arXiv subjects

Robie A. Hennigar

Publications and source records attributed to Robie A. Hennigar.

At least 19 recordsLinked to original sources

arXiv2026

Logarithmic correction to the entropy of near-extremal higher-curvature black holes

The spherically symmetric sector of broad classes of $D$-dimensional gravitational theories can be effectively described by general two-dimensional Horndeski theories. Exploiting this correspondence, we study the low-temperature dynamics of near-extremal black hole solutions of the parent theories by deriving a universal near-horizon effective description in terms of Jackiw--Teitelboim (JT) gravity. We explicitly show that the logarithmic quantum correction to the entropy, $S_{\rm JT}=\frac{3}{2}\log \left(T/T_{\rm breakdown} \right)$, previously derived for near-extremal black holes in Einstein gravity coupled to matter from the one-loop exact JT partition function, is universal across all models admitting an effective two-dimensional Horndeski description. We determine the general form of the scale at which this correction becomes dominant, $T_{\rm breakdown}$, in terms of the data of the $D$-dimensional theories. We illustrate our general results with charged black holes in Lovelock gravities and regular black holes in Quasitopological gravities.

hep-th↗

arXiv2026

Looking inside a quantum black hole

Quantum effects are expected to modify a black hole's interior structure, particularly near the singularity. We show how to extract the scaling exponent of the singularity from the quasinormal mode (QNM) spectrum of a massless scalar probe in the asymptotic large-overtone limit. We apply our method to a family of exact quantum black holes in (2+1)-dimensional anti-de Sitter (AdS) space and explicitly uncover a transition deep in the interior when quantum effects become dominant.

hep-th↗

arXiv2026

Quasinormal modes and quantum black hole interiors

Quasinormal modes (QNMs) of black hole perturbations in the large overtone limit, namely, asymptotic QNMs, are highly sensitive to a black hole's interior geometry. We probe the singularity structure of a class of exact (2+1)-dimensional quantum black hole solutions to semi-classical gravity by computing their asymptotic QNM spectra. We do this using methods of complex analysis in which the radial coordinate is analytically continued to the complex plane. The leading order QNM frequencies all fit the generic form $ω=(\text{offset})+n(\text{gap})$, for overtone number $n$, which we also confirm numerically. Under a general assumption about the global Stokes topology of the complexified radial coordinate, we show how to reconstruct the scaling exponent of more general (spacelike or timelike) black hole singularities from the offset. We then compute subleading corrections to the asymptotic QNMs, from which we provide a robust method for extracting the scaling of the metric function near the singularity. Our findings exemplify the transition between "Kasner eons", successive regimes encountered on approach to a spacelike singularity, as non-perturbative quantum effects become dominant. For charged quantum black holes, the asymptotic QNMs exhibit a crossover between two regimes in which the neutral and charge contributions to the metric function dominate, respectively. Within the crossover region, the QNMs develop an oscillatory behavior that may be a signature of the inner horizon.

hep-th↗

arXiv2026

Regular Black Holes in Nonlocal Quasitopological Gravity

We present infinite-derivative completions of Quasitopological gravities that are ghost-free, avoid strong coupling instabilities and admit exact, spherically symmetric vacuum regular-black-hole solutions satisfying a perturbative Birkhoff theorem.

gr-qc↗

arXiv2026

Quantum Solitons

We construct geometries describing the quantum backreaction of thermal fields in AdS$_3$. The solutions are obtained from branes in a four-dimensional AdS C-metric. They can be viewed as solutions of the semiclassical effective theory on the brane, which couples three-dimensional gravity to the CFT dual to the four-dimensional bulk. This brane construction is related by a double analytic continuation to earlier studies of quantum BTZ solutions. There are two families of solutions, labelled by the asymptotic mass. Solutions with negative mass correspond to the back-reaction of a thermal CFT state on global AdS$_3$. Solutions with positive mass have a horizon for zero back-reaction, which is replaced by a smooth origin in the back-reacted solution. We study the thermodynamics and first law on the brane, which we argue is realised in a two-brane setup where we include both the quantum BTZ brane and our quantum soliton brane.

