Entanglement Entropy and Thermodynamics of Dynamical Black Holes
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.