Editing Cosmic microwave background (section)

==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}}</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" />