REVIEW
Entanglement entropy in higher derivative holography
High Energy Physics - Theory · Submitted 29 May 2013 · 35 references
Pith's verdict: A useful, honest check that Lewkowycz-Maldacena reproduces the Gauss-Bonnet area functional — but the 'regime' where it works is asserted rather than derived, and the general four-derivative claim is conditional on assumptions the authors themselves call problematic.
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Referee report
The paper
The paper claims
Higher-derivative gravity gets one consistent entropy rule, but only for Gauss-Bonnet
The reading
What carries the argument
The load-bearing object is the modified area functional $S_{JM}$, which replaces the Ryu–Takayanagi area by $∫d^3x√Σ(1 + λL^2 R)$ — the area of the entangling surface plus a Gauss-Bonnet coupling times the Ricci scalar of the induced three-dimensional metric. Varying it with respect to the embedding coordinates, with the variation of the induced metric and the identity $∇_i(δL/δΣ_{ij}) = 0$, produces the surface equation $K + λL^2(RK - 2R^{ij}K_{ij}) = 0$. The second piece of machinery is the Lewkowycz–Maldacena expansion of the five-dimensional metric in Gaussian normal coordinates around the surface, $g_{ij} = h_{ij} + w{}^{(w)}K_{ij} + O(w^2)$, with $ρ = -(ε/2)log(x_1^2+x_2^2)$; the coefficients of $ε/w$ in the $ww$ component of the equations of motion then have to vanish, and for Gauss-Bonnet they vanish precisely into the same combination that $S_{JM}$ extremization produces. The third piece is the Brown-York stress tensor of Gauss-Bonnet gravity with its known Gibbons-Hawking surface term, whose $tt$ component reduces, after a 3+1 split, to $K + λL^2[(RK - 2R^{ij}K_{ij}) + (1/3)(-K^3 + 3KK_2 - 2K_3)]$.
Extended reading notes
Core claim
The central claim is that three independent constructions of the entangling surface in Gauss-Bonnet gravity agree. Extremizing the Jacobson–Myers/Hung–Myers–Smolkin area functional $S_{JM} = 2π/ℓ_P^3 ∫ d^3x √Σ (1 + λL^2 R)$ gives $K + λL^2(RK - 2R^{ij}K_{ij}) = 0$; setting the $tt$ component of the Brown-York tensor to zero on the co-dimension one surface gives the same equation up to $O(K^3)$ terms; and demanding that the $n = 1+ε$ replica solution satisfy the linearized Gauss-Bonnet equations of motion near the conical singularity gives exactly the same equation, $^{(w)}K + λL^2[^{(w)}K R - 2R_{ij}{}^{(w)}K^{ij}] = 0$. The agreement holds in the regime where the extrinsic curvature is small, so that $O(K^3)$ terms and the divergence structure in the mixed and transverse components can be dropped. Extending the same analysis to a general four-derivative Lagrangian, the authors find divergences of order $ε/w^2$ in the $ww$ component, driven by $∇∇R$ terms, that survive the approximation and produce an equation containing $∇^2{}^{(w)}K$; they state they have not been able to find an area functional yielding it and consider it unlikely that one exists. Finally, evaluating the Euclidean action outside a spherical entangling surface, the $O(1/ε^4)$ and $O(1/ε^2)$ divergences cancel only when a term proportional to $S_{JM}$ is added, with coefficients $α = κ = 1$; what remains is $4π^2L^3(1-6f_∞λ)/(f_∞^{3/2}ℓ_P^3) ln(z_0/ε)$, the universal piece of the entanglement entropy.
Load-bearing premise
The derivation assumes the extrinsic curvature of the entangling surface is small enough that $O(K^3)$ terms and the divergences in the mixed and transverse components of the equations of motion can be dropped, an approximation the paper invokes but does not quantify.
Editorial extensions
If this is right
- For Gauss-Bonnet holography, the modified area functional is the correct entropy functional in the small-extrinsic-curvature regime, since a first-principles replica derivation lands on the same surface equation.
- The universal coefficients of four-dimensional entanglement entropy — proportional to $a$ for a sphere and to $c$ for a cylinder — are reproduced by $S_{JM}$ through the logarithmic terms $S_{EE} = -4a\ln(z_0/ε)$ and $S_{EE} = -(c/2)(H/z_0)\ln(z_0/ε)$.
- The area functional can be reinterpreted as a counterterm: adding it to the Euclidean action of Gauss-Bonnet gravity removes all power-law divergences, so the finite part of the action is the universal entanglement entropy, in analogy with $TI_{tot} = E - TS_{EE}$.
- At four-derivative order, only Gauss-Bonnet appears to admit an area functional; a general combination of $R^2$, $R_{μν}R^{μν}$ and $R_{μνρσ}R^{μνρσ}$ gives a surface equation with a $\nabla^2 K$ term that no local geometric functional of the induced metric reproduces.
