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{{Short description|Distance measurement used in cosmology}}The particle horizon (also called the cosmological horizon, the comoving horizon (in Scott Dodelson's text), or the cosmic light horizon) is the maximum distance from which light from particles could have traveled to the observer in the age of the universe. Much like the concept of a terrestrial horizon, it represents the boundary between the observable and the unobservable regions of the universe,BOOK, Edward Robert Harrison, Cosmology: the science of the universe,weblink 1 May 2011, 2000, Cambridge University Press, 978-0-521-66148-5, 447–, so its distance at the present epoch defines the size of the observable universe.BOOK, Andrew R. Liddle, David Hilary Lyth, Cosmological inflation and large-scale structure,weblink 1 May 2011, 13 April 2000, Cambridge University Press, 978-0-521-57598-0, 24–, Due to the expansion of the universe, it is not simply the age of the universe times the speed of light (approximately 13.8 billion light-years), but rather the speed of light times the conformal time. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model.

Conformal time and the particle horizon

In terms of comoving distance, the particle horizon is equal to the conformal time eta that has passed since the Big Bang, times the speed of light c. In general, the conformal time at a certain time t is given by where a(t) is the scale factor of the Friedmann–Lemaître–Robertson–Walker metric, and we have taken the Big Bang to be at t=0. By convention, a subscript 0 indicates "today" so that the conformal time today eta(t_0) = eta_0 = 1.48 times 10^{18}text{ s}. Note that the conformal time is not the age of the universe, which is estimated around 4.35 times 10^{17}text{ s}. Rather, the conformal time is the amount of time it would take a photon to travel from where we are located to the furthest observable distance, provided the universe ceased expanding. As such, eta_0 is not a physically meaningful time (this much time has not yet actually passed); though, as we will see, the particle horizon with which it is associated is a conceptually meaningful distance.The particle horizon recedes constantly as time passes and the conformal time grows. As such, the observed size of the universe always increases.BOOK, Michael Paul Hobson, George Efstathiou, Anthony N. Lasenby, General relativity: an introduction for physicists,weblink 1 May 2011, 2006, Cambridge University Press, 978-0-521-82951-9, 419–, Since proper distance at a given time is just comoving distance times the scale factorJOURNAL, Davis, Tamara M., Charles H. Lineweaver, Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe, Publications of the Astronomical Society of Australia, 21, 1, 97, 2004, 10.1071/AS03040, astro-ph/0310808, 2004PASA...21...97D, 13068122, (with comoving distance normally defined to be equal to proper distance at the present time, so a(t_0) = 1 at present), the proper distance to the particle horizon at time t is given byBOOK, Massimo Giovannini, A primer on the physics of the cosmic microwave background,weblink registration, 1 May 2011, 2008, World Scientific, 978-981-279-142-9, 70–, and for today t = t_0

Evolution of the particle horizon

In this section we consider the FLRW cosmological model. In that context, the universe can be approximated as composed by non-interacting constituents, each one being a perfect fluid with density rho_i, partial pressure p_i and state equation p_i=omega_i rho_i, such that they add up to the total density rho and total pressure p.JOURNAL, Berta Margalef-Bentabol

author3=Jordi Cepa , Evolution of the cosmological horizons in a concordance universe, Journal of Cosmology and Astroparticle Physics, 21 December 2012, 2012, 12

doi=10.1088/1475-7516/2012/12/035

bibcode = 2012JCAP...12..035M, 119704554, Let us now define the following functions: Hubble function H=frac{dot a}{a} The critical density rho_c=frac{3}{8pi G}H^2 The i-th dimensionless energy density Omega_i=frac{rho_i}{rho_c} The dimensionless energy density Omega=frac rho {rho_c}=sum Omega_i The redshift z given by the formula 1+z=frac{a_0}{a(t)} Any function with a zero subscript denote the function evaluated at the present time t_0 (or equivalently z=0). The last term can be taken to be 1 including the curvature state equation.JOURNAL, Berta Margalef-Bentabol

author3=Jordi Cepa , Evolution of the cosmological horizons in a universe with countably infinitely many state equations, Journal of Cosmology and Astroparticle Physics, 8 February 2013, 2013, 015, 2

doi=10.1088/1475-7516/2013/02/015

bibcode = 2013JCAP...02..015M, 119614479, It can be proved that the Hubble function is given by H(z)=H_0sqrt{sum Omega_{i0}(1+z)^{n_i}} where the dilution exponent n_i=3(1+omega_i). Notice that the addition ranges over all possible partial constituents and in particular there can be countably infinitely many. With this notation we have: text{The particle horizon } H_p text{ exists if and only if } N>2 where N is the largest n_i (possibly infinite). The evolution of the particle horizon for an expanding universe (dot{a}>0) is: frac{dH_p}{dt}=H_p(z)H(z)+c where c is the speed of light and can be taken to be 1 (natural units). Notice that the derivative is made with respect to the FLRW-time t, while the functions are evaluated at the redshift z which are related as stated before. We have an analogous but slightly different result for event horizon. Horizon problem The concept of a particle horizon can be used to illustrate the famous horizon problem, which is an unresolved issue associated with the Big Bang model. Extrapolating back to the time of recombination when the cosmic microwave background (CMB) was emitted, we obtain a particle horizon of about{{block indent| H_p(t_text{CMB}) = ceta_text{CMB} = 284text{ Mpc} = 8.9 times 10^{-3} H_p(t_0) }}which corresponds to a proper size at that time of:{{block indent| a_text{CMB}H_p(t_text{CMB}) = 261text{ kpc} }}Since we observe the CMB to be emitted essentially from our particle horizon (284text{ Mpc} ll 14.4text{ Gpc}), our expectation is that parts of the cosmic microwave background (CMB) that are separated by about a fraction of a great circle across the sky of{{block indent|f = frac{H_p(t_text{CMB})}{H_p(t_0)}}}(an angular size of theta sim 1.7^circ)WEB, Understanding the Cosmic Microwave Background Temperature Power Spectrum,weblink 5 November 2015, should be out of causal contact with each other. That the entire CMB is in thermal equilibrium and approximates a blackbody so well is therefore not explained by the standard explanations about the way the expansion of the universe proceeds. The most popular resolution to this problem is cosmic inflation. See also Cosmological horizon Observable universe References {{reflist}}