Universe

== Physical properties ==
{{Main|Observable universe|Age of the universe|Metric expansion of space}}
Of the four [[fundamental interaction]]s, [[gravitation]] is the dominant at astronomical length scales. Gravity's effects are cumulative; by contrast, the effects of positive and negative charges tend to cancel one another, making electromagnetism relatively insignificant on astronomical length scales. The remaining two interactions, the [[weak nuclear force|weak]] and [[strong nuclear force]]s, decline very rapidly with distance; their effects are confined mainly to sub-atomic length scales.<ref name="OpenStax-college-physics"/>{{rp|1470}}

The universe appears to have much more [[matter]] than [[antimatter]], an asymmetry possibly related to the [[CP violation]].<ref>{{cite web|date=October 28, 2003 |url=http://www.pparc.ac.uk/Ps/bbs/bbs_antimatter.asp |title=Antimatter |publisher=Particle Physics and Astronomy Research Council |access-date=August 10, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20040307075727/http://www.pparc.ac.uk/Ps/bbs/bbs_antimatter.asp |archive-date=March 7, 2004 }}</ref> This imbalance between matter and antimatter is partially responsible for the existence of all matter existing today, since matter and antimatter, if equally produced at the [[Big Bang]], would have completely annihilated each other and left only [[photon]]s as a result of their interaction.<ref name="NAT-20171020">{{cite journal |author=Smorra C. |display-authors=et al |title=A parts-per-billion measurement of the antiproton magnetic moment |date=October 20, 2017 |journal=[[Nature (journal)|Nature]] |volume=550 |issue=7676 |pages=371–374 |doi=10.1038/nature24048 |pmid=29052625 |bibcode=2017Natur.550..371S |s2cid=205260736 |url=https://cds.cern.ch/record/2291601/files/nature24048.pdf |doi-access=free |access-date=August 25, 2019 |archive-date=October 30, 2018 |archive-url=https://web.archive.org/web/20181030045315/https://cds.cern.ch/record/2291601/files/nature24048.pdf |url-status=live }}</ref> The universe also appears to have neither net [[momentum]] nor [[angular momentum]], which absences follow{{clarification needed|date=February 2024}} from accepted physical laws if the universe is finite. These laws are [[Gauss's law]] and the non-divergence of the [[stress–energy–momentum pseudotensor]].<ref>{{harvtxt|Landau|Lifshitz|1975|p=361}}: "It is interesting to note that in a closed space the total electric charge must be zero. Namely, every closed surface in a finite space encloses on each side of itself a finite region of space. Therefore, the flux of the electric field through this surface is equal, on the one hand, to the total charge located in the interior of the surface, and on the other hand to the total charge outside of it, with opposite sign. Consequently, the sum of the charges on the two sides of the surface is zero."</ref>

=== Size and regions ===
{{See also|Observational cosmology}}
[[File:Extended universe logarithmic illustration (English annotated).png|thumb|upright=2.4|[[Terrestrial television|Television signals]] broadcast from Earth will never reach the edges of this image.]]

According to the general theory of relativity, far regions of [[space]] may never interact with ours even in the lifetime of the universe due to the finite [[speed of light]] and the ongoing [[expansion of space]]. For example, radio messages sent from [[Earth]] may never reach some regions of space, even if the universe were to exist forever: space may expand faster than light can traverse it.<ref name="Kaku2008">{{cite book|first=Michio|last=Kaku|title=Physics of the Impossible: A Scientific Exploration into the World of Phasers, Force Fields, Teleportation, and Time Travel|url=https://archive.org/details/physicsofimpossi00kaku|url-access=registration|date=2008|publisher=Knopf Doubleday Publishing Group|isbn=978-0-385-52544-2|pages=[https://archive.org/details/physicsofimpossi00kaku/page/202 202]–}}</ref>

