Editing Observable universe

{{Short description|All of space observable from the Earth at the present}}
{{Infobox
| bodystyle        = width:25em;
| title            = Observable universe
| image            = [[File:Observable Universe with Measurements 01.png|300px]]
| caption          = Visualization of the observable universe. The scale is such that the fine grains represent collections of large numbers of superclusters. The [[Virgo Supercluster]]—home of Milky Way—is marked at the center, but is too small to be seen.
| label1           = Diameter
| data1            = {{val|8.8|e=26|u=m}} or 880 [[Yotta-|Ym]] {{nowrap|(28.5 [[parsec|Gpc]] or 93 [[light-year|Gly]])}}<ref>{{cite book|author1=Itzhak Bars|author2=John Terning|title=Extra Dimensions in Space and Time|url=https://books.google.com/books?id=fFSMatekilIC&pg=PA27|access-date=2011-05-01|year= 2009|publisher=Springer|isbn=978-0387776378|pages=27–}}</ref>
| label2           = Circumference
| data2            = {{val| 2.764|e=27|u=m}} or 2.764 [[Ronna-|Rm]] {{nowrap|(89.6 [[parsec|Gpc]] or 292.2 [[light-year|Gly]])}}
| label3           = Volume
| data3            = {{val|3.566|e=80|u=m3}}<ref>{{Cite web|url=https://www.wolframalpha.com/|title=volume universe Wolfram{{pipe}}Alpha|website=www.wolframalpha.com}}</ref>
| label4           = Mass (ordinary matter)
| data4            = {{val|1.5|e=53|u=kg}}<ref group=note>Multiply percentage of ordinary matter given by Planck below, with total energy density given by WMAP below</ref>
| label5           = Density (of total energy)
| data5            = {{val|9.9|e=-27|u=kg/m3}} (equivalent to 6 [[proton]]s per cubic meter of space)<ref>{{cite web |url=http://map.gsfc.nasa.gov/universe/uni_matter.html |title=What is the Universe Made Of? |publisher=NASA |access-date=June 1, 2022}}</ref>
| label6           = Age
| data6            = {{val|13.787|0.020|ul=billion}} years<ref name="Planck 2018">
 {{cite journal
 |author=Planck Collaboration
 |year=2020
 |title=Planck 2018 results. VI.&nbsp;Cosmological parameters
 |journal=Astronomy & Astrophysics
 |volume=641 |at=page&nbsp;A6 (see PDF page&nbsp;15, Table&nbsp;2: "Age/Gyr", last&nbsp;column)
 |doi=10.1051/0004-6361/201833910
 |arxiv=1807.06209 |bibcode=2020A&A...641A...6P
 |s2cid=119335614
 }}</ref>
| label7           = Average temperature
| data7            = {{val|2.72548|0.00057}} [[Kelvin|K]]<ref>
 {{Cite journal
 | last1 = Fixsen
 | first1 = D. J.
 | title = The Temperature of the Cosmic Microwave Background
 | journal = The Astrophysical Journal
 | volume = 707
 | issue = 2
 | pages = 916–920
 | date = 30 November 2009
 | doi = 10.1088/0004-637X/707/2/916
 | bibcode = 2009ApJ...707..916F
 | arxiv = 0911.1955
 | s2cid = 119217397
 }}</ref>
| label8           = Contents
| data8            = {{plainlist|
* [[Baryon#Baryonic matter|Ordinary (baryonic)]] [[matter]] (4.9%)
* [[Dark matter]] (26.8%)
* [[Dark energy]] (68.3%)<ref>{{Cite web | url=https://www.esa.int/spaceinimages/Images/2013/03/Planck_cosmic_recipe | title=Planck cosmic recipe}}</ref>}}
}}

The '''observable universe''' is a [[Ball (mathematics)|spherical]] region of the [[universe]] consisting of all [[matter]] that can be [[observation|observed]] from [[Earth]]; the [[electromagnetic radiation]] from these [[astronomical object|objects]] has had time to reach the [[Solar System]] and Earth since the beginning of the [[metric expansion of space|cosmological expansion]].  Assuming the universe is [[isotropy|isotropic]], the distance to the edge of the observable universe is [[equidistant|the same]] in every direction. That is, the observable universe is a [[sphere|spherical]] region centered on the observer. Every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.

The word ''observable'' in this sense does not refer to the capability of modern technology to detect [[light]] or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the [[speed of light]] itself. No signal can travel faster than light, hence there is a maximum distance, called the [[particle horizon]], beyond which nothing can be detected, as the signals could not have reached the observer yet.

According to calculations, the current [[comoving distance]] to particles from which the [[cosmic microwave background radiation]] (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion [[parsec]]s (about 45.7 billion light-years). The comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years),<ref name="mapofuniverse">{{cite journal|last = Gott III|first = J. Richard|display-authors=4|author2=Mario Jurić |author3=David Schlegel |author4=Fiona Hoyle |author5=Michael Vogeley |author6=Max Tegmark |author7=Neta Bahcall |author8=Jon Brinkmann |title = A Map of the Universe|url=http://www.astro.princeton.edu/universe/ms.pdf|journal = The Astrophysical Journal|volume = 624|issue = 2|pages = 463–484|date = 2005|doi = 10.1086/428890|bibcode=2005ApJ...624..463G| arxiv=astro-ph/0310571|s2cid = 9654355}}</ref> about 2% larger. The [[radius]] of the observable universe is therefore estimated to be about 46.5 billion light-years.<ref>{{Cite web |title=Frequently Asked Questions in Cosmology |url=https://astro.ucla.edu/~wright/cosmology_faq.html |access-date=2023-09-15 |website=astro.ucla.edu}}</ref><ref name="ly93">{{cite journal |last1=Lineweaver |first1=Charles |last2=Davis |first2=Tamara M. |date=2005 |title=Misconceptions about the Big Bang |journal=Scientific American |volume=292 |issue=3 |pages=36–45 |bibcode=2005SciAm.292c..36L |doi=10.1038/scientificamerican0305-36}}</ref> Using the [[Friedmann equations|critical density]] and the diameter of the observable universe, the total mass of ordinary matter in the universe can be calculated to be about {{val|1.5|e=53|u=kg}}.<ref>See the "Mass of ordinary matter" section in this article.</ref> In November 2018, astronomers reported that [[extragalactic background light]] (EBL) amounted to {{val|4|e=84}} photons.<ref name="NYT-20181203">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=All the Light There Is to See? 4 x 10<sup>84</sup> Photons |url=https://www.nytimes.com/2018/12/03/science/space-stars-photons-light.html |date=3 December 2018 |work=[[The New York Times]] |access-date=4 December 2018 }}</ref><ref name="SCI-20181130">{{cite journal |author=The Fermi-LAT Collaboration |title=A gamma-ray determination of the Universe's star formation history |date=30 November 2018 |journal=[[Science (journal)|Science]] |volume=362 |issue=6418 |pages=1031–1034 |doi=10.1126/science.aat8123 |pmid=30498122 |arxiv=1812.01031 |bibcode=2018Sci...362.1031F }}</ref>

As the universe's expansion is accelerating, all currently observable objects, outside the local [[supercluster]], will eventually appear to freeze in time, while emitting progressively redder and fainter light. For instance, objects with the current [[redshift]] ''[[Redshift#Measurement, characterization, and interpretation|z]]'' from 5 to 10 will only be observable up to an age of 4–6 billion years. In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about {{convert|19|Gpc|Gly}}) will never reach Earth.<ref name=Loeb2002>{{cite journal|doi=10.1103/PhysRevD.65.047301|title=Long-term future of extragalactic astronomy|journal=Physical Review D|volume=65|issue=4|pages=047301|year=2002|last1=Loeb|first1=Abraham|arxiv=astro-ph/0107568|bibcode=2002PhRvD..65d7301L|s2cid=1791226}}</ref>

== Overview ==
{{Physical cosmology|comp/struct}}
[[File:Home in Relation to Everything-Observable Universe.png|thumb|upright=1.6|Observable Universe as a function of time and distance, in context of the [[expanding Universe]]]]
The universe's size is unknown, and it may be infinite in extent.<ref>{{cite book |first=Andrew |last=Liddle |date=2015 |publisher=John Wiley |title=An Introduction to Modern Cosmology |isbn=978-1118502143 |url=https://books.google.com/books?id=4lPWBgAAQBAJ&dq=infinite+universe+observable&pg=PA33}}</ref> Some parts of the universe are too far away for the light emitted since the [[Big Bang]] to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so one might expect that additional regions will become observable. Regions distant from observers (such as us) are expanding away faster than the speed of light, at rates estimated by [[Hubble's law]].<ref group="note"> [[Special relativity]] prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see [[Comoving and proper distances#Uses of the proper distance|uses of the proper distance]] for a discussion.</ref> The [[Accelerating expansion of the universe|expansion rate appears to be accelerating]], which [[dark energy]] was proposed to explain.

