Editing Cosmic microwave background (section)

===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}}</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>