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| {{For|a general discussion of the universe|Universe}}
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| [[File:HubbleUltraDeepFieldwithScaleComparison.jpg|thumb|300px|[[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 [[redshifted|red-shifted]] galaxies originated nearly [[Age of the universe|14]] [[1,000,000,000 (number)|billion]] years ago.]]
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| {{cosmology}}
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| The '''observable universe''' consists of the galaxies and other matter that can, in principle, be observed from [[Earth]] in the present day because light (or other signals) from those objects has had time to reach the Earth since the beginning of the [[Metric expansion of space|cosmological expansion]]. Assuming [[cosmological principle|the universe is isotropic]], the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe is a spherical volume (a [[ball (mathematics)|ball]]) centered on the observer, regardless of the shape of the universe as a whole. Every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.
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| The word ''observable'' used in this sense does not depend on whether modern [[technology]] actually permits detection of [[radiation]] from an object in this region (or indeed on whether there is any radiation to detect). It simply indicates that it is possible ''in principle'' for light or other signals from the object to reach an observer on Earth. In practice, we can see light only from as far back as the time of photon decoupling in the [[Recombination (cosmology)|recombination]] [[Epoch (astronomy)|epoch]]. That is when particles were first able to emit [[photon]]s that were not quickly re-absorbed by other particles. Before then, the universe was filled with a [[plasma (physics)|plasma]] that was opaque to photons.
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| The [[Last scattering surface|surface of last scattering]] is the collection of points in space at the exact distance that photons from the time of photon decoupling just reach us today. These are the photons we detect today as [[cosmic microwave background radiation]] (CMBR). However, with future technology, it may be possible to observe the still older [[neutrino background]], or even more distant events via [[gravitational wave]]s (which also should move at the speed of light). Sometimes astrophysicists distinguish between the ''visible'' universe, which includes only signals emitted since recombination—and the ''observable'' universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional cosmology, the end of the [[inflationary epoch]] in modern cosmology). According to calculations, the ''[[comoving distance]]'' (current proper distance) to particles from the CMBR, which represent the radius of the visible universe, is about 14.0 billion [[parsec]]s (about 45.7 billion light years), while 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|coauthors = Mario Jurić, David Schlegel, Fiona Hoyle, Michael Vogeley, Max Tegmark, Neta Bahcall, Jon Brinkmann|title = A Map of the Universe|url=http://www.astro.princeton.edu/universe/ms.pdf|journal = The Astrophysics Journal|volume = 624|issue = 2|page = 463|year = 2005|doi = 10.1086/428890|bibcode=2005ApJ...624..463G| arxiv=astro-ph/0310571}}</ref> about 2% larger.
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| The best estimate of the [[age of the universe]] as of 2013 is 13.798 ± 0.037 [[1,000,000,000 (number)|billion]] years<ref name='planck_cosmological_parameters'>{{cite journal | arxiv=1303.5076 | title=Planck 2013 results. XVI. Cosmological parameters | author=Planck collaboration | journal=Submitted to Astronomy & Astrophysics | year=2013|bibcode = 2013arXiv1303.5076P }}</ref> but due to the [[Metric expansion of space|expansion of space]] humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological [[Comoving distance#Uses of the proper distance|proper distance]], which is equal to the [[comoving distance]] at the present time) than a static 13.8 billion [[light-year]]s distance.<ref name="expandingconfusion">{{cite journal|last = Davis|first = Tamara M.|coauthors = Charles H. Lineweaver |title=Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe|journal = Publications of the Astronomical Society of Australia|volume = 21|issue = 1|page = 97|year = 2004|doi = 10.1071/AS03040 |arxiv=astro-ph/0310808|bibcode = 2004PASA...21...97D }}</ref> The [[diameter]] of the observable universe is estimated at about 28 billion parsecs (93 billion [[light-year]]s),<ref>{{cite book|author1=Itzhak Bars|author2=John Terning|title=Extra Dimensions in Space and Time|url=http://books.google.com/books?id=fFSMatekilIC&pg=PA27|accessdate=1 May 2011|date=November 2009|publisher=Springer|isbn=978-0-387-77637-8|pages=27–}}</ref> putting the edge of the observable universe at about 46–47 billion light-years away.<ref>[http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN Frequently Asked Questions in Cosmology]. Astro.ucla.edu. Retrieved on 2011-05-01.</ref><ref name=ly93>{{cite web|last = Lineweaver|first = Charles|coauthors = Tamara M. Davis|year = 2005|url = http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf|title = Misconceptions about the Big Bang|publisher = Scientific American|accessdate = 2008-11-06}}</ref>
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| == The universe versus the observable universe ==
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| Some parts of the universe may simply be too far away for the light emitted from there at any moment since the Big Bang to have had enough time to reach Earth at present, so these portions of the universe would currently lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so some regions not currently observable will become observable. However, due to [[Hubble's law]] regions sufficiently distant from us are expanding away from us much faster than the speed of light ([[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 distance#Uses of the proper distance|uses of the proper distance]] for a discussion), and the [[Accelerating universe|expansion rate appears to be accelerating]] due to [[dark energy]]. 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 our observable universe at any time in the infinite future, because light emitted by objects outside that limit would never reach us. (A subtlety is that, because the [[Hubble's law#Interpretation|Hubble parameter]] is decreasing with time, there can be cases where a galaxy that is receding from us just a bit faster than light does emit a signal that reaches us eventually<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 we can ever theoretically observe in the infinite future (leaving aside the issue that some may be impossible to observe in practice due to redshift, as discussed in the following paragraph) is only larger than the number currently observable by a factor of 2.36.<ref name="mapofuniverse">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>
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| [[File:Observable universe logarithmic illustration.png|thumb|left|250px|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.]]