Assuming dark energy remains constant (an unchanging [[cosmological constant]]) so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter the observable universe at any time in the future because light emitted by objects outside that limit could never reach the Earth. Note that, because the [[Hubble's law#Interpretation|Hubble parameter]] is decreasing with time, there can be cases where a galaxy that is receding from Earth only slightly faster than light emits a signal that eventually reaches Earth.<ref name=ly93 /><ref>[http://curious.astro.cornell.edu/question.php?number=575 Is the universe expanding faster than the speed of light?] (see the last two paragraphs).</ref> This future visibility limit is calculated at a [[comoving distance]] of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that can ever be theoretically observed in the infinite future is only larger than the number currently observable by a factor of 2.36 (ignoring redshift effects).<ref name="mapofuniverse2" group=note>The comoving distance of the future visibility limit is calculated on p. 8 of Gott et al.'s [http://www.astro.princeton.edu/universe/ms.pdf A Map of the Universe] to be 4.50 times the [[Hubble radius]], given as 4.220 billion parsecs (13.76 billion light-years), whereas the current comoving radius of the observable universe is calculated on p. 7 to be 3.38 times the Hubble radius. The number of galaxies in a sphere of a given comoving radius is proportional to the cube of the radius, so as shown on p. 8 the ratio between the number of galaxies observable in the future visibility limit to the number of galaxies observable today would be (4.50/3.38)<sup>3</sup> = 2.36.</ref>

In principle, more galaxies will become observable in the future; in practice, an increasing number of galaxies will become extremely [[redshift]]ed due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.<ref>{{cite journal |last1=Krauss |first1=Lawrence M. |last2=Scherrer |first2=Robert J. |date=2007 |title=The Return of a Static Universe and the End of Cosmology |journal=General Relativity and Gravitation |volume=39 |issue=10 |pages=1545–1550 |arxiv=0704.0221 |bibcode=2007GReGr..39.1545K |doi=10.1007/s10714-007-0472-9 |s2cid=123442313}}</ref><ref>[https://www.npr.org/templates/story/story.php?storyId=102715275 Using Tiny Particles To Answer Giant Questions]. Science Friday, 3 Apr 2009. According to the [https://www.npr.org/templates/transcript/transcript.php?storyId=102715275 transcript], [[Brian Greene]] makes the comment "And actually, in the far future, everything we now see, except for our local galaxy and a region of galaxies will have disappeared. The entire universe will disappear before our very eyes, and it's one of my arguments for actually funding cosmology. We've got to do it while we have a chance."</ref><ref>See also [[Faster than light#Universal expansion]] and [[Future of an expanding universe#Galaxies outside the Local Supercluster are no longer detectable]].</ref> A galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its history, say, a signal sent from the galaxy only 500 million years after the Big Bang. Because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future, so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang,<ref name=Loeb2002/> even though it remains at the same comoving distance less than that of the observable universe. 

This can be used to define a type of cosmic [[event horizon]] whose distance from the Earth changes over time. For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away.<ref name=ly93 />

The space before this cosmic event horizon can be called "reachable universe", that is all galaxies closer than that could be reached if we left for them today, at the speed of light; all galaxies beyond that are unreachable.<ref>{{Cite web |last=Siegel |first=Ethan |title=How Much Of The Unobservable Universe Will We Someday Be Able To See? |url=https://www.forbes.com/sites/startswithabang/2019/03/05/how-much-of-the-unobservable-universe-will-we-someday-be-able-to-see/ |access-date=2023-04-04 |website=Forbes |language=en}}</ref><ref>{{Cite web |last=Siegel |first=Ethan |date=2021-10-25 |title=94% of the universe's galaxies are permanently beyond our reach |url=https://medium.com/starts-with-a-bang/94-of-the-universes-galaxies-are-permanently-beyond-our-reach-293c29e771be |access-date=2023-04-04 |website=Starts With A Bang! |language=en}}</ref> Simple observation will show the future visibility limit (62 billion light-years) is exactly equal to the reachable limit (16 billion light-years) added to the current visibility limit (46 billion light-years).<ref>Ord, Toby. (2021). The Edges of Our Universe. [https://www.researchgate.net/publication/350647191_The_Edges_of_Our_Universe]</ref><ref name="mapofuniverse" />

[[File:Home in Relation to Everything-Reachable Universe.png|thumb|upright=1.6|The reachable Universe as a function of time and distance, in context of the expanding Universe.]]

== "The universe" versus "the observable universe" ==
Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe".{{citation needed|date=September 2015}} This can be justified on the grounds that we can never know anything by direct observation about any part of the universe that is [[Causality (physics)|causally disconnected]] from the Earth, although many credible theories require a total universe much larger than the observable universe.{{citation needed|date=September 2015}} No evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place. However, some models propose it could be finite but unbounded,<ref group=note>This does not mean "unbounded" in the mathematical sense; a finite universe would have an upper bound on the distance between two points.  Rather, it means that there is no boundary past which there is nothing. See ''[[Geodesic manifold]]''.</ref> like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.

It is plausible that the [[galaxy|galaxies]] within the observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of [[cosmic inflation]] initially introduced by [[Alan Guth]] and [[D. Kazanas]],<ref>{{cite journal|doi=10.1086/183361|title=Dynamics of the universe and spontaneous symmetry breaking|journal=The Astrophysical Journal|volume=241|pages=L59–L63|year=1980|last1=Kazanas|first1=D.|bibcode=1980ApJ...241L..59K|doi-access=free}}</ref> if it is assumed that inflation began about 10<sup>−37</sup> seconds after the Big Bang and that the pre-inflation size of the universe was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least {{val|1.5|e=34}} light-years — this is at least {{val|3|e=23}} times the radius of the observable universe.<ref>{{cite book |author=Guth |first=Alan H. |url=https://archive.org/details/inflationaryuniv0000guth |title=The inflationary universe: the quest for a new theory of cosmic origins |publisher=Basic Books |year=1997 |isbn=978-0201328400 |pages=[https://archive.org/details/inflationaryuniv0000guth/page/186 186]– |access-date=1 May 2011 |url-access=registration}}</ref>

If the universe is finite but unbounded, it is also possible that the universe is ''smaller'' than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. Bielewicz et al.<ref>{{Cite journal|last1=Bielewicz |first1=P. |last2=Banday |first2=A. J. |last3=Gorski |first3=K. M. |date=2013 |arxiv=1303.4004 |title=Constraints on the Topology of the Universe|journal=Proceedings of the XLVIIth Rencontres de Moriond|editor-first1=E. |editor-last1=Auge|editor-first2=J.|editor-last2=Dumarchez|editor-first3=J.|editor-last3=Tran Thanh Van|volume=2012 |issue=91 |bibcode=2013arXiv1303.4004B }}</ref> claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface. This value is based on matching-circle analysis of the [[WMAP]] 7-year data. This approach has been disputed.<ref>{{cite arXiv |eprint=1007.3466 |last1=Mota |first1=B. |last2=Reboucas |first2=M. J. |last3=Tavakol |first3=R. |title=Observable circles-in-the-sky in flat universes |class=astro-ph.CO |date=1 July 2010}}</ref>

