Organizers: Daniel Harlow, Manki Kim, Hotat Lam, Washington Taylor
Held every Wednesday, 3:30-4:30pm EST. (Except where noted)

#### Seminars

Wednesday, September 14

Wednesday, September 21

Wednesday, September 28

Wednesday, February 2
Jordan Cotler, Harvard
Title: Quantum space, time, and memory
Abstract: I will explain new, surprising findings about space and time in both quantum gravity and quantum information.  In quantum gravity, I will show how novel non-perturbative effects can serve as incisive probes of the quantum microstructure of black holes and their connection to random matrix theory.  For universes with a positive cosmological constant, I will use similar tools to uncover mechanisms for the emergence of space and time from a more fundamental description.  In the second part of the talk, I will establish how being bounded in space and time constrains our ability to learn about nature through experiments.  I will introduce new tools in quantum information and use them to prove strong spacetime tradeoffs for experiments leveraging a bounded “quantum memory”.  These tradeoffs have recently been corroborated experimentally.

Wednesday, February 9
Julian Munoz, Harvard
Title: New Physics at Cosmic Dawn
Abstract: The last decades have firmly established the existence of a dark sector of our universe. Yet, details of its particle content have thus far evaded all laboratory probes. In this talk I will describe how the cosmic-dawn era, during which the first galaxies formed, holds a wealth of information about dark matter and other new physics. The next decade will see detailed maps of this era with both 21-cm and space telescopes. I will explain how to use the upcoming data to test the particle nature of dark matter by measuring its clustering at smaller scales—and earlier times—than ever before. Moreover, I will introduce a new standard ruler to measure the energy content of our cosmos during unexplored eras. These studies pave the way to discovering new physics with the upcoming trove of cosmic dawn data, and provide us with a unique window to test the particle content of our universe.

Wednesday, February 16
TBA

Wednesday, February 23
Leonard Susskind, Stanford University
Title: Entanglement and Chaos in de Sitter Space
Abstract: Depending on time limitations I’ll discuss the rules of entanglement in de Sitter space, the reasons for believing that scrambling and complexity growth are hyper-fast, and the conjecture that the double-scaled limit of SYK has hyperfast behavior.

Wednesday, March 2
Sridip Pal, Institute for Advanced Study
Title: Automorphic Spectra and the Conformal Bootstrap
Abstract: We point out that the spectral geometry of hyperbolic manifolds provides a remarkably precise model of the modern conformal bootstrap. As an application, we use conformal bootstrap techniques to derive rigorous computer-assisted upper bounds on the lowest positive eigenvalue $\lambda_1(X)$ of theLaplace-Beltrami operator on closed hyperbolic surfaces and 2-orbifolds $X$. In a number of notable cases, our bounds are nearly saturated by known surfaces and orbifolds. For instance, our bound on all genus-2 surfaces $X$ is $\lambda_1(X)\leq 3.8388976481$, while the Bolza surface has $\lambda_1(X)\approx 3.838887258$.

Wednesday, March 9
Edward Witten (IAS)
Title: No Ensemble Averaging Below the Black Hole Threshold
Abstract: Since early days of the AdS/CFT correspondence, there has been a puzzle of how to interpret Euclidean signature amplitudes computed using a connected bulk manifold with disconnected boundary.   A possible interpretation involves the idea that a bulk theory with gravity is actually dual to an ensemble of boundary theories, but in important examples of the duality no ensemble is available.   I will sharpen the puzzle by showing that an important class of “sub-threshold” observables, involving states that are not black holes, is not subject to any apparent ensemble averaging. Why then are black hole amplitudes subject to apparent ensemble averaging?   I will claim that this reflects the chaos of black hole physics and the fact that the black hole Hilbert space does not have a large N limit.   (Based on arXiv:2202.01372 with J.-M. Schlenker.)