hep-th↗

arXiv2026

Buchdahl limits in theories with regular black holes

We study generalizations of Buchdahl's compactness limits for perfect-fluid star solutions of $D$-dimensional Einstein gravity coupled to higher-curvature corrections. We focus on Quasi-topological theories involving infinite towers of terms for which the unique vacuum spherically symmetric solutions correspond to regular black holes. We solve analytically the problem of constant-density stars and find that the space of solutions is bounded by: configurations with divergent central-pressure, corresponding to the most compact stars; configurations which possess zero central-pressure; and configurations for which the sizes of the stars coincide with the inner-horizon radii of the would-be regular black holes. In the more general case of perfect-fluid stars for which the mean density decreases with increasing radius, we show that, for each density profile, maximum compactness is reached when the metric becomes singular at the center. Under certain additional conditions, we find a novel Buchdahl limit for the maximum compactness of stars, attained by a specific constant-density profile. We show, in particular, that stars in these theories may be more compact than in Einstein gravity. While the vacuum solutions of these theories are such that all curvature invariants take mass-independent maximum finite values, we argue that there exist ordinary matter stars with finite central pressures for which such bounds can be violated -- namely, arbitrarily high curvatures can be reached -- unless additional constraints, such as the dominant energy condition, are imposed on the fluid.

gr-qc↗

arXiv2026

Regular Geometries from Singular Matter in Quasi-Topological Gravity

Vacuum quasi-topological gravity with infinitely many terms in the action satisfies Markov's limiting curvature hypothesis: the spherically symmetric solutions are regular and all curvature invariants are bounded by solution-independent scales. We study how this picture changes when the theory is coupled to matter. We find that minimally coupled matter spoils the scaling properties of the vacuum equations that lead to the validity of Markov's hypothesis, but the corresponding geometries often remain regular. We make this precise by developing a set of sufficient conditions on general static, spherically symmetric stress-tensors such that the corresponding solutions have bounded curvature. These conditions cover regular matter sectors but also singular matter profiles that are sufficiently singular in a sense we quantify. Our conclusions hold independently of the matter field equations and include configurations in which matter exhibits divergent energy density and pressure at finite radius or at Killing horizons, results that may have implications for mass inflation in these models. We then explore non-minimal couplings, focusing on theories with infinite towers of higher-curvature and electromagnetic terms in the action. In this class, Markov's hypothesis can be restored: we present theories admitting a universal upper bound on curvature, independent of the mass and charge. Overall, our results highlight subtleties in coupling quasi-topological gravity to matter and suggest Markov's hypothesis as a potential selection criterion for resummed gravity-matter effective theories.

gr-qc↗

arXiv2025

Birkhoff implies Quasi-topological

Quasi-topological gravities (QTGs) are higher-curvature extensions of Einstein gravity in $D\geq 5$ spacetime dimensions. Throughout the years, different notions of QTGs constructed from analytic functions of polynomial curvature invariants have been introduced in the literature. In this paper, we show that all such definitions may be reduced to three distinct inequivalent notions: type I QTGs, for which the field equations evaluated on a single-function static and spherically symmetric ansatz are second order; type II QTGs, whose field equations on general static and spherically symmetric backgrounds are second order; and type III QTGs, for which the trace of the field equations on a general background is second order. We show that type II QTGs are a subset of type I QTGs and that type III QTGs are a subset of type II QTGs modulo pure Weyl invariants. Moreover, we prove that type II QTGs possess second-order equations on general spherical backgrounds. This allows us to prove that any theory satisfying a Birkhoff theorem is a type II QTG, and that the reverse implication also holds up to a zero-measure set of theories. For every theory satisfying Birkhoff's theorem, the most general spherically symmetric solution is a generalization of the Schwarzschild spacetime characterized by a single function which satisfies an algebraic equation.

gr-qc↗

arXiv2025

Regular black hole formation in four-dimensional non-polynomial gravities

We construct four-dimensional gravity theories that resolve the Schwarzschild singularity and enable dynamical studies of nonsingular gravitational collapse. The construction employs a class of nonpolynomial curvature invariants that produce actions with (i) second-order equations of motion in spherical symmetry and (ii) a Birkhoff theorem, ensuring uniqueness of the spherically symmetric solution. Upon spherical reduction to two dimensions, these theories map to a particular subclass of Horndeski scalar-tensor models, which we use to explicitly verify the formation of regular black holes as the byproduct of the collapse of pressureless stars and thin-shells. We also show that linear perturbations on top of maximally symmetric backgrounds are governed by second-order equations.