== Size ==
[[File:HubbleUltraDeepFieldwithScaleComparison.jpg|thumb|upright=1.6|[[Hubble Ultra-Deep Field]] image of a region of the observable universe (equivalent sky area size shown in bottom left corner), near the [[Fornax|constellation Fornax]]. Each spot is a [[galaxy]], consisting of billions of stars. The light from the smallest, most [[redshift]]ed galaxies originated around 12.6 billion years ago,<ref name=Malhotra>{{cite web |title=As far as the Hubble can see |first=Sangeeta |last=Malhotra |publisher=[[Arizona State University]] |url=http://malhotra.asu.edu/Welcome_files/ASY-HI1105.pdf |access-date=October 28, 2010}}</ref> close to the [[age of the universe]].]]
The [[Comoving and proper distances|comoving distance]] from Earth to the edge of the observable universe is about 14.26 giga[[parsec]]s (46.5 [[1000000000 (number)|billion]] [[light-year]]s or {{convert|14.26|Gpc|m|disp=output only|abbr=on|sp=us}}) in any direction. The observable universe is thus a sphere with a [[diameter]] of about 28.5 gigaparsecs<ref>{{cite web|title = WolframAlpha|url=http://www.wolframalpha.com/input/?i=93+billion+light+years+in+parsecs|access-date=29 November 2011}}</ref> (93 billion light-years or {{convert|28.5|Gpc|m|disp=output only|abbr=on|sp=us}}).<ref>{{cite web|title = WolframAlpha|url=http://www.wolframalpha.com/input/?i=size+of+universe|access-date=29 November 2011}}</ref> Assuming that space is roughly [[Shape of the universe#Universe with zero curvature|flat]] (in the sense of being a [[Euclidean space]]), this size corresponds to a comoving volume of about {{val|1.22|e=4|u=Gpc<sup>3</sup>}}<!--based on a 28.5 Gpc diameter--> ({{val|4.22|e=5|u=Gly<sup>3</sup>}} or {{val|3.57|e=80|u=m3}}).<ref>{{cite web|title = WolframAlpha|url=http://www.wolframalpha.com/input/?i=%28volume+of+universe%29+%3D%3D+%283.57x10^80+m^3%29+%3D%3D+%284.21594x10^5+Gly^3%29+%3D%3D+%281.2151x10^4+Gpc^3%29|access-date=15 February 2016}}</ref>

These are distances now (in [[cosmological time]]), not distances at the time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted at the [[Recombination (cosmology)|time of photon decoupling]], estimated to have occurred about {{val|380000||fmt=commas}} years after the Big Bang,<ref name="wmap7parameters">{{cite web |title=Seven-Year Wilson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results |url=http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/basic_results/wmap_7yr_basic_results.pdf |access-date=2010-12-02 |publisher=nasa.gov}} (see p. 39 for a table of best estimates for various cosmological parameters).</ref><ref>{{cite web
|last=Abbott|first=Brian|date=May 30, 2007
|url=http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php
|title=Microwave (WMAP) All-Sky Survey
|publisher=Hayden Planetarium|access-date=2008-01-13
}}</ref> which occurred around 13.8 billion years ago. This radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from Earth.<ref name="mapofuniverse" /><ref name="ly93" /> To estimate the distance to that matter at the time the light was emitted, we may first note that according to the [[Friedmann–Lemaître–Robertson–Walker metric]], which is used to model the expanding universe, if we receive light with a [[redshift]] of ''z'', then the [[Scale factor (cosmology)|scale factor]] at the time the light was originally emitted is given by<ref>{{cite book |author=Davies |first=Paul |url=https://books.google.com/books?id=akb2FpZSGnMC&pg=PA187 |title=The new physics |publisher=Cambridge University Press |year=1992 |isbn=978-0521438315 |pages=187– |access-date=1 May 2011}}</ref><ref>{{cite book |author=Mukhanov |first=V. F. |url=https://books.google.com/books?id=1TXO7GmwZFgC&pg=PA58 |title=Physical foundations of cosmology |date=2005 |publisher=Cambridge University Press |isbn=978-0521563987 |pages=58– |access-date=1 May 2011}}</ref>
<blockquote><math> a(t) = \frac{1}{1 + z}</math>.</blockquote>

[[Wilkinson Microwave Anisotropy Probe#Nine-year data release|WMAP nine-year results]] combined with other measurements give the redshift of photon decoupling as ''z''&nbsp;=&nbsp;{{val|1091.64|0.47}},<ref name="bennet-wmap9year-2012">{{cite journal|last1=Bennett|first1=C. L.|display-authors=4|last2=Larson|first2=D.|last3=Weiland|first3=J. L.|last4=Jarosik|first4=N.|last5=Hinshaw|first5=G.|last6=Odegard|first6=N.|last7=Smith|first7=K. M.|last8=Hill|first8=R. S.|last9=Gold|first9=B.|last10=Halpern|first10=M.|last11=Komatsu|first11=E.|last12=Nolta|first12=M. R.|last13=Page|first13=L.|last14=Spergel|first14=D. N.|last15=Wollack|first15=E.|last16=Dunkley|first16=J.|last17=Kogut|first17=A.|last18=Limon|first18=M.|last19=Meyer|first19=S. S.|last20=Tucker|first20=G. S.|last21=Wright|first21=E. L.|title=Nine-year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results|journal=The Astrophysical Journal Supplement Series|date=1 October 2013|volume=208|issue=2|pages=20|doi=10.1088/0067-0049/208/2/20|bibcode=2013ApJS..208...20B|arxiv=1212.5225|s2cid=119271232}}</ref> which implies that the scale factor at the time of photon [[Decoupling (cosmology)|decoupling]] would be {{frac|1092.64}}. So if the matter that originally emitted the oldest CMBR [[photon]]s has a present distance of 46 billion light-years, then the distance would have been only about 42 million light-years at the time of decoupling.

The [[light-travel distance]] to the edge of the observable universe is the [[age of the universe]] times the [[speed of light]], 13.8 billion light years. This is the distance that a photon emitted shortly after the Big Bang, such as one from the [[cosmic microwave background]], has traveled to reach observers on Earth. Because [[spacetime]] is curved, corresponding to the [[Expansion of the universe|expansion of space]], this distance does not correspond to the true distance at any moment in time.<ref>{{Cite web |last=Wright |first=Ned |title=Light Travel Time Distance |url=https://astro.ucla.edu/~wright/Dltt_is_Dumb.html |access-date=2023-09-15 |website=astro.ucla.edu}}</ref>