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| Though 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|last = Krauss|first = Lawrence M.|coauthors = Robert J. Scherrer|title = The Return of a Static Universe and the End of Cosmology|journal = General Relativity and Gravitation|volume = 39|issue = 10|pages = 1545–1550|year = 2007|doi = 10.1007/s10714-007-0472-9|arxiv=0704.0221|bibcode = 2007GReGr..39.1545K }}</ref><ref>[http://www.npr.org/templates/story/story.php?storyId=102715275 Using Tiny Particles To Answer Giant Questions]. Science Friday, 3 Apr 2009. According to the [http://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> An additional subtlety is that 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 past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can ''never'' reach us 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>{{cite journal|last = Loeb|first = Abraham|title = The Long-Term Future of Extragalactic Astronomy|journal = Physical Review D|volume = 65|issue = 4|year = 2002|doi = 10.1103/PhysRevD.65.047301|arxiv=astro-ph/0107568|bibcode = 2002PhRvD..65d7301L }}</ref> even though it remains at the same comoving distance (comoving distance is defined to be constant with time—unlike proper distance, which is used to define recession velocity due to the expansion of space), which is less than the comoving radius of the observable universe. This fact can be used to define a type of cosmic [[event horizon]] whose distance from us 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 us in the future if the event is less than 16 billion light years away, but the signal will never reach us if the event is more than 16 billion light years away.<ref name=ly93 />
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| Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". This can be justified on the grounds that we can never know anything by direct experimentation about any part of the universe that is [[Causality (physics)|causally disconnected]] from us, although many credible theories require a total universe much larger than the observable universe. 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, though some models propose it could be finite but unbounded, 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 [[galaxies]] within our observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of [[cosmic inflation]] and its founder, [[Alan Guth]], if it is assumed that inflation began about 10<sup>−37</sup> seconds after the Big Bang, then with the plausible assumption that the size of the universe at this time 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 3x10<sup>23</sup> times larger than the size of the observable universe.<ref>{{cite book|author=Alan H. Guth|title=The inflationary universe: the quest for a new theory of cosmic origins|url=http://books.google.com/books?id=P2V1RbwvE1EC&pg=PA186|accessdate=1 May 2011|date=17 March 1998|publisher=Basic Books|isbn=978-0-201-32840-0|pages=186–}}</ref> There are also lower estimates claiming that the entire universe is in excess of 250 times larger than the observable universe.<ref>Universe Could be 250 Times Bigger Than What is Observable - by Vanessa D'Amico on February 8, 2011 http://www.universetoday.com/83167/universe-could-be-250-times-bigger-than-what-is-observable/</ref> If the entire universe is at least 250 times larger than the observable universe, then the entire universe would have a diameter in excess of 176 gigaparsecs (575 billion light years).
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| 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>[http://arxiv.org/pdf/1303.4004.pdf Constraints on the Topology of the Universe]</ref> claims to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface (since this is only a lower bound, the paper leaves open the possibility that the whole universe is much larger, even infinite). 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 |author1=Mota |author2=Reboucas |author3=Tavakol |title=Observable circles-in-the-sky in flat universes |class=astro-ph.CO |year=2010}}</ref>
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| == Size ==
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| [[File:Observable Universe with Measurements 01.png|250px|thumbnail|right|Visualization of the 93 billion light year – or 28 billion parsec – three-dimensional 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 in the image.]]
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| The [[comoving distance]] from Earth to the edge of the observable universe is about 14 [[parsec|gigaparsecs]] (46 [[1000000000 (number)|billion]] [[light year]]s or {{convert|14|Gpc|m|disp=output only|abbr=off|sp=us}}) in any direction. The observable universe is thus a sphere with a [[diameter]] of about 29 gigaparsecs<ref>{{cite web|title = WolframAlpha|url=http://www.wolframalpha.com/input/?i=93+billion+light+years+in+parsecs|accessdate=29 November 2011}}</ref> ({{convert|93|Gly|m|abbr=on|disp=or}}).<ref>{{cite web|title = WolframAlpha|url=http://www.wolframalpha.com/input/?i=size+of+universe|accessdate=29 November 2011}}</ref> Assuming that space is roughly [[Euclidean space|flat]], this size corresponds to a comoving volume of about {{val|1.3|e=4|u=Gpc<sup>3</sup>}}<!--based on a 29 Gpc diameter--> ({{val|4.1|e=5|u=Gly<sup>3</sup>}} or {{val|3.5|e=80|u=m3}}).
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| The figures quoted above 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 380,000 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|format=PDF|publisher=nasa.gov|accessdate=2010-12-02}} (see p. 39 for a table of best estimates for various cosmological parameters)</ref><ref>{{cite web
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| |last=Abbott|first=Brian|date=May 30, 2007
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| |url=http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php
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| |title=Microwave (WMAP) All-Sky Survey
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| |publisher=Hayden Planetarium|accessdate=2008-01-13
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| }}</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 us.<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 at the present time 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 the following equation.<ref>{{cite book|author=Paul Davies|title=The new physics|url=http://books.google.com/books?id=akb2FpZSGnMC&pg=PA187|accessdate=1 May 2011|date=28 August 1992|publisher=Cambridge University Press|isbn=978-0-521-43831-5|pages=187–}}</ref><ref>{{cite book|author=V. F. Mukhanov|title=Physical foundations of cosmology|url=http://books.google.com/books?id=1TXO7GmwZFgC&pg=PA58|accessdate=1 May 2011|year=2005|publisher=Cambridge University Press|isbn=978-0-521-56398-7|pages=58–}}</ref>
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| <math>\! a(t) = \frac{1}{1 + z}</math>
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| [[Wilkinson Microwave Anisotropy Probe#Nine-year data release|WMAP nine-year results]] give the redshift of photon decoupling as ''z''=1091.64 ± 0.47<ref name="wmap7parameters" /> which implies that the scale factor at the time of photon decoupling would be {{frac|1092.64}}. So if the matter that originally emitted the oldest [[CMBR]] [[photons]] has a ''present'' distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 ''million'' light-years away.