gr-qc↗

arXiv2025

Rotating Extremal Black Holes in Einstein-Born-Infeld Theory

We construct exact solutions that describe the near horizon region of extremal rotating black holes in Einstein-Born-Infeld theory. Using generalized Komar integrals, we extract the electric charge and angular momentum from the near horizon geometries and study their deviations from the Kerr-Newman solution. We identify two features that are direct consequences of the nonlinearities of Born-Infeld theory. First, we find solutions which have vanishing charge but nontrivial electric and magnetic fields. Second, we find that extremal rotating black holes do not exist for sufficiently small charge and angular momentum. Based on analogy with the static black holes, we argue that it would be particularly interesting to construct the full rotating solutions in these parameter regions as they may provide examples of rotating black holes without Cauchy horizons.

gr-qc↗

arXiv2025

Charged rotating quantum black holes

We investigate the thermodynamic and holographic properties of charged and rotating quantum black holes in a doubly holographic braneworld setup. These quantum black holes are derived from the anti-de Sitter C-metric and are exact solutions to a semi-classical gravitational theory which incorporates all orders of the backreaction of quantum fields on spacetime. The inclusion of both charge and rotation extends and generalizes previous studies. The thermodynamics and critical behavior of the black holes are examined from the bulk, brane, and boundary perspectives, and we demonstrate that the inclusion of either charge or rotation removes the re-entrant phase transitions seen in the neutral-static case. The critical exponents of the system are calculated using numerical methods and found to differ from the standard mean field theory values for the neutral-static black holes' re-entrant phase transitions, but in agreement with mean-field theory for the phase transitions of the black holes with charge and rotation. Additionally, to test the validity of the semiclassical treatment, we study a mass-gap energy scale $M_{\rm gap}$ to identify regimes where quantum fluctuations of spacetime geometry are expected to become significant and speculate about a connection with weak cosmic censorship gedankenexperiments. We also generalize the quantum Penrose inequality and the quantum reverse isoperimetric inequality to include charge and rotation. Finally, we compute a renormalized gyromagnetic ratio and analyze it in the limit of large backreaction.

hep-th↗

arXiv2025

Excising Cauchy Horizons with Nonlinear Electrodynamics

Charged and/or rotating black holes in General Relativity feature Cauchy horizons, which indicate a breakdown of predictability in the theory. Focusing on spherically symmetric charged black holes, we remark that the inevitability of Reissner-Nordstrom Cauchy horizon is due to the divergent electromagnetic self-energy of point charges. We demonstrate that any causal theory of nonlinear electrodynamics that regularizes the point charge self-energy also eliminates Cauchy horizons for weakly charged black holes. These black holes feature one (event) horizon and a spacelike singularity, analogous to the Schwarzschild metric. An example with Born-Infeld electrodynamics illustrates how this gives rise to an upper bound on the charge, which we compare with known bounds.

gr-qc↗

arXiv2025

Thermodynamics of Regular Black Holes in Anti-de Sitter Space

We construct regular black holes with anti-de Sitter asymptotics in theories incorporating infinite towers of higher-order curvature corrections in any dimension $D \ge 5$. We find that regular black branes are generically inner-extremal, potentially evading instabilities typically associated with inner horizons. Considering minimally coupled matter, we establish general criteria for the existence of singularity-free solutions. We analyze solutions coupled to Maxwell and nonlinear (Born--Infeld and RegMax) electrodynamics, demonstrating in the latter case the first examples of fully regular gravitational and electromagnetic fields for all parameter values. Here, we find that the ratio of the gravitational mass to the electrostatic self-energy determines whether the regular core is de Sitter or anti-de Sitter. We perform a detailed analysis of the black hole thermodynamics and show that the equation of state exhibits features akin to those of fluids with a finite molecular volume induced by the regularization parameter.

