Speakers and Titles/Abstracts
Workshop on Quantum Aspects of Black Holes and Spacetime
Tuesday-Friday, December 2-5, 2025 - Institute for Advanced Study
Ahmed Almheiri, Institute for Advanced Study; New York University Abu Dhabi
Title: "Black Hole Disappearance"
Abstract: Not much is known about what happens at the end of black hole evaporation. Pushing the semi-classical evolution well beyond its regime of validity suggests that the final Cauchy slice breaks into a piece that remains behind the event horizon, anchored to the point where the singularity meets the horizon, and a piece that detaches from the horizon to become a Cauchy slice for all future time evolution in slightly perturbed empty space. The evidence for this picture, however, remains scant. In fact, it is disputed by evaporating AdS black holes in two dimensions. I will offer a new scenario by analyzing evaporating black holes in a modified version of JT gravity that admits both empty AdS and black hole solutions. I will argue that the key to black hole disappearance is the emission of a baby universe that extracts the matter that formed the black hole, using the same mechanism for global symmetry violation in quantum gravity. This non-perturbative transition transforms the black hole into an excited state without a horizon that proceeds to disappear through reliable semi-classical evolution. This talk is based on work in progress with Shadi Ali Ahmad, Simon Lin, and Shoy Ouseph.
Stefano Antonini, University of California, Berkeley
Title: "A Black Hole Airy Tail: Part 2"
Abstract: Part 2 of the talk by Pratik Rath. In this talk, I will discuss a puzzle in JT (super)gravity coupled to matter, first observed by Lin-Maldacena-Rozenberg-Shan. The one-sided entropy of a two-sided black hole in the presence of many matter operator insertions becomes negative when naively computed using bulk techniques. This puzzle arises when computing annealed (instead of quenched) entropies. Using the semi-quenched entropy defined in Pratik's talk, I will explain how the positivity of entropy can be rescued. From the bulk perspective, both higher-genus topologies and wormhole effects are crucial, and the genus expansion of bulk matter diagrams must be re-summed. I will explain how this can be achieved for the LMRS setup and its generalization to non-supersymmetric JT gravity. From the matrix model perspective, the resolution relies on the statistics of eigenvalues near the edge, governed, in different regimes, by the Airy distribution or by eigenvalue instantons. Along the way, I will describe a new equivalence between many-point correlators in JT gravity with matter and pure JT gravity partition functions at low temperature, and explain its origin from a matrix model perspective.
Raphael Bousso, University of California, Berkeley
Title: "No Bounces, No Cycles: A Robust Singularity Theorem in Exact Semiclassical Gravity"
Abstract: I will review the Penrose and Wall singularity theorems and describe their limitations. Following Shahbazi-Moghaddam, I will argue that semiclassical gravity in the infinite species limit becomes an exact theory whose metric is defined on all scales and which obeys the Generalized Second Law. I prove a robust singularity theorem in this setting.
Jordan Cotler, Harvard University
Title: "Quantum Mechanics of the No-Boundary State"
Abstract: The no-boundary state in de Sitter offers a compelling framework for specifying the initial conditions of our universe. Many (but famously not all) features of the state agree beautifully with empirical observations and have served as a foundation for initial conditions in inflationary cosmology. We explore its relation to entropy puzzles in de Sitter spacetimes. We then compute the norm of the no-boundary state and show that the contributions from geometries with spherical spatial slices, including the Hartle–Hawking geometry and fluctuations around it, vanish at one loop. We present preliminary evidence that these contributions also vanish at higher loops and discuss the resulting implications for theoretical cosmology.
Anne-Catherine de la Hamette, ETH Zürich
Title: "Relational Perspectives in Quantum Gravity: The Role of Quantum Reference Frames"
Abstract: The framework of quantum reference frames (QRFs) lies at the crossroads of quantum information, gauge theory, quantum field theory, and quantum gravity, and has seen rapid progress in recent years. After introducing the perspective-neutral formulation, I will present two applications demonstrating how QRFs illuminate quantum aspects of spacetime. First, I will show how QRF methods yield concrete predictions for quantum effects in superposed spacetimes—reproducing perturbative results where applicable while extending naturally to non-perturbative regimes where no fixed background exists. Second, I will discuss recent advances showing how modeling observers as quantum systems leads to crossed product algebras and type reduction, transforming Type III algebras (which lack a well-defined trace) into Type II structures with finite entropies. This regularization reveals that different observers may measure different gravitational entropies, suggesting some entanglement entropies are fundamentally observer-dependent. I will conclude by outlining several open questions about the role of observers in quantum gravity, including whether certain divergences and paradoxes might reflect the limits of frame-independent descriptions rather than fundamental inconsistencies.
