Renormalized vacuum polarization on rotating warped AdS3 black holes
Summary (3 min read)
Introduction
- One important task is the computation of expectation values of the renormalized stress-energy tensor for a matter field in a given quantum state [7, 8].
- The contents of the paper are as follows.
II. SPACELIKE STRETCHED BLACK HOLES AND SCALAR FIELDS
- The authors first give a short description of topologically massive gravity and review the basic features of the spacelike stretched black hole solutions, including their causal structure.
- The authors then proceed to quantize the massive scalar field and outline the Hadamard renormalization procedure.
A. Spacelike stretched black holes
- The spacelike stretched black hole is one of the several types of warped AdS3 black hole solutions [13].
- There is no stationary limit surface and no observers following orbits of ∂t (the usual “static observers” in other spacetimes) anywhere.
- They are the closest to the concept of “static observers” in this spacetime.
- The authors take as the new manifold M of interest the union of regions Ĩ, II, III and ĨV (see Fig. 1).
- This allows us to analyze quantum field theory in this bounded spacetime.
B. Scalar field equation and basis modes
- In particular, a set of “up” basis modes was introduced for the exterior region, corresponding to flux coming from the black hole which is partially reflected back to the black hole and partially reflected to infinity.
- With a boundary in place, the authors define a new set of modes in Ĩ, ΦĨω̃k, with ω̃ > 0, which are the unique linearly independent solutions that satisfy the Dirichlet boundary conditions at the mirror.
- With the purpose of later defining the Hartle-Hawking state, the authors need to construct a new mode basis.
- ΦLω̃k and Φ R ω̃k are, hence, of positive frequency in the affine parameters on the horizon.
- They are further orthonormal in the Klein-Gordon inner product on spacelike hypersurfaces from mirror to mirror .
C. Quantized field and Hartle-Hawking vacuum state
- So far, only classical theory has been discussed.
- The authors now proceed to canonically quantize the scalar field using the standard Hilbert space approach.
- This is possible since, as seen above, there is a natural positive and negative frequency decomposition of the mode solutions for this spacetime.
- Furthermore, it is invariant under the spacetime isometries.
III. COMPLEX RIEMANNIAN SECTION OF THE SPACELIKE STRETCHED BLACK HOLE
- The authors first consider the complex Riemannian section of the spacelike stretched black hole and obtain the unique Green’s function associated with the Klein-Gordon equation as a mode sum.
- This is followed by a detailed account of the Hadamard renormalization procedure, in which the authors subtract the divergences in the mode sum by a sum over Minkowski modes with the same singularity structure.
- A similar subtraction procedure in a static four-dimensional black hole spacetime in with a cosmic string has been considered in [34].
A. Complex Riemannian section
- Euclidean methods are a powerful tool to study quantum field theory on static spacetimes.
- In many cases, it is much easier to perform calculations in the Riemannian section (e.g. computing the unique Green’s function associated with the scalar field equation) and then analytically continue the results back to the Lorentzian section.
- The analytic continuation procedure does unfortunately not have an immediate generalization to spacetimes that are stationary but not static.
- Both of these issues are present in Kerr (for which the absence of a real section with a positive definite metric is shown in [20]) and in their (2 + 1)-dimensional spacelike stretched black holes.
- In [23], a more general concept of ‘local Wick rotation’ is discussed for any Lorentzian manifold, even without a timelike Killing vector field, as long as its metric is a locally analytic function of the coordinates.
B. Green’s function associated with the Klein-Gordon equation
- In the real Lorentzian section, the authors defined the Feynman propagator GF ∈ D′(̃I× Ĩ) as one of the Green’s functions associated with the Klein-Gordon equation satisfied by the scalar field.
- Here, the authors find the Green’s function G ∈ D′(IC× IC) associated with the Klein-Gordon equation in the complex Riemannian section.
- In contrast to the real Lorentzian section, in the complex Riemannian section there is a unique solution to this equation which satisfies the following boundary conditions: (i) G(x, x′) is regular at r = r+, and (ii) G(x, x ′) satisfies the Dirichlet boundary conditions at r = rM.
- This is due to the fact that two of the directions of the spacetime are periodic, while the third direction is compact.
- Compare this to the situation on static spacetimes without any boundary (and suitable asymptotic properties at infinity), whose Euclidean section has a unique Euclidean Green’s function, due to the ellipticity of the Klein-Gordon operator.
D. Subtraction of the Hadamard singular part
- Consider the complex Riemannian section in (τ, z, θ̃) coordinates and suppose that x and x′ are in the region IC and are angularly separated.
- X and x′ are in a geodesically linearly convex neighborhood and the complex Synge’s world function can be obtained for small angular separation using the standard Riemannian relation.
