REVIEW
Topological black holes dressed with a conformally coupled scalar field and electric charge
High Energy Physics - Theory · Submitted 1 Dec 2005 · 62 references
Pith's verdict: A clean charged hairy AdS black hole with a non-area-law entropy and a tidy entropy-positivity/cosmic-censorship map, but the grand-canonical first law is never verified and my check suggests it may not hold for G ≠ 1.
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Referee report
The paper
The paper claims
An electrically charged black hole can carry a conformally coupled scalar field, and its entropy is not one quarter of the horizon area but that area divided by a scalar-dependent effective Newton constant, with the positivity bound on that
The reading
What carries the argument
The load-bearing object is the effective Newton constant at the horizon, $1/G_+ = (1/G)(1 - 4pi G phi_+^2/3)$, which is precisely the conformal factor $hat{g}_{munu} = (1 - 4pi G phi^2/3) g_{munu}$ of the map to the Einstein frame, together with the field redefinition $Psi = sqrt{3/(4pi G)}, tanh^{-1}sqrt{4pi G/3}, phi$. Because the same combination of the scalar appears both as the thing that rescales the entropy and as the thing that decides where the conformal frame degenerates, the requirement $S > 0$ and the requirement that the Einstein-frame geometry not degenerate into a naked singularity are literally the same inequality. The Euclidean reduced action, with its boundary term fixed by the Regge-Teitelboim variation, is the technical vehicle that produces $S = A_+/(4G_+)$ from an on-shell action rather than by assumption.
Extended reading notes
Core claim
For the hyperbolic-horizon branch ($gamma = -1$, $Lambda = -3/l^2$) the metric $ds^2 = -[r^2/l^2 - (1 + Gmu/r)^2]dt^2 + [r^2/l^2 - (1 + Gmu/r)^2]^{-1}dr^2 + r^2 dsigma^2$, with scalar $phi = sqrt{1/(2alpha l^2)}, Gmu/(r + Gmu)$ and potential $A = -(q/r)dt$, solves the Einstein-Maxwell equations with a conformally coupled, quartically self-interacting scalar, subject to the constraint $q^2 = -gamma G mu^2 (1 + 2pi Lambda G/(9alpha))$ that ties charge to mass. The authors show the scalar is regular outside the horizon, that the mass may be negative but is bounded below ($Gmu ge -l/4$), and that in the negative-mass window the causal structure contains three horizons and describes a black hole inside a black hole, with a timelike scalar singularity that is nevertheless geometrically regular. Evaluating the Euclidean action in the grand canonical ensemble gives $M = sigma mu/4pi$, $Q = sigma q/4pi$ and the non-area-law entropy $S = A_+/(4G_+)$; positivity of that entropy yields a window on $mu$, and the paper argues that under the conformal map to the Einstein frame this same window is exactly the condition that the mapped solution is not a naked singularity. A companion $alpha = 0$ solution gives a massless charged hairy black hole with $r_+ = l$, $M = 0$ and $S = (sigma l^2/4)(1/G - q^2 l^2)$, whose positivity bound $Gq^2/l^2 < 1$ plays the same double role.
Load-bearing premise
The thermodynamics is read off from a formal variation of the boundary term, but the solution family is one-parameter — the constraint $q^2 = -gamma G mu^2 (1 + 2pi Lambda G/(9alpha))$ ties charge to mass — so temperature and potential cannot really be varied independently, and the paper never writes down or checks the first law $dM = T,dS + Phi,dQ$ along that constrained family.
Editorial extensions
If this is right
- Charged, asymptotically locally AdS black holes with hyperbolic horizons exist in this theory alongside the Reissner-Nordstrom-AdS branch with the same boundary geometry, and the two branches meet only at vanishing mass and charge.
- The entropy of a black hole with a non-minimally coupled scalar is a quarter of the horizon area measured in units of a scalar-dependent effective Newton constant, so the area law is recovered only in the limit where the scalar vanishes at the horizon.
- Negative-mass black holes in this family have a causal structure with an inner and an outer event horizon, so a timelike observer can cross the outer horizon and still choose among several futures, including escaping to a mirror asymptotic region.
- The $alpha = 0$ branch produces a massless, electrically charged hairy black hole with $r_+ = l$ whose entropy still depends on the stealth scalar through the effective Newton constant.