The spatial region that can be observed with telescopes is called the [[observable universe]], which depends on the location of the observer.
The [[Comoving distance|proper distance]]—the distance as would be measured at a specific time, including the present—between Earth and the edge of the observable universe is 46 billion light-years<ref name="Extra Dimensions in Space and Time">{{cite book|first1=Itzhak|last1=Bars|first2=John|last2=Terning|title=Extra Dimensions in Space and Time|url=https://books.google.com/books?id=fFSMatekilIC&pg=PA27|access-date=October 19, 2018|date=2018|publisher=Springer|isbn=978-0-387-77637-8|pages=27–}}</ref> (14 billion [[parsecs]]), making the [[Observable universe#Size|diameter of the observable universe]] about 93 billion light-years (28 billion parsecs).<ref name="Extra Dimensions in Space and Time" /> The distance the light from the edge of the observable universe has traveled is very close to the [[age of the universe]] times the speed of light, {{convert|13.8|e9ly|e9pc}}, but this does not represent the distance at any given time because the edge of the observable universe and the Earth have since moved further apart.<ref>{{cite web |url=http://earthsky.org/space/what-is-a-light-year |title=What is a light-year? |work=EarthSky |date=February 20, 2013 |first=Christopher |last=Crockett |access-date=February 20, 2015 |archive-date=February 20, 2015 |archive-url=https://web.archive.org/web/20150220203559/http://earthsky.org/space/what-is-a-light-year |url-status=live }}</ref>

For comparison, the diameter of a typical [[galaxy]] is 30,000 light-years (9,198 [[parsecs]]), and the typical distance between two neighboring galaxies is 3 million [[light-years]] (919.8 kiloparsecs).<ref name="r196">[[#Rindler|Rindler]], p. 196.</ref> As an example, the [[Milky Way]] is roughly 100,000–180,000 light-years in diameter,<ref>{{cite web
|last1=Christian|first1=Eric
|last2=Samar|first2=Safi-Harb |author-link2=Samar Safi-Harb
|title=How large is the Milky Way?
|url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html
|archive-url=https://web.archive.org/web/19990202064645/http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980317b.html
|url-status=dead
|archive-date=February 2, 1999
|access-date=November 28, 2007}}</ref><ref>{{cite web|url=http://www.space.com/29270-milky-way-size-larger-than-thought.html|title=Size of the Milky Way Upgraded, Solving Galaxy Puzzle|publisher=Space.com|last=Hall|first=Shannon|date=May 4, 2015|access-date=June 9, 2015|archive-date=June 7, 2015|archive-url=https://web.archive.org/web/20150607104254/http://www.space.com/29270-milky-way-size-larger-than-thought.html|url-status=live}}</ref> and the nearest sister galaxy to the Milky Way, the [[Andromeda Galaxy]], is located roughly 2.5 million light-years away.<ref>{{cite journal |author=Ribas |first1=I. |last2=Jordi |first2=C. |last3=Vilardell |first3=F. |last4=Fitzpatrick |first4=E. L. |last5=Hilditch |first5=R. W. |last6=Guinan |first6=F. Edward |date=2005 |title=First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy |journal=Astrophysical Journal |volume=635 |issue=1 |pages=L37–L40 |arxiv=astro-ph/0511045 |bibcode=2005ApJ...635L..37R |doi=10.1086/499161 |s2cid=119522151}}<br />{{cite journal |author=McConnachie, A.W. |author2=Irwin, M.J. |author3=Ferguson, A.M.N. |author3-link=Annette Ferguson |author4=Ibata, R.A. |author5=Lewis, G.F. |author6=Tanvir, N. |author6-link=Nial Tanvir |date=2005 |title=Distances and metallicities for 17 Local Group galaxies |journal=Monthly Notices of the Royal Astronomical Society |volume=356 |issue=4 |pages=979–997 |arxiv=astro-ph/0410489 |bibcode=2005MNRAS.356..979M |doi=10.1111/j.1365-2966.2004.08514.x}}</ref>

