View source for Cosmological principle

{{Short description|Theory that the universe is the same in all directions}}
{{cosmology}}
{{unsolved|physics|Is the universe homogeneous and isotropic at large enough scales, as claimed by the cosmological principle and assumed by all models that use the [[Friedmann–Lemaître–Robertson–Walker metric]], including the current version of the [[Lambda-CDM model|ΛCDM model]], or is the universe [[inhomogeneous cosmology|inhomogeneous]] or anisotropic?<ref name="Snowmass21"/><ref name="Billings"/><ref name="Migkas et al"/>}}

In modern [[physical cosmology]], the '''cosmological principle''' is the notion that the spatial distribution of matter in the universe is uniformly [[Isotropy|isotropic]] and [[homogeneity (physics)|homogeneous]] when viewed on a large enough scale, since the forces are expected to act equally throughout the universe on a large scale, and should, therefore, produce no observable inequalities in the large-scale structuring over the course of evolution of the matter field that was initially laid down by the [[Big Bang]].

== Definition ==
Astronomer William Keel explains:

<blockquote>The cosmological principle is usually stated formally as 'Viewed on a sufficiently large scale, the properties of the universe are the same for all observers.' This amounts to the strongly philosophical statement that the part of the universe which we can see is a fair sample, and that the same physical laws apply throughout. In essence, this in a sense says that the universe is knowable and is playing fair with scientists.<ref name="Keel">{{cite book |author=Keel |first=William C. |title=The Road to Galaxy Formation |publisher=Springer-Praxis |year=2007 |isbn=978-3-540-72534-3 |edition=2nd |page=2}}</ref></blockquote>

As [[Andrew Liddle]] puts it, "the cosmological principle [means that] the universe looks the same whoever and wherever you are."<ref name="Liddle">{{cite book |author=Liddle |first=Andrew |url=https://archive.org/details/introductiontomo00lidd_717 |title=An Introduction to Modern Cosmology |publisher=[[John Wiley & Sons]] |year=2003 |isbn=978-0-470-84835-7 |edition=2nd |page=[https://archive.org/details/introductiontomo00lidd_717/page/n18 2] |url-access=limited}}</ref>

The two testable structural consequences of the cosmological principle are [[Homogeneity (physics)|homogeneity]] and [[isotropy]]. Homogeneity – constant density
– means that the same observational evidence is available to observers at different locations in the universe. Isotropy – looking the same in all directions – means that the same observational evidence is available by looking in any direction in the universe. Isotropy implies homogeneity, but an homogeneous universe could be anisotropic.<ref name=Peacock-1998>{{Cite book |last=Peacock |first=J. A. |url=https://www.cambridge.org/core/product/identifier/9780511804533/type/book |title=Cosmological Physics |date=1998-12-28 |publisher=Cambridge University Press |isbn=978-0-521-41072-4 |edition=1 |doi=10.1017/cbo9780511804533}}</ref>{{rp|65}}

== Origin ==
The cosmological principle is first clearly asserted in the ''[[Philosophiæ Naturalis Principia Mathematica]]'' (1687) of [[Isaac Newton]].{{dubious|it is debatable that Newton stated the cosmological principle in the form we intend at this day and age.|date=May 2018}} In contrast to some earlier classical or medieval cosmologies, in which Earth rested at the center of universe, Newton conceptualized the Earth as a sphere in orbital motion around the Sun within an empty space that extended uniformly in all directions to immeasurably large distances. He then showed, through a series of mathematical proofs on detailed observational data of the motions of planets and comets, that their motions could be explained by a single principle of "[[universal gravitation]]" that applied as well to the orbits of the [[Galilean moons]] around Jupiter, the Moon around the Earth, the Earth around the Sun, and to falling bodies on Earth. That is, he asserted the equivalent material nature of all bodies within the [[Solar System]], the identical nature of the Sun and distant stars, and thus the uniform extension of the physical laws of motion to a great distance beyond the observational location of Earth itself.

