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| [[File:Observable Universe with Measurements 01.png||right|300px|thumb|Visualization of the three-dimensional [[large-scale structure of the universe]] in the Hubble volume. The scale is such that the fine grains of light represent collections of large numbers of [[supercluster]]s. The [[Virgo Supercluster]] - home of [[Milky Way|our own galaxy]] - is at the center of our Hubble volume, but is too small to be seen in the image.]]
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| In [[Physical cosmology|cosmology]], a '''Hubble volume''', or '''Hubble sphere''', is a spherical region of the [[Universe]] surrounding an observer beyond which objects recede from that observer at a rate greater than the [[speed of light]] due to the expansion of the Universe.<ref name=Harrison>{{cite book |title=Masks of the Universe |url=http://books.google.com/?id=tSowGCP0kMIC&pg=PA206 |page=206 |author= Edward Robert Harrison |year=2003 |publisher=Cambridge University Press |isbn=0-521-77351-2}}</ref>
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| The [[comoving distance|comoving]] radius of a Hubble sphere (known as the Hubble radius or the [[Hubble length]]) is <math>c/H_0</math>, where <math>c</math> is the [[speed of light]] and <math>H_0</math> is the [[Hubble constant]]. The surface of a Hubble sphere is called the ''microphysical horizon'',<ref name= Montani>{{cite arxiv |title=Study of the Quasi-isotropic Solution near the Cosmological Singularity in Presence of Bulk-Viscosity |author= N. Carlevaro & G. Montani |arxiv=0711.1952 |year=2009}}</ref> the ''Hubble surface'', or the ''Hubble limit''.
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| More generally, the term "Hubble volume" can be applied to any region of space with a volume of order <math>(c/H_0)^3</math>.
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| However, the term is also frequently (but mistakenly) used as a [[synonym]] for the [[observable universe]]; the latter is larger than the Hubble volume.<ref name= Davis>For a discussion of why objects that are outside the Earth's Hubble sphere can be seen from Earth, see {{cite arxiv |title=Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe |author= TM Davis & CH Linewater |arxiv=astro-ph/0310808 |year=2003}}</ref><ref>For an example of mistaken usage, see {{cite book |author=Max Tegmark |authorlink=Max Tegmark |chapter=Parallel Universes |title=Science and Ultimate Reality: From Quantum to Cosmos |editor-last=Barrow |editor-first=J. D. |editor2-last=Davies |editor2-first=J. D. |editor3-last=Harper |editor3-first=C. L. |pages=459''ff'' |publisher=Cambridge University Press |year=2004 |isbn=0-521-83113-X |url=http://books.google.com/?id=K_OfC0Pte_8C&pg=PA459}}</ref>
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| ==Relationship to age of the universe==
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| The Hubble length <math>c/H_0</math> is 14 billion light years in the [[Lambda-CDM model|standard cosmological model]], somewhat larger than <math>c</math> times the [[age of the universe]], 13.8 billion years. This is because <math>1/H_0</math> gives the age of the universe by a backward extrapolation which assumes that the recession speed of each galaxy has been constant. However, modern observations indicate recession speeds are increasing slightly<ref name=Tonry>{{cite journal |title=Cosmological Results from High-z Supernovae |year=2003 |arxiv=astro-ph/0305008 |author=John L Tonry |journal=Astrophys J |volume=594 |bibcode = 2003ApJ...594....1T |doi = 10.1086/376865 |author-separator=, |display-authors=1 |last2=Schmidt |first2=Brian P. |last3=Barris |first3=Brian |last4=Candia |first4=Pablo |last5=Challis |first5=Peter |last6=Clocchiatti |first6=Alejandro |last7=Coil |first7=Alison L. |last8=Filippenko |first8=Alexei V. |last9=Garnavich |first9=Peter |pages=1 }}</ref> due to [[dark energy]], so that <math>1/H_0</math> is only an approximation to the [[age of the universe]].
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| ==Hubble limit as an event horizon==
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| Objects at the Hubble limit have an average comoving speed of ''c'' relative to an observer on the Earth so that, in a universe with constant [[Hubble parameter]], light emitted at the present time by objects outside the Hubble limit would never be seen by an observer on Earth.
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| That is, Hubble limit would coincide with a cosmological [[event horizon]] (a boundary separating events visible at some time and those that are never visible<ref name=Harrison2>{{cite book |author=Edward Robert Harrison |page=439 |url=http://books.google.com/?id=-8PJbcA2lLoC&pg=PA439 |title=Masks of the Universe |isbn=0-521-66148-X |publisher=Cambridge University Press |year=2000}}</ref>).
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| However, the [[Hubble parameter]] is not constant in various cosmological models<ref name= Davis/> so that the Hubble limit does not, in general, coincide with a cosmological event horizon. For example in a decelerating [[Friedmann equations|Friedmann universe]] the Hubble sphere expands faster than the Universe and its boundary overtakes light emitted by receding galaxies so that light emitted at earlier times by objects outside the Hubble sphere still may eventually arrive inside the sphere and be seen by us.<ref name= Davis/> Conversely, in an accelerating universe, the Hubble sphere expands more slowly than the Universe, and bodies move out of the Hubble sphere.<ref name=Harrison/>
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| Observations indicate that the universe is [[accelerating universe|accelerating]],<ref name="Tonry"/> so that some objects that we can currently exchange signals with will one day cross our Hubble limit.
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| ==See also==
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| *[[Hubble's Law]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| *[http://www.mpa-garching.mpg.de/Virgo/hubble.html The Hubble Volume Simulations]
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| [[Category:Physical cosmology]]
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The title of the author is Luther. Managing individuals is how I make money and it's something I really enjoy. Years in the past we moved to Arizona but my wife wants us to move. Camping is something that I've carried out for many years.
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