Because humans cannot observe space beyond the edge of the observable universe, it is unknown whether the size of the universe in its totality is finite or infinite.<ref name="Brian Greene 2011" /><ref>{{cite web|title=How can space travel faster than the speed of light?|first=Vanessa |last=Janek |website=Universe Today|date=February 20, 2015|url=http://www.universetoday.com/119068/how-can-space-travel-faster-than-the-speed-of-light/|access-date=June 6, 2015|archive-date=December 16, 2021|archive-url=https://web.archive.org/web/20211216061309/https://www.universetoday.com/119068/how-can-space-travel-faster-than-the-speed-of-light/|url-status=live}}</ref><ref>{{cite web |title=Is faster-than-light travel or communication possible? Section: Expansion of the Universe |url=http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#13 |work=Philip Gibbs |date=1997 |access-date=June 6, 2015 |url-status=dead |archive-url=https://web.archive.org/web/20100310205556/http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#13 |archive-date=March 10, 2010 }}</ref> Estimates suggest that the whole universe, if finite, must be more than 250 times larger than a [[Hubble volume|Hubble sphere]].<ref>{{cite journal |last1=Vardanyan |first1=M. |last2=Trotta |first2=R. |last3=Silk |first3=J. |date=January 28, 2011 |title=Applications of Bayesian model averaging to the curvature and size of the Universe |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=413 |issue=1 |pages=L91–L95 |arxiv=1101.5476 |bibcode=2011MNRAS.413L..91V |doi=10.1111/j.1745-3933.2011.01040.x |s2cid=2616287}}</ref> Some disputed<ref>{{cite web |url=https://golem.ph.utexas.edu/category/2008/06/urban_myths_in_contemporary_co.html |title=Urban Myths in Contemporary Cosmology |last=Schreiber |first=Urs |date=June 6, 2008 |website=The n-Category Café |publisher=[[University of Texas at Austin]] |access-date=June 1, 2020 |archive-date=July 1, 2020 |archive-url=https://web.archive.org/web/20200701041542/https://golem.ph.utexas.edu/category/2008/06/urban_myths_in_contemporary_co.html |url-status=live }}</ref> estimates for the total size of the universe, if finite, reach as high as <math>10^{10^{10^{122}}}</math> megaparsecs, as implied by a suggested resolution of the No-Boundary Proposal.<ref>{{cite journal|arxiv=hep-th/0610199| author=[[Don Page (physicist)|Don N. Page]]|year=2007|title=Susskind's Challenge to the Hartle-Hawking No-Boundary Proposal and Possible Resolutions| journal=Journal of Cosmology and Astroparticle Physics| volume=2007| issue=1| page=004| doi=10.1088/1475-7516/2007/01/004| bibcode=2007JCAP...01..004P| s2cid=17403084}}</ref>{{efn|name=bignumber|Although listed in [[parsec|megaparsecs]] by the cited source, this number is so vast that its digits would remain virtually unchanged for all intents and purposes regardless of which conventional units it is listed in, whether it to be [[nanometers]] or [[parsec|gigaparsecs]], as the differences would disappear into the error.}}

=== Age and expansion ===
{{Main|Age of the universe|Expansion of the universe}}
Assuming that the [[Lambda-CDM model]] is correct, the measurements of the parameters using a variety of techniques by numerous experiments yield a best value of the age of the universe at 13.799 [[Measurement uncertainty|±]] 0.021 billion years, as of 2015.<ref name="Planck 2015">{{cite journal|author=Planck Collaboration|year=2016|title=Planck 2015 results. XIII. Cosmological parameters|journal=Astronomy & Astrophysics|volume=594|page=A13, Table 4|arxiv=1502.01589|bibcode=2016A&A...594A..13P|doi=10.1051/0004-6361/201525830|s2cid=119262962}}</ref>