== Implications ==
Since the 1990s, observations assuming the cosmological principle have concluded that around 68% of the mass–energy density of the universe can be attributed to [[dark energy]], which led to the development of the [[Lambda-CDM model|ΛCDM model]].<ref name="Ellis 2009">{{cite journal|last=Ellis|first=G. F. R.|title=Dark energy and inhomogeneity|journal=Journal of Physics: Conference Series|volume=189|doi=10.1088/1742-6596/189/1/012011|year=2009|issue=1 |page=012011|bibcode=2009JPhCS.189a2011E |s2cid=250670331 |doi-access=free}}</ref><ref>{{cite journal |last1=Colin |first1=Jacques |last2=Mohayaee |first2=Roya |last3=Rameez |first3=Mohamed |last4=Sarkar |first4=Subir |date=20 November 2019 |title=Evidence for anisotropy of cosmic acceleration |url=https://www.aanda.org/articles/aa/full_html/2019/11/aa36373-19/aa36373-19.html |journal=Astronomy and Astrophysics |volume=631 |pages=L13 |arxiv=1808.04597 |bibcode=2019A&A...631L..13C |doi=10.1051/0004-6361/201936373 |s2cid=208175643 |access-date=25 March 2022}}</ref><ref>{{cite web |last1=Redd |first1=N. T. |title=What is Dark Energy? |url=https://www.space.com/20929-dark-energy.html |website=space.com |archive-url=https://archive.today/20160519162257/http://www.space.com/20929-dark-energy.html |archive-date=19 May 2016 |date=2013 |access-date=28 October 2018 }}</ref>

Observations show that more distant galaxies are closer together and have lower content of chemical elements heavier than lithium.{{Citation needed|date=December 2022}} Applying the cosmological principle, this suggests that heavier elements were not created in the Big Bang but were produced by [[nucleosynthesis]] in giant stars and expelled via a series of [[supernova]]e and new [[star formation]] from the supernova remnants, which means heavier elements would accumulate over time. Another observation is that the farthest galaxies (earlier time) are often more fragmentary, interacting and unusually shaped than local galaxies (recent time), suggesting evolution in galaxy structure as well.

A related implication of the cosmological principle is that the largest discrete structures in the universe should be in [[mechanical equilibrium]]. Homogeneity and isotropy of matter at the largest scales would suggest that the largest discrete structures are parts of a single non-discrete form, like the crumbs which make up the interior of a cake. At extreme cosmological distances, the property of mechanical equilibrium in surfaces lateral to the line of sight can be empirically tested; however, under the assumption of the cosmological principle, it cannot be detected parallel to the line of sight (see [[Timeline of cosmological epochs|timeline of the universe]]).

Cosmologists agree that in accordance with observations of distant galaxies, a universe must be non-static if it follows the cosmological principle. In 1923, [[Alexander Friedmann]] set out a variant of [[Albert Einstein]]'s equations of [[general relativity]] that describe the dynamics of a homogeneous isotropic universe.<ref name=Friedman>{{cite book |title=Die Welt als Raum und Zeit (The World as Space and Time) |series=[[Ostwalds Klassiker der exakten Wissenschaften]] |author=Alexander Friedmann |author-link=Alexander Friedmann |isbn=978-3-8171-3287-4  |year=1923|oclc=248202523 }}.</ref><ref name="Abramovitch">{{cite book |last1=Tropp |first1=Ėduard Abramovich |url=https://z-lib.io/book/15374362 |title=Alexander A. Friedmann: The Man who Made the Universe Expand |last2=Frenkel |first2=Viktor Ya. |last3=Chernin |first3=Artur Davidovich |publisher=[[Cambridge University Press]] |year=1993 |isbn=978-0-521-38470-4 |page=219}}</ref> Independently, [[Georges Lemaître]] derived in 1927 the equations of an expanding universe from the General Relativity equations.<ref name=Lemaitre1927>{{Cite journal | last=Lemaître | first=Georges | author-link=Georges Lemaître | title=Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques | year=1927 | journal=Annales de la Société Scientifique de Bruxelles | volume=A47 | issue=5 | bibcode=1927ASSB...47...49L | pages=49–56 }} ''translated by [[A. S. Eddington]]'': {{Cite journal | last=Lemaître | first=Georges | author-link=Georges Lemaître | title=Expansion of the universe, A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulæ | year=1931 | journal=[[Monthly Notices of the Royal Astronomical Society]] | volume=91 | issue=5 | bibcode=1931MNRAS..91..483L | pages=483–490 | doi=10.1093/mnras/91.5.483| doi-access=free }}</ref> Thus, a non-static universe is also implied, independent of observations of distant galaxies, as a result of applying the cosmological principle to [[general relativity]].