== Matter and mass ==
=== Number of galaxies and stars ===
The observable universe contains as many as an estimated 2 trillion galaxies<ref name="BBC-20231129">{{cite news |last=Gunn |first=Alistair |date=29 November 2023 |title=How many galaxies are there in the universe? – Do astronomers know how many galaxies exist? How many can we see in the observable Universe? |url=https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe |url-status=live |archiveurl=https://archive.today/20231203021645/https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe |archivedate=3 December 2023 |accessdate=2 December 2023 |work=[[BBC Sky at Night]]}}</ref><ref>{{cite journal |title=New Horizons spacecraft answers the question: How dark is space? |website=phys.org |url=https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |access-date=January 15, 2021 |language=en |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115110710/https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |url-status=live }}</ref><ref>{{cite news |last1=Howell |first1=Elizabeth |title=How Many Galaxies Are There? |url=https://www.space.com/25303-how-many-galaxies-are-in-the-universe.html |website=Space.com |access-date=March 5, 2021 |date=March 20, 2018 |archive-date=February 28, 2021 |archive-url=https://web.archive.org/web/20210228013433/https://www.space.com/25303-how-many-galaxies-are-in-the-universe.html |url-status=live }}</ref> and, overall, as many as an estimated 10<sup>24</sup> stars<ref name="ESA-2019">{{cite web |author=Staff |title=How Many Stars Are There In The Universe? |url=https://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |date=2019 |work=[[European Space Agency]] |access-date=September 21, 2019 |archive-date=September 23, 2019 |archive-url=https://web.archive.org/web/20190923134902/http://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |url-status=live }}</ref><ref>{{Cite book|chapter=The Structure of the Universe|doi=10.1007/978-1-4614-8730-2_10|title=The Fundamentals of Modern Astrophysics|pages=279–294|year=2015|last1=Marov|first1=Mikhail Ya.|isbn=978-1-4614-8729-6}}</ref> &ndash; more stars (and, potentially, Earth-like planets) than all the [[Sand|grains of beach sand]] on planet [[Earth]].<ref name="SU-20020201">{{cite web |last=Mackie |first=Glen |title=To see the Universe in a Grain of Taranaki Sand |url=http://astronomy.swin.edu.au/~gmackie/billions.html |date=February 1, 2002 |work=[[Centre for Astrophysics and Supercomputing]] |access-date=January 28, 2017 |archive-date=June 30, 2012 |archive-url=https://archive.today/20120630205715/http://astronomy.swin.edu.au/~gmackie/billions.html |url-status=live }}</ref><ref name="CNET-20150319">{{cite news |last=Mack |first=Eric |date=19 March 2015 |title=There may be more Earth-like planets than grains of sand on all our beaches – New research contends that the Milky Way alone is flush with billions of potentially habitable planets – and that's just one sliver of the universe. |url=https://www.cnet.com/science/the-milky-way-is-flush-with-habitable-planets-study-says/ |url-status=live |archiveurl=https://archive.today/20231201144523/https://www.cnet.com/science/the-milky-way-is-flush-with-habitable-planets-study-says/ |archivedate=1 December 2023 |accessdate=1 December 2023 |work=[[CNET]]}}</ref><ref name="MNRAS-20150313">{{cite journal |last1=Bovaird |first1=T. T. |last2=Lineweaver |first2=C. H. |last3=Jacobsen |first3=S. K. |date=13 March 2015 |title=Using the inclinations of Kepler systems to prioritize new Titius–Bode-based exoplanet predictions |url=https://academic.oup.com/mnras/article/448/4/3608/970734 |url-status=live |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=448 |issue=4 |pages=3608–3627 |arxiv=1412.6230 |doi=10.1093/mnras/stv221 |doi-access=free |archiveurl=https://archive.today/20231201151205/https://academic.oup.com/mnras/article/448/4/3608/970734 |archivedate=1 December 2023 |accessdate=1 December 2023}}</ref> Other estimates are in the hundreds of billions rather than trillions.<ref name=":0">{{cite journal |last1=Lauer |first1=T. R. |last2=Postman |first2=M. |last3=Spencer |first3=J. R. |last4=Weaver |first4=H. A. |last5=Stern |first5=S. A. |last6=Gladstone |first6=G. R. |last7=Binzel |first7=R. P. |last8=Britt |first8=D. T. |last9=Buie |first9=M. W. |last10=Buratti |first10=B. J. |last11=Cheng |first11=A. F. |last12=Grundy |first12=W. M. |last13=Horányi |first13=M. |last14=Kavelaars |first14=J. J. |last15=Linscott |first15=I. R. |last16=Lisse |first16=C. M. |last17=McKinnon |first17=W. B. |last18=McNutt |first18=R. L. |last19=Moore |first19=J. M. |last20=Núñez |first20=J. I. |last21=Olkin |first21=C. B. |last22=Parker |first22=J. W. |last23=Porter |first23=S. B. |last24=Reuter |first24=D. C. |last25=Robbins |first25=S. J. |last26=Schenk |first26=P. M. |last27=Showalter |first27=M. R. |last28=Singer |first28=K. N. |last29=Verbiscer |first29=A. J. |last30=Young |first30=L. A. |date=2022 |title=Anomalous Flux in the Cosmic Optical Background Detected with New Horizons Observations |journal=The Astrophysical Journal Letters |volume=927 |issue=1 |pages=l8 | doi=10.3847/2041-8213/ac573d|arxiv=2202.04273 |bibcode=2022ApJ...927L...8L | doi-access=free}}</ref><ref name="ann21001">{{cite news |last=Lauer |first=Todd |title= NOIRLab Scientist Finds the Universe to be Brighter than Expected |url= https://noirlab.edu/public/announcements/ann21001/ |date=12 January 2021 |work=[[NOIRLab]] |access-date=12 January 2021 }}</ref><ref name="arxiv:2011.03052">{{cite journal |last1=Lauer |first1=Tod R. |last2=Postman |first2=Marc |last3=Weaver |first3=Harold A. |last4=Spencer |first4=John R. |last5=Stern |first5=S. Alan |last6=Buie |first6=Marc W. |last7=Durda |first7=Daniel D. |last8=Lisse |first8=Carey M. |last9=Poppe |first9=A. R. |last10=Binzel |first10=Richard P. |last11=Britt |first11=Daniel T. |last12=Buratti |first12=Bonnie J. |last13=Cheng |first13=Andrew F. |last14=Grundy |first14=W. M. |last15=Horányi |first15=Mihaly |last16=Kavelaars |first16=J. J. |last17=Linscott |first17=Ivan R. |last18=McKinnon |first18=William B. |last19=Moore |first19=Jeffrey M. |last20=Núñez |first20=J. I. |last21=Olkin |first21=Catherine B. |last22=Parker |first22=Joel W. |last23=Porter |first23=Simon B. |last24=Reuter |first24=Dennis C. |last25=Robbins |first25=Stuart J. |last26=Schenk |first26=Paul |last27=Showalter |first27=Mark R. |last28=Singer |first28=Kelsi N. |last29=Verbiscer |first29=Anne J. |last30=Young |first30=Leslie A. |title=New Horizons Observations of the Cosmic Optical Background |journal=The Astrophysical Journal |date=11 January 2021 |volume=906 |issue=2 |pages=77 |doi=10.3847/1538-4357/abc881 | arxiv= 2011.03052 |bibcode=2021ApJ...906...77L |hdl=1721.1/133770 |s2cid=226277978 |doi-access=free }}</ref> The estimated total number of stars in an [[Cosmic inflation|inflationary universe]] (observed and unobserved) is 10<sup>100</sup>.<ref name="SR-20200203">{{cite journal |last=Totani |first=Tomonori |title=Emergence of life in an inflationary universe |date=3 February 2020 |journal=[[Scientific Reports]] |volume=10 |number=1671 |page=1671 |doi=10.1038/s41598-020-58060-0 |pmid=32015390 |arxiv=1911.08092 |bibcode=2020NatSR..10.1671T |doi-access=free |pmc=6997386 }}</ref>

=== <span class="anchor" id="Matter content"></span> Matter content—number of atoms ===
<!-- [[Atoms in the universe]] redirects here -->
{{Main|Abundance of the chemical elements}}
Assuming the mass of ordinary matter is about {{val|1.45|e=53|u=kg}} as discussed above, and assuming all atoms are [[hydrogen atom]]s (which are about 74% of all atoms in the Milky Way by mass), the estimated total number of atoms in the observable universe is obtained by dividing the mass of ordinary matter by the mass of a hydrogen atom. The result is approximately 10<sup>80</sup> hydrogen atoms, also known as the [[Eddington number]].

=== Mass of ordinary matter ===
The mass of the observable universe is often quoted as 10<sup>53</sup>&nbsp;kg.<ref name="Paul Davies 2006 43">{{cite book |author=Davies |first=Paul |url=https://archive.org/details/cosmicjackpotwhy0000davi/page/43 |title=The Goldilocks Enigma |date=2006 |publisher=First Mariner Books |isbn=978-0618592265 |page=[https://archive.org/details/cosmicjackpotwhy0000davi/page/43 43–]}}</ref> In this context, mass refers to ordinary (baryonic) matter and includes the [[interstellar medium]] (ISM) and the [[intergalactic medium]] (IGM). However, it excludes [[dark matter]] and [[dark energy]]. This quoted value for the mass of ordinary matter in the universe can be estimated based on critical density. The calculations are for the observable universe only as the volume of the whole is unknown and may be infinite.

