Universe

=== 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.}}
Description	What you type	What you get
Italic	''Italic text''	Italic text
Bold	'''Bold text'''	Bold text
Bold & italic	'''''Bold & italic text'''''	*Bold & italic text*
Description	What you type	What you get
Reference	Page text.<ref>[https://www.example.org/ Link text], additional text.</ref>	Page text.^[1]
Named reference	Page text.<ref name="test">[https://www.example.org/ Link text]</ref>	Page text.^[2]
Additional use of the same reference	Page text.<ref name="test" />	Page text.^[2]
Display references	<references />	↑ Link text, additional text. ↑ Link text