REVIEW
Classical instability of Kerr-AdS black holes and the issue of final state
High Energy Physics - Theory · Submitted 22 Jul 2006 · 30 references
Pith's verdict: The spin-2 instability calculation is solid and worth citing; the endpoint is an explicitly conjectural add-on that the abstract oversells.
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Referee report
The paper
The paper claims
Small rotating black holes in anti-de Sitter space are unstable even to gravitational waves, and the instability should run until the black hole's boundary rotates exactly at the speed of light.
The reading
What carries the argument
The superradiant black-hole-bomb mechanism inside a box: AdS reflects outgoing radiation back onto the hole, and a rotating horizon amplifies any wave satisfying $omega < mOmega_H$, so repeated round trips give exponential growth. The carrier of the quantitative result is the matched-asymptotics computation — a hypergeometric near-horizon solution matched to the exact Regge-Wheeler-in-AdS solution — which turns the instability question into the sign of $varpi$. The carrier of the endpoint claim is the identification of the superradiance threshold $r_+ = sqrt{a,ell}$ with the light-speed condition $Omega = 1/ell$ for the boundary Einstein universe.
Extended reading notes
Core claim
Using the Chambers-Moss-Teukolsky radial equation for spin-2 perturbations of four-dimensional Kerr-AdS, the authors solve the near-horizon region with a hypergeometric ingoing wave and match it to the exact pure-AdS far-region solution with Dirichlet boundary conditions. The resulting eigenvalue equation, expanded around the pure-AdS frequency $omega = (2n + l + 2)/ell + i,delta$, gives $delta = -P_1 P_2,varpi$ with $P_1, P_2 > 0$ and $varpi propto (omega - mOmega_H)$. So every mode with $omega < mOmega_H$ — the superradiant regime — grows exponentially, and small Kerr-AdS black holes are unstable against gravitational perturbations, not only scalar ones. The authors then follow the instability: superradiance drains angular momentum ($a$ falls) while the area law forces $r_+$ up, so the critical radius $r_c^+ = sqrt{a,ell}$ falls; the evolution should halt when the two meet, at $r_+ = sqrt{a,ell}$. That same condition is shown, via the asymptotically-AdS coordinates in which the horizon angular velocity is $Omega = a(1 + r_+^2/ell^2)/(r_+^2 + a^2)$, to be exactly the condition $Omega = 1/ell$: the boundary Einstein universe rotates at light speed.
Load-bearing premise
The endpoint claim assumes the unstable hole evolves quasi-statically through a sequence of Kerr-AdS solutions, that the area law then forces $r_+$ up while $a$ falls, and that the evolution stops exactly at $r_+ = sqrt{a,ell}$ rather than overshooting; the authors themselves write in Section III that it is in general impossible to predict the endpoint of an unstable system and that this part is an educated guess.
Editorial extensions
If this is right
- Kerr-AdS black holes with $r_+ < sqrt{a,ell}$ are dynamically unstable at the level of the metric, so their linear-mode spectrum and thermodynamic description are not the whole story.
- The instability timescale is computable in closed form, so one can estimate how fast a small spinning AdS hole loses angular momentum.
- The endpoint is a Kerr-AdS hole with $r_+ = sqrt{a,ell}$, whose conformal boundary is an Einstein universe rotating at the speed of light — a concrete target for a dual field theory description.
- Because the endpoint is expected to be slightly oblate and to coexist with trapped radiation, there should exist non-Kerr stationary solutions of Einstein's equations with AdS asymptotics, at least to first order in the deformation.
- The same instability is expected in higher dimensions; the appendix sketches the matched-asymptotics calculation for a sector of perturbations that exists only for $D geq 5$.
Reading between the lines
- If the light-speed boundary condition is the true stopping point, it suggests a general criterion: AdS black holes settle where the boundary rotation saturates causality, which could be checked against other rotating AdS solutions with different asymptotics.
- The paper's own caveat that the endpoint is an educated guess could be tested numerically: a time-domain evolution of a perturbed small Kerr-AdS hole would show whether the trajectory in the $(a, r_+)$ plane actually terminates on $r_+ = sqrt{a,ell}$ or crosses it.
- The coexistence with outside radiation suggests the endpoint is not a vacuum solution but a hairy, radiation-dressed configuration; that would put it in the same family as other AdS black holes with scalar or gravitational hair, and its stability would itself need checking.
- The quoted first-order oblate deformation is a concrete prediction that a perturbative construction could confirm or refute independently of the dynamical argument.
