Editing Observable universe (section)

== Large-scale structure ==
[[File:Galactic treasure chest RXC J0142.9+4438.jpg|thumb|Galaxy clusters, like [[RXC J0142.9+4438]], are the nodes of the cosmic web that permeates the entire Universe.<ref>{{cite web |title=Galactic treasure chest |url=http://www.spacetelescope.org/images/potw1833a/ |website=www.spacetelescope.org |access-date=13 August 2018}}</ref>]]
[[File:Constrained_Local_Universe_Evolution_Simulation_(spherical).webm|thumb|upright=1.8|Video of a <!--[[:Category:Cosmological simulation|simulation]]-->[[cosmology|cosmological]] [[computer simulation|simulation]] of the local universe, showing the large-scale structure of galaxy clusters and dark matter.<ref>{{cite web |title=Blueprints of the Universe |url=https://www.eso.org/public/videos/cluesAdler-cylindrical/ |website=www.eso.org |access-date=31 December 2020 |language=en}}</ref>]]{{See also|List of largest cosmic structures}}
[[Redshift survey|Sky surveys]] and mappings of the various [[wavelength]] bands of [[electromagnetic radiation]] (in particular [[Hydrogen line|21-cm emission]]) have yielded much information on the content and character of the [[universe]]'s structure. The organization of structure appears to follow a [[hierarchy|hierarchical]] model with organization up to the [[scale (spatial)|scale]] of [[supercluster]]s and [[Galaxy filament|filaments]]. Larger than this (at scales between 30 and 200 megaparsecs),<ref>{{Cite book|url=https://books.google.com/books?id=RLwangEACAAJ|title=An Introduction to Modern Astrophysics|last1=Carroll|first1=Bradley W.|last2=Ostlie|first2=Dale A.|year=2013|publisher=Pearson|isbn=978-1292022932|edition=International|page=1178|language=en}}</ref> there seems to be no continued structure, a phenomenon that has been referred to as the ''End of Greatness''.<ref name=Kirshner /> The shape of the large scale structure can be summarized by the [[matter power spectrum]].

=== Cosmic Web: walls, filaments, nodes, and voids ===
[[File:Map of the Cosmic Web Generated from Slime Mould Algorithm.jpg|thumb|upright=1.8|Map of the cosmic web generated from a slime mould-inspired algorithm<ref>{{Cite web|url=https://www.spacetelescope.org/images/heic2003a/|title=Map of the Cosmic Web Generated from Slime Mould Algorithm|website=www.spacetelescope.org}}</ref>]]
The organization of structure arguably begins at the stellar level, though most cosmologists rarely address [[astrophysics]] on that scale. [[Star]]s are organized into [[galaxy|galaxies]], which in turn form [[galaxy group]]s, [[galaxy cluster]]s, [[supercluster]]s, sheets, [[galaxy filament|walls and filaments]], which are separated by immense [[void (astronomy)|voids]], creating a vast foam-like structure<ref>{{Cite book|url=https://books.google.com/books?id=RLwangEACAAJ|title=An Introduction to Modern Astrophysics|last1=Carroll|first1=Bradley W.|last2=Ostlie|first2=Dale A.|year=2013|publisher=Pearson|isbn=978-1292022932|edition=International|pages=1173–1174|language=en}}</ref> sometimes called the "cosmic web". Prior to 1989, it was commonly assumed that [[virial theorem|virialized]] galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, since the early 1980s, more and more structures have been discovered. In 1983, Adrian Webster identified the [[Webster LQG]], a [[large quasar group]] consisting of 5 quasars. The discovery was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe.

In 1987, [[R. Brent Tully|Robert Brent Tully]] identified the [[Pisces–Cetus Supercluster Complex]], the galaxy filament in which the [[Milky Way]] resides. It is about 1 billion light-years across. That same year, an unusually large region with a much lower than average distribution of galaxies was discovered, the [[Giant Void]], which measures 1.3 billion light-years across. Based on [[redshift survey]] data, in 1989 [[Margaret Geller]] and [[John Huchra]] discovered the "[[CfA2 Great Wall|Great Wall]]",<ref name="redshift">{{cite journal |author=Geller |first1=M. J. |last2=Huchra |first2=J. P. |date=1989 |title=Mapping the universe. |journal=Science |volume=246 |issue=4932 |pages=897–903 |bibcode=1989Sci...246..897G |doi=10.1126/science.246.4932.897 |pmid=17812575 |s2cid=31328798}}</ref> a sheet of galaxies more than 500 million [[light-year]]s long and 200 million light-years wide, but only 15 million light-years thick. The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from [[redshift]]s.