[[File:Galactic Cntr full cropped.jpg|thumb|upright=2|Astronomers have discovered stars in the [[Milky Way]] galaxy that are almost 13.6 billion years old.]]
Over time, the universe and its contents have evolved. For example, the relative population of [[quasar]]s and galaxies has changed<ref>{{cite news |url=https://www.science.org/content/article/galaxy-collisions-give-birth-quasars |work=Science News |title=Galaxy Collisions Give Birth to Quasars |date=March 25, 2010 |first=Phil |last=Berardelli |access-date=July 30, 2022 |archive-date=March 25, 2022 |archive-url=https://web.archive.org/web/20220325005200/https://www.science.org/content/article/galaxy-collisions-give-birth-quasars |url-status=live }}</ref> and the [[expansion of the universe|universe has expanded]]. This expansion is inferred from the observation that the light from distant galaxies has been [[redshift]]ed, which implies that the galaxies are receding from us. Analyses of [[Type Ia supernova]]e indicate that the [[accelerating expansion of the Universe|expansion is accelerating]].<ref name="riess">{{cite journal|author=Riess, Adam G.|year=1998|title=Observational evidence from supernovae for an accelerating universe and a cosmological constant|journal=Astronomical Journal|volume=116|issue=3|pages=1009–1038|arxiv=astro-ph/9805201 |doi=10.1086/300499|bibcode=1998AJ....116.1009R|last2=Filippenko|last3=Challis|last4=Clocchiatti|last5=Diercks|last6=Garnavich|last7=Gilliland|last8=Hogan|last9=Jha|last10=Kirshner|last11=Leibundgut|last12=Phillips|last13=Reiss|last14=Schmidt|last15=Schommer|last16=Smith|last17=Spyromilio|last18=Stubbs|last19=Suntzeff|last20=Tonry|s2cid=15640044|author-link=Adam Riess}}</ref><ref name="perlmutter">{{cite journal|author=Perlmutter, S. |journal=Astrophysical Journal|volume=517|issue=2|pages=565–586|year=1999|title=Measurements of Omega and Lambda from 42 high redshift supernovae|arxiv=astro-ph/9812133 |doi=10.1086/307221|bibcode=1999ApJ...517..565P|last2=Aldering|last3=Goldhaber|last4=Knop|last5=Nugent|last6=Castro|last7=Deustua|last8=Fabbro|last9=Goobar|last10=Groom|last11=Hook|last12=Kim|last13=Kim|last14=Lee|last15=Nunes|last16=Pain|last17=Pennypacker|last18=Quimby|last19=Lidman|last20=Ellis|last21=Irwin|last22=McMahon|last23=Ruiz-Lapuente|last24=Walton|last25=Schaefer|last26=Boyle|last27=Filippenko|last28=Matheson|last29=Fruchter|last30=Panagia|s2cid=118910636|display-authors=29|author-link=Saul Perlmutter}}</ref>

The more matter there is in the universe, the stronger the mutual [[gravitational]] pull of the matter. If the universe were ''too'' dense then it would re-collapse into a [[gravitational singularity]]. However, if the universe contained too ''little'' matter then the self-gravity would be too weak for astronomical structures, like galaxies or planets, to form. Since the Big Bang, the universe has expanded [[monotonic]]ally. [[Anthropic principle#Anthropic 'coincidences'|Perhaps unsurprisingly]], our universe has [[Critical Mass Density of the Universe|just the right mass–energy density]], equivalent to about 5 protons per cubic metre, which has allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today.<ref>{{cite book|first1=Raymond A. |last1=Serway |first2=Clement J. |last2=Moses |first3=Curt A. |last3=Moyer |title=Modern Physics |publisher=Cengage Learning |year=2004 |isbn=978-1-111-79437-8 |page=21}}</ref><ref>{{cite book |url=https://openstax.org/books/astronomy-2e/pages/29-7-the-anthropic-principle |title=Astronomy 2e |publisher=OpenStax |isbn=978-1-951-69350-3 |first1=Andrew |last1=Fraknoi |display-authors=etal |year=2022 |page=1017 |access-date=February 14, 2023 |archive-date=February 14, 2023 |archive-url=https://web.archive.org/web/20230214122906/https://openstax.org/books/astronomy-2e/pages/29-7-the-anthropic-principle |url-status=live }}</ref>