- Bounds obtained from demanding positive entropy in the conformal frame coincide with bounds that forbid naked singularities in the minimally coupled Einstein frame.
Reading between the lines
- The entropy formula suggests a general rule worth testing: whenever a scalar couples conformally, replace $G$ by $G_+$ at the horizon and the first law may still close, which would let one read off thermodynamic stability without redoing the Euclidean integral.
- The 'black hole inside a black hole' causal structure raises a concrete question the paper sets aside: whether the inner horizon contributes an extra channel of Hawking radiation to asymptotic observers, which would make the single-horizon thermodynamic treatment an approximation rather than the full answer.
- Because the conformal factor vanishes exactly where the effective Newton constant changes sign, the map between frames acts as a dictionary in which a negative-entropy solution is not merely unphysical but geometrically singular; this could be turned into a practical diagnostic for other conformally coupled hairy solutions.
- The stealth solutions, whose stress-energy tensor vanishes while the scalar is nontrivial, suggest that scalar profiles can be invisible to the geometry but not to the entropy, a distinction worth checking in other dimensions.
Original abstract
Electrically charged solutions for gravity with a conformally coupled scalar field are found in four dimensions in the presence of a cosmological constant. If a quartic self-interaction term for the scalar field is considered, there is a solution describing an asymptotically locally AdS charged black hole dressed with a scalar field that is regular on and outside the event horizon, which is a surface of negative constant curvature. This black hole can have negative mass, which is bounded from below for the extremal case, and its causal structure shows that the solution describes a "black hole inside a black hole". The thermodynamics of the non-extremal black hole is analyzed in the grand canonical ensemble. The entropy does not follow the area, and there is an effective Newton constant which depends on the value of the scalar field at the horizon. If the base manifold is locally flat, the solution has no electric charge, and the scalar field has a vanishing stress-energy tensor so that it dresses a locally AdS spacetime with a nut at the origin. In the case of vanishing selfinteraction, the solutions also dress locally AdS spacetimes, and if the base manifold is of negative constant curvature a massless electrically charged hairy black hole is obtained. The thermodynamics of this black hole is also analyzed. It is found that the bounds for the black holes parameters in the conformal frame obtained from requiring the entropy to be positive are mapped into the ones that guarantee cosmic censorship in the Einstein frame.
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
Electrically charged solutions for gravity with a conformally coupled scalar field are found in four dimensions in the presence of a cosmological constant. If a quartic self-interaction term for the scalar field is considered, there is a solution describing an asymptotically locally AdS charged black hole dressed with a scalar field that is regular on and outside the event horizon, which is a surface of negative constant curvature. This black hole can have negative mass, which is bounded from below for the extremal case, and its causal structure shows that the solution describes a "black hole inside a black hole". The thermodynamics of the non-extremal black hole is analyzed in the grand canonical ensemble. The entropy does not follow the area, and there is an effective Newton constant which depends on the value of the scalar field at the horizon. If the base manifold is locally flat, the solution has no electric charge, and the scalar field has a vanishing stress-energy tensor so that it dresses a locally AdS spacetime with a nut at the origin. In the case of vanishing selfinteraction, the solutions also dress locally AdS spacetimes, and if the base manifold is of negative constant curvature a massless electrically charged hairy black hole is obtained. The thermodynamics of this black hole is also analyzed. It is found that the bounds for the black holes parameters in the conformal frame obtained from requiring the entropy to be positive are mapped into the ones that guarantee cosmic censorship in the Einstein frame.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | Electrically charged solutions for gravity with a conformally coupled scalar field are found in four dimensions in the presence of a cosmological constant. | contribution 0.54 | 0.91 | 0.14 | asserted 1.00 |
| s1 | If a quartic self-interaction term for the scalar field is considered, there is a solution describing an asymptotically locally AdS charged black hole dressed with a scalar field that is regular on and outside the event horizon, which is a surface of negative constant curvature. | result 0.85 | 0.88 | 0.26 | asserted 0.98 |
| s2 | This black hole can have negative mass, which is bounded from below for the extremal case, and its causal structure shows that the solution describes a "black hole inside a black hole". | result 0.92 | 0.88 | 0.21 | asserted 0.77 |
| s3 | The thermodynamics of the non-extremal black hole is analyzed in the grand canonical ensemble. | contribution 0.57 | 0.91 | 0.05 | asserted 0.99 |
| s4 | The entropy does not follow the area, and there is an effective Newton constant which depends on the value of the scalar field at the horizon. | result 1.00 | 0.88 | 0.11 | asserted 1.00 |
| s5 | If the base manifold is locally flat, the solution has no electric charge, and the scalar field has a vanishing stress-energy tensor so that it dresses a locally AdS spacetime with a nut at the origin. | result 0.99 | 0.86 | 0.05 | asserted 1.00 |
| s6 | In the case of vanishing selfinteraction, the solutions also dress locally AdS spacetimes, and if the base manifold is of negative constant curvature a massless electrically charged hairy black hole is obtained. | result 0.95 | 0.87 | 0.09 | asserted 1.00 |
| s7 | The thermodynamics of this black hole is also analyzed. | contribution 0.68 | 0.89 | 0.04 | asserted 0.89 |
| s8 | It is found that the bounds for the black holes parameters in the conformal frame obtained from requiring the entropy to be positive are mapped into the ones that guarantee cosmic censorship in the Einstein frame. | result 0.87 | 0.93 | 0.40 | asserted 1.00 |
What was examined, and what was not
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Figures
Figures reproduced from arXiv:hep-th/0512022 by the authors.
