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 the 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".
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From Schwarzschild to Vaidya metrics
The Schwarzschild metric as the static and spherically symmetric solution to Einstein's equation reads
To remove the coordinate singularity of this metric at
and Eq(1) could be transformed into the "retarded(/outgoing) Schwarzschild metric"
or, we could instead employ the "advanced(/ingoing)" null coordinate
so Eq(1) becomes the "advanced(/ingoing) Schwarzschild metric"
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
The extended metrics Eq(6) and Eq(7) are respectively the "retarded(/outgoing)" and "advanced(/ingoing)" Vaidya metrics. It is also interesting and sometimes useful to recast the Vaidya metrics Eqs(6)(7) into the form
where
Outgoing Vaidya with pure Emitting field
As for the "retarded(/outgoing)" Vaidya metric Eq(6), the Ricci tensor has only one nonzero component
while the Ricci curvature scalar vanishes,
where
we have
Following the calculations using Newman–Penrose (NP) formalism in Box A, the outgoing Vaidya spacetime Eq(6) is of Petrov-type D, and the nonzero components of the Weyl-NP and Ricci-NP scalars are
It is notable that, the Vaidya field is a pure radiation field rather than electromagnetic fields. The emitted particles or energy-matter flows have zero rest mass and thus are generally called "null dusts", typically such as photons and neutrinos, but cannot be electromagnetic waves because the Maxwell-NP equations are not satisfied. By the way, the outgoing and ingoing null expansion rates for the line element Eq(6) are respectively
Ingoing Vaidya with pure absorbing field
As for the "advanced/ingoing" Vaidya metric Eq(7), the Ricci tensors again have one nonzero component
and therefore
This is a pure radiation field with energy density
Also, the outgoing and ingoing null expansion rates for the line element Eq(7) are respectively
The advanced/ingoing Vaidya solution Eq(7) is especially useful in black-hole physics as it is one of the few existing exact dynamical solutions. For example, it is often employed to investigate the differences between different definitions of the dynamical black-hole boundaries, such as the classical event horizon and the quasilocal trapping horizon; and as shown by Eq(17), the evolutionary hypersurface
Comparison with the Schwarzschild metric
As a natural and simplest extension of the Schwazschild metric, the Vaidya metric still has a lot in common with it:
However, there are three clear differences between the Schwarzschild and Vaidya metric:
Kinnersley metric
While the Vaidya metric is an extension of the Schwarzschild metric to include a pure radiation field, the Kinnersley metric constitutes a further extension of the Vaidya metric; it describes a massive object that accelerates in recoil as it emits massless radiation anisotropically. The Kinnersley metric is a special case of the Kerr-Schild metric, and in cartesian spacetime coordinates
where for the duration of this section all indices shall be raised and lowered using the "flat space" metric
In the special case where
Vaidya-Bonner metric
Since the radiated or absorbed matter might be electrically non-neutral, the outgoing and ingoing Vaidya metrics Eqs(6)(7) can be naturally extended to include varying electric charges,
Eqs(18)(19) are called the Vaidya-Bonner metrics, and apparently, they can also be regarded as extensions of the Reissner–Nordström metric, as opposed to the corresponce between Vaidya and Schwarzschild metrics.