Two years later, astronomers Roger G. Clowes and Luis E. Campusano discovered the [[Clowes–Campusano LQG]], a [[large quasar group]] measuring two billion light-years at its widest point, which was the largest known structure in the universe at the time of its announcement. In April 2003, another large-scale structure was discovered, the [[Sloan Great Wall]]. In August 2007, a possible supervoid was detected in the constellation [[Eridanus (constellation)|Eridanus]].<ref>{{Cite web |title=Biggest void in space is 1 billion light years across |url=https://www.newscientist.com/article/dn12546-biggest-void-in-space-is-1-billion-light-years-across/ |access-date=2023-09-15 |website=New Scientist |language=en-US}}</ref> It coincides with the '[[CMB cold spot]]', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model. This supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a billion light-years across, almost as big as the Giant Void mentioned above.

{{unsolved|physics|The largest structures in the universe are larger than expected. Are these actual structures or random density fluctuations?}}

[[File:Large-scale structure of light distribution in the universe.jpg|thumb|upright=2|Computer simulated image of an area of space more than 50 million light-years across, presenting a possible large-scale distribution of light sources in the universe—precise relative contributions of galaxies and [[quasar]]s are unclear.]]
Another large-scale structure is the [[SSA22 Protocluster]], a collection of galaxies and enormous gas bubbles that measures about 200 million light-years across.

In 2011, a large quasar group was discovered, [[U1.11]], measuring about 2.5 billion light-years across. On January 11, 2013, another large quasar group, the [[Huge-LQG]], was discovered, which was measured to be four billion light-years across, the largest known structure in the universe at that time.<ref>{{cite web | last = Wall | first = Mike | url = https://www.foxnews.com/science/largest-structure-in-universe-discovered/ | title = Largest structure in universe discovered | date = 2013-01-11 | publisher = [[Fox News]]}}</ref> In November 2013, astronomers discovered the [[Hercules–Corona Borealis Great Wall]],<ref name="2014paper">{{cite journal |last1=Horváth |first1=I. |last2=Hakkila |first2=Jon |last3=Bagoly |first3=Z. |date=2014 |title=Possible structure in the GRB sky distribution at redshift two |journal=Astronomy & Astrophysics |volume=561 |pages=L12 |arxiv=1401.0533 |bibcode=2014A&A...561L..12H |doi=10.1051/0004-6361/201323020 |s2cid=24224684}}</ref><ref name=original>{{cite arXiv |last1 = Horvath |first1 = I. |last2= Hakkila |first2=J. |last3=Bagoly |first3=Z. |title = The largest structure of the Universe, defined by Gamma-Ray Bursts |date = 2013 |eprint=1311.1104 |class=astro-ph.CO}}</ref> an even bigger structure twice as large as the former. It was defined by the mapping of [[gamma-ray burst]]s.<ref name=2014paper/><ref>{{cite web | last = Klotz | first = Irene | url = http://news.discovery.com/space/galaxies/universes-largest-structure-is-a-cosmic-conundrum-131119.htm | title = Universe's Largest Structure is a Cosmic Conundrum | date = 2013-11-19 | work = Discovery | access-date = 2013-11-20 | archive-date = 2016-05-16 | archive-url = https://web.archive.org/web/20160516172545/http://news.discovery.com/space/galaxies/universes-largest-structure-is-a-cosmic-conundrum-131119.htm | url-status = dead }}</ref>

In 2021, the [[American Astronomical Society]] announced the detection of the [[The Giant Arc|Giant Arc]]; a crescent-shaped string of galaxies that span 3.3 billion light years in length, located 9.2 billion light years from Earth in the constellation [[Boötes]] from observations captured by the [[Sloan Digital Sky Survey]].<ref>{{cite web | last = Ferreira | first = Becky | url = https://www.vice.com/en/article/a-structure-in-deep-space-is-so-giant-its-challenging-standard-physics/ | title = A Structure In Deep Space Is So Giant It's Challenging Standard Physics | date = 2021-06-23 | work = Vice}}</ref>

