Editing Cosmic microwave background (section)

==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>