Editing Cosmological principle (section)

== CMB dipole ==
{{unsolved|physics|Is the CMB dipole purely kinematic, or does it signal anisotropy of the universe, resulting in the breakdown of the FLRW metric and the cosmological principle?<ref name="Snowmass21"/>}}
The [[cosmic microwave background]] (CMB) provides a snapshot of a largely isotropic and homogeneous universe. The largest-scale feature of the CMB is the [[dipole]] [[anisotropy]]; it is typically subtracted from maps due to its large amplitude. The standard interpretation of the dipole is that it is [[Kinematics|kinematic]], due to the [[Doppler effect]] caused by the motion of the solar system with respect to the CMB rest-frame.

Several studies have reported dipoles in the large-scale distribution of galaxies that align with the CMB dipole direction but that indicate a larger amplitude than would be caused by the CMB dipole velocity.<ref>{{cite journal |last1=Secrest |first1=Nathan |last2=von Hausegger |first2=Sebastian |last3=Rameez |first3=Mohamed |last4=Mohayaee |first4=Roya |last5=Sarkar |first5=Subir |last6=Colin |first6=Jacques |date=25 February 2021 |title=A Test of the Cosmological Principle with Quasars |journal=The Astrophysical Journal |volume=908 |issue=2 |pages=L51 |arxiv=2009.14826 |bibcode=2021ApJ...908L..51S |doi=10.3847/2041-8213/abdd40 |issn=2041-8213 |s2cid=222066749 |doi-access=free }}</ref> A similar dipole is seen in data of [[Radio galaxy|radio galaxies]]; however, the amplitude of the dipole depends on the observing frequency, showing that these anomalous features cannot be purely kinematic.<ref>{{cite journal |last1=Siewert |first1=Thilo M. |last2=Schmidt-Rubart |first2=Matthias |last3=Schwarz |first3=Dominik J. |year=2021 |title=Cosmic radio dipole: Estimators and frequency dependence |journal=Astronomy & Astrophysics |volume=653 |pages=A9 |arxiv=2010.08366 |bibcode=2021A&A...653A...9S |doi=10.1051/0004-6361/202039840 |s2cid=223953708}}</ref> Other authors have found radio dipoles consistent with the CMB expectation.<ref>{{Cite journal |last=Darling |first=Jeremy |date=2022-06-01 |title=The Universe is Brighter in the Direction of Our Motion: Galaxy Counts and Fluxes are Consistent with the CMB Dipole |journal=The Astrophysical Journal |volume=931 |issue=2 |pages=L14 |doi=10.3847/2041-8213/ac6f08 |issn=0004-637X|arxiv=2205.06880 |bibcode=2022ApJ...931L..14D |doi-access=free }}</ref> Further claims of anisotropy along the CMB dipole axis have been made with respect to the [[Hubble diagram]] of [[Type Ia supernovae]]<ref>{{cite journal |last1=Singal |first1=Ashok K. |year=2022 |title=Peculiar motion of Solar system from the Hubble diagram of supernovae Ia and its implications for cosmology |journal=Monthly Notices of the Royal Astronomical Society |volume=515 |issue=4 |pages=5969–5980 |arxiv=2106.11968 |doi=10.1093/mnras/stac1986|doi-access=free }}</ref> and [[quasars]].<ref>{{cite journal |last1=Singal |first1=Ashok K. |year=2022 |title=Solar system peculiar motion from the Hubble diagram of quasars and testing the cosmological principle |journal=Monthly Notices of the Royal Astronomical Society |volume=511 |issue=2 |pages=1819–1829 |arxiv=2107.09390 |doi=10.1093/mnras/stac144|doi-access=free }}</ref> Separately, the CMB dipole direction has emerged as a preferred direction in some studies of alignments in quasar polarizations<ref>{{cite journal |last1=Hutsemekers |first1=D. |last2=Cabanac |first2=R. |last3=Lamy |first3=H. |last4=Sluse |first4=D. |date=October 2005 |title=Mapping extreme-scale alignments of quasar polarization vectors |journal=Astronomy & Astrophysics |volume=441 |issue=3 |pages=915–930 |arxiv=astro-ph/0507274 |bibcode=2005A&A...441..915H |doi=10.1051/0004-6361:20053337 |issn=0004-6361 |s2cid=14626666}}</ref> and [[strong lensing]] time delay,<ref>{{cite journal |last1=Krishnan |first1=Chethan |last2=Mohayaee |first2=Roya |last3=Colgáin |first3=Eoin Ó |last4=Sheikh-Jabbari |first4=M. M. |last5=Yin |first5=Lu |date=16 September 2021 |title=Does Hubble Tension Signal a Breakdown in FLRW Cosmology? |journal=Classical and Quantum Gravity |volume=38 |issue=18 |page=184001 |arxiv=2105.09790 |bibcode=2021CQGra..38r4001K |doi=10.1088/1361-6382/ac1a81 |issn=0264-9381 |s2cid=234790314}}</ref> and in Type Ia supernovae<ref>{{cite journal |last1=Krishnan |first1=Chethan |last2=Mohayaee |first2=Roya |last3=Colgáin |first3=Eoin Ó |last4=Sheikh-Jabbari |first4=M. M. |last5=Yin |first5=Lu |year=2022 |title=Hints of FLRW breakdown from supernovae |journal=Physical Review D |volume=105 |issue=6 |page=063514 |arxiv=2106.02532 |bibcode=2022PhRvD.105f3514K |doi=10.1103/PhysRevD.105.063514 |s2cid=235352881}}</ref> and other [[standard candles]].<ref>{{cite journal |last1=Luongo |first1=Orlando |last2=Muccino |first2=Marco |last3=Colgáin |first3=Eoin Ó |last4=Sheikh-Jabbari |first4=M. M. |last5=Yin |first5=Lu |year=2022 |title=Larger H0 values in the CMB dipole direction |journal=Physical Review D |volume=105 |issue=10 |page=103510 |arxiv=2108.13228 |bibcode=2022PhRvD.105j3510L |doi=10.1103/PhysRevD.105.103510 |s2cid=248713777}}</ref> Some authors have argued that the correlation of distant effects with the dipole direction may indicate that its origin is not kinematic.

