Cosmic microwave background: Difference between revisions

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. Its total energy density exceeds that of all the photons emitted by all the stars in the history of the universe. 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 very smooth and uniform, but maps by sensitive detectors detect small but important temperature variations. 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 influenced by various interactions of matter and photons up to the point of decoupling, which results in a characteristic pattern of tiny ripples 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.

{{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>

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  (10⁹) the number density of matter in the universe. The total energy density of the CMB exceeds that of all the photons emitted by all the stars in the history of the universe.<ref name=Ryden-2016>{{Cite book |last=Ryden |first=Barbara |title=Introduction to cosmology |date=2017 |publisher=Cambridge University Press |isbn=978-1-107-15483-4 |edition=Second |location=New York, NY}}</ref>{{rp|69}} 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

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>

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 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=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>

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>

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]] (10⁻³⁰) 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

In the [[Big Bang]] model for the formation of the [[universe]], [[inflation (cosmology)|inflationary cosmology]] predicts that after about 10⁻³⁷ 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 10⁻⁶ 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>

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>

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>

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}}

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 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>

While 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}} subsequent reanalysis with compensation for foreground dust show limits in agreement with results from [[Lambda-CDM]] models.<ref>{{Cite journal |last=BICEP/Keck Collaboration |last2=Ade |first2=P. A. R. |last3=Ahmed |first3=Z. |last4=Amiri |first4=M. |last5=Barkats |first5=D. |last6=Thakur |first6=R. Basu |last7=Bischoff |first7=C. A. |last8=Beck |first8=D. |last9=Bock |first9=J. J. |last10=Boenish |first10=H. |last11=Bullock |first11=E. |last12=Buza |first12=V. |last13=Cheshire |first13=J. R. |last14=Connors |first14=J. |last15=Cornelison |first15=J. |date=2021-10-04 |title=Improved Constraints on Primordial Gravitational Waves using Planck, WMAP, and BICEP/Keck Observations through the 2018 Observing Season |url=https://link.aps.org/doi/10.1103/PhysRevLett.127.151301 |journal=Physical Review Letters |volume=127 |issue=15 |pages=151301 |doi=10.1103/PhysRevLett.127.151301|arxiv=2110.00483 }}</ref>

[[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>

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 |arxiv=astro-ph/0406168 }}</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>

{{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>

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>

  }}</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">

  }}</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>

* 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>

* 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>

* 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>

<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 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>

* {{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}}

[[Category:Inflation (cosmology)]]