String/Gravity Theory Seminar Spring 2025

Wednesday, September 11
Albert Schwarz, U.C. Davis
Title: Inclusive scattering matrix
Abstract: Recently I introduced the notion of inclusive scattering matrix
closely related to inclusive cross sections. If a  theory has particle
interpretation  the inclusive scattering matrix contains precisely the
same information as conventional scattering matrix, it can be expressed
in terms of generalized Green functions that appear in Keldysh
formalism of non-equilibrium statistical physics. However, the
inclusive scattering matrix makes sense also in theories that do not
have particle interpretation.  One of the ways to define the inclusive
scattering matrix is based on consideration of L-functionals that were
introduced to get rid of problems related to existence of non-equivalent
 irreducible representations of CCR in the case of infinite number of 
degrees of freedom.  I’ll describe the formalism of
L-functionals and its use in QED.

Wednesday, September 18
Joaquin Turiaci, University of Washington (*Joint seminar with Harvard)
Title: Spin-statistics for black hole microstates 
Abstract: I will describe how to use the gravitational path integral to make universal predictions regarding the distribution of black hole microstates according to whether they have bosonic or fermionic statistics. The gravitational analysis will involve rotating complex solutions that were recently introduced in the context of supersymmetric black holes. I will also describe the consequences of spacetime wormhole contributions to the correlations between the bosonic and fermionic energy levels.

Wednesday, September 25
Gabriel Cuomo, NYU & Princeton
Title: The EFT of Large Spin Mesons
Abstract: As well known, mesons with large spin J in large Nc QCD can be described as rotating open strings using effective field theory (EFT). However, some subtleties arise for light quarks, due to the breakdown of the derivative expansion near the endpoints. I will describe a consistent treatment of such endpoints’ singularities and obtain results, in a systematic 1/J expansion, for the spectrum of the leading and daughter Regge trajectories. Interestingly, the redshift factor associated with the quarks’ acceleration implies that the applicability regime of the EFT is smaller than for static fluxtubes. Depending on time, I will also mention some extensions of the EFT of phenomenological interests, such as the quarks’ spin, and the
worldsheet axion, a massive string mode identified in lattice simulations of 4d fluxtubes. Finally, I will comment on the comparison with data in 4d QCD, and discuss the prospects for using a similar EFT to study the glueball spectrum (closed strings) in Yang-Mills theory.

Wednesday, October 2
Nathan Benjamin, USC
Title:  Angular fractals in thermal CFT
Abstract:  I will discuss universal properties of partition functions at high temperature and large angular fugacity in d>2 conformal field theories (CFTs). This provides spin-refined information — namely the statistics of spins of local operators — valid for all CFTs, that in some sense generalizes modular invariance of d=2 CFTs. As an example application, I will show that the effective free energy of even-spin minus odd-spin operators at high temperature is smaller than the usual free energy by a factor of 1/2^d. Based on arXiv:2405.17562.


Wednesday, October 9
Yiming Chen, Stanford
Title: Fortuity and Chaos of BPS States
Abstract: It was recently suggested that holographic BPS states can be divided into two categories: monotone and fortuitous. Various pieces of evidence suggest that monotone states correspond to horizonless microstate geometries, while fortuitous states are dual to supersymmetric black holes. In this talk, I will connect this classification to the fine-grained features of BPS states using concepts from quantum chaos. In concrete models such as the N=4 SYM theory, explicit calculations demonstrate that monotone states are only weakly chaotic, whereas fortuitous states can exhibit strong chaos due to an “invasion” mechanism. We also propose using the N=2 SUSY SYK model as a toy model to explore many of these ideas.

Wednesday, October 16
Andrea Puhm, University of Amsterdam, Netherlands
Title: Spinning up the black hole – string correspondence
Abstract: The correspondence principle between strings and black holes is a general framework for matching black holes and massive states of fundamental strings at a point where their physical properties (such as mass, entropy and temperature) smoothly agree with each other. As such it offers a statistical interpretation of black hole entropy. I will discuss the extension of this correspondence principle to rotating black holes and strings. Several puzzles arise when attempting to include rotation, but they can be resolved by adding novel ingredients to the correspondence: dynamical features, non-stationary configurations and shapes of strings and black holes. In upcoming work we compare the sizes of rotating objects for small, typical, and large values of the angular momentum and find further support for our proposal.