- The equivalence between the minimal-surface condition and $T^{BY}_{tt} = 0$ holds in Gauss-Bonnet only up to $O(K^3)$ terms; the residual difference is $2f_∞λ(z + f(z)f'(z))(f'(z) + f'(z)^3 - zf''(z))$ for the sphere.
Reading between the lines
- If the small-extrinsic-curvature restriction is the real boundary of the method rather than a technical convenience, then holographic entanglement entropy for small regions (large extrinsic curvature at the turning point) in Gauss-Bonnet gravity is not yet derived from first principles, and the agreement found here is a large-surface statement only.
- The counterterm reading of $S_{JM}$ suggests a diagnostic for other higher-derivative theories: any candidate entropy functional should be the object whose addition cancels power-law divergences in the on-shell Euclidean action, which is a check independent of the replica construction.
- The authors' remark that a field redefinition can recast four-derivative terms into Gauss-Bonnet order-by-order in the coupling hints that the obstruction found for Weyl-squared may be an artifact of treating the coupling non-perturbatively; a perturbative expansion might restore an area functional at each order.
- The failure to find an area functional for general four-derivative gravity, if confirmed, would mean that entanglement entropy in such theories is not determined by the induced geometry alone, paralleling how Wald entropy for black holes depends on the full curvature tensor and not just on area.
Original abstract
We consider holographic entanglement entropy in higher derivative gravity theories. Recently Lewkowycz and Maldacena arXiv:1304.4926 have provided a method to derive the equations for the entangling surface from first principles. We use this method to compute the entangling surface in four derivative gravity. Certain interesting differences compared to the two derivative case are pointed out. For Gauss-Bonnet gravity, we show that in the regime where this method is applicable, the resulting equations coincide with proposals in the literature as well as with what follows from considerations of the stress tensor on the entangling surface. Finally we demonstrate that the area functional in Gauss-Bonnet holography arises as a counterterm needed to make the Euclidean action free of power law divergences.
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
We consider holographic entanglement entropy in higher derivative gravity theories. Recently Lewkowycz and Maldacena arXiv:1304.4926 have provided a method to derive the equations for the entangling surface from first principles. We use this method to compute the entangling surface in four derivative gravity. Certain interesting differences compared to the two derivative case are pointed out. For Gauss-Bonnet gravity, we show that in the regime where this method is applicable, the resulting equations coincide with proposals in the literature as well as with what follows from considerations of the stress tensor on the entangling surface. Finally we demonstrate that the area functional in Gauss-Bonnet holography arises as a counterterm needed to make the Euclidean action free of power law divergences.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | We consider holographic entanglement entropy in higher derivative gravity theories. | background 0.72 | 0.58 | 0.04 | asserted 0.64 |
| s1 | Recently Lewkowycz and Maldacena arXiv:1304.4926 have provided a method to derive the equations for the entangling surface from first principles. | background 0.97 | 0.03 | 0.03 | asserted 1.00 |
| s2 | We use this method to compute the entangling surface in four derivative gravity. | method 0.53 | 0.95 | 0.11 | asserted 0.99 |
| s3 | Certain interesting differences compared to the two derivative case are pointed out. | result 0.89 | 0.77 | 0.08 | asserted 0.87 |
| s4 | For Gauss-Bonnet gravity, we show that in the regime where this method is applicable, the resulting equations coincide with proposals in the literature as well as with what follows from considerations of the stress tensor on the entangling surface. | result 1.00 | 0.89 | 0.22 | asserted 1.00 |
| s5 | Finally we demonstrate that the area functional in Gauss-Bonnet holography arises as a counterterm needed to make the Euclidean action free of power law divergences. | result 0.95 | 0.93 | 0.28 | asserted 1.00 |
What was examined, and what was not
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- relations to other papers
- P abstract
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Figures
Figures reproduced from arXiv:1305.6694 by the authors.
References
How the literature uses it
52 arXiv papers cite this in 83 sentences
Papers citing it · 51 sentences without a confident label
Contests it 2 papers
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This argument would lead to the proposal ([ref]) beyond the spherical shape [ref] [The subsequent papers [ref] [ref] point out there are some mismatches for higher derivative gravity theory, but the mismatch is removed by the paper [ref] ].
Holographic R\'enyi Entropy and Generalized Entropy method · Introduction · contest 0.71
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We will explain that a modification to the order of limits needs to be done to the use of the LM method in [ref] for deriving the surface equation.
On generalized gravitational entropy, squashed cones and holography · Introduction · contest 0.80
Uses its method or tool 2 papers
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In order to proceed, we could assume the following as in [ref] that $O() ∼αr/ϵ$ where $α≪1$, then the $ij$, $ir$ components go to zero.