=== Estimates based on critical density ===
Critical density is the energy density for which the universe is flat.<ref>See [[Friedmann equations#Density parameter]].</ref> If there is no dark energy, it is also the [[density]] for which the expansion of the universe is poised between continued expansion and collapse.<ref>{{cite book |author=Kaku |first=Michio |url=https://books.google.com/books?id=cKULZJpcJBwC |title=Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos |publisher=Knopf Doubleday |year=2006 |isbn=978-0307276988 |page=385 |language=en-us}}</ref> From the [[Friedmann equations]], the value for <math>\rho_\text{c}</math> critical density, is:<ref>{{cite book |author=Schutz |first=Bernard F. |url=https://books.google.com/books?id=iEZNXvYwyNwC&pg=PA361 |title=Gravity from the ground up |date=2003 |publisher=Cambridge University Press |isbn=978-0521455060 |pages=361– |language=en-uk}}</ref>
: <math>\rho_\text{c} = \frac{3 H^2}{8 \pi G},</math>
where ''G'' is the [[gravitational constant]] and {{nowrap|1=''H'' = ''H''<sub>0</sub>}} is the present value of the [[Hubble constant]]. The value for ''H''<sub>0</sub>, as given by the European Space Agency's Planck Telescope, is ''H''<sub>0</sub> = 67.15 kilometres per second per megaparsec. This gives a critical density of {{val|0.85|e=-26|u=kg/m3}}, or about 5 hydrogen atoms per cubic metre. This density includes four significant types of energy/mass: ordinary matter (4.8%), neutrinos (0.1%), [[cold dark matter]] (26.8%), and [[dark energy]] (68.3%).<ref name="planck_cosmological_parameters">{{cite journal | arxiv=1303.5076 | title=Planck 2013 results. XVI. Cosmological parameters | author=Planck collaboration | journal=Astronomy & Astrophysics | date=2013|bibcode = 2014A&A...571A..16P | doi=10.1051/0004-6361/201321591 | volume=571 | pages=A16| s2cid=118349591 }}</ref> 

Although neutrinos are [[Standard Model]] particles, they are listed separately because they are [[Scale factor (cosmology)#Radiation-dominated era|ultra-relativistic]] and hence [[Equation of state (cosmology)#Ultra-relativistic particles|behave]] like radiation rather than like matter. The density of ordinary matter, as measured by Planck, is 4.8% of the total critical density or {{val|4.08|e=-28|u=kg/m3}}. To convert this density to mass we must multiply by volume, a value based on the radius of the "observable universe". Since the universe has been expanding for 13.8 billion years, the [[Comoving and proper distances|comoving distance]] (radius) is now about 46.6 billion light-years. Thus, volume ({{sfrac|4|3}}''πr''<sup>3</sup>) equals {{val|3.58|e=80|u=m3}} and the mass of ordinary matter equals density ({{val|4.08|e=-28|u=kg/m3}}) times volume ({{val|3.58|e=80|u=m3}}) or {{val|1.46|e=53|u=kg}}.

== Large-scale structure ==
[[File:Galactic treasure chest RXC J0142.9+4438.jpg|thumb|Galaxy clusters, like [[RXC J0142.9+4438]], are the nodes of the cosmic web that permeates the entire Universe.<ref>{{cite web |title=Galactic treasure chest |url=http://www.spacetelescope.org/images/potw1833a/ |website=www.spacetelescope.org |access-date=13 August 2018}}</ref>]]
[[File:Constrained_Local_Universe_Evolution_Simulation_(spherical).webm|thumb|upright=1.8|Video of a <!--[[:Category:Cosmological simulation|simulation]]-->[[cosmology|cosmological]] [[computer simulation|simulation]] of the local universe, showing the large-scale structure of galaxy clusters and dark matter.<ref>{{cite web |title=Blueprints of the Universe |url=https://www.eso.org/public/videos/cluesAdler-cylindrical/ |website=www.eso.org |access-date=31 December 2020 |language=en}}</ref>]]{{See also|List of largest cosmic structures}}
[[Redshift survey|Sky surveys]] and mappings of the various [[wavelength]] bands of [[electromagnetic radiation]] (in particular [[Hydrogen line|21-cm emission]]) have yielded much information on the content and character of the [[universe]]'s structure. The organization of structure appears to follow a [[hierarchy|hierarchical]] model with organization up to the [[scale (spatial)|scale]] of [[supercluster]]s and [[Galaxy filament|filaments]]. Larger than this (at scales between 30 and 200 megaparsecs),<ref>{{Cite book|url=https://books.google.com/books?id=RLwangEACAAJ|title=An Introduction to Modern Astrophysics|last1=Carroll|first1=Bradley W.|last2=Ostlie|first2=Dale A.|year=2013|publisher=Pearson|isbn=978-1292022932|edition=International|page=1178|language=en}}</ref> there seems to be no continued structure, a phenomenon that has been referred to as the ''End of Greatness''.<ref name=Kirshner /> The shape of the large scale structure can be summarized by the [[matter power spectrum]].

=== Cosmic Web: walls, filaments, nodes, and voids ===
[[File:Map of the Cosmic Web Generated from Slime Mould Algorithm.jpg|thumb|upright=1.8|Map of the cosmic web generated from a slime mould-inspired algorithm<ref>{{Cite web|url=https://www.spacetelescope.org/images/heic2003a/|title=Map of the Cosmic Web Generated from Slime Mould Algorithm|website=www.spacetelescope.org}}</ref>]]
The organization of structure arguably begins at the stellar level, though most cosmologists rarely address [[astrophysics]] on that scale. [[Star]]s are organized into [[galaxy|galaxies]], which in turn form [[galaxy group]]s, [[galaxy cluster]]s, [[supercluster]]s, sheets, [[galaxy filament|walls and filaments]], which are separated by immense [[void (astronomy)|voids]], creating a vast foam-like structure<ref>{{Cite book|url=https://books.google.com/books?id=RLwangEACAAJ|title=An Introduction to Modern Astrophysics|last1=Carroll|first1=Bradley W.|last2=Ostlie|first2=Dale A.|year=2013|publisher=Pearson|isbn=978-1292022932|edition=International|pages=1173–1174|language=en}}</ref> sometimes called the "cosmic web". Prior to 1989, it was commonly assumed that [[virial theorem|virialized]] galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, since the early 1980s, more and more structures have been discovered. In 1983, Adrian Webster identified the [[Webster LQG]], a [[large quasar group]] consisting of 5 quasars. The discovery was the first identification of a large-scale structure, and has expanded the information about the known grouping of matter in the universe.

In 1987, [[R. Brent Tully|Robert Brent Tully]] identified the [[Pisces–Cetus Supercluster Complex]], the galaxy filament in which the [[Milky Way]] resides. It is about 1 billion light-years across. That same year, an unusually large region with a much lower than average distribution of galaxies was discovered, the [[Giant Void]], which measures 1.3 billion light-years across. Based on [[redshift survey]] data, in 1989 [[Margaret Geller]] and [[John Huchra]] discovered the "[[CfA2 Great Wall|Great Wall]]",<ref name="redshift">{{cite journal |author=Geller |first1=M. J. |last2=Huchra |first2=J. P. |date=1989 |title=Mapping the universe. |journal=Science |volume=246 |issue=4932 |pages=897–903 |bibcode=1989Sci...246..897G |doi=10.1126/science.246.4932.897 |pmid=17812575 |s2cid=31328798}}</ref> a sheet of galaxies more than 500 million [[light-year]]s long and 200 million light-years wide, but only 15 million light-years thick. The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from [[redshift]]s.

Two years later, astronomers Roger G. Clowes and Luis E. Campusano discovered the [[Clowes–Campusano LQG]], a [[large quasar group]] measuring two billion light-years at its widest point, which was the largest known structure in the universe at the time of its announcement. In April 2003, another large-scale structure was discovered, the [[Sloan Great Wall]]. In August 2007, a possible supervoid was detected in the constellation [[Eridanus (constellation)|Eridanus]].<ref>{{Cite web |title=Biggest void in space is 1 billion light years across |url=https://www.newscientist.com/article/dn12546-biggest-void-in-space-is-1-billion-light-years-across/ |access-date=2023-09-15 |website=New Scientist |language=en-US}}</ref> It coincides with the '[[CMB cold spot]]', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model. This supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a billion light-years across, almost as big as the Giant Void mentioned above.