Wednesday, October 23
Dionysius Annios, Kings College, London
Title: Comments on gravitational boundaries
Abstract: We discuss recent progress on gravitational boundaries of finite spatial size. This is motivated by recent theorems/conjectures on well-posedness for such boundaries, tied with the necessity to understand finite gravitating regions for a variety of questions. We take both a Euclidean and Lorentzian perspective and, time permitting discuss different values of the cosmological constant. Part of the talk is based on 2402.04305 2310.08648

Wednesday, October 30
Theo Jacobson, UCLA
Title: Gauging C on the Lattice
Abstract: We will discuss general aspects of charge conjugation symmetry in Euclidean lattice field theories including its dynamical gauging. As an application, we construct O(2) gauge theory on the lattice using a non-abelian generalization of the Villain formulation. This lattice discretization preserves the non-invertible and higher-group global symmetries of the continuum theory, whose symmetry operators we construct explicitly at the lattice level. We discuss their implications, including selection rules for extended operators and the structure of the phase diagram. 

Wednesday, November 6
Kristan Jensen, U. of Victoria
Title: Quantizing Carrollian field theories
Abstract: Conformal Carrollian field theories have recently emerged as a candidate holographic dual to flat space quantum gravity. The basic idea is that if there is a dual description along null infinity, it ought to be invariant under the conformal Carroll group, the ultrarelativistic c->0 limit of the relativistic conformal group, since that is how Poincare acts at null infinity. It is straightforward to write down Carrollian field theories, but as I will explain their quantum mechanical behavior is quite subtle. For example, two-derivative Carrollian theories often exhibit UV/IR mixing reminiscent of the recent literature on fractons, and in a sense I will explain such theories are ones of generalized free fields and infinite effective central charge. These results pose challenges for the Carrollian holography program.

Wednesday, November 13
Daine Danielson, University of Chicago
Title: Asymptotic Symmetry Induced Decoherence and Superselection in QED and Quantum Gravity
Abstract: We use asymptotic symmetry arguments to show that the only physical states in QED scattering theory are maximally entangled states, in which electron (or positron) momenta at timelike infinity are entangled with soft radiation with memory at null infinity. This radiation is in addition to the electron’s Coulomb field, which is not radiation and accompanies the electron to timelike infinity. States lacking this soft, entangling radiation fail to localize in the bulk, due to superselection in the momentum basis. This has not obstructed collider predictions, because the central-momentum dependence of scattering cross sections is insensitive to such delocalization. Nevertheless, this soft radiation is required for the consistency of the local theory. In quantum gravity, an analogous argument suggest that all gauge-invariant observables of the theory must be nonlocal in nature. This points to a theory of relational observables, or extended objects. Some resulting physical phenomena are discussed.

Wednesday, November 20
Hong Zhe (Vincent) Chen, UC Santa Barbara
Title: Disentanglement as a strong cosmic censor
Abstract: If entanglement builds spacetime, then conversely, disentanglement ought to destroy spacetime. From the quantum null energy condition and quantum focusing conjecture, I derive disentanglement criteria which necessitate infinite energies and strong spacetime singularities. These results are applied to the strong cosmic censorship proposal, where singularities at the Cauchy horizons in black holes are desirable. Using the disentanglement criteria and without resorting to any detailed calculations, I provide an exceedingly general and physically transparent discussion of strong cosmic censorship in semiclassical black holes. I argue that strong cosmic censorship is enforced in asymptotically flat and de Sitter black holes by disentanglement across putative Cauchy horizons and describe how similar disentanglement might be avoided in some anti-de Sitter cases. I will also present calculations of mutual information in two-dimensional CFTs to explicitly verify some intuition about (dis)entanglement on black hole backgrounds developed in the more general argument.