- The authors shall accomplish this by comparing GBHHad to the Hadamard singular part for a scalar field in rotating Minkowski spacetime in the complex Riemannian section.
- The advantage of using the Minkowski spacetime is that its symmetries allow us to compute the Green’s function in both closed form and as a mode sum.
E. Fixing of the Minkowski free parameters
- At least some of the parameters of the Minkowski Green’s function must be fixed such that the double sum in (3.27) is convergent in the coincidence limit.
- The authors now claim that, with this choice of parameters, the double sum in (3.27) is convergent in the coincidence limit.
IV. NUMERICAL EVALUATION OF THE VACUUM POLARIZATION
- As described previously, the sums in (4.1) are convergent.
- For the numerical evaluation of the sums, cutoffs are imposed appropriately.
- The numerical results for selected values of the parameters are presented in Fig.
- Note that 〈Φ2(x)〉 gets arbitrarily large and negative as the mirror is approached, as expected (see e.g. chapter 4.3 of [7]).
- The authors reemphasize that the result shown in Fig. 2 is the full renormalized vacuum polarization in the Hartle-Hawking state.
V. CONCLUSIONS
- The authors have employed a ‘quasi-Euclidean’ method to obtain a complex Riemannian section of the original spacetime, in which they found the unique Green’s function associated with the Klein-Gordon equation.
- This renormalization procedure renders a smooth function whose coincidence limit is precisely the renormalized value of 〈Φ2(x)〉.
- A key ingredient in their implementation of the Hadamard renormalization was to match the mode sum for the Green’s function on the complex Riemannian section of the black hole to a mode sum on the complex Riemannian section of a rotating Minkowski spacetime.
- The implementation of their method in Kerr would, hence, seem feasible in principle, and it should prove interesting to attempt the implementation in practice.
Did you find this useful? Give us your feedback
Citations
25 citations
21 citations
16 citations
9 citations
Cites background from "Renormalized vacuum polarization on..."
...With the notable exception of [23, 24, 29, 34], most of the works cited above concerned either asymptotically flat or asymptotically de Sitter black holes....
[...]
7 citations
Cites background from "Renormalized vacuum polarization on..."
...(An analytic approximation good for fields with large mass is available, however [8], and exact results are obtainable in d = 3 with AdS asymptotics [9, 10]....
[...]
References
6,464 citations
1,806 citations
1,793 citations
"Renormalized vacuum polarization on..." refers background in this paper
...One important task is the computation of expectation values of the renormalized stressenergy tensor for a matter field in a given quantum state [7, 8]....
[...]
460 citations
Related Papers (5)
Frequently Asked Questions (9)
Q2. What are the future works mentioned in the paper "Renormalized vacuum polarization on rotating warped ads3 black holes" ?
In the future, the authors intend to extend this method to compute the expectation value of the stress-energy tensor. The authors anticipate that this method can be extended to wider classes of rotating black hole spacetimes, and in particular in four dimensions to the Kerr spacetime.
Q3. What is the advantage of using the Minkowski spacetime?
The advantage of using the Minkowski spacetime is that its symmetries allow us to compute the Green’s function in both closed form and as a mode sum.
Q4. What is the function for x ?
After this matching is performed, the vacuum polarization is just given by〈Φ2(x)〉 = lim θ̃→0 GBHren(x, x ′) , (3.28)which is a well-defined smooth function for x ∈ Ĩ.
Q5. What is the definition of a spacetime with a Killing field?
In the context of quantum field theory, as it is detailed below, the nonexistence of an everywhere timelike Killing vector field in the exterior region of the spacetime is directly related to the nonexistence of a well defined quantum vacuum state which is regular at the horizon and is invariant under the isometries of the spacetime.
Q6. What is the metric of the spacelike stretched black hole?
The dimensionless coupling ν ∈ (1,∞) is the warp factor, and in the limit ν → 1 the above metric reduces to the metric of the BTZ black hole in a rotating frame.
Q7. What is the renormalization procedure for the Feynman propagator?
This is followed by a detailed account of the Hadamard renormalization procedure, in which the authors subtract the divergences in the mode sum by a sum over Minkowski modes with the same singularity structure.
Q8. What is the renormalized vacuum polarization in other Hadamard states of interest?
To find the renormalized vacuum polarization in other Hadamard states of interest, such as the Boulware vacuum state, it would suffice to use the Hartle-Hawking state as a reference and just to calculate the difference, which is finite without further renormalization.
Q9. what is the singular part of the Feynman propagator?
the singular, state-independent part of the Feynman propagator isGHad(x, x ′) :=i 4 √ 2π U(x, x′)√ σ(x, x′) + i . (2.21)This is known as the “Hadamard singular part” and it is singular at x′ → x.