== Criticism ==
[[Karl Popper]] criticized the cosmological principle on the grounds that it makes "our ''lack'' of knowledge a principle of ''knowing something''". He summarized his position as:
:"The 'cosmological principles' were, I fear, dogmas that should not have been proposed."<ref>Helge Kragh: [http://philsci-archive.pitt.edu/9062/1/Popper_%26_cosmology_PhilSci.pdf "The most philosophically of all the sciences": Karl Popper and physical cosmology] {{Webarchive|url=https://web.archive.org/web/20130720014110/http://philsci-archive.pitt.edu/9062/1/Popper_%26_cosmology_PhilSci.pdf |date=2013-07-20 }} (2012)</ref>

== Observations ==
Although the universe is inhomogeneous at smaller scales, according to the [[Lambda-CDM model|ΛCDM model]] it ought to be isotropic and statistically homogeneous on scales larger than 250 million light years. However, recent findings (the [[Axis of evil (cosmology)|Axis of Evil]] for example) have suggested that violations of the cosmological principle exist in the universe and thus have called the ΛCDM model into question, with some authors suggesting that the cosmological principle is now obsolete and the [[Friedmann–Lemaître–Robertson–Walker metric]] breaks down in the late universe.<ref name="Snowmass21">{{citation |last1=Abdalla |first1=Elcio |title=Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies |date=11 Mar 2022 |journal=Journal of High Energy Astrophysics |volume=34 |page=49 |arxiv=2203.06142v1 |bibcode=2022JHEAp..34...49A |doi=10.1016/j.jheap.2022.04.002 |s2cid=247411131 |last2=Abellán |first2=Guillermo Franco |last3=Aboubrahim |first3=Armin}}</ref>