Wednesday, November 27
No Talk

Wednesday, December 4
 Hsin-Yuan Huang (Robert), CTP

Speaker:  Hsin-Yuan  (Robert) Huang, CTP

Title: What cannot be learned in the quantum universe

Abstract: Recent advances have significantly enhanced our understanding of what can be efficiently learned in the quantum universe. However, certain fundamental aspects remain resistant to efficient learning using known algorithms. This talk explores several fundamental properties—including time, causal structure, topological order, noise—and demonstrates how they can be provably hard to learn. These results stem from our recent work on how to construct random unitaries (with Ma, arxiv:2410.10116) and generate them in extremely low depth (with Schuster and Haferkamp, arXiv:2407.07754). Examining these “unlearnable” aspects of our world sheds light on the fundamental limits of scientific inquiry in the quantum realm.

Wednesday, December 11
Damian van de Heisteeg, Harvard
Title: Finiteness and the Emergence of Dualities

Abstract: We argue that the finiteness of quantum gravity (QG) amplitudes in fully compactified theories (at least in supersymmetric cases) leads to a bottom-up prediction for the existence of non-trivial dualities. In particular, finiteness requires the moduli space of massless fields to be compactifiable, meaning that its volume must be finite or at least grow no faster than that of Euclidean space. Moreover, we relate the compactifiability of moduli spaces to the condition that the lattice of charged objects transform in a semisimple representation under the action of the duality group. These ideas are supported by a wide variety of string theory examples.

Wednesday, February 14
Matthew Headrick, Brandeis
Title: Geometric Surprises in the Python’s Lunch Conjecture
Abstract: A bulge surface, on a time reflection-symmetric Cauchy slice of a holographic spacetime, is a non-minimal extremal surface that occurs between two locally minimal surfaces homologous to a given boundary region. The python’s lunch conjecture of Brown et al. relates the bulge area to the complexity of the state.
We study the geometry of bulges in a variety of classical spacetimes, and discover a number of surprising features that distinguish them from more familiar extremal surfaces such as Ryu-Takayanagi surfaces: they spontaneously break spatial isometries, both continuous and discrete; they are sensitive to the choice of boundary infrared regulator; they can self-intersect; and they probe entanglement shadows, orbifold singularities, and compact spaces such as the sphere in AdS_p x S^q. These features imply, according to the python’s lunch conjecture, novel qualitative differences between complexity and entanglement in the holographic context. We also find, surprisingly, that extended black brane interiors have a non-extensive complexity; similarly, for multi-boundary wormhole states, the complexity pleateaus after a certain number of boundaries have been included.

Wednesday, February 28
Ryan Unger, Princeton
Title: Formation of extremal black holes: critical collapse and the third law
Abstract: According to the third law of black hole thermodynamics, extremal black holes are a physically inaccessible ideal limit. In this talk, I will present a definitive disproof of the third law by showing that extremal black holes can form in gravitational collapse. In fact, we show that extremal black holes take on a central role in gravitational collapse: they arise on the black hole formation threshold, giving rise to a phenomenon that we call extremal critical collapse. This is joint work with Christoph Kehle (ETH Zürich, soon to be AP at MIT in math).

Wednesday, March 6
Edward Mazenc, ETH Zurich
Title: Strings From Feynman Diagrams
Abstract: How are bulk strings related to boundary Feynman diagrams? I will give an overview of my work with Rajesh Gopakumar on deriving the closed string dual to the simplest possible gauge theory, a Hermitian matrix integral. Working in the conventional ‘t Hooft limit, we extract topological string theories which replace the minimal string away from the double-scaling limit. We show how to exactly reconstruct both the closed string worldsheet and its embedding into the emergent target space, purely from the matrix Feynman diagrams. Along the way, we will encounter the notion of open-closed-open triality which allows us to establish this dictionary, and predicts multiple open string descriptions of the same bulk physics. I’ll close by embedding our results in the broader context of AdS/CFT.

Wednesday, March 13
Gautam Satishchandran
Title: Generalized Black Hole Entropy is von Neumann Entropy
Abstract: It was recently shown that the von Neumann algebras of observables dressed to the mass of a Schwarzschild-AdS black hole or an observer in de Sitter are Type II, and thus admit well-defined traces. The von Neumann entropies of “semi-classical” states were found to be generalized entropies. However, these arguments relied on the existence of an equilibrium (KMS) state and thus do not apply to, for example, black holes in asymptotically flat spacetimes or asymptotically de Sitter spacetime. In this talk, I will show that the algebra of observables in the “exterior” of any black horizon always contains a Type II factor “localized” on the horizon and the entropy of semi-classical states is the generalized entropy.

