Submitted manuscript}}</ref>
* 2003 – E-mode polarization spectrum obtained by the CBI.<ref>{{cite journal|last=Readhead|first=A. C. S.|year=2004|title=Polarization Observations with the Cosmic Background Imager|journal=[[Science (journal)|Science]]|pmid=15472038|volume=306|issue=5697|pages=836–844|doi=10.1126/science.1105598|bibcode=2004Sci...306..836R|arxiv = astro-ph/0409569 |s2cid=9234000|display-authors=etal}}</ref> The [[Cosmic Background Imager|CBI]] and the [[Very Small Array]] produces yet higher quality maps at high resolution (covering small areas of the sky).
* 2003 – The [[Wilkinson Microwave Anisotropy Probe]] spacecraft produces an even higher quality map at low and intermediate resolution of the whole sky (WMAP provides {{em|no}} high-resolution data, but improves on the intermediate resolution maps from [[BOOMERanG experiment|BOOMERanG]]).
* 2004 – E-mode polarization spectrum obtained by the [[Cosmic Background Imager|CBI]].<ref>A. Readhead et al., "Polarization observations with the Cosmic Background Imager", Science 306, 836–844 (2004).</ref>
* 2004 – The [[Arcminute Cosmology Bolometer Array Receiver]] produces a higher quality map of the high resolution structure not mapped by WMAP.
* 2005 – The [[Arcminute Microkelvin Imager]] and the [[Sunyaev–Zel'dovich Array]] begin the first surveys for very high redshift [[galaxy cluster|clusters of galaxies]] using the [[Sunyaev–Zel'dovich effect]].
* 2005 – [[Ralph A. Alpher]] is awarded the [[National Medal of Science]] for his groundbreaking work in nucleosynthesis and prediction that the universe expansion leaves behind background radiation, thus providing a model for the Big Bang theory.
* 2006 – The long-awaited three-year [[WMAP]] results are released, confirming previous analysis, correcting several points, and including [[Cosmic microwave background radiation#Polarization|polarization]] data.
* 2006 – Two of COBE's principal investigators, [[George F. Smoot|George Smoot]] and [[John C. Mather|John Mather]], received the [[Nobel Prize in Physics]] in 2006 for their work on precision measurement of the CMBR.
* 2006–2011 – Improved measurements from [[WMAP]], new supernova surveys ESSENCE and SNLS, and baryon acoustic oscillations from [[Sloan Digital Sky Survey|SDSS]] and [[Astronomical survey#List of sky surveys|WiggleZ]], continue to be consistent with the standard [[Lambda-CDM model]].
* 2010 – The first all-sky map from the [[Planck (spacecraft)|Planck telescope]] is released.
* 2013 – An improved all-sky map from the [[Planck (spacecraft)|Planck telescope]] is released, improving the measurements of WMAP and extending them to much smaller scales.
* 2014 – On March 17, 2014, astrophysicists of the [[BICEP and Keck Array|BICEP2]] collaboration announced the detection of inflationary [[gravitational waves]] in the [[B-modes|B-mode]] [[power spectrum]], which if confirmed, would provide clear experimental evidence for the [[Inflation (cosmology)|theory of inflation]].<ref name="BICEP2-2014"/><ref name="NASA-20140317">{{cite web |last=Clavin |first=Whitney |title=NASA Technology Views Birth of the Universe |url=http://www.jpl.nasa.gov/news/news.php?release=2014-082 |date=March 17, 2014 |website=[[NASA]] |access-date=March 17, 2014 }}</ref><ref name="NYT-20140317">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=March 17, 2014 |title=Space Ripples Reveal Big Bang's Smoking Gun |work=[[The New York Times]] |url=https://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-space-buttresses-landmark-theory-of-big-bang.html |url-access=registration |access-date=March 17, 2014}}</ref><ref name="NYT-20140324">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Ripples From the Big Bang |url=https://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html |archive-date=2022-01-01 |url-access=limited |date=March 24, 2014 |work=[[The New York Times]] |access-date=March 24, 2014 }}{{cbignore}}</ref><ref name="PRL-20140619">
{{cite journal |author=Ade, P.A.R. (BICEP2 Collaboration) |title=Detection of B-Mode Polarization at Degree Angular Scales by BICEP2 |year=2014 |journal=[[Physical Review Letters]] |volume=112 |issue=24 |page=241101 |doi=10.1103/PhysRevLett.112.241101 |pmid=24996078|arxiv = 1403.3985 |bibcode = 2014PhRvL.112x1101B |s2cid=22780831 }}</ref><ref>{{cite web | url=http://www.math.columbia.edu/~woit/wordpress/?p=6865 | title=BICEP2 News {{pipe}} Not Even Wrong}}</ref> However, on 19 June 2014, lowered confidence in confirming the [[cosmic inflation]] findings was reported.<ref name="PRL-20140619" /><ref name="NYT-20140619">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Astronomers Hedge on Big Bang Detection Claim |url=https://www.nytimes.com/2014/06/20/science/space/scientists-debate-gravity-wave-detection-claim.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2014/06/20/science/space/scientists-debate-gravity-wave-detection-claim.html |archive-date=2022-01-01 |url-access=limited |date=June 19, 2014 |work=[[The New York Times]] |access-date=June 20, 2014 }}{{cbignore}}</ref><ref name="BBC-20140619">{{cite news |last=Amos |first=Jonathan |title=Cosmic inflation: Confidence lowered for Big Bang signal |url=https://www.bbc.com/news/science-environment-27935479 |date=June 19, 2014 |work=[[BBC News]] |access-date=June 20, 2014 }}</ref>
* 2015 – On January 30, 2015, the same team of astronomers from BICEP2 withdrew the claim made on the previous year. Based on the combined data of BICEP2 and Planck, the [[European Space Agency]] announced that the signal can be entirely attributed to [[Cosmic dust|dust]] in the Milky Way.<ref name="nature-20150130">{{cite journal|title=Gravitational waves discovery now officially dead|last=Cowen|first=Ron|date=2015-01-30|journal=Nature|doi=10.1038/nature.2015.16830|s2cid=124938210}}<!--|access-date=2015-10-26--></ref>
* 2018 – The final data and maps from the [[Planck (spacecraft)|Planck telescope]] is released, with improved measurements of the polarization on large scales.<ref>{{Cite journal |author1=Planck Collaboration |display-authors=etal |title=Planck 2018 results. I. Overview and the cosmological legacy of Planck |journal=Astronomy and Astrophysics |year=2020 |volume=641 |pages=A1 |doi=10.1051/0004-6361/201833880 |arxiv=1807.06205 |bibcode = 2020A&A...641A...1P|s2cid=119185252 }}</ref>
* 2019 – [[Planck (spacecraft)|Planck telescope]] analyses of their final 2018 data continue to be released.<ref>{{Cite journal |author1=Planck Collaboration |display-authors=etal |title=Planck 2018 results. V. CMB power spectra and likelihoods |journal=Astronomy and Astrophysics |year=2020 |volume=641 |pages=A5 |doi=10.1051/0004-6361/201936386 |arxiv=1907.12875 |bibcode=2020A&A...641A...5P |s2cid=198985935 }}</ref>