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| === Misconceptions === <!-- This section is linked from [[Universe]] -->
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| [[File:Incorrect plaque at the Rose Center for Earth and Space, April 2011.jpg|540px|thumb|left|An example of one of the most common misconceptions about the size of the observable universe. Despite the fact that the universe is 13.8 billion years old, the distance to the edge of the observable universe is '''not''' 13.8 billion light-years, because [[Metric expansion of space|the universe is expanding]]. This plaque appears at the [[Rose Center for Earth and Space]] in [[New York City]].]]
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| Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these figures are listed below, with brief descriptions of possible reasons for misconceptions about them.
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| ;13.8 billion light-years
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| :The [[age of the universe]] is estimated to be 13.8 billion years. While it is commonly understood that nothing can accelerate to velocities equal to or greater than that of light, it is a common misconception that the radius of the observable universe must therefore amount to only 13.8 billion light-years. This reasoning would only make sense if the flat, static [[Minkowski space]]time conception under special relativity were correct. In the real universe, [[spacetime]] is curved in a way that corresponds to the [[Metric expansion of space|expansion of space]], as evidenced by [[Hubble's law]]. Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance.<ref>Ned Wright, [http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html "Why the Light Travel Time Distance should not be used in Press Releases"].</ref>
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| ;15.8 billion light-years
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| :This is obtained in the same way as the 13.8 billion light year figure, but starting from an incorrect age of the universe that the popular press reported in mid-2006.<ref>[http://www.space.com/scienceastronomy/060807_mm_huble_revise.html Universe Might be Bigger and Older than Expected]. Space.com (2006-08-07). Retrieved on 2011-05-01.</ref><ref>[http://space.newscientist.com/article/dn9676-big-bang-pushed-back-two-billion-years.html Big bang pushed back two billion years – space – 04 August 2006 – New Scientist]. Space.newscientist.com. Retrieved on 2011-05-01.</ref> For an analysis of this claim and the paper that prompted it, see the following reference at the end of this article.<ref>Edward L. Wright, [http://www.astro.ucla.edu/~wright/old_new_cosmo.html#05Aug06 "An Older but Larger Universe?"]</ref>
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| ;27.6 billion light-years
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| :This is a diameter obtained from the (incorrect) radius of 13.8 billion light-years.
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| ;78 billion light-years
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| :In 2003, Cornish et al.<ref name = "cornish">{{cite journal|author1=Cornish|author2=Spergel|author3=Starkman|author4=Eiichiro Komatsu|doi=10.1103/PhysRevLett.92.201302|journal=Phys. Rev. Lett. | year = 2004 | issue = 02|volume=92|title=Constraining the Topology of the Universe|issue=20|year=2003|arxiv=astro-ph/0310233|bibcode = 2004PhRvL..92t1302C }}</ref> found this lower bound for the diameter of the ''whole'' universe (not just the observable part), if we postulate that the universe is finite in size due to its having a nontrivial [[topology]],<ref>{{cite web|last=Levin |first=Janna |url=http://plus.maths.org/issue10/features/topology/ |title=In space, do all roads lead to home? |publisher=plus.maths.org |date= |accessdate=2012-08-15}}</ref><ref>http://cosmos.phy.tufts.edu/~zirbel/ast21/sciam/IsSpaceFinite.pdf</ref> with this lower bound based on the estimated current distance between points that we can see on opposite sides of the [[cosmic microwave background radiation]] (CMBR). If the whole universe is smaller than this sphere, then light has had time to circumnavigate it since the big bang, producing multiple images of distant points in the CMBR, which would show up as patterns of repeating circles.<ref>[http://www.etsu.edu/physics/etsuobs/starprty/120598bg/section7.htm Bob Gardner's "Topology, Cosmology and Shape of Space" Talk, Section 7]. Etsu.edu. Retrieved on 2011-05-01.</ref> Cornish et al. looked for such an effect at scales of up to 24 gigaparsecs ({{convert|78|Gly|m|abbr=on|disp=or}}) and failed to find it, and suggested that if they could extend their search to all possible orientations, they would then "be able to exclude the possibility that we live in a universe smaller than 24 Gpc in diameter". The authors also estimated that with "lower noise and higher resolution CMB maps (from [[Wilkinson Microwave Anisotropy Probe|WMAP]]'s extended mission and from [[Planck satellite|Planck]]), we will be able to search for smaller circles and extend the limit to ~28 Gpc."<ref name="cornish"/> This estimate of the maximum lower bound that can be established by future observations corresponds to a radius of 14 gigaparsecs, or around 46 billion light years, about the same as the figure for the radius of the visible universe (whose radius is defined by the CMBR sphere) given in the opening section. A 2012 preprint by most of the same authors as the Cornish et al. paper has extended the current lower bound to a diameter of 98.5% the diameter of the CMBR sphere, or about 26 Gpc.<ref name = "Vaudrevange preprint">{{cite journal|author1=Vaudrevange|author2=Starkmanl|author3=Cornish|author4=Spergel|title=Constraints on the Topology of the Universe: Extension to General Geometries|arxiv=1206.2939 |bibcode = 2012arXiv1206.2939V }}</ref>
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| ;156 billion light-years
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| :This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius.<ref name="spacedotcom156">[http://web.archive.org/web/20080822013053/http://www.space.com/scienceastronomy/mystery_monday_040524.html SPACE.com – Universe Measured: We're 156 Billion Light-years Wide!]</ref> Since 78 billion light-years is already a diameter (the original paper by Cornish et al. says, "By extending the search to all possible orientations, we will be able to exclude the possibility that we live in a universe smaller than 24 Gpc in diameter," and 24 Gpc is 78 billion light years),<ref name="cornish" /> the doubled figure is incorrect. This figure was very widely reported.<ref name="spacedotcom156" /><ref>Roy, Robert. (2004-05-24) [http://www.msnbc.msn.com/id/5051818/ New study super-sizes the universe – Technology & science – Space – Space.com – msnbc.com]. MSNBC. Retrieved on 2011-05-01.</ref><ref>{{cite news| url=http://news.bbc.co.uk/2/hi/science/nature/3753115.stm|work=BBC News|title=Astronomers size up the Universe|date=2004-05-28|accessdate=2010-05-20}}</ref> A press release from [[Montana State University – Bozeman]], where Cornish works as an astrophysicist, noted the error when discussing a story that had appeared in [[Discover (magazine)|''Discover'' magazine]], saying "''Discover'' mistakenly reported that the universe was 156 billion light-years wide, thinking that 78 billion was the radius of the universe instead of its diameter."<ref>{{cite news|url=http://www.montana.edu/cpa/news/nwview.php?article=2108|title=MSU researcher recognized for discoveries about universe|date=2004-12-21|accessdate=2011-02-08}}</ref>
