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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Particle horizon ： ウィキペディア英語版 Particle horizon The particle horizon (also called the cosmological horizon, the light horizon, or the cosmic light horizon) is the maximum distance from which particles could have traveled to the observer in the age of the universe. 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 (approximately 13.8 billion light years), but rather the speed of light times the conformal time (see below). 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 $\backslash eta$ 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, :$\backslash eta\; =\; \backslash int\_^\; \backslash frac$ 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 $\backslash eta(t\_0)\; =\; \backslash eta\_0\; =\; 1.48\; \backslash times\; 10^\backslash$. Note that the conformal time is not the age of the universe. 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, $\backslash eta\_0$ 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 fraction of the universe always increases.〔 Since proper distance at a given time is just comoving distance times the scale factor (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 by :$a(t)\; H\_p(t)\; =\; a(t)\; \backslash int\_^\; \backslash frac$ and for today $t\; =\; t\_0$ :$H\_p(t\_0)\; =\; c\backslash eta\_0\; =\; 14.4\backslash \; =\; 46.9\backslash$. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Particle horizon」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.