Wednesday, March 20 (Virtual Seminar)
Sergey Alexandrov
Title: Modular bootstrap for compact Calabi-Yau threefolds
Abstract: BPS indices encoding entropy of supersymmetric black holes in compactifications of Type II string theory on compact Calabi-Yau threefolds coincide with generalized Donaldson-Thomas invariants whose computation represents an outstanding problem.
I’ll show how this problem can be solved for a set of one-parameter threefolds by combining a direct integration of topological string, modular properties of rank 0 DT invariants counting D4-D2-D0 BPS states, and wall-crossing relations between rank 1 and rank 0 DT invariants. As a result, one obtains explicit (mock) modular functions encoding infinite sets of D4-D2-D0 BPS indices, which in turn can be used to generate new boundary conditions for the holomorphic anomaly equation of topological string allowing to overcome the limitations of the direct integration method.

Wednesday, April 10
Aidan Herderschee, Princeton
Title: Three Point Amplitudes and Emergent Symmetries in Matrix Theory
Abstract: The Banks-Fischler-Shenker-Susskind (BFSS) theory is a matrix model conjecturally dual to M-theory in eleven dimensional asymptotically flat space when the size of the matrices is taken large. The model has many desirable features due to its relative simplicity. For example, as a quantum mechanical model, the model avoids many of the usual issues plaguing quantum field theories. However, the model is nevertheless poorly understood. After giving a review of the BFSS model, I will give an overview of some recent results. First, I will argue that three point amplitudes in the matrix model can be related to a supersymmetric index. This index-amplitude relation can be used to compute three point amplitudes in regimes that are naively not holographic. I will then leverage the three point amplitude result, in combination with other assumptions, to argue for an emergent Lorentz symmetry in the holographic regime.

Zahra Zahradee, CERN
Title: Bootstrapping N=4 SYM using integrability
Abstract: In this talk we use integrability data to bootstrap correlation functions of planar maximally supersymmetric Yang- Mills theory, focusing on four-point correlation function of stress-tensor. First, we start by demonstrating why the conventional bootstrap approach fails and new techniques are required. Next, we introduce a set of sum rules that are tailored for this problem as there are only sensitive to single-traces in the OPE expansion. Integrability enters at this stage and provides information on the spectrum of these operators. Their OPE coefficients however remain unknown. We then discuss how these sum rules can be employed in numerical bootstrap to nonperturbatively bound the OPE coefficients. We show, for the first time, rigorous bounds for the planar OPE coefficient of single-trace operators at various t’Hooft couplings well outside the perturbative regime. This talk is based on an upcoming paper and 2207.01615.

Wednesday, 5/1
Savdeep Sethi, U. Chicago
Title: The Heterotic String Potential Energy
Abstract: In the first part of my talk, I will explore the potential energy for the $O(16)\times O(16)$ non-supersymmetric string compactified on a circle. Even in this simplest of compactifications, there are new surprises. For example, a distinguished critical point of the potential energy that was believed to be a minimum for many years was instead found to be a saddle point. In fact all the critical points we currently know on the circle are saddle points. I will then describe how to stabilize the dilaton and build perturbatively stable solutions of this non-supersymmetric string with $AdS_3$ spacetimes. If time permits, I will end by sketching a speculative connection with quantum field theories deformed by special irrelevant operators with features similar to the TT operator.

Organizers: Jonathan Sorce, Sergio Hernandez-Cuenca, Ho Tat Lam, Manki Kim, Daniel Harlow, Washington Taylo

Wednesday, September 20 *4:15 pm talk
Richard Nally, Cornell
Title: Evidence for KKLT de Sitter Vacua
Abstract: The construction of de Sitter vacua remains a core problem for string theory.  A leading proposal, put forth 20 years ago by Kachru, Kallosh, Linde, and Trivedi, calls for the insertion of antibranes into a supersymmetric AdS vacuum with a conifold.  In this talk, we provide evidence for the existence of such vacua.  We begin by constructing the first examples of conifold AdS vacua with all moduli stabilized.  We then incorporate antibranes, working to leading order in the alpha’ expansion, and construct an ensemble of candidate de Sitter vacua. We conclude with comments on the validity of the EFT, including the future prospects of finding dS candidates whose order parameters are such that they demonstrably survive alpha’ corrections.

