Editing Cosmic microwave background (section)

==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}}</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}}</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}}