=== Violations of isotropy ===
The [[cosmic microwave background]] (CMB) is predicted by the ΛCDM model to be isotropic, that is to say that its intensity is about the same whichever direction we look at.<ref>{{Cite web | url=http://www.gizmag.com/universe-homogeneous-300-million-light-years/24149/ | title=Australian study backs major assumption of cosmology| date=17 September 2012}}</ref> Data from the [[Planck Mission]] shows hemispheric bias in two respects: one with respect to average temperature (i.e. temperature fluctuations), the second with respect to larger variations in the degree of perturbations (i.e. densities).<ref name="Planck">{{cite web | url=http://sci.esa.int/planck/51551-simple-but-challenging-the-universe-according-to-planck/ | title=Simple but challenging: the Universe according to Planck | work=[[ESA Science & Technology]] | orig-date=March 21, 2013 |date= October 5, 2016 | access-date=October 29, 2016}}</ref><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=Bersanelli |first12=M. |last13=Bielewicz |first13=P. |last14=Bock |first14=J. J. |last15=Bond |first15=J. R. |date=2020-09-01 |title=Planck 2018 results. VII. Isotropy and statistics of the CMB |url=https://ui.adsabs.harvard.edu/abs/2020A&A...641A...7P |journal=Astronomy and Astrophysics |volume=641 |pages=A7 |doi=10.1051/0004-6361/201935201 |arxiv=1906.02552 |bibcode=2020A&A...641A...7P |issn=0004-6361|hdl=10138/320318 |hdl-access=free }}</ref> The collaboration noted that these features are not strongly statistically inconsistent with isotropy.<ref>{{Cite journal |last1=Planck Collaboration |last2=Aghanim |first2=N. |last3=Akrami |first3=Y. |last4=Arroja |first4=F. |last5=Ashdown |first5=M. |last6=Aumont |first6=J. |last7=Baccigalupi |first7=C. |last8=Ballardini |first8=M. |last9=Banday |first9=A. J. |last10=Barreiro |first10=R. B. |last11=Bartolo |first11=N. |last12=Basak |first12=S. |last13=Battye |first13=R. |last14=Benabed |first14=K. |last15=Bernard |first15=J. -P. |date=2020-09-01 |title=Planck 2018 results. I. Overview and the cosmological legacy of Planck |url=https://ui.adsabs.harvard.edu/abs/2020A&A...641A...1P |journal=Astronomy and Astrophysics |volume=641 |pages=A1 |doi=10.1051/0004-6361/201833880 |arxiv=1807.06205 |bibcode=2020A&A...641A...1P |issn=0004-6361|hdl=10138/320876 |s2cid=119185252 |hdl-access=free }}</ref> Some authors say that the universe around Earth is isotropic at high significance by studies of the [[cosmic microwave background]] temperature maps.<ref name="Saadeh">{{cite journal| vauthors = Saadeh D, Feeney SM, Pontzen A, Peiris HV, McEwen, JD|title=How Isotropic is the Universe?|journal=Physical Review Letters|date=2016|volume=117|number=13|page= 131302 |doi=10.1103/PhysRevLett.117.131302|pmid=27715088|arxiv=1605.07178|bibcode = 2016PhRvL.117m1302S |s2cid=453412}}</ref> There are however claims of isotropy violations from [[galaxy cluster]]s,<ref name="Billings">{{cite web |author=Billings |first=Lee |date=April 15, 2020 |title=Do We Live in a Lopsided Universe? |url=https://www.scientificamerican.com/article/do-we-live-in-a-lopsided-universe1/ |access-date=March 24, 2022 |website=[[Scientific American]]}}</ref><ref name="Migkas et al">{{cite journal |author1=Migkas, K. |author2=Schellenberger, G. |author3=Reiprich, T. H. |author4=Pacaud, F. |author5=Ramos-Ceja, M. E. |author6=Lovisari, L. |date=8 April 2020 |title=Probing cosmic isotropy with a new X-ray galaxy cluster sample through the LX-T scaling relation |url=https://www.aanda.org/articles/aa/full_html/2020/04/aa36602-19/aa36602-19.html |journal=Astronomy & Astrophysics |volume=636 |issue=April 2020 |page=42 |arxiv=2004.03305 |bibcode=2020A&A...636A..15M |doi=10.1051/0004-6361/201936602 |s2cid=215238834 |access-date=24 March 2022}}</ref> [[quasar]]s,<ref>{{cite journal |last1=Secrest |first1=Nathan J. |last2=von Hausegger |first2=Sebastian |last3=Rameez |first3=Mohamed |last4=Mohayaee |first4=Roya |last5=Sarkar |first5=Subir |last6=Colin |first6=Jacques |date=February 25, 2021 |title=A Test of the Cosmological Principle with Quasars |journal=The Astrophysical Journal Letters |volume=908 |issue=2 |pages=L51 |arxiv=2009.14826 |bibcode=2021ApJ...908L..51S |doi=10.3847/2041-8213/abdd40 |s2cid=222066749 |doi-access=free }}</ref> and [[type Ia supernova]]e.<ref>{{cite journal |last1=Javanmardi |first1=B. |last2=Porciani |first2=C. |last3=Kroupa |first3=P. |last4=Pflamm-Altenburg |first4=J. |date=August 27, 2015 |title=Probing the Isotropy of Cosmic Acceleration Traced By Type Ia Supernovae |url=https://iopscience.iop.org/article/10.1088/0004-637X/810/1/47 |journal=The Astrophysical Journal Letters |volume=810 |issue=1 |page=47 |arxiv=1507.07560 |bibcode=2015ApJ...810...47J |doi=10.1088/0004-637X/810/1/47 |s2cid=54958680 |access-date=March 24, 2022}}</ref>

