Editing Physical cosmology (section)

===Formation and evolution of large-scale structure===
{{Main|Large-scale structure of the cosmos|Structure formation|Galaxy formation and evolution}}
Understanding the formation and evolution of the largest and earliest structures (i.e., quasars, galaxies, [[galaxy groups and clusters|clusters]] and [[supercluster]]s) is one of the largest efforts in cosmology. Cosmologists study a model of '''hierarchical structure formation''' in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling.<ref name=Hess2013>{{cite journal | title=Simulating structure formation of the Local Universe | last1=Heß | first1=Steffen | last2=Kitaura | first2=Francisco-Shu | last3=Gottlöber | first3=Stefan | journal=Monthly Notices of the Royal Astronomical Society | volume=435 | issue=3 | pages=2065–2076 | date=November 2013 | doi=10.1093/mnras/stt1428 | doi-access=free | bibcode=2013MNRAS.435.2065H | arxiv=1304.6565 | s2cid=119198359 }}</ref> One way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matter [[power spectrum]]. This is the approach of the ''[[Sloan Digital Sky Survey]]'' and the [[2dF Galaxy Redshift Survey]].<ref name=Cole2005>{{cite journal | title=The 2dF Galaxy Redshift Survey: power-spectrum analysis of the final data set and cosmological implications | display-authors=6 | last1=Cole | first1=Shaun | last2=Percival | first2=Will J. | last3=Peacock | first3=John A. | last4=Norberg | first4=Peder | last5=Baugh | first5=Carlton M. | last6=Frenk | first6=Carlos S. | last7=Baldry | first7=Ivan | last8=Bland-Hawthorn | first8=Joss | last9=Bridges | first9=Terry | last10=Cannon | first10=Russell | last11=Colless | first11=Matthew | last12=Collins | first12=Chris | last13=Couch | first13=Warrick | last14=Cross | first14=Nicholas J. G. | last15=Dalton | first15=Gavin | last16=Eke | first16=Vincent R. | last17=De Propris | first17=Roberto | last18=Driver | first18=Simon P. | last19=Efstathiou | first19=George | last20=Ellis | first20=Richard S. | last21=Glazebrook | first21=Karl | last22=Jackson | first22=Carole | last23=Jenkins | first23=Adrian | last24=Lahav | first24=Ofer | last25=Lewis | first25=Ian | last26=Lumsden | first26=Stuart | last27=Maddox | first27=Steve | last28=Madgwick | first28=Darren | last29=Peterson | first29=Bruce A. | last30=Sutherland | first30=Will | last31=Taylor | first31=Keith | journal=Monthly Notices of the Royal Astronomical Society | volume=362 | issue=2 | pages=505–534 | year=2005 | doi=10.1111/j.1365-2966.2005.09318.x | doi-access=free | bibcode=2005MNRAS.362..505C | arxiv=astro-ph/0501174 | s2cid=6906627 }}</ref><ref name=Nichol2007>{{cite journal | title=The Shape of the Sloan Digital Sky Survey Data Release 5 Galaxy Power Spectrum | display-authors=1 | last1=Percival | first1=Will J. | last2=Nichol | first2=Robert C. | last3=Eisenstein | first3=Daniel J. | last4=Frieman | first4=Joshua A. | last5=Fukugita | first5=Masataka | last6=Loveday | first6=Jon | last7=Pope | first7=Adrian C. | last8=Schneider | first8=Donald P. | last9=Szalay | first9=Alex S. | last10=Tegmark | first10=Max | last11=Vogeley | first11=Michael S. | last12=Weinberg | first12=David H. | last13=Zehavi | first13=Idit | last14=Bahcall | first14=Neta A. | last15=Brinkmann | first15=Jon | last16=Connolly | first16=Andrew J. | last17=Meiksin | first17=Avery | journal=The Astrophysical Journal | volume=657 | issue=2 | pages=645–663 | year=2007 | doi=10.1086/510615 | bibcode=2007ApJ...657..645P | arxiv=astro-ph/0608636 | s2cid=53141475 }}</ref>

Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into [[Galaxy filament|filaments]], superclusters and [[void (astronomy)|voids]]. Most simulations contain only non-baryonic [[cold dark matter]], which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.<ref name=Kuhlen2012>{{cite journal | title=Numerical simulations of the dark universe: State of the art and the next decade | journal=Physics of the Dark Universe | volume=1 | issue=1–2 | date=November 2012 | pages=50–93 | first1=Michael | last1=Kuhlen | first2=Mark | last2=Vogelsberger | first3=Raul | last3=Angulo | doi=10.1016/j.dark.2012.10.002 | arxiv=1209.5745 | bibcode=2012PDU.....1...50K | s2cid=119232040 }}</ref>
[[File:Gravitational lens found in the DESI Legacy Surveys data (noirlab2104c).jpg|thumb|upright=1.2|An example of a gravitational lens found in the DESI Legacy Surveys data. There are four sets of lensed images in DESI-090.9854-35.9683, corresponding to four distinct background galaxies—from the outermost giant red arc to the innermost bright blue arc, arranged in four concentric circles. All of them are gravitationally warped—or lensed—by the orange galaxy at the very center. Dark matter is expected to produce gravitational lensing also.]]
Other, complementary observations to measure the distribution of matter in the distant universe and to probe [[reionization]] include:
* The [[Lyman-alpha forest]], which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early universe, by measuring the absorption of light from distant quasars by the gas.<ref name=Weinberg2003>{{Cite book | last1=Weinberg | first1=David H. | last2=Davé | first2=Romeel | last3=Katz | first3=Neal | last4=Kollmeier | first4=Juna A. | chapter=The Lyman-α Forest as a Cosmological Tool | title=AIP Conference Proceedings: The Emergence of Cosmic Structure | date=May 2003 | series=AIP Conference Series | volume=666 | issue=2003 | editor1-first=S.H. | editor1-last=Holt | editor2-first=C. S. | editor2-last=Reynolds | pages=157–169 | arxiv=astro-ph/0301186 | doi=10.1063/1.1581786 | bibcode=2003AIPC..666..157W | citeseerx=10.1.1.256.1928 | s2cid=118868536 }}</ref>
* The [[Hydrogen line|21-centimeter]] [[Absorption (electromagnetic radiation)|absorption]] line of neutral atomic hydrogen also provides a sensitive test of cosmology.<ref name=Furlanetto2006>{{cite journal | title=Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe | last1=Furlanetto | first1=Steven R. | last2=Oh | first2=S. Peng | last3=Briggs | first3=Frank H. | journal=Physics Reports | volume=433 | issue=4–6 | pages=181–301 | date=October 2006 | doi=10.1016/j.physrep.2006.08.002 | bibcode=2006PhR...433..181F | arxiv=astro-ph/0608032 | citeseerx=10.1.1.256.8319 | s2cid=118985424 }}</ref>
* [[Weak lensing]], the distortion of a distant image by [[gravitational lensing]] due to dark matter.<ref name=Munshi2008>{{cite journal | title=Cosmology with weak lensing surveys | last1=Munshi | first1=Dipak | last2=Valageas | first2=Patrick | last3=van Waerbeke | first3=Ludovic | last4=Heavens | first4=Alan | journal=Physics Reports | volume=462 | issue=3 | pages=67–121 | year=2008 | doi=10.1016/j.physrep.2008.02.003 | pmid=16286284 | bibcode=2008PhR...462...67M | arxiv=astro-ph/0612667 | citeseerx=10.1.1.337.3760 | s2cid=9279637 }}</ref>

These will help cosmologists settle the question of when and how structure formed in the universe.