Wednesday, March 16
Javier Magan, University of Pennsylvania
Title: Charged density of states in QFT from entropic certainties
Abstract: In this talk we will describe a proof of a recent conjecture by Harlow and Ooguri concerning a universal formula for the charged density of states at high energies in QFT’s  with finite-group global symmetries. To this end we will first present a recent new theorem concerning relative entropy for non-commuting algebras. This relation is called the certainty principle due to its intimate connection with quantum complementarity. We will remark that although the immediate application of this result concerns charged states, the origin and physics of such density can be understood by looking at the vacuum neutral sector only. We will also comment on certain generalizations.

Wednesday, March 23, Spring Break (no seminar)

Wednesday, March 30
Title: On encoding beyond the cosmological horizon
Abstract: Black hole event horizons and cosmological event horizons share many properties, making it natural to ask whether our recent advances in understanding black holes generalize to cosmology. To this end, we discuss a paradox that occurs if observers can access what lies beyond their cosmological horizon in the same way that they can access what lies beyond a black hole horizon. In particular, distinct observers with distinct horizons may encode the same portion of spacetime, violating the no-cloning theorem of quantum mechanics. This paradox is due precisely to the observer-dependence of the cosmological horizon — the sharpest difference from a black hole horizon — although we will argue that the gravity path integral avoids the paradox in controlled examples. We also comment on how our construction might be utilized to encode inflating universes in a dual quantum system.

**Starting April 6, all String/Gravity Theory seminars will start at 2:30pm**

Wednesday, April 6
Scott Collier, Princeton
Title: Harnessing S-duality in N=4 SYM and supergravity as SL(2,Z)-averaged strings
Abstract: I will describe an approach to extracting the physical consequences of S-duality of four-dimensional N = 4 super Yang-Mills (SYM) and its string theory dual based on SL(2,Z) spectral theory. I will show that processing S-duality in this way leads to strong consequences for the CFT data, both perturbatively and non-perturbatively in all parameters. In large-N limits, I will argue for the existence and scaling of non-perturbative effects, both at large N and at strong ‘t Hooft coupling. An elegant benchmark for these techniques is a certain integrated stress-tensor multiplet four-point function, whose form I will elucidate. I will explain how the ensemble average of CFT observables over the N = 4 supersymmetric conformal manifold with respect to the Zamolodchikov measure is cleanly isolated by the spectral decomposition, and will show that the large-N limit of the ensemble average is equal to the strong-coupling limit of the observable in the planar theory, which is its value in type IIB supergravity on AdS_5 x S^5. This embeds an emergent averaged holographic duality within the conventional holographic paradigm.

Wednesday, April 13
Pavel Kovtun​, University of Victoria
Title: Hydrodynamics beyond hydrodynamics
Abstract: In this talk, I will discuss two questions. First, do the equations of relativistic hydrodynamics make sense? And second, how universal are the long-distance, late-time predictions of classical hydrodynamics?

Wednesday, April 20
Clifford V. Johnson, University of Southern California
**CANCELLED**
Title: Embracing both Wigner and `t Hooft in matrix models of 2D gravity
Abstract: The main purpose of this talk is to urge us to more carefully consider how random matrix models capture theories of 2D gravity. The key is to emphasize a Wignerian view alongside the more standard ’t Hooftian approach used in this area. A central observation is that without doing so, the usual narratives about the physics are incomplete. The  results prompt a proposal for an overhaul of how we think about  the subject, and an immediate consequence is a new proposed duality that makes holography for JT gravity (and probably other 2D gravities) much more like traditional holography in other dimensions. This provides a simple solution to the factorization puzzle, and opens up some interesting avenues of research.