Matthew Dodelson, Harvard University
Title: "Bouncing off a Stringy Singularity"
Abstract: In AdS/CFT, the black hole singularity is encoded as a divergence in a certain analytic continuation of the two-point function. This divergence corresponds to a null geodesic that bounces off the singularity. In this talk, we will study the fate of this null geodesic at finite 't Hooft coupling, finding that it is smoothed out by stringy effects in the example of the SYK model. I will then explain how this computation is related to classical dynamical systems. Finally, I will comment on the generalization of this computation to matrix quantum mechanics.
Anatoly Dymarsky, Institute for Advanced Study; University of Kentucky
Title: "TQFT Gravity and Sum over Topologies"
Abstract: A simple model of topological gravity can be defined by summing a 3d TQFT over all bulk topologies. I will explain how this sum is formulated so as to be holographically dual to a well-defined boundary ensemble. I will then present several explicit examples and discuss their implications for pure 3d quantum gravity.
Elliott Gesteau, Massachusetts Institute of Technology / Harvard University
Title: "Failures of Spacetime Emergence"
Abstract: In holography, an effective spacetime description emerges from the fundamental boundary description. But can all spacetime geometries emerge from a holographic theory? In this talk, I will argue that in some situations, a region of spacetime can fail to emerge from a traditional application of the rules of holography. This conclusion can be reached both from large N arguments and from tensor network models. I will also comment on various proposed methods to restore the emergence of spacetime, and their relation to the notion of observer complementarity.
Daniel Green, University of California, San Diego
Title: "Quantum Walks and RG in de Sitter Space"
Abstract: Classical random walks have long served as useful intuition for the dynamics of light scalar fields in de Sitter space. This description can be shown to arise from QFT on a fixed dS background, via RG flow. The RG approach also permits corrections to the stochastic formalism to be calculated systematically, but fails to describe the tails of the probability distribution. After reviewing these results, I will show how to improve these results using exact RG and the dS density matrix. We will find this gives rise to a quantum walk and will use this perspective to resolve some open issues with the stochastic formalism.
Daniel Harlow, Massachusetts Institute of Technology
Title: "Observers, Alpha Parameters, and the Hartle-Hawking State"
Abstract: It has recently been understood from several points of view that the Hilbert space of quantum gravity in a closed universe has dimension one. It has also been argued that this problem can be ameliorated by explicitly including the observer in the fundamental laws of physics. In this talk I will explain how this proposal is related to the Hartle Hawking state and alpha parameters of Coleman, Giddings, Strominger, Marolf, and Maxfield. I will also point out a new tension between the gravitational path integral and standard assumptions about the landscape of string theory. Based on work with Ying Zhao.
Thomas Hartman, Cornell University
Title: "Triangulating Quantum Gravity in AdS$_3$"
Abstract: The path integral of pure 3D gravity is formulated on a finite region of spacetime, with boundary conditions that fix dihedral angles or geodesic lengths. This amplitude calculates statistics of black hole matrix elements in the dual CFT. The fixed-length path integral is related to Virasoro TQFT; the fixed-angle path integral is the partition function of Conformal Turaev-Viro theory, a novel topological theory based on triangulations; and the two are related by a modular S-transform.
Sean Hartnoll, University of Cambridge
Title: "Quantum Cosmology in the BKL Regime and Dual Primon Gases"
Abstract: Over 50 years ago, Belinski-Khalatnikov-Lifshitz (BKL) argued that the dynamics of spacetime close to a space like singularity is chaotic and inhomogeneous. I will revisit the BKL scenario within a modern understanding of quantum chaos and holographic duality. I will argue that the remarkable modular symmetries that arise in the near-singularity dynamics suggests a dual description of the start of time as a so-called “primon gas”, a description that is at once both simple and also connects with deep results from number theory.
Luca Iliesiu, University of California, Berkeley
Title: "On the Hilbert Space of Quantum Gravity from the Gravitational Path Integral"
Abstract: Does the gravitational path integral define a valid Hilbert space for open and closed universes? In this talk, I will review the positivity criteria that the gravitational path integral has to satisfy in order for all open and closed universe states to have positive norm. Although the positivity criteria appear to be highly constraining, I will describe several cases in which they are miraculously satisfied. This leads to a well defined Hilbert space where we can meaningfully discuss transition probabilities among different closed and open universe states. Using the no-boundary state as a case study, I will explain how to compute transition probabilities from this state when properly accounting for non-perturbative corrections in the gravitational path integral. Surprisingly, the resulting probabilities drastically differ from the traditional approach of Hartle and Hawking.