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| ;180 billion light-years
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| :This estimate accompanied the age estimate of 15.8 billion years in some sources;<ref>[http://www.space.com/scienceastronomy/060807_mm_huble_revise.html Space.com – Universe Might be Bigger and Older than Expected]</ref> it was obtained by adding 15% to the figure of 156 billion light years.
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| == Large-scale structure ==
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| [[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 as a [[hierarchy|hierarchical]] model with organization up to the [[scale (spatial)|scale]] of [[supercluster]]s and [[Galaxy filament|filament]]s. Larger than this, there seems to be no continued structure, a phenomenon that has been referred to as the '''End of Greatness'''.
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| === Walls, filaments, and voids ===
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| [[Image:2dfdtfe.gif|thumb|300px|left|[[Dtfe|DTFE reconstruction]] of the inner parts of the [[2dF Galaxy Redshift Survey]]]]
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| 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 groups]], [[galaxy cluster]]s, [[supercluster]]s, sheets, walls and [[Galaxy filament|filaments]], which are separated by immense [[void (astronomy)|void]]s, creating a vast foam-like structure 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, based on [[redshift survey]] data, in 1989 [[Margaret Geller]] and [[John Huchra]] discovered the "[[CfA2 Great Wall|Great Wall]]",<ref name="redshift">[http://www.sciencemag.org/cgi/content/abstract/246/4932/897 M. J. Geller & J. P. Huchra, ''Science'' '''246''', 897 (1989).]</ref> a sheet of galaxies more than 500 million [[light-year]]s long and 200 million 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.
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| 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, and 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.<ref>[http://space.newscientist.com/article/dn12546-biggest-void-in-space-is-1-billion-light-years-across.html Biggest void in space is 1 billion light years across – space – 24 August 2007 – New Scientist]. Space.newscientist.com. Retrieved on 2011-05-01.</ref> It coincides with the '[[WMAP cold spot|WMAP 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.
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| Another large-scale structure is the [[Himiko (Lyman-alpha blob)|Newfound Blob]], a collection of galaxies and enormous gas bubbles that measures about 200 million light years across.
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| In recent studies the universe appears as a collection of giant bubble-like [[void (astronomy)|voids]] separated by sheets and [[Galaxy filament|filaments of galaxies]], with the [[supercluster]]s appearing as occasional relatively dense nodes. This network is clearly visible in the [[2dF Galaxy Redshift Survey]]. In the figure, a three dimensional reconstruction of the inner parts of the survey is shown, revealing an impressive view of the cosmic structures in the nearby universe. Several superclusters stand out, such as the Sloan Great Wall.
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| 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 that time.<ref>{{cite web | last = Wall | first = Mike | url = http://www.foxnews.com/science/2013/01/11/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 web | last = Horvath I., Hakkila J., and Bagoly Z. | first = | url = http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:1401.0533 | title = Possible structure in the GRB sky distribution at redshift two | date = 2014 | publisher = |accessdate= }}</ref><ref name=original>{{cite journal|last = Horvath I., Hakkila J., and Bagoly Z. |first = |coauthors = |title = The largest structure of the Universe, defined by Gamma-Ray Bursts|journal = |volume = |issue = |pages = |year = 2013|doi = |arxiv=1311.1104|bibcode = 2013arXiv1311.1104H}}</ref> an even bigger structure twice as large as the former. It was defined by mapping of [[gamma-ray burst]]s.<ref name=2014paper/><ref>{{cite web | last = | first = Irene Klotz | 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 | publisher = [[discovery]] }}</ref>
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| === End of Greatness ===
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| The '''End of Greatness''' is an observational scale discovered at roughly 100 [[Megaparsec|Mpc]] (roughly 300 million [[lightyear]]s) where the lumpiness seen in the large-scale structure of the [[universe]] is [[wiktionary:homogeneous|homogenized]] and [[isotropic|isotropized]] in accordance with the [[Cosmological Principle]]. At this scale, no pseudo-random [[fractal]]ness is apparent.<ref>LiveScience.com, [http://news.yahoo.com/universe-isnt-fractal-study-finds-215053937.html "The Universe Isn't a Fractal, Study Finds"], Natalie Wolchover,22 August 2012</ref>
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| 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 |title=The Extravagant Universe: Exploding Stars, Dark Energy and the Accelerating Cosmos |author=Robert P Kirshner |url=http://books.google.com/?id=qQ_mV2prqNYC&pg=PA71 |page=71 |isbn=0-691-05862-8 |year=2002 |publisher=Princeton University Press}}</ref>
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| === Observations ===
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| [[Image:2MASS LSS chart-NEW Nasa.jpg|right|399px|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 astronomy|K band]] (2.2 um). Blue are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center)." <ref>[http://spider.ipac.caltech.edu/staff/jarrett/papers/LSS/ "Large Scale Structure in the Local Universe: The 2MASS Galaxy Catalog"], Jarrett, T.H. 2004, PASA, 21, 396</ref>]]
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| Another indicator of large-scale structure is the '[[Lyman-alpha forest]]'. This is a collection of [[absorption line]]s 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 be associated with the formation of new galaxies.