=== End of Greatness ===
The ''End of Greatness'' is an observational scale discovered at roughly 100&nbsp;[[Megaparsec|Mpc]] (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the [[universe]] is [[wikt:homogeneous|homogenized]] and [[isotropic|isotropized]] in accordance with the [[cosmological principle]].<ref name=Kirshner /> At this scale, no pseudo-random [[fractal]]ness is apparent.<ref>Natalie Wolchover, [https://news.yahoo.com/universe-isnt-fractal-study-finds-215053937.html "The Universe Isn't a Fractal, Study Finds"], LiveScience.com, 22 August 2012.</ref>

The [[supercluster]]s and [[Galaxy filament|filaments]] seen in smaller surveys are [[random]]ized to the extent that the smooth distribution of the universe is visually apparent. It was not until the [[redshift survey]]s of the 1990s were completed that this scale could accurately be observed.<ref name="Kirshner">{{cite book |author=Kirshner |first=Robert P. |url=https://archive.org/details/extravagantunive00kirs |title=The Extravagant Universe: Exploding Stars, Dark Energy and the Accelerating Cosmos |date=2002 |publisher=Princeton University Press |isbn=978-0691058627 |page=[https://archive.org/details/extravagantunive00kirs/page/71 71] |language=en-us |url-access=registration}}</ref>

=== Observations ===
[[File:2MASS LSS chart-NEW Nasa.jpg|right|upright=2.5|thumb|"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the [[Milky Way]]. The image is derived from the [[2MASS|2MASS Extended Source Catalog (XSC)]]—more than 1.5 million galaxies, and the Point Source Catalog (PSC)—nearly 0.5 billion Milky Way stars. The galaxies are color-coded by '[[redshift]]' obtained from the [[Uppsala General Catalogue|UGC]], [[Harvard-Smithsonian Center for Astrophysics|CfA]], Tully NBGC, LCRS, [[2dF Galaxy Redshift Survey|2dF]], 6dFGS, and [[Sloan Digital Sky Survey|SDSS]] surveys (and from various observations compiled by the [[NASA/IPAC Extragalactic Database|NASA Extragalactic Database]]), or photo-metrically deduced from the [[K band (infrared)|K band]] (2.2&nbsp;μm). Blue are the nearest sources ({{nowrap|''z'' < 0.01}}); green are at moderate distances ({{nowrap|0.01 < ''z'' < 0.04}}) and red are the most distant sources that 2MASS resolves ({{nowrap|0.04 < ''z'' < 0.1}}). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center)."<ref>{{cite journal |last1=Jarrett |first1=T. H. |date=2004 |title=Large Scale Structure in the Local Universe: The 2MASS Galaxy Catalog |journal=Publications of the Astronomical Society of Australia |volume=21 |issue=4 |pages=396–403 |arxiv=astro-ph/0405069 |bibcode=2004PASA...21..396J |doi=10.1071/AS04050 |s2cid=56151100}}</ref>]]

[[File:Galactic+celestial quads.jpg|thumb|upright=2.5|Constellations grouped in galactic quadrants (N/S, 1–4) and their approximate divisions vis-a-vis celestial quadrants (NQ/SQ)]]

Another indicator of large-scale structure is the '[[Lyman-alpha forest]]'. This is a collection of [[Spectral line|absorption lines]] that appear in the spectra of light from [[quasar]]s, which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly [[hydrogen]]) gas. These sheets appear to collapse into filaments, which can feed galaxies as they grow where filaments either cross or are dense. An early direct evidence for this cosmic web of gas was the 2019 detection, by astronomers from the RIKEN Cluster for Pioneering Research in Japan and Durham University in the U.K., of light from the brightest part of this web, surrounding and illuminated by a cluster of forming galaxies, acting as cosmic flashlights for intercluster medium hydrogen fluorescence via Lyman-alpha emissions.<ref>{{cite journal |last1=Hamden |first1=Erika |date=4 October 2019 |title=Observing the cosmic web |url=https://www.science.org/doi/abs/10.1126/science.aaz1318 |journal=Science |volume=366 |issue=6461 |pages=31–32 |bibcode=2019Sci...366...31H |doi=10.1126/science.aaz1318 |pmid=31604290 |s2cid=203717729|url-access=subscription }}</ref><ref>{{cite web |last1=Byrd |first1=Deborah |title=Cosmic Web Fuels Stars And Supermassive Black Holes |url=https://earthsky.org/space/cosmic-web-gas-reservoir-fuel-galaxies-growth/ |website=earthsky.org |date=6 October 2019}}</ref>

