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1. Beyond Hawking evaporation of black holes formed by dark matter in compact stars

   https://doi.org/10.1103/PhysRevD.111.L041306  

   Basumatary, Ujjwal; Raj, Nirmal; Ray, Anupam (February 2025, Physical Review D)

   The memory burden effect is an explicit resolution to the information paradox by which an evaporating black hole acquires quantum hair, which then suppresses its rate of mass loss with respect to the semiclassical Hawking rate. We show that this has significant implications for particle dark matter that captures in neutron stars and forms black holes that go on to consume the host star. In particular, we show that constraints on the nucleon scattering cross section and mass of spin-0 and spin- $1/2$dark matter would be extended by several orders of magnitude. Published by the American Physical Society2025 

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2. Dark dimension, the swampland, and the dark matter fraction composed of primordial near-extremal black holes

   https://doi.org/10.1103/PhysRevD.109.095008  

   Anchordoqui, Luis A; Antoniadis, Ignatios; Lüst, Dieter (May 2024, Physical Review D)

   In a recent publication we studied the decay rate of primordial black holes perceiving the dark dimension, an innovative five-dimensional (5D) scenario that has a compact space with characteristic length scale in the micron range. We demonstrated that the rate of Hawking radiation of 5D black holes slows down compared to 4D black holes of the same mass. Armed with our findings we showed that for a species scale of $\mathcal{O}\left({10}^{10}\mathrm{GeV}\right)$, an all-dark-matter interpretation in terms of primordial black holes should be feasible for black hole masses in the range ${10}^{14}\lesssim M/\mathrm{g}\lesssim {10}^{21}$. As a natural outgrowth of our recent study, herein we calculate the Hawking evaporation of near-extremal 5D black holes. Using generic entropy arguments we demonstrate that Hawking evaporation of higher-dimensional near-extremal black holes proceeds at a slower rate than the corresponding Schwarzschild black holes of the same mass. Assisted by this result we show that if there were 5D primordial near-extremal black holes in nature, then a primordial black hole all-dark-matter interpretation would be possible in the mass range ${10}^5\sqrt{\beta }\lesssim M/\mathrm{g}\lesssim {10}^{21}$, where $\beta$is a parameter that controls the difference between mass and charge of the associated near-extremal black hole. Published by the American Physical Society2024 

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3. On the lapse contour in the gravitational path integral

   https://doi.org/10.1103/PhysRevD.111.066014  

   Banihashemi, Batoul; Jacobson, Ted (March 2025, Physical Review D)

   The gravitational path integral is usually implemented with a covariant action by analogy with other gauge field theories, but the gravitational case is different in important ways. A key difference is that the integrand has an essential singularity, which occurs at zero lapse where the spacetime metric degenerates. The lapse integration contour required to impose the local time reparametrization constraints must run from $-\infty$to $+\infty$, yet must not pass through zero. This raises the question: for an application—such as a partition function—where the constraints should be imposed, what is the correct integration contour, and why? We study that question by starting with the reduced phase space path integral, which involves no essential singularity. We observe that if the momenta are to be integrated before the lapse, to obtain a configuration space path integral, the lapse contour should pass below the origin in the complex lapse plane. This contour is also consistent with the requirement that quantum field fluctuation amplitudes have the usual short distance vacuum form, and with obtaining the Bekenstein-Hawking horizon entropy from a Lorentzian path integral. Published by the American Physical Society2025 

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4. Entropy of dynamical black holes

   https://doi.org/10.1103/PhysRevD.110.024070  

   Hollands, Stefan; Wald, Robert M; Zhang, Victor G (July 2024, Physical Review D)

   We propose a new formula for the entropy of a dynamical black hole—valid to leading order for perturbations off of a stationary black hole background—in an arbitrary classical diffeomorphism covariant Lagrangian theory of gravity in $n$dimensions. In stationary eras, this formula agrees with the usual Noether charge formula, but in nonstationary eras, we obtain a nontrivial correction term. In particular, in general relativity, our formula for the entropy of a dynamical black hole differs from the standard Bekenstein-Hawking formula $A/4$by a term involving the integral of the expansion of the null generators of the horizon. We show that, to leading perturbative order, our dynamical entropy in general relativity is equal to $1/4$of the area of the apparent horizon. Our formula for entropy in a general theory of gravity is obtained from the requirement that a “local physical process version” of the first law of black hole thermodynamics hold for perturbations of a stationary black hole. It follows immediately that for first order perturbations sourced by external matter that satisfies the null energy condition, our entropy obeys the second law of black hole thermodynamics. For vacuum perturbations, the leading-order change in entropy occurs at second order in perturbation theory, and the second law is obeyed at leading order if and only if the modified canonical energy flux is positive (as is the case in general relativity but presumably would not hold in more general theories of gravity). Our formula for the entropy of a dynamical black hole differs from a formula proposed independently by Dong and by Wall. We obtain the general relationship between their formula and ours. We then consider the generalized second law in semiclassical gravity for first order perturbations of a stationary black hole. We show that the validity of the quantum null energy condition (QNEC) on a Killing horizon is equivalent to the generalized second law using our notion of black hole entropy but using a modified notion of von Neumann entropy for matter. On the other hand, the generalized second law for the Dong-Wall entropy is equivalent to an integrated version of QNEC, using the unmodified von Neumann entropy for the entropy of matter. Published by the American Physical Society2024 

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5. Scalarization of isolated black holes in scalar Gauss-Bonnet theory in the fixing-the-equations approach

   https://doi.org/10.1103/PhysRevD.110.024033  

   Lara, Guillermo; Pfeiffer, Harald P; Wittek, Nikolas A; Vu, Nils L; Nelli, Kyle C; Carpenter, Alexander; Lovelace, Geoffrey; Scheel, Mark A; Throwe, William (July 2024, Physical Review D)

   One of the most promising avenues to perform numerical evolutions in theories beyond general relativity is the approach, a proposal in which new “driver” equations are added to the evolution equations in a way that allows for stable numerical evolutions. In this direction, we extend the numerical relativity code p to evolve a “fixed” version of scalar Gauss-Bonnet theory in the decoupling limit, a phenomenologically interesting theory that allows for hairy black hole solutions in vacuum. We focus on isolated black hole systems both with and without linear and angular momentum, and propose a new driver equation to improve the recovery of such stationary solutions. We demonstrate the effectiveness of the latter by numerically evolving black holes that undergo spontaneous scalarization using different driver equations. Finally, we evaluate the accuracy of the obtained solutions by comparing with the original unaltered theory. Published by the American Physical Society2024 

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