There are dynamical forces acting on the particles in the universe which affect the expansion rate. Before 1998, it was expected that the expansion rate would be decreasing as time went on due to the influence of gravitational interactions in the universe; and thus there is an additional observable quantity in the universe called the [[deceleration parameter]], which most cosmologists expected to be positive and related to the matter density of the universe. In 1998, the deceleration parameter was measured by two different groups to be negative, approximately −0.55, which technically implies that the second derivative of the cosmic [[scale factor cosmology|scale factor]] <math> \ddot{a}</math> has been positive in the last 5–6 billion years.<ref name="nobel_2011">{{cite web |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/2011/ |title=The Nobel Prize in Physics 2011 |access-date=April 16, 2015 |archive-date=April 17, 2015 |archive-url=https://web.archive.org/web/20150417023358/http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/ |url-status=live }}</ref><ref>{{cite news|last=Overbye|first=Dennis|title=A 'Cosmic Jerk' That Reversed the Universe|url=https://www.nytimes.com/2003/10/11/us/a-cosmic-jerk-that-reversed-the-universe.html?pagewanted=all&src=pm|newspaper=New York Times|date=October 11, 2003|access-date=February 20, 2017|archive-date=July 1, 2017|archive-url=https://web.archive.org/web/20170701114952/http://www.nytimes.com/2003/10/11/us/a-cosmic-jerk-that-reversed-the-universe.html?pagewanted=all&src=pm|url-status=live}}</ref>

=== Spacetime ===
{{Main|Spacetime|World line}}
{{See also|Lorentz transformation}}
Modern physics regards [[event (relativity)|events]] as being organized into [[spacetime]].<ref>{{Cite book
 |author=Schutz, Bernard
 |title=A First Course in General Relativity
 |publisher=Cambridge University Press
 |edition=2nd
 |date= 2009
 |isbn=978-0-521-88705-2
 |pages=[https://archive.org/details/firstcourseingen00bern_0/page/142 142, 171]
 |author-link=Bernard Schutz
 |url=https://archive.org/details/firstcourseingen00bern_0/page/142
 }}</ref> This idea originated with the [[special theory of relativity]], which predicts that if one observer sees two events happening in different places at the same time, a second observer who is moving relative to the first will see those events happening at different times.<ref name="Mermin2005">{{cite book|first=N. David |last=Mermin |author-link=N. David Mermin |title=It's About Time: Understanding Einstein's Relativity |publisher=Princeton University Press |year=2021 |orig-year=2005 |edition=Princeton Science Library paperback |isbn=978-0-691-12201-4 |oclc=1193067111}}</ref>{{rp|45–52}} The two observers will disagree on the time <math>T</math> between the events, and they will disagree about the distance <math>D</math> separating the events, but they will agree on the [[speed of light]] <math>c</math>, and they will measure the same value for the combination <math>c^2T^2 - D^2</math>.<ref name="Mermin2005"/>{{rp|80}} The square root of the [[absolute value]] of this quantity is called the ''interval'' between the two events. The interval expresses how widely separated events are, not just in space or in time, but in the combined setting of spacetime.<ref name="Mermin2005"/>{{rp|84,136}}<ref>{{cite journal |doi=10.1007/s10714-006-0254-9 |bibcode=2006GReGr..38..643B |arxiv=gr-qc/0407022 |title=Spacetime and Euclidean geometry |journal=General Relativity and Gravitation |volume=38 |issue=4 |year=2006 |pages=643–651 |last1=Brill |first1=Dieter |last2=Jacobsen |first2=Ted |citeseerx=10.1.1.338.7953 |s2cid=119067072 }}</ref>

