Universe

=== Shape ===
{{Main|Shape of the universe}}
[[File:End of universe.jpg|thumb|The three possible options for the shape of the universe]]

General relativity describes how spacetime is curved and bent by mass and energy (gravity). The [[topology]] or [[geometry]] of the universe includes both [[Shape of the universe#Local geometry (spatial curvature)|local geometry]] in the [[observable universe]] and [[Shape of the universe#Global geometry|global geometry]]. Cosmologists often work with a given [[space-like]] slice of spacetime called the [[Comoving distance|comoving coordinates]]. The section of spacetime which can be observed is the backward [[light cone]], which delimits the [[cosmological horizon]]. The cosmological horizon, also called the particle horizon or the light horizon, is the maximum distance from which [[Elementary particle|particles]] can have traveled to the [[observation|observer]] in the [[age of the universe]]. This horizon represents the boundary between the observable and the unobservable regions of the universe.<ref name="books.google.com">{{cite book |author=Harrison |first=Edward Robert |url=https://books.google.com/books?id=kNxeHD2cbLYC&pg=PA447 |title=Cosmology: the science of the universe |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-66148-5 |pages=447– |access-date=May 1, 2011 |archive-url=https://web.archive.org/web/20160826075123/https://books.google.com/books?id=kNxeHD2cbLYC&pg=PA447 |archive-date=August 26, 2016 |url-status=live}}</ref><ref>{{cite book |last1=Liddle |first1=Andrew R. |url=https://books.google.com/books?id=XmWauPZSovMC&pg=PA24 |title=Cosmological inflation and large-scale structure |last2=Lyth |first2=David Hilary |date=2000 |publisher=Cambridge University Press |isbn=978-0-521-57598-0 |pages=24– |access-date=May 1, 2011 |archive-url=https://web.archive.org/web/20131231164745/http://books.google.com/books?id=XmWauPZSovMC&pg=PA24 |archive-date=December 31, 2013 |url-status=live}}</ref> The existence, properties, and significance of a cosmological horizon depend on the particular [[cosmological model]].

An important parameter determining the future evolution of the universe theory is the [[density parameter]], Omega (Ω), defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible [[Shape of the universe|geometries]] depending on whether Ω is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes.<ref name=FateOfTheUniverse>{{cite web|title=What is the Ultimate Fate of the Universe?|url=http://map.gsfc.nasa.gov/universe/uni_fate.html|publisher=National Aeronautics and Space Administration |access-date=August 23, 2015|archive-date=December 22, 2021|archive-url=https://web.archive.org/web/20211222195155/https://map.gsfc.nasa.gov/universe/uni_fate.html|url-status=live}}</ref>

Observations, including the [[Cosmic Background Explorer]] (COBE), [[Wilkinson Microwave Anisotropy Probe]] (WMAP), and [[Planck (spacecraft)|Planck]] maps of the CMB, suggest that the universe is infinite in extent with a finite age, as described by the [[Friedmann–Lemaître–Robertson–Walker metric|Friedmann–Lemaître–Robertson–Walker]] (FLRW) models.<ref name="nasa_popular_uni_curv">{{Cite web |title=WMAP – Shape of the Universe |url=https://map.gsfc.nasa.gov/universe/uni_shape.html |access-date=February 14, 2023 |website=map.gsfc.nasa.gov |archive-date=March 31, 2019 |archive-url=https://web.archive.org/web/20190331105235/https://map.gsfc.nasa.gov/universe/uni_shape.html |url-status=live }}</ref><ref name="Nat03" /><ref name="RBSG08">{{cite journal|last1=Roukema|first1=Boudewijn|first2=Zbigniew |last2=Buliński |first3=Agnieszka |last3=Szaniewska |first4=Nicolas E. |last4=Gaudin |title=A test of the Poincare dodecahedral space topology hypothesis with the WMAP CMB data|journal=Astronomy and Astrophysics|volume=482|issue=3 |pages=747–753|date=2008|arxiv=0801.0006|doi=10.1051/0004-6361:20078777|bibcode=2008A&A...482..747L|s2cid=1616362}}</ref><ref name="Aurich0403597">{{cite journal|last=Aurich|first=Ralf|author2=Lustig, S. |author3=Steiner, F. |author4=Then, H. |title=Hyperbolic Universes with a Horned Topology and the CMB Anisotropy|journal=Classical and Quantum Gravity|volume=21 |issue=21 |pages=4901–4926|date=2004 |doi=10.1088/0264-9381/21/21/010 |arxiv=astro-ph/0403597|bibcode=2004CQGra..21.4901A|s2cid=17619026}}</ref> These FLRW models thus support inflationary models and the standard model of cosmology, describing a [[Minkowski space|flat]], homogeneous universe presently dominated by [[dark matter]] and [[dark energy]].<ref name="planck_cosmological_parameters">{{cite journal |arxiv=1303.5076 |title=Planck 2013 results. XVI. Cosmological parameters |author=Planck Collaboration |journal=Astronomy & Astrophysics |date=2014 |bibcode=2014A&A...571A..16P |doi=10.1051/0004-6361/201321591 |volume=571 |page=A16|s2cid=118349591 }}</ref><ref>{{cite web
|title=Planck reveals 'almost perfect' universe
|url=http://physicsworld.com/cws/article/news/2013/mar/21/planck-reveals-almost-perfect-universe
|work=Michael Banks
|publisher=Physics World
|date=March 21, 2013
|access-date=March 21, 2013
|archive-date=March 24, 2013
|archive-url=https://web.archive.org/web/20130324022238/http://physicsworld.com/cws/article/news/2013/mar/21/planck-reveals-almost-perfect-universe
|url-status=live
}}</ref>