Wednesday, September 27
Naomi Gendler, Harvard
Title: Glimmers from the Axiverse
Abstract: We study axion-photon couplings in compactifications of type IIB string theory. We find that these couplings are systematically suppressed compared to the inverse axion periodicity, as a result of two effects.  First, couplings to the QED theta angle are suppressed for axion mass eigenstates that are light compared to the mass scale set by stringy instantons on the cycle supporting QED.  Second, in compactifications with many axions the intersection matrix is sparse, making kinetic mixing weak. We study the resulting phenomenology in an ensemble of  explicit models constructed from the Kreuzer-Skarke database up to the maximum Hodge number $h^{1,1}=491$. We conclude that compactifications in this corner of the landscape involve many invisible axions, as well as a handful that may be detectable via photon couplings.


Hirosi Ooguri, Caltech
Title: Symmetry Resolution at High Energy
Abstract: The density of states of a unitary conformal field theory is known to have a universal behavior at high energy. In two dimensions, this behavior is described by the Cardy formula. If the theory has symmetry, it is interesting to understand the symmetry resolution, namely the decomposition of its Hilbert space into irreducible representations of the symmetry. In this talk, I will present universal formulas for the symmetry resolution with respect to both internal global symmetry and spacetime symmetry. I will also discuss the entanglement entropy, the entanglement spectra, and the high energy properties of the three-point functions. The formulas are derived by using thermal effective field theory and are applicable to any unitary conformal field theory in arbitrary spacetime dimensions. 

Wednesday, October 11
Jinzhao Wang, Stanford University
Title: What exactly does Bekenstein Bound?
Abstract: The Bekenstein bound posits a maximum entropy for matter with finite energy confined to a spacetime region. It is often interpreted as a fundamental limit on the information that can be stored by physical objects. In this work, we test this interpretation by asking whether the Bekenstein bound imposes constraints on a channel’s communication capacity, a context in which information can be given a mathematically rigorous and operationally meaningful definition. We first derive a bound on the accessible information and demonstrate that the Bekenstein bound constrains the decoding instead of the encoding. Then we study specifically the Unruh channel that describes a stationary Alice exciting different species of free scalar fields to send information to an accelerating Bob, who is therefore confined to a Rindler wedge and exposed to the noise of Unruh radiation. We show that the classical and quantum capacities of the Unruh channel obey the Bekenstein bound. In contrast, the entanglement-assisted capacity is as large as the input size even at arbitrarily high Unruh temperatures. This reflects that the Bekenstein bound can be violated if we do not properly constrain the decoding operation in accordance with the bound. We further find that the Unruh channel can transmit a significant number of zero-bits, which are communication resources that can be used as minimal substitutes for the classical/quantum bits needed for many primitive information processing protocols, such as dense coding and teleportation. We show that the Unruh channel has a large zero-bit capacity even at high temperatures, which underpins the capacity boost with entanglement assistance and allows Alice and Bob to perform quantum identification. Therefore, unlike classical bits and qubits, zero-bits and their associated information processing capability are not constrained by the Bekenstein bound.

(This talk is based on the recent work (https://arxiv.org/abs/2309.07436) with Patrick Hayden.)

Wednesday, October 18
Yangrui Hu, Perimeter Institute
Title: The Fate of Celestial Symmetries
Abstract: In this talk, we start by introducing celestial symmetries and using these symmetries to organize the data in phase space. Specifically, a systematic cataloging of the generators of celestial symmetries on phase space is presented, which contains a semi-infinite tower of higher-spin light-ray operators. Moreover, in the phase space representation, the emergence of two-particle and multi-particle operators is a natural consequence, essential for the algebra to close. To study the multi-particle states in celestial CFT, in the second part of this talk, we discuss the holomorphic multi-collinear limit of amplitudes and derive celestial 3-OPEs. In particular, in $\phi^3$ theory, celestial 3-OPE contains a term with a branch cut. This term addresses an ongoing debate in the literature about Jacobi identity and associativity of the celestial OPEs. We further use this result to deduce a new (multi-particle) term in celestial two-particle OPE. 