Original abstract
It is now established that small Kerr-Anti-de Sitter (Kerr-AdS) black holes are unstable against scalar perturbations, via superradiant amplification mechanism. We show that small Kerr-AdS black holes are also unstable against gravitational perturbations and we compute the features of this instability. We also describe with great detail the evolution of this instability. In particular, we identify its endpoint state. It corresponds to a Kerr-AdS black hole whose boundary is an Einstein universe rotating with the light velocity. This black hole is expected to be slightly oblate and to co-exist in equilibrium with a certain amount of outside radiation.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
It is now established that small Kerr-Anti-de Sitter (Kerr-AdS) black holes are unstable against scalar perturbations, via superradiant amplification mechanism. We show that small Kerr-AdS black holes are also unstable against gravitational perturbations and we compute the features of this instability. We also describe with great detail the evolution of this instability. In particular, we identify its endpoint state. It corresponds to a Kerr-AdS black hole whose boundary is an Einstein universe rotating with the light velocity. This black hole is expected to be slightly oblate and to co-exist in equilibrium with a certain amount of outside radiation.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | It is now established that small Kerr-Anti-de Sitter (Kerr-AdS) black holes are unstable against scalar perturbations, via superradiant amplification mechanism. | background 1.00 | 0.05 | 0.08 | asserted 1.00 |
| s1 | We show that small Kerr-AdS black holes are also unstable against gravitational perturbations and we compute the features of this instability. | contribution 0.58 | 0.96 | 0.38 | asserted 1.00 |
| s2 | We also describe with great detail the evolution of this instability. | contribution 0.99 | 0.92 | 0.06 | asserted 0.99 |
| s3 | In particular, we identify its endpoint state. | contribution 0.64 | 0.93 | 0.16 | asserted 0.99 |
| s4 | It corresponds to a Kerr-AdS black hole whose boundary is an Einstein universe rotating with the light velocity. | result 1.00 | 0.83 | 0.52 | asserted 1.00 |
| s5 | This black hole is expected to be slightly oblate and to co-exist in equilibrium with a certain amount of outside radiation. | result 0.67 | 0.39 | 0.17 | hedged possibility 0.95 |
What was examined, and what was not
- arXiv listing abstract v2
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- relations to other papers
- P abstract
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References
How later papers used it
47 later papers cite it
1 paper by the same authors is listed below: 1 compared its own results with it.
Compares with it 1 of 4 papers with this label · 1 by the same authors
By the same authors
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Reported that its own numerical results for small black holes agree with this paper's analytic estimate derived assuming Mω_R ≪ 1.
Breit-Wigner resonances and the quasinormal modes of anti-de Sitter black holes (2009) (same authors) · 0.98 · paraphrased: the licence does not allow quoting
How the literature uses it
45 arXiv papers cite this in 67 sentences
Papers citing it · 42 sentences without a confident label
Uses its method or tool 1 paper
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We do this by matching asymptotic expansions of the solution in an intermediate region, see e.g., [ref] for asymptotically flat spacetimes and [ref] for asymptotically AdS.
Quasinormal modes of Proca and Maxwell fields in $d$-dimensional Schwarzschild-AdS black holes · Introduction · 0.93
Compares with it 4 papers
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We estimate the growth-timescale to be of order $4×10^6M$, which agrees with the estimate of Ref. [ref] and with frequency-domain calculations [ref] .
Black holes in anti-de Sitter: quasinormal modes, tails and tales of flat spacetime · Results · 0.98
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Our results have been benchmarked using previous results in the literature, specifically for scalar field perturbations of Kerr-AdS BHs [ref] .
Holographic thermalization, quasinormal modes and superradiance in Kerr-AdS · Numerical procedures · 0.99
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For small BHs our numerical results agree with the following analytic estimate, derived under the assumption that $Mω_R ≪1$ [ref] :
Breit-Wigner resonances and the quasinormal modes of anti-de Sitter black holes · Gravitational perturbations · 0.98
Cites it as support 1 paper
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Finally, we have have indications – directly in the time-domain – that small, rotating BHs in AdS are unstable against a superradiant mechanism in the sense that small non-axisymmetric fluctuations grow [ref] .
Black holes in anti-de Sitter: quasinormal modes, tails and tales of flat spacetime · Conclusions · 0.98
Cites it as background 18 papers
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This is usually known as a BH bomb or superradiant instability [ref] .
Black holes in anti-de Sitter: quasinormal modes, tails and tales of flat spacetime · Results · 0.84
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Recent prosperity of AdS spacetimes follows the historic claiming of the AdS/CFT duality [ref] , and for rotating AdS black holes, rotational superradiant instability has been widely studied [ref] .
Energy extraction from AdS black holes via superradiance · Introduction · 0.98
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Several different reflection mechanisms have been considered: for example, a “mirror” surrounding the hole imposed as a boundary condition [ref] , the mass of perturbation fields [ref] , nonminimal couplings [ref] , and the spacetime curvature itself for asymptotically AdS black holes [ref] .
Superradiant instability of the Kerr brane · Introduction · 0.95
Discussion
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