Wednesday, April 27
Beatrix Muehlmann, McGill University
Title: Timelike Liouville Theory and the cosmological horizon
Abstract: While the Euclidean two-dimensional gravitational path integral is in general highly fluctuating, it admits a semiclassical two-sphere saddle if coupled to a matter CFT with large and positive central charge. In Weyl gauge this gravity theory is known as timelike Liouville theory, and is conjectured to be a non-unitary two-dimensional CFT. I will discuss the semiclassical computation of the timelike Liouville sphere partition function around the two-sphere saddle and propose an all-loop order result. If time allows I will report on ongoing work studying higher genus contributions as well as a supersymmetric extension of timelike Liouville theory. Since the two-sphere is the geometry of Euclidean two-dimensional de Sitter space our discussion is tied to the conjecture of Gibbons-Hawking, according to which the dS entropy is encoded in the Euclidean gravitational path integral over compact manifolds.

Wednesday, May 4 (NOTE: Talk will be at 9am instead of 3pm)
Yuji Okawa, Tokyo University
Title: Correlation functions of scalar field theories from homotopy algebras
Abstract: When actions are written in terms of homotopy algebras such as $A_\infty$ algebras and $L_\infty$ algebras, expressions of on-shell scattering amplitudes in perturbation theory are universal for both string field theories and ordinary field theories. We thus expect that homotopy algebras can be useful in gaining insights into quantum aspects of string field theories from ordinary field theories. In addition to on-shell scattering amplitudes we find that correlation functions can also be described in terms of homotopy algebras, and in this talk we explain explicit expressions for correlation functions of scalar field theories using quantum $A_\infty$ algebras presented in arXiv:2203.05366. Then we further discuss the application to the renormalization group.

September 8
Jordan Cotler, Harvard
Title: Black hole microstate statistics from Euclidean wormholes

September 15
Geoff Penington, Berkeley
Title:  Pythons and Tensor Networks are a Few of My Favorite Things
Abstract: I’ll solve quantum gravity with pythons and/or tensor networks.

September 22
Daniel Mayerson, IPhT, Saclay
Title: Fuzzball Shadows: Emergent Horizons from Microstructure
Abstract: The advent of black hole imaging has opened a new window into probing the horizon scale of black holes. An important question is whether string theory results for black hole physics can predict interesting and observable features that current and future experiments can probe. I will discuss the physical properties of four-dimensional, string-theoretical, horizonless “fuzzball” geometries by means of imaging their shadows. Their microstructure traps light rays straying near the would-be horizon on long-lived, highly redshifted chaotic orbits. In fuzzballs sufficiently near the scaling limit this creates a shadow much like that of a black hole, while avoiding the paradoxes associated with an event horizon. Finally, I will consider comparing such fuzzball images to their black hole counterparts. In particular, detailed measurements of higher order photon rings have the potential to discriminate between fuzzballs and black holes in future observations.

September 29
Ted Erler, CEICO (remote)
Title: Relating covariant and lightcone string field theories
Abstract: We describe recent work which aims to construct a field transformation which relates Witten’s open bosonic string field theory and the lightcone string field theory of Kaku and Kikkawa.

October 6
Ashoke Sen, Harish-Chandra Institute (remote)
Title: D-instanton amplitudes in string theory
Abstract: I shall review the problems in computing D-instanton
contribution to string amplitudes using the usual world-sheet methods, and
recent progress in overcoming these difficulties using insights from
string field theory.

Tuesday, October 12
Jennifer Lin, Oxford (remote)
Title: A new look at the gravitational entropy formula
Abstract: The Ryu-Takayanagi formula and its generalizations have led to a surprising amount of progress in our understanding of quantum gravity in the last fifteen years, culminating in the recent derivation of the Page curve in toy models of evaporating black holes.  However, we still don’t understand whythese formulas are true from a canonical point of view. In this talk, I will attempt to make progress on this problem by developing an analogy between gravitational entropy formulas in low-dimensional examples of holography and similar-looking formulas that have appeared in the study of entanglement entropy in emergent gauge theories. This talk will be based on 1807.06575, 2107.11872, and 2107.12634.