Jonah Kudler-Flam, Institute for Advanced Study
Title: "Emergent Mixed States for Baby Universes and Black Holes"
Abstract: I will discuss the fate of quantum states in the semiclassical limit of the AdS/CFT correspondence, focusing on cases where the gravitational side involves black holes or “baby universes.” Typically, these states do not converge to simple, pure states. For black holes—where the entropy diverges as G→0—they can instead approach mixed states, reflecting an effective loss of information in the semiclassical limit. For configurations involving baby universes, wormhole effects prevent a conventional semiclassical limit from existing. I will comment on extensions of the AdS/CFT correspondence involving averaging.
Hong Liu, Massachusetts Institute of Technology
Title: " "Filtering'' CFTs at large N: Euclidean Wormholes, Closed Universes, and Black Hole Interiors"
Abstract: We argue that Euclidean wormhole configurations in AdS/CFT--central to the factorization problem--originate from the erratic $N$-dependence that arises in the large-$N$ limit of conformal field theories (CFTs). We propose that constructing the gravitational dual of a large-$N$ CFT in the semiclassical limit requires a filtering procedure that smooths out this erratic $N$-dependence. Euclidean wormholes--both the external ones connecting disjoint boundaries and the internal ones corresponding to handles--encode correlations of this erratic behavior in Euclidean partition functions. Moreover, internal wormholes do not appear to induce random couplings in the low-energy effective theory of gravity.
The Lorentzian manifestations of these erratic-$N$ correlations include the emergence of closed universes, their transitions, and black hole interiors. By relating correlations among the erratic components of certain Euclidean partition functions to transitions among closed universes, we derive an infinite tower of inequalities constraining wormhole actions.
We argue that an AdS spacetime entangled with a baby universe is quantum-volatile, reflecting the underlying erratic behavior at large $N$. Furthermore, local operators in a closed universe and in the interior of a black hole are intrinsically erratic, and the black hole interior is quantum-volatile.
Mehrdad Mirbabayi, ICTP, Trieste
Title: "An Observer's Measure of De Sitter Entropy"
Abstract: The two-point correlation function of a massive field, measured along an observer's worldline in de Sitter (dS), decays exponentially in time. Meanwhile, every dS observer is surrounded by a horizon and the holographic interpretation of the horizon entropy S_dS suggests that the correlation function should stop decaying, and start behaving erratically at late times. We find evidence for this expectation in Jackiw-Teitelboim gravity by finding a topologically nontrivial saddle, which is suppressed by exp(-S_dS), and which gives a constant contribution to the norm squared of the correlator. This constant might have the interpretation of the late-time average over all microscopic theories that have the same low-energy effective description. Based on arxiv-2311.07724.
Beatrix Muehlmann, Institute for Advanced Study
Title: "Higher Spin Theories in de Sitter Space"
Abstract: I will discuss a two-dimensional higher-spin model and the lessons it offers—as well as the obstacles it helps us overcome—toward a microscopic completion of four-dimensional higher-spin theory on the four-sphere.
Geoff Penington, University of California, Berkeley
Title: "Holographic Algebras at Null Infinity"
Abstract: TBA
Pratik Rath, University of California, Berkeley
Title: "A Black Hole Airy Tail: Part 1"
Abstract: Part 1: In this talk, I will discuss an entropic puzzle in JT gravity and its resolution, which requires taking into account non-perturbative effects in the gravitational path integral. In JT gravity, which is dual to a random matrix ensemble, the gravitational thermal entropy becomes negative at very low temperatures. This puzzle arises when computing the annealed (instead of quenched) entropy, corresponding to an incorrect averaging procedure in the dual matrix model. We define an “intermediate” quantity, the semi-quenched entropy, to show how the positivity of the quenched entropy can be rescued. From the bulk perspective, both a resummation of higher-genus topologies and wormhole effects are crucial. From the matrix model perspective, the resolution relies on the statistics of eigenvalues near the edge, governed, in different regimes, by the Airy distribution or by 1-eigenvalue instantons. A more general version of this puzzle and its resolution will be discussed by Stefano Antonini in Part 2 of this talk.
Martin Sasieta, University of California, Berkeley
Title: "ER Toward Typical EPR"
Abstract: Do typical black hole microstates have semiclassical interiors? I will present a constructive approach to this question, where we prepare ensembles of black hole interiors with long wormholes using Brownian matter sources. Evaluating the frame potentials of the ensembles with the gravitational path integral, I will show that the ensembles become quantitatively generic. I will derive a correspondence between the degree of randomness of the ensembles and the average geometric length of the wormhole. This correspondence constitutes a ``complexity=geometry'' relation.