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| Caution is required in describing structures on a cosmic scale because things are often different than they appear. [[Gravitational lens]]ing (bending of light by gravitation) can make an image appear to originate in a different direction from its real source. This is caused when foreground objects (such as galaxies) 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]] (gravitational shear) by the intervening universe in general also subtly changes the observed large-scale structure. As of 2004, measurements of this subtle shear showed considerable promise as a test of cosmological models.
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| The large-scale structure of the universe also looks different if one only uses [[redshift]] to measure distances to galaxies. For example, galaxies behind a galaxy cluster are attracted to it, and so fall towards it, and so are slightly blueshifted (compared to how they would be if there were no cluster) On the near side, things are slightly redshifted. Thus, the environment of the cluster looks a bit squashed if using redshifts to measure distance. An opposite effect works on the 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 "''[[Fingers of God|finger of God]]''"—the illusion of a long chain of galaxies pointed at the Earth.
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| === Cosmography of our cosmic neighborhood ===
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| 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.
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| The Great Attractor, discovered in 1986, lies at a distance of between 150 million and 250 million light-years (250 million is the most recent estimate), 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, and/or radiating large amounts of radio waves.
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| 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 was embedded.<ref>[http://www.nytimes.com/1987/11/10/science/massive-clusters-of-galaxies-defy-concepts-of-the-universe.html?pagewanted=all Massive Clusters of Galaxies Defy Concepts of the Universe N.Y. Times Tue. November 10, 1987:]</ref><ref>[http://plasmauniverse.info/LargeScale.html Map of the Pisces-Cetus Supercluster Complex:]</ref>
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| ==Mass of ordinary matter==
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| The mass of the universe is often quoted as 10<sup>50</sup> tons or 10<sup>53</sup> kg.<ref>{{cite book|author=Paul Davies|title=The Goldilocks Enigma| url= http://www.amazon.com/Goldilocks-Enigma-Universe-Just-Right/dp/0547053584/ref=sr_1_1?s=books&ie=UTF8&qid=1372701918&sr=1-1&keywords=goldilocks+enigma |accessdate=1 July 2013 |year= 2006|publisher=First Mariner Books|isbn=978-0-618-59226-5|page=43–}}</ref> In this context, mass refers to ordinary matter and includes the [[interstellar medium]] (ISM) and the [[intergalactic medium]] (IGM). However, it excludes [[dark matter]] and [[dark energy]]. Three calculations substantiate this quoted value for the mass of ordinary matter in the universe: Estimates based on critical density, extrapolations from number of stars, and estimates based on steady-state. The calculations obviously assume a '''finite''' universe.
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| === Estimates based on critical density ===
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| Critical Density is the energy density where the expansion of the universe is poised between continued expansion and collapse.<ref>{{cite book|author=Michio Kaku|title=Parallel Worlds| url= http://www.amazon.com/s/ref=nb_sb_ss_i_4_8?url=search-alias%3Dstripbooks&field-keywords=parallel+worlds&sprefix=Parallel%2Cstripbooks%2C271| accessdate=1 July 2013 |year=2005|publisher=Anchor Books|isbn=978-1-4000-3372-0|page=385}}</ref> Observations of the cosmic microwave background from the [[Wilkinson Microwave Anisotropy Probe]] suggest that the [[spatial curvature]] of the universe is very close to zero, which in current cosmological models implies that the value of the [[density parameter]] must be very close to a certain critical density value. At this condition, the calculation for <math>\rho_c</math> critical density, is):<ref>{{cite book|author=Bernard F. Schutz|title=Gravity from the ground up|url=http://books.google.com/books?id=iEZNXvYwyNwC&pg=PA361|accessdate=1 May 2011|year=2003|publisher=Cambridge University Press|isbn=978-0-521-45506-0|pages=361–}}</ref>
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| <math>\rho_c = \frac{3H_0^2}{8 \pi G}</math>
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| where G is the [[gravitational constant]]. From The European Space Agency's Planck Telescope results: <math>H_0</math>, is 67.15 kilometers per second per mega parsec. This gives a critical density of {{val|0.85|e=-26|u=kg/m<sup>3</sup>}} (commonly quoted as about 5 hydrogen atoms/m<sup>3</sup>). 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=Submitted to Astronomy & Astrophysics | year=2013}}</ref> Note that although neutrinos are defined as particles like electrons, they are listed separately because they are difficult to detect and so different from ordinary matter. Thus, the density of ordinary matter is 4.8% times the total critical density calculated or {{val|4.08|e=-28|u=kg/m<sup>3</sup>}}.