References
How the literature uses it
52 arXiv papers cite this in 105 sentences
Papers citing it · 68 sentences without a confident label
Builds on it 2 papers
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We begin by considering the case of a conformally coupled scalar field, presenting the non-Abelian extension of the MTZ black hole [ref] .
(Super-)renormalizable hairy meronic black holes · Discussion · 0.93
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Par exemple, en utilisant les techniques d�crites dans ce chapitre, il est possible de construire en dimension quatre des trous noirs axioniques en th�orie d'Einstein-Maxwell-AdS en pr�sence d'un champ scalaire conform�ment coupl� � la gravitation et avec deux champs axioniques, conduisant ainsi � une axionisation des trous noirs de [ref] .
Black hole solutions of modified gravity theories · Conclusion · 0.98
Uses its method or tool 1 paper
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Here we observe a linear relation between temperature and heat capacity [equation] with the entropy for the MTZ black hole [equation] However, since the conformal coupling term appears in the action, the entropy should be obtained by making use of the Euclidean action approach [ref] and the Wald method [ref] as [equation] where an effective Newtonian constant at the event horizon is given by [equation] The positivity of entropy $S_M ≥0$ requires a bound [equation] It requires lower bounds for being the positive entropy.
Holographic superconductor in the exact hairy black hole · CMTZ black holes · 0.84
Compares with it 8 papers
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Les solutions que nous pr�sentons ici poss�dent un horizon plat, un cheveu scalaire secondaire et partagent des propri�t�s quasiment identiques � celles des trous noirs avec horizons hyperboliques �tudi�s dans [ref] .
Black hole solutions of modified gravity theories · Introduction · 0.99
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The causal structure of the solution is notoriously enhanced with respect to the MTZ family [ref] including regular black holes, wormholes and cosmological spacetimes with bounces[The vanishing cosmological constant case was early discussed in [ref] and it represents a traversable wormhole.].
AdS-Taub-NUT spacetimes and exact black bounces with scalar hair · Introduction · 0.93
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The temperature $T_H$ is identical to the Hawking temperature of the MTZ black hole as determined in [ref] .
Hawking Radiation via Gravitational Anomalies in Non-spherical Topologies · Hawking Radiation from a TBH Conformally Coupled to a Scalar Field · 0.86
Cites it as background 21 papers
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Fortunately, the charged MTZ (CMTZ) black hole was found with scalar hair when the coefficient $α$ of $ϕ^4$-term is between 0 and $2πG/3l^2$ [ref] , where the upper bound corresponds to the MTZ black hole [ref] and the lower bound is not allowed.
Holographic superconductor in the exact hairy black hole · Introduction · 0.81
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In particular, when gravitational theories are supplemented with conformally coupled scalar fields, a broad class of solutions has been constructed [ref] , including gravitational instantons [ref] .
(Super-)renormalizable hairy meronic black holes · Introduction · 0.80
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In the case of a negative cosmological constant, stable solutions were found numerically for spherical geometries [ref] and an exact solution in asymptotically AdS space with hyperbolic geometry was presented in [ref] and generalized later to include charge [ref] .
A New Class of Exact Hairy Black Hole Solutions · Introduction · 0.93
Discussion
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