Wednesday, October 25 *virtual seminar*
Gonzalo Torroba, Bariloche
Title: Hyperbolic compactifications and de Sitter quantum gravity
Abstract: In this talk we present de Sitter solutions obtained by compactifications of M-theory with minimal ingredients. We argue that M-theory on a hyperbolic manifold with small closed geodesics supporting Casimir energy, along with 7-form flux, contains a 3-term structure for volume stabilization at positive potential energy. Field deformations are highly constrained by hyperbolic rigidity and warping effects, leading to calculations giving strong indications of a positive Hessian. We test this via explicit backreacted solutions and perturbations in patches, and initiate a neural network study of further aspects of the internal fields. As a relatively simple de Sitter uplift of the large-N M2-brane theory, the construction provides an explicit setup for de Sitter holography, and introduces new connections between mathematics and the physics of string/M theory compactifications.
Based on SciPost Phys. 12 (2022) 083.

Wednesday, November 1
Suzanne Bintanja, U. Amsterdam
Title: A toolbox for holographic CFTs
Abstract: Holographic CFTs are CFTs whose gravitational dual have a low energy effective description of semiclassical general relativity minimally coupled to (possibly interacting) matter. Such CFTs must satisfy stringent constraints. In this talk I will review these constraints and discuss how to find holographic CFTs using conformal manifolds. The conformal manifold can be explored using exactly marginal deformations, which come in many different flavours. I will discuss the effects of the different types of marginal deformations and in doing so we will see how this toolbox can be used to explore the space of holographic CFTs.

Wednesday, November 8
Nima Lashkari, Perdue University
Title: Chaos in Observable Algebras of Quantum Gravity
Abstract: The Hawking-Page phase transition in holography is associated with the emergence of new symmetries and dynamical properties of correlators. Above the transition point: (1) Correlators cluster in time (Maldacena’s information loss) (2) There exists a coarse-grained entropy that grows monotonically in time (Second law) (3) A large class of correlators decay exponentially (Quasi-normal modes) (4) There is an emergent approximate Lie group  (near-horizon symmetries).

I discuss the properties above from the point of view of the modular flow of the observable algebra of quantum gravity on the boundary. I prove that property (1) implies that the observable algebra is a type III_1 von Neumann factor. I point out that there is a class of quantum ergodic systems (K-systems) characterized by the existence of future and past subalgebras that satisfy all the four properties above. In other words, modular K-systems are maximally chaotic. I comment on the implications of the results above for the emergence of spacetime in holography, the generalizations of these results beyond modular flow, and the quantum ergodic hierarchy.

Wednesday, November 15
Hofie Hannesdottir, Princeton
Title: What can be measured asymptotically?
Abstract: We consider asymptotic observables in quantum field theories in which the S-matrix makes sense. We argue that in addition to scattering amplitudes, a whole compendium of inclusive observables exists where the time ordering is relaxed. These include expectation values of electromagnetic or gravitational radiation fields as well as out-of-time-order amplitudes. We explain how to calculate them in different ways: by relating them to amplitudes and products of amplitudes, and by using generalizations of the LSZ reduction formula. We show how the generalized reduction formulas lead to constraints on discontinuities and cuts of asymptotic observables. Finally, we discuss how to relate asymptotic observables to one another through new versions of crossing symmetry. As an application, we discuss computations of gravitational waveforms emitted in black-hole scattering, emphasizing the role of classical cut contributions and highlighting the infrared physics of in-in observables.