=== Violations of homogeneity ===
The cosmological principle implies that at a sufficiently large scale, the universe is [[homogeneous]]. Based on [[N-body simulation]]s in a ΛCDM universe, Jaswant Yadav and his colleagues showed that the spatial distribution of galaxies is statistically homogeneous if averaged over scales of 260[[Parsec#Megaparsecs and gigaparsecs|/''h'' Mpc]] or more.<ref name="Yadav">{{cite journal |last1=Yadav |first1=Jaswant |last2=Bagla |first2=J. S. |last3=Khandai |first3=Nishikanta |date=25 February 2010 |title=Fractal dimension as a measure of the scale of homogeneity |journal=Monthly Notices of the Royal Astronomical Society |volume=405 |issue=3 |pages=2009–2015 |arxiv=1001.0617 |bibcode=2010MNRAS.405.2009Y |doi=10.1111/j.1365-2966.2010.16612.x |doi-access=free |s2cid=118603499}}</ref>

A number of observations have been reported to be in conflict with predictions of maximal structure sizes:
* The [[Clowes–Campusano LQG]], discovered in 1991, has a length of 580 Mpc, and is marginally larger than the consistent scale.
* The [[Sloan Great Wall]], discovered in 2003, has a length of 423 Mpc,<ref name=apj624_2_463>{{Cite journal | display-authors=1 | last1=Gott | first1=J. Richard III | last2=Jurić | first2=Mario | last3=Schlegel| first3=David | last4=Hoyle | first4=Fiona | last5=Vogeley | first5=Michael | last6=Tegmark | first6=Max | last7=Bahcall | first7=Neta |last8=Brinkmann | first8=Jon | title=A Map of the Universe | journal=The Astrophysical Journal | volume=624 | issue=2 | pages=463–484 |date=May 2005 | doi=10.1086/428890 | bibcode=2005ApJ...624..463G |arxiv = astro-ph/0310571| s2cid=9654355 }}</ref> which is just barely consistent with the cosmological principle.
* [[U1.11]], a [[large quasar group]] discovered in 2011, has a length of 780 Mpc, two times larger than the upper limit of the homogeneity scale.
* The [[Huge-LQG]], discovered in 2012, is three times longer and two times wider than is predicted to be possible by current models.
* In November 2013, a new structure 10 billion light years away measuring 2000–3000 Mpc (more than seven times that of the Sloan Great Wall) was discovered, the [[Hercules–Corona Borealis Great Wall]], putting further doubt on the validity of the cosmological principle.<ref>{{citation|arxiv=1311.1104|last1= Horvath|first1= I.|title= The largest structure of the Universe, defined by Gamma-Ray Bursts|last2=  Hakkila|first2= J.|last3=  Bagoly|first3= Z.|year= 2013}}</ref>
* In September 2020, a 4.9{{px2}}''σ'' conflict was found between the kinematic explanation of the CMB dipole and the measurement of the dipole in the angular distribution of a flux-limited, all-sky sample of 1.36 million quasars.<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=2021-02-01|title=A Test of the Cosmological Principle with Quasars|journal=The Astrophysical Journal Letters|volume=908|issue=2|pages=L51|doi=10.3847/2041-8213/abdd40|arxiv=2009.14826|bibcode=2021ApJ...908L..51S|s2cid=222066749|issn=2041-8205 |doi-access=free }}</ref>
* In June 2021, the [[The Giant Arc|Giant Arc]] was discovered, a structure spanning approximately 1000 Mpc.<ref>{{Cite web|url=https://www.newscientist.com/article/2280076-line-of-galaxies-is-so-big-it-breaks-our-understanding-of-the-universe/|title = Line of galaxies is so big it breaks our understanding of the universe}}</ref> It is located 2820 Mpc away and consists of galaxies, galactic clusters, gas, and dust.
* In January 2024, the [[Big Ring]] was discovered. It is located 9.2 billion light years away from Earth and has a diameter of 1.3 billion light years, giving it an angular size of 15 full moons as seen from Earth.<ref>{{Cite web |title=A Big Cosmological Mystery |url=https://www.uclan.ac.uk/news/big-ring-in-the-sky |access-date=2024-01-15 |website=University of Central Lancashire |language=en}}</ref>