In 2021, an international team, headed by Roland Bacon from the Centre de Recherche Astrophysique de Lyon (France), reported the first observation of diffuse extended Lyman-alpha emission from redshift 3.1 to 4.5 that traced several cosmic web filaments on scales of 2.5−4&nbsp;[[cMpc]] (comoving mega-parsecs), in filamentary environments outside massive structures typical of web nodes.<ref>{{cite journal |author=Bacon, R.; Mary, D.; Garel, T.; Blaizot, J.; Maseda, M.; Schaye, J.; Wisotzki, L.; Conseil, S.; Brinchmann, J.; Leclercq, F.; Abril-Melgarejo, V.; Boogaard, L.; Bouché, N. F.; Contini, T.; Feltre, A.; Guiderdoni, B.; Herenz, C.; Kollatschny, W.; Kusakabe, H.; Matthee, J.; Michel-Dansac, L.; Nanayakkara, T.; Richard, J.; Roth, M.; Schmidt, K. B.; Steinmetz, M.; Tresse, L.; Urrutia, T.; Verhamme, A.; Weilbacher, P. M.; Zabl, J.; and Zoutendijk, S. L. |title=The MUSE Extremely Deep Field: The cosmic web in emission at high redshift |journal=Astronomy & Astrophysics |date=18 March 2021 |volume=647 |issue=A107 |pages=A107 |doi=10.1051/0004-6361/202039887 |url=https://www.aanda.org/articles/aa/full_html/2021/03/aa39887-20/aa39887-20.html#S1 |quote=This first detection of the cosmic web structure in Lyα emission in typical filamentary environments, namely outside massive structures typical of web nodes, is a milestone in the long search for the cosmic web signature at high z. This has been possible because of the unprecedented faint surface brightness of 5 × 10−20 erg s−1 cm−2 arcsec−2 achieved by 140 h MUSE observations on the VLT.|arxiv=2102.05516 |bibcode=2021A&A...647A.107B |s2cid=231861819 }}</ref>

Some caution is required in describing structures on a cosmic scale because they are often different from how they appear. [[Gravitational lens]]ing can make an image appear to originate in a different direction from its real source, when foreground objects curve surrounding spacetime (as predicted by [[general relativity]]) and deflect passing light rays. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. [[Weak gravitational lensing|Weak lensing]] by the intervening universe in general also subtly changes the observed large-scale structure.

The large-scale structure of the universe also looks different if only redshift is used to measure distances to galaxies. For example, galaxies behind a galaxy cluster are attracted to it and fall towards it, and so are [[blueshift]]ed (compared to how they would be if there were no cluster). On the near side, objects are redshifted. Thus, the environment of the cluster looks somewhat pinched if using redshifts to measure distance. The opposite effect is observed on galaxies already within a cluster: the galaxies have some random motion around the cluster center, and when these random motions are converted to redshifts, the cluster appears elongated. This creates a "''[[Redshift-space distortions|finger of God]]''"—the illusion of a long chain of galaxies pointed at Earth.

=== Cosmography of Earth's cosmic neighborhood ===
At the centre of the [[Hydra–Centaurus Supercluster]], a gravitational anomaly called the [[Great Attractor]] affects the motion of galaxies over a region hundreds of millions of light-years across. These galaxies are all [[redshift]]ed, in accordance with [[Hubble's law]]. This indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies.

The Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years in the direction of the [[Hydra (constellation)|Hydra]] and [[Centaurus]] [[constellation]]s. In its vicinity there is a preponderance of large old galaxies, many of which are colliding with their neighbours, or radiating large amounts of radio waves.

In 1987, [[astronomer]] [[R. Brent Tully]] of the [[University of Hawaii]]'s Institute of Astronomy identified what he called the [[Pisces–Cetus Supercluster Complex]], a structure one billion [[light-year]]s long and 150 million light-years across in which, he claimed, the Local Supercluster is embedded.<ref>{{Cite news|url=https://www.nytimes.com/1987/11/10/science/massive-clusters-of-galaxies-defy-concepts-of-the-universe.html|title=Massive Clusters of Galaxies Defy Concepts of the Universe|first=John Noble|last=Wilford|newspaper=The New York Times|date=November 10, 1987}}</ref>