*Monday, November 20 (Virtual speaker) at 1:00 PM
Carlo Maccaferri, Turin University
Title: On open-closed duality in string field theory 
Abstract:  I approach the problem of open-closed duality through the  perspective of string field theory (SFT) where I provide a  description of the backreaction of a large N stack of D-branes as a new closed string background, possibly without D-branes anymore. To achieve this, I first of all give a new convenient formulation of open-closed SFT based on a single open-closed nilpotent structure which captures the consistency of the theory. Then I perform the ‘t Hooft large N limit, obtaining at the leading order a classical closed SFT plus a quantum but planar open SFT. I then discuss  the integration-out the open string sector applying the so-called homotopy transfer to this new nilpotent structure, ending up with a purely classical closed SFT. The obtained closed string theory have tadpoles, whose strength is controlled by the ‘t Hooft coupling. To get rid of the tadpoles I finally perform a vacuum shift, which describes closed strings in the backreacted new background, without D-branes. I discuss this construction in the context of minimal string theory where, following well-known results by Gaiotto and Rastelli, I show how we can move in the space of $(2,p)$ closed string backgrounds by computing the backreaction of a large number of FZZT branes in the $(2,1)$ background. This talk is based on  2305.02843, 2305.02844 and work in progress.


Wednesday, November 29
Yin-Chen He, Perimeter University
Title: Fuzzy sphere regularization of 3D CFTs
Abstract: Conformal Field Theory (CFT) represents a class of quantum field theories that has profound applications across various physics domains, from critical phenomena in statistical mechanics to quantum matter, quantum gravity, and string theory. In this talk, I will introduce our recently proposed ‘fuzzy (non-commutative) sphere regularization’ scheme, a method that addresses and offers a solution to the longstanding need for a non-perturbative approach to 3D CFTs. I will first elucidate its fundamental concepts and then diving into illustrative examples, particularly, the 3D Ising transition. Specifically, we showcase that this scheme is not only potent—revealing a wealth of universal data on 3D CFTs otherwise inaccessible through existing methods—but also efficient, as the necessary computations can be performed on a laptop within an hour. Our innovative scheme not only heralds a new era for the study of CFTs but also hints at a profound interplay between non-commutative geometry and both CFTs and QFTs at large.

*Friday, December 8 (note different day)
Muyang Liu, University of Southern Denmark
Title: Families of T-dual Little String Theories from Heterotic Strings on ALE spaces
Abstract: 6D Little string theories (LSTs) are UV-complete non-local theories that are decoupled from gravity with an intrinsic string scale. In this talk, I will revisit the properties of 6D (1,0) LSTs obtained from heterotic string compactifications on ALE spaces. So far, instantonic heterotic E8 × E8 theories are rarely visited due to the lack of the Lagrangian description, with the exception of a few cases explored by Aspinwall and Morrison as well as Blum and Intriligator. I will present the construction of novel heterotic E8 × E8 ALE instantonic theories using the 6D conformal matter approach. All LSTs have a continuous 2-group symmetry structure; the relevant 2-group data, combined with Coulomb and tensor branch data, provide matching criteria for those novel theories to be predicted as T-dual partners with the known Spin(32)/Z2 ALE instantonic LSTs from the theoretical perspective. These predictions are proven by utilizing duality with the geometric engineering in F-theory, where the T-dual system is realized as the inequivalent elliptic fibration structure of the same geometry in the toric construction. I will also explore the geometric engineering limit for the Heterotic string on an ALE singularity. In the end, the Heterotic/F-theory duality shows that the Calabi-Yau threefold in our consideration also admits a nested elliptic K3 fibration structure. The underlying K3 fiber determines the flavor group and the corresponding global forms, and inequivalent elliptic fibration structure read from the K3 has the potential of building much more enriched families of T-dual 

Wednesday, December 13
Seminar cancelled
Wednesday, September 20 *4:15 pm talk
Richard Nally, Cornell
Title: Evidence for KKLT de Sitter Vacua
Abstract: The construction of de Sitter vacua remains a core problem for string theory.  A leading proposal, put forth 20 years ago by Kachru, Kallosh, Linde, and Trivedi, calls for the insertion of antibranes into a supersymmetric AdS vacuum with a conifold.  In this talk, we provide evidence for the existence of such vacua.  We begin by constructing the first examples of conifold AdS vacua with all moduli stabilized.  We then incorporate antibranes, working to leading order in the alpha’ expansion, and construct an ensemble of candidate de Sitter vacua. We conclude with comments on the validity of the EFT, including the future prospects of finding dS candidates whose order parameters are such that they demonstrably survive alpha’ corrections.