However, as pointed out by Seshadri Nadathur in 2013 using statistical properties,<ref name=Nadathur>{{cite journal|last=Nadathur|first=Seshadri|title=Seeing patterns in noise: gigaparsec-scale 'structures' that do not violate homogeneity|journal=Monthly Notices of the Royal Astronomical Society|date=2013|volume=434|issue=1|pages=398–406|doi=10.1093/mnras/stt1028|doi-access=free |arxiv=1306.1700|bibcode =2013MNRAS.434..398N|s2cid=119220579}}</ref> the existence of structures larger than the homogeneous scale (260[[Parsec#Megaparsecs and gigaparsecs|/''h'' Mpc]] by Yadav's estimation)<ref name=Yadav /> does not necessarily violate the cosmological principle in the ΛCDM model (see ''{{section link|Huge-LQG#Dispute}}'').<ref name="Labini">{{cite journal| vauthors = Sylos-Labini F, Tekhanovich D, Baryshev Y|title=Spatial density fluctuations and selection effects in galaxy redshift surveys|journal=Journal of Cosmology and Astroparticle Physics|date=2014|volume=7|number=13|page= 35|doi=10.1088/1475-7516/2014/07/035|arxiv=1406.5899|bibcode = 2014JCAP...07..035S|s2cid=118393719}}</ref>

== 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]].

== Perfect cosmological principle ==
The '''perfect cosmological principle''' is an extension of the cosmological principle, and states that the universe is homogeneous and isotropic in space ''and'' time. In this view the universe looks not only the same everywhere in space (on large scales), but also the same as it always has and always will. The perfect cosmological principle underpins [[steady state theory]] and emerges{{clarify|underpins emerging? does this mean underpins the emergence of something (unstated) from chaotic inflation theory?|date=April 2017}} from [[chaotic inflation theory]].<ref>{{cite journal |author=Aguirre |first1=Anthony |last2=Gratton |first2=Steven |name-list-style=amp |date=2003 |title=Inflation without a beginning: A null boundary proposal |journal=Physical Review D |volume=67 |issue=8 |page=083515 |arxiv=gr-qc/0301042 |bibcode=2003PhRvD..67h3515A |doi=10.1103/PhysRevD.67.083515 |s2cid=37260723}}</ref><ref>{{cite journal |author=Aguirre |first1=Anthony |last2=Gratton |first2=Steven |name-list-style=amp |date=2002 |title=Steady-State Eternal Inflation |journal=Physical Review D |volume=65 |issue=8 |page=083507 |arxiv=astro-ph/0111191 |bibcode=2002PhRvD..65h3507A |doi=10.1103/PhysRevD.65.083507 |s2cid=118974302}}</ref><ref>{{cite web|url=http://www.lifesci.sussex.ac.uk/home/John_Gribbin/cosmo.htm|title=Inflation for Beginners|author=Gribbin, John|author-link=John Gribbin|access-date=2017-02-01|archive-date=2010-03-26|archive-url=https://web.archive.org/web/20100326145452/http://www.lifesci.sussex.ac.uk/home/John_Gribbin/cosmo.htm|url-status=dead}}</ref>

== See also ==
{{div col|colwidth=20em}}
* [[Background independence]]
* [[Copernican principle]]
* [[Cosmic variance]]
* [[End of Greatness]]
* [[Friedmann–Lemaître–Robertson–Walker metric]]
* [[Large-scale structure of the cosmos]]
* [[Expansion of the universe]]
* [[Redshift]]
{{div col end}}

== References ==
{{reflist}}

[[Category:Physical cosmological concepts]]
[[Category:Principles]]
[[Category:Concepts in astronomy]]