Editing Universe (section)

=== Size and regions ===
{{See also|Observational cosmology}}
[[File:Extended universe logarithmic illustration (English annotated).png|thumb|upright=2.3|Illustration of the observable universe, centered on the Sun. The distance scale is [[logarithmic scale|logarithmic]]. Due to the finite speed of light, we see more distant parts of the universe at earlier times.]]

Due to the finite [[speed of light]], there is a limit (known as the [[particle horizon]]) to how far light can travel over the [[age of the universe]].
The spatial region from which we can receive light is called the [[observable universe]]. The [[Comoving distance|proper distance]] (measured at a fixed time) 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><ref>{{Cite news |last=Crane |first=Leah |date=29 June 2024 |editor-last=de Lange |editor-first=Catherine |title=How big is the universe, really? |work=New Scientist |page=31}}</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" /> Although the distance traveled by light from the edge of the observable universe is close to the [[age of the universe]] times the speed of light, {{convert|13.8|e9ly|e9pc}}, the proper distance is larger 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">{{harvnb|Rindler|1986||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|doi-access=free }}</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> An estimate from 2011 suggests that if the [[cosmological principle]] holds, the whole universe 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 |doi-access=free |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 [[Hartle–Hawking state|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.}}