{{unsolved|physics|The largest structures in the universe are larger than expected. Are these actual structures or random density fluctuations?}}

[[File:Large-scale structure of light distribution in the universe.jpg|thumb|upright=2|Computer simulated image of an area of space more than 50 million light-years across, presenting a possible large-scale distribution of light sources in the universe—precise relative contributions of galaxies and [[quasar]]s are unclear.]]
Another large-scale structure is the [[SSA22 Protocluster]], a collection of galaxies and enormous gas bubbles that measures about 200 million light-years across.

In 2011, a large quasar group was discovered, [[U1.11]], measuring about 2.5 billion light-years across. On January 11, 2013, another large quasar group, the [[Huge-LQG]], was discovered, which was measured to be four billion light-years across, the largest known structure in the universe at that time.<ref>{{cite web | last = Wall | first = Mike | url = https://www.foxnews.com/science/largest-structure-in-universe-discovered/ | title = Largest structure in universe discovered | date = 2013-01-11 | publisher = [[Fox News]]}}</ref> In November 2013, astronomers discovered the [[Hercules–Corona Borealis Great Wall]],<ref name="2014paper">{{cite journal |last1=Horváth |first1=I. |last2=Hakkila |first2=Jon |last3=Bagoly |first3=Z. |date=2014 |title=Possible structure in the GRB sky distribution at redshift two |journal=Astronomy & Astrophysics |volume=561 |pages=L12 |arxiv=1401.0533 |bibcode=2014A&A...561L..12H |doi=10.1051/0004-6361/201323020 |s2cid=24224684}}</ref><ref name=original>{{cite arXiv |last1 = Horvath |first1 = I. |last2= Hakkila |first2=J. |last3=Bagoly |first3=Z. |title = The largest structure of the Universe, defined by Gamma-Ray Bursts |date = 2013 |eprint=1311.1104 |class=astro-ph.CO}}</ref> an even bigger structure twice as large as the former. It was defined by the mapping of [[gamma-ray burst]]s.<ref name=2014paper/><ref>{{cite web | last = Klotz | first = Irene | url = http://news.discovery.com/space/galaxies/universes-largest-structure-is-a-cosmic-conundrum-131119.htm | title = Universe's Largest Structure is a Cosmic Conundrum | date = 2013-11-19 | work = Discovery | access-date = 2013-11-20 | archive-date = 2016-05-16 | archive-url = https://web.archive.org/web/20160516172545/http://news.discovery.com/space/galaxies/universes-largest-structure-is-a-cosmic-conundrum-131119.htm | url-status = dead }}</ref>

In 2021, the [[American Astronomical Society]] announced the detection of the [[The Giant Arc|Giant Arc]]; a crescent-shaped string of galaxies that span 3.3 billion light years in length, located 9.2 billion light years from Earth in the constellation [[Boötes]] from observations captured by the [[Sloan Digital Sky Survey]].<ref>{{cite web | last = Ferreira | first = Becky | url = https://www.vice.com/en/article/a-structure-in-deep-space-is-so-giant-its-challenging-standard-physics/ | title = A Structure In Deep Space Is So Giant It's Challenging Standard Physics | date = 2021-06-23 | work = Vice}}</ref>

=== End of Greatness ===
The ''End of Greatness'' is an observational scale discovered at roughly 100&nbsp;[[Megaparsec|Mpc]] (roughly 300 million light-years) where the lumpiness seen in the large-scale structure of the [[universe]] is [[wikt:homogeneous|homogenized]] and [[isotropic|isotropized]] in accordance with the [[cosmological principle]].<ref name=Kirshner /> At this scale, no pseudo-random [[fractal]]ness is apparent.<ref>Natalie Wolchover, [https://news.yahoo.com/universe-isnt-fractal-study-finds-215053937.html "The Universe Isn't a Fractal, Study Finds"], LiveScience.com, 22 August 2012.</ref>

The [[supercluster]]s and [[Galaxy filament|filaments]] seen in smaller surveys are [[random]]ized to the extent that the smooth distribution of the universe is visually apparent. It was not until the [[redshift survey]]s of the 1990s were completed that this scale could accurately be observed.<ref name="Kirshner">{{cite book |author=Kirshner |first=Robert P. |url=https://archive.org/details/extravagantunive00kirs |title=The Extravagant Universe: Exploding Stars, Dark Energy and the Accelerating Cosmos |date=2002 |publisher=Princeton University Press |isbn=978-0691058627 |page=[https://archive.org/details/extravagantunive00kirs/page/71 71] |language=en-us |url-access=registration}}</ref>

=== Observations ===
[[File:2MASS LSS chart-NEW Nasa.jpg|right|upright=2.5|thumb|"Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the [[Milky Way]]. The image is derived from the [[2MASS|2MASS Extended Source Catalog (XSC)]]—more than 1.5 million galaxies, and the Point Source Catalog (PSC)—nearly 0.5 billion Milky Way stars. The galaxies are color-coded by '[[redshift]]' obtained from the [[Uppsala General Catalogue|UGC]], [[Harvard-Smithsonian Center for Astrophysics|CfA]], Tully NBGC, LCRS, [[2dF Galaxy Redshift Survey|2dF]], 6dFGS, and [[Sloan Digital Sky Survey|SDSS]] surveys (and from various observations compiled by the [[NASA/IPAC Extragalactic Database|NASA Extragalactic Database]]), or photo-metrically deduced from the [[K band (infrared)|K band]] (2.2&nbsp;μm). Blue are the nearest sources ({{nowrap|''z'' < 0.01}}); green are at moderate distances ({{nowrap|0.01 < ''z'' < 0.04}}) and red are the most distant sources that 2MASS resolves ({{nowrap|0.04 < ''z'' < 0.1}}). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center)."<ref>{{cite journal |last1=Jarrett |first1=T. H. |date=2004 |title=Large Scale Structure in the Local Universe: The 2MASS Galaxy Catalog |journal=Publications of the Astronomical Society of Australia |volume=21 |issue=4 |pages=396–403 |arxiv=astro-ph/0405069 |bibcode=2004PASA...21..396J |doi=10.1071/AS04050 |s2cid=56151100}}</ref>]]

[[File:Galactic+celestial quads.jpg|thumb|upright=2.5|Constellations grouped in galactic quadrants (N/S, 1–4) and their approximate divisions vis-a-vis celestial quadrants (NQ/SQ)]]

Another indicator of large-scale structure is the '[[Lyman-alpha forest]]'. This is a collection of [[Spectral line|absorption lines]] that appear in the spectra of light from [[quasar]]s, which are interpreted as indicating the existence of huge thin sheets of intergalactic (mostly [[hydrogen]]) gas. These sheets appear to collapse into filaments, which can feed galaxies as they grow where filaments either cross or are dense. An early direct evidence for this cosmic web of gas was the 2019 detection, by astronomers from the RIKEN Cluster for Pioneering Research in Japan and Durham University in the U.K., of light from the brightest part of this web, surrounding and illuminated by a cluster of forming galaxies, acting as cosmic flashlights for intercluster medium hydrogen fluorescence via Lyman-alpha emissions.<ref>{{cite journal |last1=Hamden |first1=Erika |date=4 October 2019 |title=Observing the cosmic web |url=https://www.science.org/doi/abs/10.1126/science.aaz1318 |journal=Science |volume=366 |issue=6461 |pages=31–32 |bibcode=2019Sci...366...31H |doi=10.1126/science.aaz1318 |pmid=31604290 |s2cid=203717729|url-access=subscription }}</ref><ref>{{cite web |last1=Byrd |first1=Deborah |title=Cosmic Web Fuels Stars And Supermassive Black Holes |url=https://earthsky.org/space/cosmic-web-gas-reservoir-fuel-galaxies-growth/ |website=earthsky.org |date=6 October 2019}}</ref>