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| 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 13.7 billion years, the [[comoving distance]] (radius) is now about 46.6 billion light years. Thus, volume (4/3 π r<sup>3</sup>) equals {{val|3.58|e=80|u=m<sup>3</sup>}} and mass of ordinary matter equals density ({{val|4.08|e=-28|u=kg/m<sup>3</sup>}}) times volume ({{val|3.58|e=80|u=m<sup>3</sup>}}) or {{val|1.46|e=53|u=kg}}.
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| === Extrapolation from number of stars ===
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| There is currently no way to know exactly the number of stars, but from current literature, the range of 10<sup>22</sup> to 10<sup>24</sup> is normally quoted.<ref>{{cite news|title=Astronomers count the stars|publisher=BBC News|date=July 22, 2003|url=http://news.bbc.co.uk/2/hi/science/nature/3085885.stm|accessdate=2006-07-18 }}</ref><ref>[http://www.npr.org/blogs/thetwo-way/2010/12/01/131730552/ "trillions-of-earths-could-be-orbiting-300-sextillion-stars"]</ref><ref>{{cite journal|last = van Dokkum|first =Pieter G.|coauthors = Charlie Conroy|title = A substantial population of low-mass stars in luminous elliptical galaxies|url = http://www.nature.com/nature/journal/vaop/ncurrent/abs/nature09578.html|journal = Nature|volume = 468|issue = 7326|pages = 940–942|year = 2010|pmid = 21124316|doi = 10.1038/nature09578 |bibcode = 2010Natur.468..940V |arxiv = 1009.5992 }}</ref><ref>[http://www.universetoday.com/24328/how-many-stars/ "How many stars?"]</ref>
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| One way to substantiate this range is to estimate the number of galaxies and multiply by the number of stars in an average galaxy. The 2004 [[Hubble Ultra-Deep Field]] image contains an estimated 10,000 galaxies.<ref>[url= http://hubblesite.org/newscenter/archive/releases/2004/28/text/]| NASA, Hubble News Release STSci - 2004-7</ref> The patch of sky in this area, is 3.4 arc minutes on each side. For a relative comparison, it would require over 50 of these images to cover the full moon. If this area is typical for the entire sky, there are over 100 billion galaxies in the universe.<ref>{{cite book|author=James R Johnson|title=Comprehending the Cosmos, a Macro View of the Universe|url= http://www.amazon.com/s/ref=nb_sb_ss_i_0_17?url=search-alias%3Dstripbooks&field-keywords=comprehending+the+cosmos&sprefix=Comprehending+the%2Cstripbooks%2C308| accessdate=1 July 2013 |isbn=978-1-477-64969-5|page=36}}</ref> More recently, in 2012, Hubble scientists produced the [[Hubble Extreme Deep Field]] image which showed slightly more galaxies for a comparable area.<ref>{{Cite press release | url= http://hubblesite.org/newscenter/archive/releases/2012/37/image/a/ | accessdate=1 July 2013 | title=Hubble Goes to the eXtreme to Assemble Farthest Ever View of the Universe | date=25 September 2012}}</ref> However, in order to compute the number of stars based on these images, we would need additional assumptions: the percent of both large and dwarf galaxies; and, their average number of stars. Thus, a reasonable option is to assume 100 billion average galaxies and 100 billion stars per average galaxy. This results in 10 <sup>22</sup> stars.
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| Next, we need average star mass which can be calculated from the distribution of stars in the Milky Way. Within the Milky Way, if a large number of stars are counted by spectral class, 73% are class M stars which contain only 30% of the Sun's mass. Considering mass and number of stars in each spectral class, the average star is 51.5% of the Sun's mass.<ref>{{cite book|author=James R Johnson|title=Comprehending the Cosmos, a Macro View of the Universe|url= http://www.amazon.com/s/ref=nb_sb_ss_i_0_17?url=search-alias%3Dstripbooks&field-keywords=comprehending+the+cosmos&sprefix=Comprehending+the%2Cstripbooks%2C308| accessdate=1 July 2013 |isbn=978-1-477-64969-5|page=34}}</ref> The Sun's mass is {{val|2|e=30|u=kg}}. so a reasonable number for the mass of an average star in the universe is 10<sup>30</sup> kg. Thus, the mass of all stars equals the number of stars (10<sup>22</sup>) times an average mass of star (10<sup>30</sup> kg) or 10<sup>52</sup> kg.
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| The next calculation adjusts for Interstellar Medium (ISM) and Intergalactic Medium (IGM). ISM is material between stars: gas (mostly hydrogen) and dust. IGM is material between galaxies, mostly hydrogen. Ordinary matter (protons, neutrons and electrons) exists in ISM and IGM as well as in stars. In the reference, "The Cosmic Energy Inventory“, the percentage of each part is defined: stars - 5.9%, Interstellar Medium (ISM) - 1.7%, and Intergalactic Medium (IGM) - 92.4%.<ref>{{cite journal|authors= Fukugita, Masataka, Peebles, P. J. E.|title= "The Cosmic Energy Inventory", Astro Physics Review 18 Aug 2004 |url= http://arxiv.org/abs/astro-ph/0406095| accessdate=1 July 2013}}</ref>
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| Thus, to extrapolate the mass of the universe from the star mass, divide the 10<sup>52</sup> kg mass calculated for stars by 5.9%. The result is {{val|1.7|e=53|u=kg}} for all the ordinary matter.
