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| A '''cosmological horizon''' is a measure of the distance from which one could possibly retrieve information.<ref>{{cite journal
| | The writer is known by the name of Figures Wunder. For many years he's been operating as a meter reader and it's some thing he truly appreciate. California is where her house is but she requirements to move simply because of her family members. To collect coins is a factor that I'm completely addicted to.<br><br>my blog :: over the counter std test ([http://xn--299ay03byycca57h.kr/zbxe/?document_srl=319947 relevant web page]) |
| |last=Margalef-Bentabol
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| |first=Berta
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| |coauthors=Margalef-Bentabol, Juan; Cepa, Jordi
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| |title=Evolution of the cosmological horizons in a universe with countably infinitely many state equations
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| |journal=Journal of Cosmology and Astroparticle Physics
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| |date=8 February 2013
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| |volume=2013
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| |series=015
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| |issue=02
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| |doi=10.1088/1475-7516/2013/02/015
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| |url=http://iopscience.iop.org/1475-7516/2013/02/015
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| |arxiv=1302.2186}}</ref> This observable constraint is due to various properties of [[general relativity]], the [[metric expansion of space|expanding universe]], and the physics of [[Big Bang]] [[physical cosmology|cosmology]]. Cosmological horizons set the size and scale of the [[observable universe]]. This article will explain a number of these horizons. This article will report distances in units of [[kiloparsec]]s (kpc), [[megaparsec]]s (Mpc), and [[gigaparsec]]s (Gpc).
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| ==Particle horizon==
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| {{main|Particle horizon}}
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| ==Hubble horizon==
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| One can define a so-called "Hubble Horizon" which shows roughly how far light would travel if space were not expanding. This size is
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| :<math>\chi = c t</math>
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| where <math>t</math> is the [[lookback time]] since the Big Bang (otherwise known as the [[age of the universe]]) which, according to the [[Friedmann Equations]], is: | |
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| :<math>t = \int^{a}_{0}{\frac{da}{H_0 \sqrt{\Omega_R a^{-2} + \Omega_m a^{-1} + \Omega_k +\Omega_\Lambda a^2}}}</math>
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| where <math>H_0</math> is the [[Hubble Constant]] and the <math>\Omega</math> density parameters are, in order, the density of [[radiation]], [[matter]], [[curvature]], and [[dark energy]] scaled to the [[Friedmann equations#Density parameter|critical density of the universe]].
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| Today, roughly:
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| :<math>\chi_0 = \frac{c}{H_0}</math>,
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| yielding a Hubble horizon of some 4.2 Gpc. 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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| ==Event horizon==
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| {{main|Event Horizon}}
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| The particle horizon differs from the cosmic [[event horizon]], in that the particle horizon represents the largest comoving distance from which light could have reached the observer by a specific time, while the event horizon is the largest comoving distance from which light emitted now can ''ever'' reach the observer in the future.<ref>Lars Bergström and Ariel Goobar: "Cosmology and Particle Physics", ''WILEY'' (1999), page 65.ISBN 0-471-97041-7</ref> At present, this cosmic event horizon is thought to be at a comoving distance of about 46.6 billion light years.<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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| In general, the proper distance to the event horizon at time <math>t</math> is given by<ref name="Giovannini">{{cite book|author=Massimo Giovannini|title=A primer on the physics of the cosmic microwave background|url=http://books.google.com/books?id=4ziIOYR1qZQC&pg=PA70|accessdate=1 May 2011|year=2008|publisher=World Scientific|isbn=978-981-279-142-9|pages=70–}}</ref>
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| :<math>d_e(t) = a(t) \int_{t}^{t_{max}} \frac{cdt'}{a(t')}</math>
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| where <math>t_{max}</math> 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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| For our case, assuming that [[dark energy]] is due to a [[cosmological constant]], <math>d_e(t_0) \rightarrow \infin</math>.
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| ==Future horizon==
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| In an [[accelerating universe]], there are events which will be unobservable as <math>t \rightarrow \infin</math> as signals from future events become [[redshift]]ed to arbitrarily long wavelengths in the exponentially expanding [[de Sitter space]]. 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 [[spacelike|spatially separated]] for a certain frame of reference happening [[simultaneity|simultaneously]] with the event occurring right now for which no signal will ever reach us, even though we can observe events that occurred [[timelike|at the same location in space that happened in the distant past]]. 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. Additionally, the signals coming from that location will have less and less energy and be less and less frequent until the location, for all practical purposes, becomes unobservable. In a universe that is dominated by [[dark energy]] which is undergoing an exponential expansion of the [[cosmic scale factor|scale factor]], all objects that are [[Virial theorem#In astrophysics|gravitationally unbound]] with respect to the Milky Way will become unobservable, in a futuristic version of [[Kapteyn's Universe]].<ref>http://arxiv.org/abs/0704.0221</ref>
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| ==Practical horizons== | |
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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 [[surface of last scattering]]. This is the farthest distance that any photon can freely stream. Similarly, there is a "neutrino horizon" set for the [[cosmic neutrino background|farthest distance a neutrino can freely stream]] and a gravitational wave horizon at the farthest distance that [[gravitational wave background|gravitational waves can freely stream]]. The latter is predicted to be a direct probe of the end of [[cosmic inflation]].
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| ==References==
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| {{reflist}}
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| [[Category:Physical cosmology]]
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| [[Category:Universe]]
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The writer is known by the name of Figures Wunder. For many years he's been operating as a meter reader and it's some thing he truly appreciate. California is where her house is but she requirements to move simply because of her family members. To collect coins is a factor that I'm completely addicted to.
my blog :: over the counter std test (relevant web page)