In 2021, an international team, headed by Roland Bacon from the Centre de Recherche Astrophysique de Lyon (France), reported the first observation of diffuse extended Lyman-alpha emission from redshift 3.1 to 4.5 that traced several cosmic web filaments on scales of 2.5−4&nbsp;[[cMpc]] (comoving mega-parsecs), in filamentary environments outside massive structures typical of web nodes.<ref>{{cite journal |author=Bacon, R.; Mary, D.; Garel, T.; Blaizot, J.; Maseda, M.; Schaye, J.; Wisotzki, L.; Conseil, S.; Brinchmann, J.; Leclercq, F.; Abril-Melgarejo, V.; Boogaard, L.; Bouché, N. F.; Contini, T.; Feltre, A.; Guiderdoni, B.; Herenz, C.; Kollatschny, W.; Kusakabe, H.; Matthee, J.; Michel-Dansac, L.; Nanayakkara, T.; Richard, J.; Roth, M.; Schmidt, K. B.; Steinmetz, M.; Tresse, L.; Urrutia, T.; Verhamme, A.; Weilbacher, P. M.; Zabl, J.; and Zoutendijk, S. L. |title=The MUSE Extremely Deep Field: The cosmic web in emission at high redshift |journal=Astronomy & Astrophysics |date=18 March 2021 |volume=647 |issue=A107 |pages=A107 |doi=10.1051/0004-6361/202039887 |url=https://www.aanda.org/articles/aa/full_html/2021/03/aa39887-20/aa39887-20.html#S1 |quote=This first detection of the cosmic web structure in Lyα emission in typical filamentary environments, namely outside massive structures typical of web nodes, is a milestone in the long search for the cosmic web signature at high z. This has been possible because of the unprecedented faint surface brightness of 5 × 10−20 erg s−1 cm−2 arcsec−2 achieved by 140 h MUSE observations on the VLT.|arxiv=2102.05516 |bibcode=2021A&A...647A.107B |s2cid=231861819 }}</ref>

Some caution is required in describing structures on a cosmic scale because they are often different from how they appear. [[Gravitational lens]]ing can make an image appear to originate in a different direction from its real source, when foreground objects curve surrounding spacetime (as predicted by [[general relativity]]) and deflect passing light rays. Rather usefully, strong gravitational lensing can sometimes magnify distant galaxies, making them easier to detect. [[Weak gravitational lensing|Weak lensing]] by the intervening universe in general also subtly changes the observed large-scale structure.

The large-scale structure of the universe also looks different if only redshift is used to measure distances to galaxies. For example, galaxies behind a galaxy cluster are attracted to it and fall towards it, and so are [[blueshift]]ed (compared to how they would be if there were no cluster). On the near side, objects are redshifted. Thus, the environment of the cluster looks somewhat pinched if using redshifts to measure distance. The opposite effect is observed on galaxies already within a cluster: the galaxies have some random motion around the cluster center, and when these random motions are converted to redshifts, the cluster appears elongated. This creates a "''[[Redshift-space distortions|finger of God]]''"—the illusion of a long chain of galaxies pointed at Earth.

=== Cosmography of Earth's cosmic neighborhood ===
At the centre of the [[Hydra–Centaurus Supercluster]], a gravitational anomaly called the [[Great Attractor]] affects the motion of galaxies over a region hundreds of millions of light-years across. These galaxies are all [[redshift]]ed, in accordance with [[Hubble's law]]. This indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies.

The Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years in the direction of the [[Hydra (constellation)|Hydra]] and [[Centaurus]] [[constellation]]s. In its vicinity there is a preponderance of large old galaxies, many of which are colliding with their neighbours, or radiating large amounts of radio waves.

In 1987, [[astronomer]] [[R. Brent Tully]] of the [[University of Hawaii]]'s Institute of Astronomy identified what he called the [[Pisces–Cetus Supercluster Complex]], a structure one billion [[light-year]]s long and 150 million light-years across in which, he claimed, the Local Supercluster is embedded.<ref>{{Cite news|url=https://www.nytimes.com/1987/11/10/science/massive-clusters-of-galaxies-defy-concepts-of-the-universe.html|title=Massive Clusters of Galaxies Defy Concepts of the Universe|first=John Noble|last=Wilford|newspaper=The New York Times|date=November 10, 1987}}</ref>

== Most distant objects ==
{{main|List of the most distant astronomical objects}}
The most distant [[astronomical object]] identified (as of May of 2025) is a galaxy classified as [[MoM-z14]]<ref>{{Citation |last=Naidu |first=Rohan P. |title=<nowiki>A Cosmic Miracle: A Remarkably Luminous Galaxy at $z_{\rm{spec}}=14.44$ Confirmed with JWST</nowiki> |date=2025-05-16 |url=http://arxiv.org/abs/2505.11263 |access-date=2025-05-31 |publisher=arXiv |doi=10.48550/arXiv.2505.11263 |id=arXiv:2505.11263 |last2=Oesch |first2=Pascal A. |last3=Brammer |first3=Gabriel |last4=Weibel |first4=Andrea |last5=Li |first5=Yijia |last6=Matthee |first6=Jorryt |last7=Chisholm |first7=John |last8=Pollock |first8=Clara L. |last9=Heintz |first9=Kasper E.}}</ref>, at a redshift of 14.44. In 2009, a [[gamma ray burst]], [[GRB 090423]], was found to have a [[redshift]] of 8.2, which indicates that the collapsing star that caused it exploded when the universe was only 630 million years old.<ref name="NASAGRB">{{Cite web |title=New Gamma-Ray Burst Smashes Cosmic Distance Record {{!}} Science Mission Directorate |url=https://science.nasa.gov/science-news/science-at-nasa/2009/28apr_grbsmash/ |access-date=2023-09-15 |website=science.nasa.gov}}</ref> The burst happened approximately 13 billion years ago,<ref>{{Cite web |last=Atkinson |first=Nancy |date=2009-10-28 |title=More Observations of GRB 090423, the Most Distant Known Object in the Universe |url=https://www.universetoday.com/43517/more-observations-of-grb-090423-the-most-distant-known-object-in-the-universe/ |access-date=2023-09-15 |website=Universe Today |language=en-US}}</ref> so a distance of about 13 billion light-years was widely quoted in the media, or sometimes a more precise figure of 13.035 billion light-years.<ref name="NASAGRB" /> 

This would be the "light travel distance" (see [[Distance measures (cosmology)]]) rather than the "[[Comoving and proper distances#Uses of the proper distance|proper distance]]" used in both [[Hubble's law]] and in defining the size of the observable universe. Cosmologist [[Edward L. Wright|Ned Wright]] argues against using this measure.<ref>{{Cite web |title=Light Travel Time Distance |url=https://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html |access-date=2023-07-01 |website=www.astro.ucla.edu}}</ref> The proper distance for a redshift of 8.2 would be about 9.2 [[Megaparsecs|Gpc]],<ref>{{cite journal |author=Meszaros, Attila |display-authors=etal |journal=Baltic Astronomy |volume=18 |title=Impact on cosmology of the celestial anisotropy of the short gamma-ray bursts |pages=293–296 |date=2009 |arxiv=1005.1558 |bibcode=2009BaltA..18..293M }}</ref> or about 30 billion light-years.

