Editing Observable universe (section)

== 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>