It was recently shown that (near)extremal Kerr black holes are sensitive probes of small higherderivative corrections to general relativity. In particular, these corrections produce diverging tidal forces on the horizon in the extremal limit. We show that adding a black hole charge makes this effect qualitatively stronger. Higherderivative corrections to the KerrNewman solution produce tidal forces that scale inversely in the black hole temperature. We find that, unlike the Kerr case, for realistic values of the black hole charge large tidal forces can arise before quantum corrections due to the Schwarzian mode become important, so that the nearhorizon behavior of the black hole is dictated by higherderivative terms in the effective theory.
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A<sc>bstract</sc> Free, publiclyaccessible full text available May 1, 2025 
A<sc>bstract</sc> Due to the conformal factor problem, the definition of the Euclidean gravitational path integral requires a nontrivial choice of contour. The present work examines a generalization of a recently proposed ruleofthumb [1] for selecting this contour at quadratic order about a saddle. The original proposal depended on the choice of an indefinitesignature metric on the space of perturbations, which was taken to be a DeWitt metric with parameter
α = – 1. This choice was made to match previous results, but was otherwise admittedlyad hoc . To begin to investigate the physics associated with the choice of such a metric, we now explore contours defined using analogous prescriptions forα ≠ – 1. We study such contours for Euclidean gravity linearized about AdSSchwarzschild black holes in reflecting cavities with thermal (canonical ensemble) boundary conditions, and we compare pathintegral stability of the associated saddles with thermodynamic stability of the classical spacetimes. While the contour generally depends on the choice of DeWitt parameterα , the precise agreement between these two notions of stability found atα = – 1 continues to hold over the finite interval (– 2, – 2/d ), whered is the dimension of the bulk spacetime. This agreement manifestly fails forα > – 2/d when the DeWitt metric becomes positive definite. However, we also find dramatic failures forα < – 2 that correlate with breakdowns of the de Donderlike gauge condition defined byα , and at which the relevant fluctuation operator fails to be diagonalizable. This provides criteria that may be useful in predicting metrics on the space of perturbations that give physicallyuseful contours in more general settings. Along the way, we also identify an interesting error in [1], though we show this error to be harmless.Free, publiclyaccessible full text available May 1, 2025 
A trace inequality for Euclidean gravitational path integrals (and a new positive action conjecture)
A<sc>bstract</sc> The AdS/CFT correspondence states that certain conformal field theories are equivalent to string theories in a higherdimensional antide Sitter space. One aspect of the correspondence is an equivalence of density matrices or, if one ignores normalizations, of positive operators. On the CFT side of the correspondence, any two positive operators
A, B will satisfy the trace inequality Tr(AB ) ≤ Tr(A )Tr(B ). This relation holds on any Hilbert space and is deeply associated with the fact that the algebra$$ \mathcal{H} $$ $H$B ( ) of bounded operators on$$ \mathcal{H} $$ $H$ is a type I von Neumann factor. Holographic bulk theories must thus satisfy a corresponding condition, which we investigate below. In particular, we argue that the Euclidean gravitational path integral respects this inequality at all orders in the semiclassical expansion and with arbitrary higherderivative corrections. The argument relies on a conjectured property of the classical gravitational action, which in particular implies a positive action conjecture for quantum gravity wavefunctions. We prove this conjecture for JackiwTeitelboim gravity and we also motivate it for more general theories.$$ \mathcal{H} $$ $H$Free, publiclyaccessible full text available April 1, 2025 
A<sc>bstract</sc> We study black holes in two and three dimensions that have spacelike curvature singularities behind horizons. The 2D solutions are obtained by dimensionally reducing certain 3D black holes, known as quantum BTZ solutions. Furthermore, we identify the corresponding dilaton potential and show how it can arise from a higherdimensional theory. Finally, we show that the rotating BTZ black hole develops a singular inner horizon once quantum effects are properly accounted for, thereby solidifying strong cosmic censorship for all known cases.
Free, publiclyaccessible full text available December 1, 2024 
A<sc>bstract</sc> We determine tree level, allorder celestial operator product expansions (OPEs) of gluons and gravitons in the maximally helicity violating (MHV) sector. We start by obtaining the allorder collinear expansions of MHV amplitudes using the inverse soft recursion relations that they satisfy. These collinear expansions are recast as celestial OPE expansions in bases of momentum as well as boost eigenstates. This shows that inverse soft recursion for MHV amplitudes is dual to OPE recursion in celestial conformal field theory.
Free, publiclyaccessible full text available October 1, 2024 
A<sc>bstract</sc> In AdS/CFT, observables on the boundary are invariant under renormalization group (RG) flow in the bulk. In this paper, we study holographic entanglement entropy under bulk RG flow and find that it is indeed invariant. We focus on treelevel RG flow, where massive fields in a UV theory are integrated out to give the IR theory. We explicitly show that in several simple examples, holographic entanglement entropy calculated in the UV theory agrees with that calculated in the IR theory. Moreover, we give an argument for this agreement to hold for general treelevel RG flow. Along the way, we generalize the replica method of calculating holographic entanglement entropy to bulk theories that include matter fields with nonzero spin.
Free, publiclyaccessible full text available November 1, 2024 
A<sc>bstract</sc> Using covariant expansions, recent work showed that pole skipping happens in general holographic theories with bosonic fields at frequencies i(
l _{b}−s )2πT , wherel _{b}is the highest integer spin in the theory ands takes all positive integer values. We revisit this formalism in theories with gauge symmetry and upgrade the poleskipping condition so that it works without having to remove the gauge redundancy. We also extend the formalism by incorporating fermions with general spins and interactions and show that their presence generally leads to a separate tower of poleskipping points at frequencies i(l _{f}−s )2πT ,l _{f}being the highest halfinteger spin in the theory ands again taking all positive integer values. We also demonstrate the practical value of this formalism using a selection of examples with spins 0, $$ \frac{1}{2} $$ $\frac{1}{2}$, 1, $$ \frac{3}{2} $$ $\frac{3}{2}$, 2.Free, publiclyaccessible full text available December 1, 2024 
Matter falling into a SchwarzschildAdS black hole from the left causes increased focussing of ingoing geodesics from the right, and, as a consequence, they reach the singularity sooner. In a standard Penrose diagram, the singularity “bends down”. We show how to detect this feature of the singularity holographically, using a boundary twopoint function. We model the matter with a shock wave, and show that this bending down of the singularity can be read off from a novel analytic continuation of the boundary twopoint function. Along the way, we obtain a generalization of the recently proposed thermal product formula for twopoint correlators.
Free, publiclyaccessible full text available January 1, 2025 
Free, publiclyaccessible full text available August 1, 2024