==In popular culture==
* In the ''[[Stargate Universe]]'' TV series (2009–2011), an [[Ancient (Stargate)|ancient]] spaceship, ''Destiny'', was built to study patterns in the CMBR which is a sentient message left over from the beginning of time.<ref>{{cite AV media | date=November 10, 2010 | title=Stargate Universe - Robert Carlyle talks about background radiation and Destiny's mission | type=Video | url=https://www.youtube.com/watch?v=ID6QSX9CAsI | access-date=2023-02-28 | publisher=YouTube }}</ref>
* In ''[[Wheelers (novel)|Wheelers]]'', a novel (2000) by [[Ian Stewart (mathematician)|Ian Stewart]] & [[Jack Cohen (biologist)|Jack Cohen]], CMBR is explained as the encrypted transmissions of an ancient civilization. This allows the Jovian "blimps" to have a society older than the currently-observed age of the universe.<ref>{{cite book |title=Wheelers |url=https://archive.org/details/wheelers0000stew/page/494/mode/2up?q=%22age+of+the+universe%22}}</ref>
* In ''[[The Three-Body Problem (novel)|The Three-Body Problem]]'', a 2008 novel by [[Liu Cixin]], a probe from an alien civilization compromises instruments monitoring the CMBR in order to deceive a character into believing the civilization has the power to manipulate the CMBR itself.<ref>{{Cite web |last=Liu |first=Cixin |date=2014-09-23 |title=The Three-Body Problem: "The Universe Flickers" |url=https://www.tor.com/2014/09/23/the-three-body-problem-the-universe-flickers/ |access-date=2023-01-23 |website=Tor.com |language=en-US}}</ref>
* The 2017 issue of the [[Banknotes of the Swiss franc#Ninth series|Swiss 20 francs bill]] lists several astronomical objects with their distances – the CMB is mentioned with 430 · 10<sup>15</sup> [[light-second]]s.<ref>{{Cite web |title=Astronomy in your wallet - NCCR PlanetS |url=https://nccr-planets.ch/blog/2017/12/07/astronomy-in-your-wallet/ |access-date=2023-01-23 |website=nccr-planets.ch |language=en-US}}</ref>
* In the 2021 Marvel series ''[[WandaVision]]'', a mysterious television broadcast is discovered within the Cosmic Microwave Background.<ref>{{Cite web |date=2021-02-03 |title=WandaVision's 'cosmic microwave background radiation' is real, actually |url=https://www.syfy.com/syfy-wire/wandavision-cosmic-microwave-background-radiation-science-real-marvel-tv |access-date=2023-01-23 |website=SYFY Official Site |language=en-US}}</ref>