== Horizons ==
{{Main|Cosmological horizon}}
The limit of observability in the universe is set by cosmological horizons which limit—based on various physical constraints—the extent to which information can be obtained about various events in the universe. The most famous horizon is the [[particle horizon]] which sets a limit on the precise distance that can be seen due to the finite [[age of the universe]]. Additional horizons are associated with the possible future extent of observations, larger than the particle horizon owing to the [[expansion of space]], an "optical horizon" at the [[Cosmic microwave background|surface of last scattering]], and associated horizons with the surface of last scattering for [[cosmic neutrino background|neutrinos]] and [[gravitational wave background|gravitational waves]].
{{multiple image
 | align             = center
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 | background color  =
 | width             =
 | caption_align     = center
 | header_background =
 | header_align      = center
 | header            = [[Location of Earth]] in the [[Universe]]
 | image1            = The Earth seen from Apollo 17.jpg
 | width1            = 82
 | caption1          = [[Earth]]
 | image2            = Solar System true color.jpg
 | width2            = 146
 | caption2          = [[Solar&nbsp;System]]
 | image3            = Galaxymap.com, map 1000 parsecs (2022).png
 | width3            = 82
 | caption3          = [[Molecular cloud]]s around the Sun inside the [[Orion-Cygnus Arm]]
 | image4            = Milky Way Arms ssc2008-10.svg
 | width4            = 93
 | caption4          = [[Orion Arm]]
 | image5            = Artist's impression of the Milky Way (updated - annotated).jpg
 | width5            = 83
 | caption5          = [[Milky&nbsp;Way]]
 | image6            = Local Group and nearest galaxies.jpg
 | width6            = 111
 | caption6          = [[Local Group|Local&nbsp;Group]]
 | image7            = Local supercluster-ly.jpg
 | width7            = 86
 | caption7          = [[Virgo Supercluster|Virgo SCl]]
 | image8            = Observable universe r2.jpg
 | width8            = 83
 | caption8          = [[Laniakea Supercluster|Laniakea SCl]]
 | image9            = Observable Universe with Measurements 01.png
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 | footer_background =
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}}
{{wide image|Location of Earth (9x1-English Annot-small).png|2250px|A diagram of the Earth's location in the observable universe. (''[[:File:Location of Earth (3x3-English Annot-small).png|Alternative image]].'')}}

{{wide image|Observable_Universe_Logarithmic_Map_(horizontal_layout_english_annotations).png|2250px|A logarithmic map of the observable universe. From left to right, spacecraft and celestial bodies are arranged according to their proximity to the Earth.}}

== Gallery ==
{{multiple image|heading |direction=horizontal |align=center |width= |image1=Observable universe logarithmic illustration.png |caption1=Artist's [[logarithmic scale]] conception of the observable universe with the [[Solar System]] at the center, inner and outer [[planets]], [[Kuiper belt]], [[Oort cloud]], [[Alpha Centauri]], [[Perseus Arm]], [[Milky Way galaxy]], [[Andromeda Galaxy]], nearby [[Galaxy|galaxies]], [[Cosmic web]], [[Cosmic microwave radiation]] and the Big Bang's invisible plasma on the edge. Celestial bodies appear enlarged to appreciate their shapes. |width1=300 |image2=2dfdtfe.gif |caption2=[[Dtfe|DTFE reconstruction]] of the inner parts of the [[2dF Galaxy Redshift Survey]] |width2=475 |footer= }}
<!---
[[File:Observable universe logarithmic illustration.png|thumb|left|upright=1.8|Artist's [[logarithmic scale]] conception of the observable universe with the [[Solar System]] at the center, inner and outer [[planets]], [[Kuiper belt]], [[Oort cloud]], [[Alpha Centauri]], [[Perseus Arm]], [[Milky Way galaxy]], [[Andromeda Galaxy]], nearby [[Galaxy|galaxies]], [[Cosmic web]], [[Cosmic microwave radiation]] and the Big Bang's invisible plasma on the edge. Celestial bodies appear enlarged to appreciate their shapes.]]
[[File:2dfdtfe.gif|thumb|upright=1.6|left|[[Dtfe|DTFE reconstruction]] of the inner parts of the [[2dF Galaxy Redshift Survey]]]]
--->
{{clear}}

== See also ==
{{div col|colwidth=30em}}
* {{annotated link|Bolshoi cosmological simulation}}
* {{annotated link|Causality (physics)}}
* {{annotated link|Chronology of the universe}}
* {{annotated link|Dark flow}}
* {{annotated link|Hubble volume}}
* {{annotated link|Illustris project}}
* {{annotated link|Multiverse}}
* {{annotated link|Orders of magnitude (length)}}
* {{annotated link|UniverseMachine}}
{{div col end}}

== Notes ==
{{reflist|group=note}}

== References ==
{{reflist|30em}}

== Further reading ==
* {{cite journal|title = Morphology Of The Galaxy Distribution From Wavelet Denoising|author = Vicent J. Martínez|display-authors = 4|author2 = Jean-Luc Starck|author3 = Enn Saar|author4 = David L. Donoho |author4-link = David Donoho|author5 = Simon Reynolds|author6=Pablo de la Cruz|author7 = Silvestre Paredes|name-list-style = amp|arxiv = astro-ph/0508326|journal = The Astrophysical Journal|date = 2005|volume = 634|issue = 2|pages = 744–755|bibcode = 2005ApJ...634..744M|doi = 10.1086/497125|s2cid = 14905675}}
* {{cite journal|author = Mureika, J. R.|author2 = Dyer, C. C.|name-list-style = amp
|title = Review: Multifractal Analysis of Packed Swiss Cheese Cosmologies
|journal = General Relativity and Gravitation
|arxiv = gr-qc/0505083
|date = 2004
|volume = 36|issue = 1
|pages = 151–184
|doi = 10.1023/B:GERG.0000006699.45969.49
|bibcode = 2004GReGr..36..151M|s2cid = 13977714}}
* {{cite journal|author = Gott, III, J. R.|display-authors = etal|title = A Map of the Universe
|journal = The Astrophysical Journal
|arxiv = astro-ph/0310571
|date = May 2005
|volume = 624|issue = 2
|pages = 463–484
|doi = 10.1086/428890
|bibcode = 2005ApJ...624..463G|s2cid = 9654355}}
* {{cite journal |title=Scale-invariance of galaxy clustering |author=Labini |first1=F. Sylos |last2=Montuori |first2=M. |last3=Pietronero |first3=L. |name-list-style=amp |journal=Physics Reports |date=1998 |volume=293 |issue=1 |pages=61–226 |doi=10.1016/S0370-1573(97)00044-6 |bibcode=1998PhR...293...61S |arxiv=astro-ph/9711073 |s2cid=119519125}}

== External links ==
* [http://www.mpa-garching.mpg.de/galform/millennium/ "Millennium Simulation" of structure forming] – Max Planck Institute of Astrophysics, Garching, Germany
* {{APOD |date=7 November 2007 |title=The Sloan Great Wall: Largest Known Structure?}}
* [http://www.astro.ucla.edu/~wright/cosmology_faq.html Cosmology FAQ]
* [https://www.sciencedaily.com/releases/2007/04/070419125240.htm Forming Galaxies Captured In The Young Universe By Hubble, VLT & Spitzer]
* [http://www.phys.ksu.edu/personal/gahs/phys191/horizon.html Animation of the cosmic light horizon]
* [https://arxiv.org/abs/astro-ph/0305179 Inflation and the Cosmic Microwave Background by Charles Lineweaver]
* [http://www.astro.princeton.edu/universe/ Logarithmic Maps of the Universe]
* [http://www.mso.anu.edu.au/2dFGRS/ List of publications of the 2dF Galaxy Redshift Survey]
* [http://www.atlasoftheuniverse.com/universe.html The Universe Within 14 Billion Light Years – NASA Atlas of the Universe] – Note, this map only gives a rough cosmographical estimate of the expected distribution of superclusters within the observable universe; very little actual mapping has been done beyond a distance of one billion light-years.
* [https://www.youtube.com/watch?v=17jymDn0W6U Video: ''The Known Universe'', from the American Museum of Natural History]
* [http://ned.ipac.caltech.edu/ NASA/IPAC Extragalactic Database]
* [http://irfu.cea.fr/cosmography Cosmography of the Local Universe] at irfu.cea.fr (17:35) ([https://arxiv.org/abs/1306.0091 arXiv])
* [https://www.livescience.com/how-many-atoms-in-universe.html There are about 10<sup>82</sup> atoms in the observable universe] – ''[[LiveScience]]'', July 2021
* [https://www.forbes.com/sites/startswithabang/2019/05/21/this-is-why-we-will-never-know-everything-about-our-universe/ Limits to knowledge about Universe] – ''[[Forbes]]'', May 2019

{{Earth's location}}
{{Portal bar|Stars|Outer space}}

[[Category:Concepts in astronomy]]
[[Category:Physical cosmological concepts]]