The special theory of relativity cannot account for [[gravity]]. Its successor, the [[general theory of relativity]], explains gravity by recognizing that spacetime is not fixed but instead dynamical. In general relativity, gravitational force is reimagined as curvature of [[spacetime]]. A curved path like an orbit is not the result of a force deflecting a body from an ideal straight-line path, but rather the body's attempt to fall freely through a background that is itself curved by the presence of other masses. A remark by [[John Archibald Wheeler]] that has become proverbial among physicists summarizes the theory: "Spacetime tells matter how to move; matter tells spacetime how to curve",<ref name="Wheeler">{{Cite book|last=Wheeler|first=John Archibald|url=https://books.google.com/books?id=zGFkK2tTXPsC&pg=PA235|title=Geons, Black Holes, and Quantum Foam: A Life in Physics|date=2010|publisher=W. W. Norton & Company|isbn=978-0-393-07948-7|language=en|author-link=John Archibald Wheeler|access-date=February 17, 2023|archive-date=February 17, 2023|archive-url=https://web.archive.org/web/20230217135729/https://books.google.com/books?id=zGFkK2tTXPsC&pg=PA235|url-status=live}}</ref><ref>{{Cite journal|last=Kersting|first=Magdalena|date=May 2019|title=Free fall in curved spacetime – how to visualise gravity in general relativity|journal=[[Physics Education]] |volume=54|issue=3|pages=035008|doi=10.1088/1361-6552/ab08f5|bibcode=2019PhyEd..54c5008K |s2cid=127471222 |issn=0031-9120|doi-access=free|hdl=10852/74677|hdl-access=free}}</ref> and therefore there is no point in considering one without the other.<ref name="Hawking" /> The [[Newton's law of universal gravitation|Newtonian theory of gravity]] is a good approximation to the predictions of general relativity when gravitational effects are weak and objects are moving slowly compared to the speed of light.<ref>{{Cite book |last1=Goldstein |first1=Herbert |title=Classical Mechanics |title-link=Classical Mechanics (Goldstein) |last2=Poole |first2=Charles P. |last3=Safko |first3=John L. |date=2002 |publisher=Addison Wesley |isbn=0-201-31611-0 |edition=3rd |location=San Francisco |oclc=47056311 |author-link=Herbert Goldstein |author2-link=Charles P. Poole}}</ref>{{Rp|page=327}}<ref>{{Cite book |last=Goodstein |first=Judith R. |url=https://www.worldcat.org/oclc/1020305599 |title=Einstein's Italian Mathematicians: Ricci, Levi-Civita, and the Birth of General Relativity |date=2018 |publisher=American Mathematical Society |isbn=978-1-4704-2846-4 |location=Providence, Rhode Island |pages=143 |oclc=1020305599 |author-link=Judith R. Goodstein}}</ref>

The relation between matter distribution and spacetime curvature is given by the [[Einstein field equations]], which require [[tensor calculus]] to express.<ref>{{Cite book |last=Choquet-Bruhat |first=Yvonne |url=https://www.worldcat.org/oclc/317496332 |title=General Relativity and the Einstein Equations |date=2009 |publisher=Oxford University Press |isbn=978-0-19-155226-7 |location=Oxford |oclc=317496332 |author-link=Yvonne Choquet-Bruhat}}</ref>{{Rp|page=43}}<ref>{{Cite book |last=Prescod-Weinstein |first=Chanda |url=https://www.worldcat.org/oclc/1164503847 |title=The Disordered Cosmos: A Journey into Dark Matter, Spacetime, and Dreams Deferred |date=2021 |publisher=Bold Type Books |isbn=978-1-5417-2470-9 |location=New York, New York |language=en-us |oclc=1164503847 |author-link=Chanda Prescod-Weinstein |access-date=February 17, 2023 |archive-date=February 21, 2022 |archive-url=https://web.archive.org/web/20220221214240/http://www.worldcat.org/oclc/1164503847 |url-status=live }}</ref> The solutions to these equations include not only the spacetime of special relativity, [[Minkowski spacetime]], but also [[Schwarzschild metric|Schwarzschild spacetimes]], which describe [[black hole]]s; [[Friedmann–Lemaître–Robertson–Walker metric|FLRW spacetime]], which describes an expanding universe; and more.