==See also==
{{Div col}}
* {{Annotated link|Axis of evil (cosmology)}}
* {{Annotated link|Cosmic neutrino background}}
* {{Annotated link|Cosmic microwave background spectral distortions}}
* {{Annotated link|Cosmological perturbation theory}}
* {{Annotated link|Gravitational wave background}}
* {{Annotated link|Heat death of the universe}}
* {{annotated link|Horizons: Exploring the Universe}}
* {{Annotated link|Lambda-CDM model}}
* {{Annotated link|List of cosmological computation software}}
* {{Annotated link|Non-standard cosmology}}
* {{Annotated link|Observational cosmology}}
* {{Annotated link|Galaxy#Observation history|Observation history of galaxies}}
* {{Annotated link|Physical cosmology}}
* {{Annotated link|Timeline of cosmological theories}}
{{div col end}}

== Notes ==
{{notelist}}

==References==
{{Reflist|30em|refs=
<ref name="Apeiron2_3_79">{{Cite journal| last1     = Assis
 | first1    = A. K. T.
 | last2     = Paulo
 | first2    = São
 | last3     = Neves
 | first3    = M. C. D.
 | title     = History of the 2.7&nbsp;K Temperature Prior to Penzias and Wilson
 | journal   = Apeiron
 | volume    = 2
 | issue     = 3
 | pages      = 79–87
 |date=July 1995
 | url       = http://redshift.vif.com/JournalFiles/Pre2001/V02NO3PDF/V02N3ASS.PDF
 }}</ref>

<ref name="BICEP2-2014">{{cite web |author=Staff |title=BICEP2 2014 Results Release |url=http://bicepkeck.org |date=17 March 2014 |website=[[National Science Foundation]] |access-date=18 March 2014 }}</ref>

<ref name=cs>{{cite news |title=POLARBEAR project offers clues about origin of universe's cosmic growth spurt |url=http://www.csmonitor.com/Science/2014/1021/POLARBEAR-project-offers-clues-about-origin-of-universe-s-cosmic-growth-spurt |newspaper=Christian Science Monitor | date=October 21, 2014}}</ref>

<ref name=pc1>{{Cite journal|title = A Measurement of the Cosmic Microwave Background B-Mode Polarization Power Spectrum at Sub-Degree Scales with POLARBEAR |author = The Polarbear Collaboration|year = 2014|journal = [[The Astrophysical Journal]]|doi = 10.1088/0004-637X/794/2/171|bibcode=2014ApJ...794..171P |volume=794 |issue = 2 |pages=171|arxiv = 1403.2369 |s2cid = 118598825}}</ref>

<ref name="NYT-20140922">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Study Confirms Criticism of Big Bang Finding |url=https://www.nytimes.com/2014/09/23/science/space/study-confirms-criticism-of-big-bang-finding.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2014/09/23/science/space/study-confirms-criticism-of-big-bang-finding.html |archive-date=2022-01-01 |url-access=limited |date=22 September 2014 |work=[[The New York Times]] |access-date=22 September 2014 }}{{cbignore}}</ref>

}}

==Further reading==
* {{cite book|last1=Balbi|first1=Amedeo|title=The music of the big bang : the cosmic microwave background and the new cosmology|date=2008|publisher=Springer|location=Berlin|isbn=978-3-540-78726-6}}
* {{cite book|last=Durrer |first=Ruth |author-link=Ruth Durrer |title=The Cosmic Microwave Background |publisher=Cambridge University Press |year=2008 |isbn=978-0-521-84704-9}}
* {{cite book|last1=Evans|first1=Rhodri|title=The Cosmic Microwave Background: How It Changed Our Understanding of the Universe|publisher=Springer|isbn=978-3-319-09927-9|date=2015 |language=en}}

==External links==
{{Commons category}}
{{Wikiquote}}
* [http://www.quantumfieldtheory.info/cmb.pdf Student Friendly Intro to the CMB] A pedagogic, step-by-step introduction to the cosmic microwave background power spectrum analysis suitable for those with an undergraduate physics background. More in depth than typical online sites. Less dense than cosmology texts.
* [http://xstructure.inr.ac.ru/x-bin/theme3.py?level=3&index1=87807 CMBR Theme on arxiv.org]
* [http://www.astronomycast.com/cosmology/the-big-bang-and-cosmic-microwave-background/ Audio: Fraser Cain and Dr. Pamela Gay – Astronomy Cast. The Big Bang and Cosmic Microwave Background – October 2006]
* [http://thecmb.org Visualization of the CMB data from the Planck mission]
* {{cite web|author=Copeland, Ed|title=CMBR: Cosmic Microwave Background Radiation|url=http://www.sixtysymbols.com/videos/CMBR.htm|website=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]}}

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