Arvin Shahbazi-Moghaddam, Stanford University
Title: "Exact Semiclassical Gravity and Partial Cauchy Slice Holography"
Abstract: This talk has two parts. In the first, I will discuss a well-known Lorentzian axiomatic framework in which quantum gravity theorems can be rigorously established, with the quantum extremal surface prescription as a central highlight. I will describe a recently sharpened understanding of its regime of validity—namely the limit $G\to0$ with cG held fixed, where c counts the number of matter species. Exactness in cG allows the framework to reach non-perturbatively beyond Einstein gravity. To exemplify its power, I will explain constraints on possible singularity-resolution scenarios, presenting two explicit geometric solutions: one in which the singularity resolves and one in which it does not. In the second part, I will discuss preliminary work on a “holographic” encoding of Lorentzian subregions via the gravitational path integral. As an example, I will present a computation of black hole entropy obtained directly from bulk partial Cauchy-slice preparations in the Lorentzian sector. I will explain potential applications of this approach.
Eva Silverstein, Stanford University
Title: "The Yes Boundaries Wavefunctions of the Universe and Late Time Cosmology"
Abstract: The topology of the universe is unknown, and a typical manifold will contain nontrivial cycles and boundaries. This is quite consistent with the observed Lambda CDM cosmology, which does not imply that we live in a closed universe (including observers). We start by briefly summarizing recently derived constraints on a population of timelike boundaries in the observable universe using recent CMB and BAO data, incorporating UV sensitive effects. We then turn to their role in holography, where they incorporate a multiplicity of states. In recent years this led to a concrete microstate count for the timelike-bounded de Sitter static patch in four dimensions, via a combination of solvable and fine-tuned deformations of AdS/CFT. We next construct the thermofield double of the system to generate the cosmological future wedge in this framework. We then show how to introduce constraints in this Hamiltonian system in order to capture tall states further down in the spectrum, where the boundaries are in causal contact. (We also note a separate method in progress for reaching the future wedge, deforming the timelike bounded theory to Cauchy slice holography.)
Douglas Stanford, Stanford University
Title: "On the Phase of the de Sitter Density of States"
Abstract: The one-loop gravitational path integral around Euclidean de Sitter space S^D has a complex phase that casts doubt on a state counting interpretation. Recently, it was proposed to cancel this phase by including an observer. We explore this proposal in the case where the observer is a charged black hole in equilibrium with the de Sitter horizon. We compute the phase of the one-loop determinant within a two-dimensional dilaton gravity reduction, using both numerical and analytical methods. Our results interpolate between previous studies of a probe geodesic observer and the Nariai solution. We also revisit the prescription for going from the Euclidean path integral to the state-counting partition function, finding a positive sign in the final density of states. Based on work with Yiming Chen, Haifeng Tang, Zhenbin Yang.
Joaquin Turiaci, University of Washington
Title: "The Hartle-Hawking Wavefunction and de Sitter Black Holes"
Abstract: The Schwarzschild-de Sitter solution can be used to compute the Hartle-Hawking wavefunction of the universe on a spatial circle times a sphere. Its norm can be evaluated in the semiclassical limit and compared to the on-shell action of the Euclidean Nariai geometry. I will describe new results clarifying the role that quantum corrections to the wavefunction play in this derivation. Next, I will present a calculation of the phase of the path integral on Nariai to test whether, at the quantum level, it can be interpreted as a norm. In the process, I will derive a simple formula for the phase of the path integral on any solution of pure gravity. Finally, I will describe ongoing work based on an alternative interpretation of the Nariai geometry as computing the nucleation rate of black holes in de Sitter.
Mykhaylo Usatyuk, University of California, Santa Barbara
Title: "Near-Extremal Black Hole Evaporation"
Abstract: TBA
Zhenbin Yang, Tsinghua University
Title: "Comments on the de Sitter Double Cone"
Abstract: We study the double cone geometry proposed by Saad, Shenker, and Stanford in de Sitter space. We demonstrate that with the inclusion of static patch observers, the double cone leads to a linear ramp consistent with random matrix behavior. This ramp arises from the relative time shift between two clocks located in opposite static patches.
Ying Zhao, Massachusetts Institute of Technology
Title: "Observers, Alpha Parameters, and the Hartle-Hawking State: Part II"
Abstract: It was argued that any fixed holographic theory contains only one closed universe state and hence fails to give semi-classical physics. It was proposed that this problem can be resolved by including a classical observer living inside the universe. Earlier works focused on closed universes connected with asymptotic Euclidean boundaries. In this talk we examine the case of Hartle-Hawking state where the dominant Euclidean topology is a sphere. We show that different features emerge. We comment on the potential implications for the understanding of de Sitter space. Based on work with Daniel Harlow.