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315:), Hubble volume, or Hubble horizon is a conceptual horizon defining the boundary between particles that are moving slower and faster than the speed of light relative to an observer at one given time. Note that this does not mean the particle is unobservable; the light from the past is reaching and will continue to reach the observer for a while. Also, more importantly, in the current expansion models, light emitted from the Hubble radius will reach us in a finite amount of time.
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342:), while particles on the Hubble radius recede from us with the speed of light, the Hubble radius gets larger over time, so light emitted towards us from a particle on the Hubble radius will be inside the Hubble radius some time later. In such models, only light emitted from the cosmic event horizon or further will never reach us in a finite amount of time.
68:. It represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe. Due to the expansion of the universe, it is not simply the age of the universe times the speed of light, as in the Hubble horizon, but rather the
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While we will continue to receive signals from this location in space, even if we wait an infinite amount of time, a signal that left from that location today will never reach us. The signals coming from that location will have less and less energy and be less and less frequent until the location,
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In an ever-accelerating universe, if two particles are separated by a distance greater than the Hubble radius, they cannot talk to each other from now on (as they are now, not as they have been in the past). However, if they are outside of each other's particle horizon, they could have never
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1086:. This sets a limit on the farthest distance that we can possibly see as measured in units of proper distance today. Or, more precisely, there are events that are spatially separated for a certain frame of reference happening
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In terms of comoving distance, the particle horizon is equal to the conformal time that has passed since the Big Bang, times the speed of light. In general, the conformal time at a certain time is given in terms of the
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yielding a Hubble horizon of some 4.1 gigaparsecs. This horizon is not really a physical size, but it is often used as useful length scale as most physical sizes in cosmology can be written in terms of those factors.
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The particle horizon is the boundary between two regions at a point at a given time: one region defined by events that have already been observed by an observer, and the other by events which cannot be observed
180:
63:
The particle horizon, also called the cosmological horizon, the comoving horizon, or the cosmic light horizon, is the maximum distance from which light from particles could have traveled to the observer in the
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reach the observer in the future. The current distance to our cosmic event horizon is about five gigaparsecs (16 billion light-years), well within our observable range given by the particle horizon.
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While not technically "horizons" in the sense of an impossibility for observations due to relativity or cosmological solutions, there are practical horizons which include the optical horizon, set at the
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with the event occurring right now for which no signal will ever reach us, even though we can observe events that occurred at the same location in space that happened in the distant past.
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Margalef-Bentabol, Berta; Margalef-Bentabol, Juan; Cepa, Jordi (8 February 2013). "Evolution of the cosmological horizons in a universe with countably infinitely many state equations".
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from which light could have reached the observer by a specific time, while the cosmic event horizon is the largest comoving distance from which light emitted now can
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multiplied by the conformal time. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model.
1022:{\displaystyle \max(d_{e})={\frac {c}{H_{e}}}=c{\sqrt {\frac {3}{\Lambda }}}={\frac {c}{{\sqrt {\Omega _{\Lambda }}}H_{0}}}=17.55\ {\textrm {Gly}}.}
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is a measure of the distance from which one could possibly retrieve information. This observable constraint is due to various properties of
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1261:. Wiley-Praxis series in astronomy and astrophysics. Chichester ; New York : Chichester: Wiley ; Praxis Pub. p. 65.
297:. It represents the furthest distance from which we can retrieve information from the past, and so defines the observable universe.
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communicated. Depending on the form of expansion of the universe, they may be able to exchange information in the future. Today,
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is the time-coordinate of the end of the universe, which would be infinite in the case of a universe that expands forever.
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Krauss, Lawrence M.; Scherrer, Robert J.; Cepa, Jordi (2007). "The return of a static universe and the end of cosmology".
1123:. This is the farthest distance that any photon can freely stream. Similarly, there is a "neutrino horizon" set for the
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It is a common misconception that light from the Hubble radius can never reach us. In models assuming decreasing
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One can also define a comoving Hubble horizon by simply dividing the Hubble radius by the scale factor
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Lineweaver, Charles H.; Davis, Tamara M. (March 2005). "Misconceptions about the Big Bang".
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with respect to the Milky Way will become unobservable, in a futuristic version of
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677:{\displaystyle r_{{\text{HS}},\mathrm {comoving} }(t)={\frac {c}{a(t)H(t)}}\,.}
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838:{\displaystyle d_{e}(t)=a(t)\int _{t}^{t_{\text{max}}}{\frac {cdt'}{a(t')}}}
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as a function of time and distance, in context of the expanding
Universe.
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1333:. Singapore ; Hackensack, NJ: World Scientific. p. 70.
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and a gravitational wave horizon at the farthest distance that
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to arbitrarily long wavelengths in the exponentially expanding
1131:. The latter is predicted to be a direct probe of the end of
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In general, the proper distance to the event horizon at time
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1329:
A primer on the physics of the cosmic microwave background
566:{\displaystyle r_{\text{HS}}(t_{0})={\frac {c}{H_{0}}}\,,}
175:{\displaystyle \eta (t)=\int _{0}^{t}{\frac {dt'}{a(t')}}}
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which is often referred to as the only particle horizon:
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Hubble radius, Hubble sphere (not to be confused with a
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45:. Cosmological horizons set the size and scale of the
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which is undergoing an exponential expansion of the
49:. This article explains a number of these horizons.
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697:The particle horizon differs from the cosmic
345:The Hubble velocity of an object is given by
886:, there will be a minimum Hubble parameter
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1422:Klauber, Robert D. (October 9, 2018).
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1257:Bergström, L.; Goobar, Ariel (1999).
1129:gravitational waves can freely stream
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1078:as signals from future events become
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1071:{\displaystyle t\rightarrow \infty }
1259:Cosmology and particle astrophysics
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875:For our case, assuming that
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338:with time (some cases of
1228:Dodelson, Scott (2003).
19:Not to be confused with
1424:"Horizons in Cosmology"
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1284:Scientific American
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66:age of the universe
47:observable universe
1538:Physical cosmology
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877:dark energy
1175:(2): 015.
1151:References
1080:redshifted
381:Replacing
1462:Astronomy
1405:123442313
1380:0704.0221
1349:191658608
1312:0036-8733
1215:119614479
1207:1475-7516
1182:1302.2186
1066:∞
1063:→
981:Λ
977:Ω
959:Λ
776:∫
206:∫
190:η
123:∫
107:η
43:cosmology
1532:Category
1139:See also
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40:Big Bang
1522:Science
1450:Physics
1436:Portals
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1187:Bibcode
1171:. 015.
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1048:In an
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845:where
34:, the
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1427:(PDF)
1401:S2CID
1375:arXiv
1211:S2CID
1177:arXiv
1004:17.55
1345:OCLC
1335:ISBN
1308:ISSN
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