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| === Estimates based on steady-state universe ===
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| Sir [[Fred Hoyle]] calculated the mass of an observable [[steady-state universe]] using the formula:<ref>{{cite book|title=Cosmology and Controversy: The Historical Development of Two Theories of the Universe|author=Helge Kragh|page=212, Chapter 5|url=http://books.google.com/?id=GhVkQwv9ZesC&pg=PA212|isbn=0-691-00546-X|date=1999-02-22|publisher=Princeton University Press}}</ref>
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| :<math>\frac{4}{3}\pi\rho\left(\frac{c}{H}\right)^3</math>
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| which can also be stated as <ref>http://arxiv.org/abs/1004.1035 Valev,Dimitar, Estimation of the total mass and energy of the universe, arXiv:1004.1035v [physics. gen-ph] 7 Apr 2010, pages= 3-4, Recently, Valev has derived by dimensional analysis of fundamental parameters c, G, and H the equation c^3/(GH) that is close to the Hoyle formula.</ref>
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| :<math>\frac{c^3}{2GH} \ </math>
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| Here ''H'' = [[Hubble constant]], ρ = Hoyle's value for the density, ''G'' = [[gravitational constant]], and ''c'' = [[speed of light]].
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| This calculation yields approximately {{val|0.92|e=53|u=kg}}; however, this represents '''all''' energy/matter and is based on the [[Hubble volume]] (the volume of a sphere with radius equal to the [[Hubble length]] of about 13.7 billion light years). The critical density calculation above was based on the [[comoving distance]] radius of 46.6 billion light years. Thus, the Hoyle equation mass/energy result must be adjusted for increased volume. The comoving distance radius gives a volume about 39 times greater (46.7 cubed divided by 13.7 cubed). However, as volume increases, ordinary matter and dark matter would not increase; only dark energy increases with volume. Thus, assuming ordinary matter and dark matter are 27.9% of the total mass/energy, and dark energy is 72.1%, the amount of total mass/energy for the steady-state calculation would be: mass of ordinary matter and dark matter (27.9% times {{val|0.92|e=53|u=kg}}) plus the mass of dark energy ((72.1% times {{val|0.92|e=53|u=kg}}) times increased volume (39)). This equals: {{val|2.61|e=54|u=kg}}. As noted above for the Critical Density method, ordinary matter is 4.8% of all energy/matter. If the Hoyle result is multiplied by this percent, the result for ordinary matter is {{val|1.25|e=53|u=kg}}.
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| ===Comparison of results===
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| In summary, the three independent calculations produced reasonably close results: {{val|1.46|e=53|u=kg}}, {{val|1.7|e=53|u=kg}}, and {{val|1.25|e=53|u=kg}}. The average is {{val|1.47|e=53|u=kg}}.
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| The key assumptions using the Extrapolation from Star Mass method were the number of stars (10<sup>22</sup>) and the percentage of ordinary matter in stars (5.9%). The key assumptions using the Critical Density method were the comoving distance radius of the universe (46.6 billion light years) and the percentage of ordinary matter in all matter (4.8%). The key assumptions using the Hoyle steady-state method were the comoving distance radius and the percentage of dark energy in all mass (72.1%). Both the Critical Density and the Hoyle steady-state equations also used the Hubble constant (67.15 km/s/Mpc).
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| == Matter content — number of atoms ==
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| Assuming the mass of ordinary matter is about {{val|1.47|e=53|u=kg}} (reference previous section) and assuming all atoms are [[hydrogen atom]]s (which in reality make up about 74% of all atoms in our galaxy by mass, see [[Abundance of the chemical elements]]), calculating the estimated total number of atoms in the universe is straight forward. Divide the mass of ordinary matter by the mass of a hydrogen atom ({{val|1.47|e=53|u=kg}} divided by {{val|1.67|e=-27|u=kg}}). The result is approximately 10<sup>80</sup> hydrogen atoms.
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| == Most distant objects ==
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| The most distant [[astronomical object]] yet announced as of January 2011 is a galaxy candidate classified [[UDFj-39546284]]. 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">[http://science.nasa.gov/science-news/science-at-nasa/2009/28apr_grbsmash/ New Gamma-Ray Burst Smashes Cosmic Distance Record – NASA Science]. Science.nasa.gov. Retrieved on 2011-05-01.</ref> The burst happened approximately 13 billion years ago,<ref>[http://www.universetoday.com/43517/more-observations-of-grb-090423-the-most-distant-known-object-in-the-universe/ More Observations of GRB 090423, the Most Distant Known Object in the Universe]. Universetoday.com (2009-10-28). Retrieved on 2011-05-01.</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" /> though this would be the "light travel distance" (''see'' [[Distance measures (cosmology)]]) rather than the "[[Comoving distance#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 the common use of light travel distance in astronomical press releases on [http://www.astro.ucla.edu/~wright/Dltt_is_Dumb.html this page], and at the bottom of the page offers online calculators that can be used to calculate the current proper distance to a distant object in a flat universe based on either the redshift ''z'' or the light travel time). The proper distance for a redshift of 8.2 would be about 9.2 [[Megaparsecs|Gpc]],<ref>{{cite journal |authors=Meszaros, Attila et al. |journal=Baltic Astronomy |volume=18 |title=Impact on cosmology of the celestial anisotropy of the short gamma-ray bursts |pages=293–296 |year=2009 |arxiv=1005.1558 |bibcode=2009BaltA..18..293M }}</ref> or about 30 billion light years. Another record-holder for most distant object is a galaxy observed through and located beyond [[Abell 2218]], also with a light travel distance of approximately 13 billion light years from Earth, with observations from the [[Hubble telescope]] indicating a redshift between 6.6 and 7.1, and observations from [[W. M. Keck Observatory|Keck]] telescopes indicating a redshift towards the upper end of this range, around 7.<ref>[http://www.spacetelescope.org/news/heic0404/ Hubble and Keck team up to find farthest known galaxy in the Universe|Press Releases|ESA/Hubble]. Spacetelescope.org (2004-02-15). Retrieved on 2011-05-01.</ref> The galaxy's light now observable on Earth would have begun to emanate from its source about 750 million years after the [[Big Bang]].<ref>[http://www.msnbc.msn.com/id/4274187/ MSNBC: "Galaxy ranks as most distant object in cosmos"]</ref>
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| ==Horizons==
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| {{main|cosmological horizon}}
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| The limit of observability in our universe is set by a set of cosmological horizons which limit, based on various physical constraints, the extent to which we can obtain information 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 [[surface of last scattering]], and associated horizons with the surface of last scattering for [[cosmic neutrino background|neutrinos]] and [[gravitational wave background|gravitational waves]].