The universe appears to be a smooth spacetime continuum consisting of three [[space|spatial]] [[dimension]]s and one temporal ([[time]]) dimension. Therefore, an event in the spacetime of the physical universe can therefore be identified by a set of four coordinates: {{nowrap begin}}(''x'', ''y'', ''z'', ''t''){{nowrap end}}. On average, [[3-space|space]] is observed to be very nearly [[Shape of the universe|flat]] (with a [[curvature]] close to zero), meaning that [[Euclidean geometry]] is empirically true with high accuracy throughout most of the universe.<ref name="Shape">{{Cite web |title=WMAP Mission – Age of the Universe |url=https://map.gsfc.nasa.gov/m_mm/mr_content.html |access-date=February 14, 2023 |website=map.gsfc.nasa.gov |archive-date=December 4, 2022 |archive-url=https://web.archive.org/web/20221204182149/https://map.gsfc.nasa.gov/m_mm/mr_content.html |url-status=live }}</ref> Spacetime also appears to have a [[simply connected space|simply connected]] [[topology]], in analogy with a sphere, at least on the length scale of the observable universe. However, present observations cannot exclude the possibilities that the universe has more dimensions (which is postulated by theories such as the [[string theory]]) and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or [[toroid]]al topologies of two-dimensional [[space]]s.<ref name="Nat03">{{cite journal
 |last1        = Luminet
 |first1       = Jean-Pierre
 |author-link  = Jean-Pierre Luminet
 |last2        = Weeks
 |first2       = Jeffrey R.
 |last3        = Riazuelo
 |first3       = Alain
 |last4        = Lehoucq
 |first4       = Roland
 |last5        = Uzan
 |first5       = Jean-Philippe
 |title        = Dodecahedral space topology as an explanation for weak wide-angle temperature correlations in the cosmic microwave background
 |journal      = [[Nature (journal)|Nature]]
 |volume       = 425
 |issue        = 6958
 |pages        = 593–595
 |date         = October 9, 2003
|pmid         = 14534579
 |arxiv        = astro-ph/0310253
 |doi          = 10.1038/nature01944
 |bibcode      = 2003Natur.425..593L
 |s2cid        = 4380713
 |url          = https://cds.cern.ch/record/647738
 |type         = Submitted manuscript
 |access-date  = August 21, 2018
|archive-date = May 17, 2021
|archive-url  = https://web.archive.org/web/20210517180259/https://cds.cern.ch/record/647738
 |url-status   = live
}}</ref><ref name="_spacetime_topology">{{cite conference
|first1=Jean-Pierre
|last1=Luminet
|first2=Boudewijn F.
 |last2=Roukema
 |title=Topology of the Universe: Theory and Observations
|book-title=Proceedings of Cosmology School held at Cargese, Corsica, August 1998
|date=1999
|arxiv=astro-ph/9901364
|bibcode=1999ASIC..541..117L }}</ref>

=== 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>

=== Support of life ===
{{Main|Fine-tuned universe}}
The fine-tuned universe hypothesis is the proposition that the conditions that allow the existence of observable [[life]] in the universe can only occur when certain universal [[physical constant|fundamental physical constants]] lie within a very narrow range of values. According to this hypothesis, if any of several fundamental constants were only slightly different, the universe would have been unlikely to be conducive to the establishment and development of [[matter]], astronomical structures, elemental diversity, or life as it is understood. Whether this is true, and whether that question is even logically meaningful to ask, are subjects of much debate.<ref name=stanford_encylopedia>{{cite web
|url=https://plato.stanford.edu/entries/fine-tuning/
|title=Fine-Tuning
|website=[[The Stanford Encyclopedia of Philosophy]]
|publisher=Center for the Study of Language and Information (CSLI), Stanford University
|access-date=February 15, 2022
|date=November 12, 2021
|first=Simon
|last=Friederich
|archive-date=October 10, 2023
|archive-url=https://web.archive.org/web/20231010234820/https://plato.stanford.edu/entries/fine-tuning/
|url-status=live
}}</ref> The proposition is discussed among [[philosophy|philosophers]], [[scientist]]s, [[theology|theologians]], and proponents of [[creationism]].<ref name=toa>{{cite web |url=http://www.talkorigins.org/indexcc/CI/CI301.html |title=CI301: The Anthropic Principle |access-date=October 31, 2007 |editor-first=Mark |editor-last=Isaak |date=2005 |work=Index to Creationist Claims |publisher=[[TalkOrigins Archive]] |archive-date=July 1, 2014 |archive-url=https://web.archive.org/web/20140701145811/http://www.talkorigins.org/indexcc/CI/CI301.html |url-status=live }}</ref>