October 20
Daniel Jafferis, Harvard
Title: TBA

October 27
Ben Heidenreich, Amherst (remote)
Title: TBA

November 3
Fabian Ruehle, Northeastern University
Title: A Physics and a Math Conjecture
Abstract: Recently, many conjectures about the nature of phenomena that can or cannot arise in String Theory or Quantum Gravity have been put forward. One of them, the Swampland Distance Conjecture, relates geodesic motion in the moduli space of compactifications to a change in the spectrum of the theory. Motivated by this, we study geodesics in the Kähler or vector moduli space of Calabi-Yau threefolds described as hypersurfaces or complete intersections in projective or toric ambient spaces. We discuss how geometric flop transitions in these spaces can lead to isomorphic or non-isomorphic Calabi-Yau manifolds. We find that there exist infinite flop chains of isomorphic geometries, but only a finite number of flops to inequivalent manifolds. The latter is expected based on the swampland distance conjecture, and mathematically fits to a conjecture due to Kawamata and Morrison. We also present a classification and analytic solution of all geodesics in 2D (vector) moduli spaces of Calabi-Yau threefolds. The talk is based on 2104.03325 and  2108.10323.

November 9 (Note–this is a Tuesday seminar at 12:15)
Herman Verlinde, Princeton
Title:  Chaos in Celestial CFT
Abstract: Celestial holography proposes that scattering in flat spacetime is captured by a CFT living on the celestial sphere.  I argue that the Hilbert space of celestial CFT can be identified with the Hilbert space of an accelerating Rindler observer in the bulk. Rindler particles exhibit Lyapunov behavior when shockwaves shift the observer horizon.  I show how this chaotic behavior is encoded in celestial CFT, giving evidence that it describes a maximally chaotic system. I comment on how particles can cross the Rindler horizon via a GWJ quantum teleportation protocol.

November 17
Brian Swingle, Brandeis
Title: Effective Field Theory of Chaotic Spectral Correlations
Abstract: Ensembles of quantum chaotic systems are expected to exhibit random matrix universality in their energy spectrum. The presence of this universality can be diagnosed by looking for a linear in time ‘ramp’ in the spectral form factor, but for realistic systems this feature is typically only visible after a sufficiently long time. Given the wide prevalence of this random matrix behavior, it is natural to ask for an effective field theory which predicts the ramp and computes corrections to it arising from physical constraints. I will present such an effective theory based on fluctuating hydrodynamics.

December 1
Xi Yin, Harvard
Title: Some aspects of D-instantons in string theories
Abstract:  I will discuss the effect of D-instantons in type IIB superstring theory, as well as in the non-critical type 0B string theory. I will present new tests of dualities, some previously unknown pieces of string amplitudes, and comment on the role of string field theory. This is based on upcoming works with Agmon, Balthazar, Cho, and Rodriguez.​

February 24
Roberto Emparan
Title: Quantum BTZ black hole

March 3
Kenneth Intriligator, UCSD
Title: RG flows, anomalies, and 2-group global symmetries in 4d and 6d

March 10
Simeon Hellerman, IPMU Tokyo
Title: Quantum Information Theory of the Gravitational Anomaly

March 17
Dieter Luest, LMU Munich
Title: The swampland at a large number of space-time dimensions

March 24
Yasunori Lee, IPMU
Title: Some comments on 6d global gauge anomalies

March 31
Edgar Shaghoulian

April 7
Jorrit Kruthoff, Stanford
Title: Classical aspects of black hole interiors

April 14
Emily Nardoni, UCLA
Title: From SU(N) Seiberg-Witten Theory to Adjoint QCD

April 21
Suvrat Raju, ICTS

April 28
Alejandra Castro

May 5
Xiaoliang Qi, Stanford
Title: Entanglement island, miracle operators and the firewall

May 12
Dan Freed, UT Austin​​

May 19
Sergio Hernandez–Cuenca , UCSB