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arxiv: 2509.05700 · v3 · pith:SUKYIO5Vnew · submitted 2025-09-06 · ✦ hep-th · gr-qc

Entanglement Entropy and Thermodynamics of Dynamical Black Holes

Pith reviewed 2026-05-21 23:07 UTC · model grok-4.3

classification ✦ hep-th gr-qc
keywords dynamical black holesentanglement entropyf(R) gravityWald entropyreplica trickapparent horizongeneralized second lawphysical process first law
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The pith

In f(R) theories the dynamical black hole entropy equals Wald entropy on the generalized apparent horizon at first order.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that for dynamical black holes the thermodynamic entropy defined by Hollands-Wald-Zhang coincides with the Wald entropy computed on the generalized apparent horizon when only first-order perturbations are considered. Using the replica method to compute entanglement entropy, the authors demonstrate that selecting the apparent horizon as the entangling surface is the only choice that recovers this entropy and obeys the physical-process first law. They further reinterpret the generalized second law in terms of matter entanglement entropy across the apparent horizon, allowing the total entropy to be written as a renormalized generalized entropy at the leading area-law term.

Core claim

Under first-order perturbations, the dynamical black hole entropy in any f(R) theory equals the Wald entropy evaluated on the generalized apparent horizon. Only the apparent-horizon prescription in the replica method reproduces this entropy and satisfies the physical-process first law.

What carries the argument

The generalized apparent horizon used as the entangling surface in the replica trick for computing gravitational entropy in dynamical spacetimes.

If this is right

  • The apparent horizon must be used instead of the event horizon to obtain consistent entropy in evolving black holes.
  • The modified von Neumann entropy represents matter entanglement across the apparent horizon.
  • The total entropy reduces to the renormalized generalized entropy on the apparent horizon at leading order.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • This choice of surface may generalize to other higher-curvature theories beyond f(R).
  • Numerical simulations of perturbed black holes could test whether apparent-horizon replica entropy matches thermodynamic expectations.
  • Similar horizon prescriptions might apply to other dynamical systems like collapsing stars or cosmological horizons.

Load-bearing premise

The replica trick remains valid when applied to a dynamical spacetime with the apparent horizon chosen as the entangling surface and still satisfies the first law to linear order.

What would settle it

An explicit computation in a specific f(R) model where the event-horizon replica entropy fails to match the dynamical black hole entropy or violates the first law at linear order.

read the original abstract

We explore the thermodynamic and entanglement properties of dynamical black holes based on the recently proposed dynamical black hole entropy by Hollands-Wald-Zhang. We first provide direct proof that, under first-order perturbations, the dynamical black hole entropy in any $f(R)$ theory equals the Wald entropy evaluated on the generalized apparent horizon. Then, we compute the gravitational entropy explicitly from the replica method using both the event horizon and the apparent horizon as the entangling surfaces, and we show that only the apparent horizon prescription reproduces the correct dynamical black hole entropy satisfying the physical process first law. Furthermore, we reinterpret the generalized second law by identifying the modified von Neumann entropy as the matter entanglement across the apparent horizon. This allows us to express the total entropy as the renormalized generalized entropy evaluated on this surface at the level of the leading local area-law term.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The paper claims to provide a direct proof that, under first-order perturbations, the dynamical black hole entropy of Hollands-Wald-Zhang equals the Wald entropy evaluated on the generalized apparent horizon in any f(R) theory. It then computes gravitational entropy via the replica method, comparing event-horizon and apparent-horizon choices as entangling surfaces, and shows that only the apparent-horizon prescription reproduces the dynamical entropy while satisfying the physical-process first law at linear order. It further reinterprets the generalized second law by equating the modified von Neumann entropy to matter entanglement across the apparent horizon, expressing the total entropy as the renormalized generalized entropy on that surface at leading order.

Significance. If the central claims hold, the work is significant because it supplies an explicit bridge between the replica-trick definition of entanglement entropy and the Hollands-Wald-Zhang dynamical entropy in modified gravity, thereby furnishing evidence that the apparent horizon is the physically preferred surface for thermodynamic relations in non-stationary spacetimes. The direct proof of equality to Wald entropy and the first-law match at linear order constitute concrete, falsifiable statements that can be checked against existing constructions.

major comments (1)
  1. [Abstract, paragraph 2] Abstract, paragraph 2 and the replica-method derivation: the claim that selecting the apparent horizon as the entangling surface allows a consistent n-fold cover, analytic continuation, and extraction of the linear entropy variation requires explicit verification that the conical singularity produces no O(ε²) surface terms or stress-tensor contributions that would invalidate the first-law match. Because the apparent horizon is not a Killing horizon, the standard Euclidean replica construction does not apply directly; the manuscript must demonstrate that all higher-order corrections vanish at the order needed for the physical-process first law.
minor comments (1)
  1. [Notation and definitions] Clarify the precise definition of the generalized apparent horizon used in the Wald-entropy comparison and ensure it is stated identically in the proof and in the replica calculation.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for the constructive major comment. The point concerning the justification of the replica construction on the apparent horizon is well taken, and we will revise the paper to supply the requested explicit verification at linear order.

read point-by-point responses
  1. Referee: [Abstract, paragraph 2] Abstract, paragraph 2 and the replica-method derivation: the claim that selecting the apparent horizon as the entangling surface allows a consistent n-fold cover, analytic continuation, and extraction of the linear entropy variation requires explicit verification that the conical singularity produces no O(ε²) surface terms or stress-tensor contributions that would invalidate the first-law match. Because the apparent horizon is not a Killing horizon, the standard Euclidean replica construction does not apply directly; the manuscript must demonstrate that all higher-order corrections vanish at the order needed for the physical-process first law.

    Authors: We agree that the apparent horizon is not a Killing horizon and that the standard Euclidean replica construction therefore requires additional care in the dynamical case. Our derivation is performed entirely within a first-order perturbative expansion around a stationary background, in which the apparent horizon coincides with the event horizon at zeroth order. Within this framework the n-fold cover is constructed perturbatively, the conical singularity is introduced at linear order in the replica parameter, and the entropy variation is extracted from the coefficient of the linear term. We will revise the manuscript to add an explicit perturbative calculation (in a new subsection or appendix) demonstrating that all O(ε²) contributions from the conical singularity to the surface terms and to the stress-tensor expectation value vanish identically at the order relevant for the physical-process first law. This will make the linear-order match with the Hollands-Wald-Zhang entropy fully rigorous. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation is self-contained against external benchmarks

full rationale

The paper builds directly on the external Hollands-Wald-Zhang dynamical black hole entropy definition (cited as prior independent work) and performs explicit first-order perturbation proofs for f(R) theories plus replica-method comparisons between event and apparent horizons. No load-bearing step reduces by construction to a self-definition, fitted input renamed as prediction, or self-citation chain from the current authors. The replica-trick application to dynamical spacetimes is treated as a calculational choice whose validity is checked against the external entropy and first-law condition rather than assumed to force the outcome. This is the most common honest finding for papers that reference external constructions while adding independent computations.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The paper rests on the validity of the replica trick in dynamical spacetimes, the definition of dynamical black hole entropy from Hollands-Wald-Zhang, and standard assumptions of f(R) gravity at linear order.

axioms (2)
  • domain assumption The replica trick yields the correct gravitational entropy when the entangling surface is chosen as the apparent horizon.
    Invoked when the authors compute entropy on the apparent horizon and compare to the dynamical definition.
  • domain assumption First-order perturbation theory is sufficient to compare dynamical entropy with Wald entropy.
    Stated in the first result of the abstract.

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discussion (0)

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Reference graph

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