The first step in a backreaction problem is to find a regularized energy-momentum tensor of the quantum
fields using reasonable techniques, since the expectation value of this serves as the source in the
semiclassical Einstein equation. For this, much work started in the 1980s (and still ongoing
sparingly) is concerned with finding the right approximations for the regularized energy-momentum
tensor [6, 7
, 8
, 165
, 217, 260
, 292]. Even in the simplest spherically symmetric spacetime, including the
important Schwarzschild metric, it is technically quite involved. To name a few of the important
landmarks in this endeavor (this is adopted from [165]), Howard and Candelas [177
, 178
] have
computed the stress-energy of a conformally-invariant scalar field in the Schwarzschild geometry;
Jensen and Ottewill [218] have computed the vacuum stress-energy of a massless vector field in
Schwarzschild. Approximation methods have been developed by Page, Brown, and Ottewill [41, 42, 287
]
for conformally-invariant fields in Schwarzschild spacetime, by Frolov and Zel’nikov [120] for
conformally-invariant fields in a general static spacetime, and by Anderson, Hiscock and Samuel [7
, 8
] for
massless arbitrarily-coupled scalar fields in a general static spherically-symmetric spacetime. Furthermore,
the DeWitt-Schwinger approximation has been derived by Frolov and Zel’nikov [118, 119] for
massive fields in Kerr spacetime, and by Anderson, Hiscock and Samuel [7, 8
] for a general
(arbitrary curvature coupling and mass) scalar field in a general, static, spherically-symmetric
spacetime. And they have applied their method to the Reissner–Nordström geometry [6].
Though arduous and demanding, the effort continues on because of its importance in finding the
backreaction effects of Hawking radiation on the evolution of black holes and the quantum structure of
spacetime.
Here we wish to address the black hole backreaction problem with new insights and methods provided
by stochastic gravity. (For the latest developments, see, e.g., [183, 187
, 207, 208].) It is not our intention to
seek better approximations for the regularized energy-momentum tensor, but to point out new ingredients
lacking in the existing semiclassical-gravity framework. In particular one needs to consider both the
dissipation and the fluctuation aspects in the backreaction of particle creation and vacuum
polarization.
In a short note Hu, Raval and Sinha [199] first used the stochastic gravity formalism to address the
backreaction of evaporating black holes. A more detailed analysis is given by the recent work of Hu and
Roura [201, 202
]. For the class of quasi-static black holes, the formulation of the problem in
this new light was sketched out by Sinha, Raval, and Hu [332
]. We follow these two latter
works in the stochastic gravity theory approach to the black-hole fluctuations and backreaction
problems.
From the statistical field-theory perspective provided by stochastic gravity, one can understand that
backreaction effect is the manifestation of a fluctuation-dissipation relation [48, 49, 103, 104, 274, 365]. This
was first conjectured by Candelas and Sciama [76, 324
, 325
] for a dynamic Kerr black hole emitting
Hawking radiation and Mottola [268
] for a static black hole (in a box) in quasi-equilibrium
with its radiation via linear-response theory [33, 234, 235, 236, 238]. This postulate was shown
to hold for fully dynamical spacetimes. From the cosmological-backreaction problem Hu and
Sinha [206
] derived a generalized fluctuation-dissipation relation relating dissipation (of anisotropy in
Bianchi Type I universes) and fluctuations (measured by particle numbers created in neighboring
histories).
While the fluctuation-dissipation relation in linear-response theory captures the response of the system (e.g., dissipation of the black hole) to the environment (in these cases the quantum matter field), linear-response theory (in the way it is commonly presented in statistical thermodynamics) cannot provide a full description of self-consistent backreaction on at least two counts:
First, because it is usually based on the assumption of a specified background spacetime (static in this
case) and state (thermal) of the matter field(s) (e.g., [268]). The spacetime and the state of matter should
be determined in a self-consistent manner by their dynamics and mutual influence. Second, the
fluctuation part represented by the noise kernel is amiss, e.g., [10, 11]. This is also a problem in the
fluctuation-dissipation relation proposed by Candelas and Sciama [76
, 324
, 325
] (see below). As
demonstrated by many authors [73
, 206
] backreaction is intrinsically a dynamic process. The
Einstein–Langevin equation in stochastic gravity overcomes both of these deficiencies.
For Candelas and Sciama [76, 324
, 325
], the classical formula they showed relating the dissipation in
area linearly to the squared absolute value of the shear amplitude is suggestive of a fluctuation-dissipation
relation. When the gravitational perturbations are quantized (they choose the quantum state to
be the Unruh vacuum) they argue that it approximates a flux of radiation from the hole at
large radii. Thus the dissipation in area due to the Hawking flux of gravitational radiation
is allegedly related to the quantum fluctuations of gravitons. The criticism in [199
] is that
their’s is not a fluctuation-dissipation relation in the truly statistical mechanical sense because it
does not relate dissipation of a certain quantity (in this case, horizon area) to the fluctuations
of the same quantity. To do so would require one to compute the two-point function of the
area, which, being a four-point function of the graviton field, is related to a two-point function
of the stress tensor. The stress tensor is the true “generalized force” acting on the spacetime
via the equations of motion, and the dissipation in the metric must eventually be related to
the fluctuations of this generalized force for the relation to qualify as a fluctuation-dissipation
relation.
From this reasoning, we see that the vacuum expectation value of the stress-energy bitensor, known as the
noise kernel, is the necessary new ingredient in addition to the dissipation kernel, and that stochastic
gravity as an extension of semiclassical gravity is the appropriate framework for backreaction
considerations. The noise kernel for quantum fields in Minkowski and de Sitter spacetime has been carried
out by Martin, Roura and Verdaguer [257, 259, 316], and for thermal fields in black-hole spacetimes and
scalar fields in general spacetimes by Campos, Hu and Phillips [69
, 70
, 304
, 305
].
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