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| {{wide image|Earth's Location in the Universe (JPEG).jpg|2000px|A diagram of our location in the observable universe. (''[[:File:Earth's Location in the Universe SMALLER (JPEG).jpg|Click here for an alternate image]].'')}}
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| == See also ==
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| * [[Big Bang]]
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| * [[Bolshoi Cosmological Simulation]]
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| * [[Causality (physics)]]
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| * [[Chronology of the universe]]
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| * [[Dark flow]]
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| * [[Event horizon#Particle horizon of the observable universe|Event horizon of the universe]]
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| * [[Hubble volume]]
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| * [[Multiverse]]
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| * [[Orders of magnitude (length)]]
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| * [[Timeline of the Big Bang]]
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| == References ==
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| {{Reflist|colwidth=30em}}
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| == Further reading ==
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| * {{cite journal|title=Morphology Of The Galaxy Distribution From Wavelet Denoising|author=Vicent J. Martínez, Jean-Luc Starck, Enn Saar, [[David Donoho|David L. Donoho]], Simon Reynolds, Pablo de la Cruz, and Silvestre Paredes|arxiv=astro-ph/0508326| journal = The Astrophysical Journal|year = 2005|volume = 634|issue=2|pages = 744–755|bibcode=2005ApJ...634..744M| doi = 10.1086/497125}}
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| * {{cite journal|author = Mureika, J. R. and Dyer, C. C.
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| |title = Review: Multifractal Analysis of Packed Swiss Cheese Cosmologies
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| |journal = General Relativity and Gravitation
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| |arxiv = gr-qc/0505083
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| |year = 2004
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| |volume = 36|issue = 1
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| |pages = 151–184
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| |doi = 10.1023/B:GERG.0000006699.45969.49
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| |bibcode = 2004GReGr..36..151M}}
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| * {{cite journal| author = Gott, III, J. R. et al.|title = A Map of the Universe
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| |journal = The Astrophysical Journal
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| |arxiv = astro-ph/0310571
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| |date=may 2005
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| |volume = 624| issue = 2
| |
| |pages = 463–484
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| |doi = 10.1086/428890
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| |bibcode = 2005ApJ...624..463G}}
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| * {{cite journal|title=Scale-invariance of galaxy clustering|author=F. Sylos Labini, M. Montuori and L. Pietronero| journal = Physics Reports| year = 1998
| |
| |volume = 293|issue=1
| |
| |pages = 61–226|doi = 10.1016/S0370-1573(97)00044-6
| |
| |bibcode = 1998PhR...293...61S |arxiv=astro-ph/9711073}}
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| | |
| == External links ==
| |
| * [http://www.youtube.com/watch?v=K8V8Iy9Tozk Calculating the total mass of ordinary matter in the universe, what you always wanted to know]
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| * [http://www.mpa-garching.mpg.de/galform/millennium/ "Millennium Simulation" of structure forming] Max Planck Institute of Astrophysics, Garching, Germany
| |
| * [http://www.physics.usyd.edu.au/sifa/MSPM/An Visualisations of large-scale structure: animated spins of groups, clusters, filaments and voids], identified in SDSS data by MSPM (Sydney Institute for Astronomy)
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| * [http://apod.nasa.gov/apod/ap071107.html The Sloan Great Wall: Largest Known Structure?] on [http://apod.nasa.gov APOD]
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| * [http://www.astro.ucla.edu/~wright/cosmology_faq.html Cosmology FAQ]
| |
| * [http://www.sciencedaily.com/releases/2007/04/070419125240.htm Forming Galaxies Captured In The Young Universe By Hubble, VLT & Spitzer]
| |
| * [http://www.nasa.gov/multimedia/imagegallery NASA featured Images and Galleries]
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| * [http://www.cnn.com/2003/TECH/space/07/22/stars.survey/ Star Survey reaches 70 sextillion]
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| * [http://www.phys.ksu.edu/personal/gahs/phys191/horizon.html Animation of the cosmic light horizon]
| |
| * [http://arxiv.org/abs/astro-ph/0305179 Inflation and the Cosmic Microwave Background by Charles Lineweaver]
| |
| * [http://www.astro.princeton.edu/~mjuric/universe/ Logarithmic Maps of the Universe]
| |
| * [http://www.mso.anu.edu.au/2dFGRS/ List of publications of the 2dF Galaxy Redshift Survey]
| |
| * [http://www.aao.gov.au/local/www/6df/Publications/index.html List of publications of the 6dF Galaxy Redshift and peculiar velocity 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):]
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| * [http://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) ([http://arxiv.org/abs/1306.0091 arXiv])
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| {{Earth's location}}
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| {{DEFAULTSORT:Observable Universe}}
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| [[Category:Universe]]
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| [[Category:Physical cosmology]]
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