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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 Distance measures (cosmology) ： ウィキペディア英語版 Distance measures (cosmology) Distance measures are used in physical cosmology to give a natural notion of the distance between two objects or events in the universe. They are often used to tie some ''observable'' quantity (such as the luminosity of a distant quasar, the redshift of a distant galaxy, or the angular size of the acoustic peaks in the CMB power spectrum) to another quantity that is not ''directly'' observable, but is more convenient for calculations (such as the comoving coordinates of the quasar, galaxy, etc.). The distance measures discussed here all reduce to the common notion of Euclidean distance at low redshift. In accord with our present understanding of cosmology, these measures are calculated within the context of general relativity, where the Friedmann–Lemaître–Robertson–Walker solution is used to describe the universe. ==Overview== There are a few different definitions of "distance" in cosmology which all coincide for sufficiently small redshifts. The expressions for these distances are most practical when written as functions of redshift $z$, since redshift is always the observable. They can easily be written as functions of scale factor $a=1/(1+z)$, cosmic $t$ or conformal time $\backslash eta$ as well by performing a simple transformation of variables. By defining the dimensionless Hubble parameter :$E(z)=\backslash sqrt$ and the Hubble distance $d\_H\; =\; c/H\_0$, the relation between the different distances becomes apparent. Here, $\backslash Omega\_m$ is the total matter density, $\backslash Omega\_\backslash Lambda$ is the dark energy density, $\backslash Omega\_k\; =\; 1-\backslash Omega\_m-\backslash Omega\_\backslash Lambda$ represents the curvature, $H\_0$ is the Hubble parameter today and $c$ is the speed of light. The following measures for distances from the observer to an object at redshift $z$ along the line of sight are commonly used in cosmology: Comoving distance: :$d\_C(z)\; =\; d\_H\; \backslash int\_0^z\; \backslash frac$ Transverse comoving distance: : d_C(z) & \text\Omega_k=0\\ \frac \sin\left(\sqrtd_C(z)/d_H\right) & \text\Omega_k<0\end\right. Angular diameter distance: :$d\_A(z)\; =\; \backslash frac$ Luminosity distance: :$d\_L(z)\; =\; (1+z)\; d\_M(z)$ Light-travel distance: :$d\_T(z)\; =\; d\_H\; \backslash int\_0^z\; \backslash frac$ Note that the comoving distance is recovered from the transverse comoving distance by taking the limit $\backslash Omega\_k\; \backslash to\; 0$, such that the two distance measures are equivalent in a flat universe. 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「Distance measures (cosmology)」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.