gr-qc↗

arXiv2025

Regular black holes from Oppenheimer-Snyder collapse

It has been recently shown that regular black holes arise as the unique spherically symmetric solutions of broad families of generalizations of Einstein gravity involving infinite towers of higher-curvature corrections in $D\geq 5$ spacetime dimensions. In this paper we argue that such regular black holes arise as the byproduct of the gravitational collapse of pressureless dust stars. We show that, just like for Einstein gravity, the modified junction conditions for these models impose that the dust particles on the star surface follow geodesic trajectories on the corresponding black hole background. Generically, in these models the star collapses until it reaches a minimum size (and a maximum density) inside the inner horizon of the black hole it creates. Then, it bounces back and reappears through a white hole in a different universe, where it eventually reaches its original size and restarts the process. Along the way, we study FLRW cosmologies in the same theories that regularize black hole singularities. We find that the cosmological evolution is completely smooth, with the big bang and big crunch singularities predicted by Einstein gravity replaced by cosmological bounces.

gr-qc↗

arXiv2025

Quantum censors: backreaction builds horizons

Cosmic censorship posits spacetime singularities remain concealed behind event horizons, preserving the determinism of General Relativity. While quantum gravity is expected to resolve singularities, we argue that cosmic censorship remains necessary whenever spacetime has a reliable semi-classical description. Using holography to construct exact solutions to semi-classical gravity, we show backreaction of quantum matter generates horizons -- quantum censors -- to thwart potential violations of censorship. Along with a quantum Penrose inequality, this provides compelling evidence cosmic censorship is robust, even nonperturbatively, in semi-classical gravity.

hep-th↗

arXiv2024

Spectrum of the Laplacian on the Page metric

We numerically construct the spectrum of the Laplacian on Page's inhomogeneous Einstein metric on $\mathbb{CP}^2 \# \overline{\mathbb{CP}}^2$ by reducing the problem to a (singular) Sturm-Liouville problem in one dimension. We perform a perturbative analysis based upon a closely related, exactly solvable problem that strongly supports our results. We also study the spectrum of the Lichnerowicz Laplacian on symmetric traceless transverse two-tensors. The method relies on both the isometries of the Page metric and pseudospectral methods to numerically solve the resulting ODEs.

math.SP↗

arXiv2024

Regular black holes from thin-shell collapse

We establish that regular black holes can form from gravitational collapse. Our model builds on a recent construction that realized regular black holes as exact solutions to purely gravitational theories that incorporate an infinite tower of higher curvature corrections in any dimension $D \ge 5$ [arXiv:2403.04827]. We identify a two-dimensional Horndeski theory that captures the spherically symmetric dynamics of the theories in question and use this to prove a Birkhoff theorem and obtain the generalized Israel junction conditions. Armed with these tools, we consider the collapse of thin shells of pressureless matter, showing that this leads generically to the formation of regular black holes. The interior dynamics we uncover is intricate, consisting of shell bounces and white hole explosions into a new universe. The result is that regular black holes are the unique spherically symmetric solutions of the corresponding theories and also the endpoint of gravitational collapse of matter. Along the way, we establish evidence for a solution-independent upper bound on the curvature, suggestive of Markov's limiting curvature hypothesis.

gr-qc↗

arXiv2024

Dynamical Formation of Regular Black Holes

We study dynamical gravitational collapse in a theory with an infinite tower of higher-derivative corrections to the Einstein-Hilbert action and we show that, under very general conditions, it leads to the formation of regular black holes. Our results are facilitated by the use of a class of theories that possess second-order equations on spherically symmetric metrics, but which are general enough to provide a basis for the gravitational effective action. We analytically solve the collapse of a thin shell of dust and show that it inevitably experiences a bounce at small radius and that its motion can be extended to arbitrary proper time. The collapse of the shell always gives rise to a singularity-free, geodesically complete spacetime that contains horizons if the total mass is above a critical value. In that case, the shell bounces into a new universe through a white hole explosion. Our construction provides, to the best of our knowlege, the first fully dynamical description of formation of regular black holes, and it suggests that higher-derivative corrections may be the most natural way to resolve the singularities of Einstein's theory.

gr-qc↗