Editing Observable universe (section)

=== 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.