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Vaidya metric : ウィキペディア英語版
Vaidya metric
In general relativity, the Vaidya metric describes the non-empty external spacetime of a spherically symmetric and nonrotating star which is either emitting or absorbing null dusts. It is named after the Indian physicist Prahalad Chunnilal Vaidya and constitutes a simplest non-static generalization of the non-radiative Schwarzschild solution to Einstein's field equation, and therefore is also called the "radiating(/shining) Schwarzschild metric".
== From Schwarzschild to Vaidya metrics ==

The Schwarzschild metric as the static and spherically symmetric solution to Einstein's equation reads
(1)\quad ds^2=-\Big( 1-\frac \Big) dt^2+\Big( 1-\frac \Big)^dr^2+r^2(d\theta^2+\sin^2\theta\,d\phi^2)\;.
To remove the coordinate singularity of this metric at r=2M, one could switch to the Eddington–Finkelstein coordinates. Thus, introduce the "retarded(/outgoing)" null coordinate u by
(2)\quad t=u+r+2M\ln\Big(\frac-1\Big)\qquad\Rightarrow\quad dt=du+\Big( 1-\frac \Big)^dr\;,
and Eq(1) could be transformed into the "retarded(/outgoing) Schwarzschild metric"
(3)\quad ds^2=-\Big( 1-\frac \Big) du^2-2dudr+r^2(d\theta^2+\sin^2\theta\,d\phi^2)\;;
or, we could instead employ the "advanced(/ingoing)" null coordinate v by
(4)\quad t=v-r-2M\ln\Big(\frac-1\Big)\qquad\Rightarrow\quad dt=dv-\Big( 1-\frac \Big)^dr\;,
so Eq(1) becomes the "advanced(/ingoing) Schwarzschild metric"
(5)\quad ds^2=-\Big( 1-\frac \Big) dv^2+2dvdr+r^2(d\theta^2+\sin^2\theta\,d\phi^2)\;.
Eq(3) and Eq(5), as static and spherically symmetric solutions, are valid for both ordinary celestial objects with finite radii and singular objects such as black holes. It turns out that, it is still physically reasonable if one extends the mass parameter M in Eqs(3) and Eq(5) from a constant to functions of the corresponding null coordinate, M(u) and M(v) respectively, thus
(6)\quad ds^2=-\Big( 1-\frac \Big) du^2-2dudr+r^2(d\theta^2+\sin^2\theta\,d\phi^2)\;,
(7)\quad ds^2=-\Big( 1-\frac \Big) dv^2+2dvdr+r^2(d\theta^2+\sin^2\theta\,d\phi^2)\;.
The extended metrics Eq(6) and Eq(7) are respectively the "retarded(/outgoing)" and "advanced(/ingoing)" Vaidya metrics.〔Eric Poisson. ''A Relativist's Toolkit: The Mathematics of Black-Hole Mechanics''. Cambridge: Cambridge University Press, 2004. Section 4.3.5 and Section 5.1.8.〕〔Jeremy Bransom Griffiths, Jiri Podolsky. ''Exact Space-Times in Einstein's General Relativity''. Cambridge: Cambridge University Press, 2009. Section 9.5.〕 It is also interesting and sometimes useful to recast the Vaidya metrics Eqs(6)(7) into the form
(8)\quad ds^2 =\fracdu^2 +ds^2(\text)=\fracdv^2 +ds^2(\text)\,,
where ds^2(\text)=- du^2-2dudr+r^2 (d\theta^2+\sin^2\theta\, d\phi^2 )=- dv^2+2dvdr+r^2 (d\theta^2+\sin^2\theta\, d\phi^2 )=-dt^2+dr^2+r^2 (d\theta^2+\sin^2\theta\, d\phi^2 ) represents the metric of flat spacetime.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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