Editing Cosmological principle (section)

== 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>