On generalized gravitational entropy, squashed cones and holography · Equation for the entangling surface · 0.91
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Here we will use the gravitational entropy proposed by Dong [ref] and Camps [ref] , which motivated to describe the gravitational dual of the entanglement entropy of CFT in the AdS/CFT setup (see also [ref] ).
Cutoff Scale of Quadratic Gravity from Quantum Focusing Conjecture · Gravitational Entropy · 0.80
Compares with it 3 papers
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Then the difference ([ref]) becomes -2ł[ K_(2) K_(2) K_(2) -3K_(2) K_(2)ij K_(2)^ij +2K_(2)ij K_(2)^jk K_(2)k^(2)i ], which is in accord with the result in [ref] .
Note on generalized gravitational entropy in Lovelock gravity · Further investigation · 0.81
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* We also showed that the entangling surface equations are the same as what comes from the JM entropy functional without the small extrinsic curvature condition needed in [ref] .
On generalized gravitational entropy, squashed cones and holography · Discussion · 0.88
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In [ref] the same approach was investigated for Gauss-Bonnet gravity, and it was shown that demanding $T_tt=0$ imposes the following equation on the profile of the co-dimension one hypersurface: [equation] Although not equal, this equation bears a remarkable similarity with the divergent part of the $qq$-component [ref] derived above from the conical boundary condition approach.
Conditions on holographic entangling surfaces in higher curvature gravity · Testing the conditions · 0.96
Cites it as background 21 papers
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Remarkably enough, a general formula for any theory involving arbitrary contractions of the Riemann tensor $ℒ(R_μνρσ)$, which seems to satisfy several consistency checks, has been recently proposed by Dong [ref] (see also, e.g. [ref] )).
Higher-curvature corrections to holographic entanglement entropy in geometries with hyperscaling violation · Introduction · 0.84
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Interest in CFTs with bulk Lovelock duals extends to holographic computations of entanglement entropy [ref] .
Extended First Law for Entanglement Entropy in Lovelock Gravity · Introduction · 0.89
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In adapted coordinates —see subsection [ref] below for our conventions— these two terms read[Interesting additional developments and explorations include [ref] .][Generalizations of dongf to the case in which covariant derivatives of the Riemann appear in the action have also been presented [ref] .] [equation] In principle, the generalized holographic surface $Γ_A$ should be obtained by extremizing the new functional [ref] .
Holographic entanglement entropy for perturbative higher-curvature gravities · Introduction · 0.89
What it cites · 29 sentences without a confident label
Contests 2 papers
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Fursaev suggested a derivation [ref] of the Ryu-Takayanagi prescription which appeared to have some problems since it led to the wrong Renyi entropies [ref] (there is a proposed resolution to this in [ref] ).
Entanglement Renyi entropies in holographic theories · Introduction · contest 0.87
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Fursaev suggested a derivation [ref] of the Ryu-Takayanagi prescription which appeared to have some problems since it led to the wrong Renyi entropies [ref] (there is a proposed resolution to this in [ref] ).
Entanglement Renyi Entropies in Conformal Field Theories and Holography · Introduction · contest 0.87
Uses the method or tool of 2 papers
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Since the metric perturbation is periodic, we will need to argue that the contribution from the perturbations are zero [ref] .
Generalized gravitational entropy · Generalized gravitational entropy method · 0.83
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At this stage, following [ref] , we choose the following coordinate system [equation] where $y^l$ denotes the two transverse directions to the surface.
A Covariant Holographic Entanglement Entropy Proposal · General minimal surface condition · 0.95
Compares with 3 papers
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In particular they were in agreement with [ref] .
Holographic Entanglement Entropy in Lovelock Gravities · Discussion · 0.99
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In particular they were in agreement with [ref] .
On Holographic Entanglement Entropy and Higher Curvature Gravity · Discussion · 0.99
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Further in this regime they also agreed with the $T_tt^BY=0$ condition [ref] .
Entanglement entropy from the holographic stress tensor · Discussion · 0.98
Cites as background 7 papers
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In the context of holography, Ryu and Takayanagi [ref] proposed a remarkably simple prescription to compute entanglement entropy (EE) for the field theory dual to Einstein gravity.
Holographic Entanglement Entropy: An Overview · Introduction · 0.98
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Recently, Lewkowycz and Maldacena [ref] have proposed a way to set up the calculation of entanglement entropy in what appears to be a self-consistent manner.
Generalized gravitational entropy · Introduction · 0.84
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For Gauss-Bonnet gravity, $λ_1=λ ,λ_2=-4λ , λ_3=λ .$ The equations of motion (these can be found in many places, eg [ref] ) are, G_αβ-6/L^2g_αβ- L^2/2H_αβ=T_αβ where, [equation] and [equation] [equation] [equation] Here $∇̂$ is the usual covariant derivative made of the full five dimensional metric.
Effect of curvature squared corrections in AdS on the viscosity of the dual gauge theory · Generalized gravitational entropy method · 0.87
Discussion
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