Alternatively, [[Planck (spacecraft)|Planck]] data has been used to estimate the velocity with respect to the CMB independently of the dipole, by measuring the subtle aberrations and distortions of fluctuations caused by [[relativistic beaming]]<ref>{{Cite journal |last1=Planck Collaboration |last2=Aghanim |first2=N. |last3=Armitage-Caplan |first3=C. |last4=Arnaud |first4=M. |last5=Ashdown |first5=M. |last6=Atrio-Barandela |first6=F. |last7=Aumont |first7=J. |last8=Baccigalupi |first8=C. |last9=Banday |first9=A. J. |last10=Barreiro |first10=R. B. |last11=Bartlett |first11=J. G. |last12=Benabed |first12=K. |last13=Benoit-Lévy |first13=A. |last14=Bernard |first14=J. -P. |last15=Bersanelli |first15=M. |date=2014-11-01 |title=Planck 2013 results. XXVII. Doppler boosting of the CMB: Eppur si muove |url=https://ui.adsabs.harvard.edu/abs/2014A&A...571A..27P |journal=Astronomy and Astrophysics |volume=571 |pages=A27 |doi=10.1051/0004-6361/201321556 |arxiv=1303.5087 |bibcode=2014A&A...571A..27P |issn=0004-6361|hdl=10138/233688 |s2cid=5398329 |hdl-access=free }}</ref> and separately using the [[Sunyaev–Zeldovich effect]].<ref>{{Cite journal |last1=Planck Collaboration |last2=Akrami |first2=Y. |last3=Ashdown |first3=M. |last4=Aumont |first4=J. |last5=Baccigalupi |first5=C. |last6=Ballardini |first6=M. |last7=Banday |first7=A. J. |last8=Barreiro |first8=R. B. |last9=Bartolo |first9=N. |last10=Basak |first10=S. |last11=Benabed |first11=K. |last12=Bernard |first12=J. -P. |last13=Bersanelli |first13=M. |last14=Bielewicz |first14=P. |last15=Bond |first15=J. R. |date=2020-12-01 |title=Planck intermediate results. LVI. Detection of the CMB dipole through modulation of the thermal Sunyaev-Zeldovich effect: Eppur si muove II |url=https://ui.adsabs.harvard.edu/abs/2020A&A...644A.100P |journal=Astronomy and Astrophysics |volume=644 |pages=A100 |doi=10.1051/0004-6361/202038053 |arxiv=2003.12646 |bibcode=2020A&A...644A.100P |issn=0004-6361|hdl=10138/324269 |s2cid=214713774 |hdl-access=free }}</ref> These studies found a velocity consistent with the value obtained from the dipole, indicating that it is consistent with being entirely kinematic. Measurements of the [[velocity field]] of galaxies in the local universe show that on short scales galaxies are moving with the [[Local Group]], and that the average mean velocity decreases with increasing distance.<ref>{{Cite journal |last1=Avila |first1=Felipe |last2=Oliveira |first2=Jezebel |last3=Dias |first3=Mariana L. S. |last4=Bernui |first4=Armando |date=2023-02-01 |title=The bulk flow motion and the Hubble-Lemaître law in the Local Universe with the ALFALFA survey |url=https://ui.adsabs.harvard.edu/abs/2023BrJPh..53...49A |journal=Brazilian Journal of Physics |volume=53 |issue=2 |page=49 |doi=10.1007/s13538-023-01259-z |issn=0103-9733|arxiv=2302.04978 |bibcode=2023BrJPh..53...49A |s2cid=256631872 }}</ref> This follows the expectation if the CMB dipole were due to the local peculiar velocity field, it becomes more homogeneous on large scales. Surveys of the local volume have been used to reveal a low-density region in the opposite direction to the CMB dipole,<ref>{{Cite journal |last1=Hoffman |first1=Yehuda |last2=Pomarède |first2=Daniel |last3=Tully |first3=R. Brent |last4=Courtois |first4=Hélène M. |date=2017-01-01 |title=The dipole repeller |url=https://ui.adsabs.harvard.edu/abs/2017NatAs...1E..36H |journal=Nature Astronomy |volume=1 |issue=2 |page=0036 |doi=10.1038/s41550-016-0036 |issn=2397-3366|arxiv=1702.02483 |bibcode=2017NatAs...1E..36H |s2cid=7537393 }}</ref> potentially explaining the origin of the local [[Peculiar_velocity#Bulk_flow|bulk flow]].