Wednesday, September 27
Naomi Gendler, Harvard
Title: Glimmers from the Axiverse
Abstract: We study axion-photon couplings in compactifications of type IIB string theory. We find that these couplings are systematically suppressed compared to the inverse axion periodicity, as a result of two effects.  First, couplings to the QED theta angle are suppressed for axion mass eigenstates that are light compared to the mass scale set by stringy instantons on the cycle supporting QED.  Second, in compactifications with many axions the intersection matrix is sparse, making kinetic mixing weak. We study the resulting phenomenology in an ensemble of  explicit models constructed from the Kreuzer-Skarke database up to the maximum Hodge number $h^{1,1}=491$. We conclude that compactifications in this corner of the landscape involve many invisible axions, as well as a handful that may be detectable via photon couplings.

Wednesday, October 4
Hirosi Ooguri, Caltech
Title: Symmetry Resolution at High Energy
Abstract: The density of states of a unitary conformal field theory is known to have a universal behavior at high energy. In two dimensions, this behavior is described by the Cardy formula. If the theory has symmetry, it is interesting to understand the symmetry resolution, namely the decomposition of its Hilbert space into irreducible representations of the symmetry. In this talk, I will present universal formulas for the symmetry resolution with respect to both internal global symmetry and spacetime symmetry. I will also discuss the entanglement entropy, the entanglement spectra, and the high energy properties of the three-point functions. The formulas are derived by using thermal effective field theory and are applicable to any unitary conformal field theory in arbitrary spacetime dimensions. 

Wednesday, October 11
Jinzhao Wang, Stanford University
Title: What exactly does Bekenstein Bound?
Abstract: The Bekenstein bound posits a maximum entropy for matter with finite energy confined to a spacetime region. It is often interpreted as a fundamental limit on the information that can be stored by physical objects. In this work, we test this interpretation by asking whether the Bekenstein bound imposes constraints on a channel’s communication capacity, a context in which information can be given a mathematically rigorous and operationally meaningful definition. We first derive a bound on the accessible information and demonstrate that the Bekenstein bound constrains the decoding instead of the encoding. Then we study specifically the Unruh channel that describes a stationary Alice exciting different species of free scalar fields to send information to an accelerating Bob, who is therefore confined to a Rindler wedge and exposed to the noise of Unruh radiation. We show that the classical and quantum capacities of the Unruh channel obey the Bekenstein bound. In contrast, the entanglement-assisted capacity is as large as the input size even at arbitrarily high Unruh temperatures. This reflects that the Bekenstein bound can be violated if we do not properly constrain the decoding operation in accordance with the bound. We further find that the Unruh channel can transmit a significant number of zero-bits, which are communication resources that can be used as minimal substitutes for the classical/quantum bits needed for many primitive information processing protocols, such as dense coding and teleportation. We show that the Unruh channel has a large zero-bit capacity even at high temperatures, which underpins the capacity boost with entanglement assistance and allows Alice and Bob to perform quantum identification. Therefore, unlike classical bits and qubits, zero-bits and their associated information processing capability are not constrained by the Bekenstein bound.

(This talk is based on the recent work (https://arxiv.org/abs/2309.07436) with Patrick Hayden.)

Wednesday, October 18
Yangrui Hu, Perimeter Institute
Title: The Fate of Celestial Symmetries
Abstract: In this talk, we start by introducing celestial symmetries and using these symmetries to organize the data in phase space. Specifically, a systematic cataloging of the generators of celestial symmetries on phase space is presented, which contains a semi-infinite tower of higher-spin light-ray operators. Moreover, in the phase space representation, the emergence of two-particle and multi-particle operators is a natural consequence, essential for the algebra to close. To study the multi-particle states in celestial CFT, in the second part of this talk, we discuss the holomorphic multi-collinear limit of amplitudes and derive celestial 3-OPEs. In particular, in $\phi^3$ theory, celestial 3-OPE contains a term with a branch cut. This term addresses an ongoing debate in the literature about Jacobi identity and associativity of the celestial OPEs. We further use this result to deduce a new (multi-particle) term in celestial two-particle OPE. 

Wednesday, October 25 *virtual seminar*
Gonzalo Torroba, Bariloche
Title: Hyperbolic compactifications and de Sitter quantum gravity
Abstract: In this talk we present de Sitter solutions obtained by compactifications of M-theory with minimal ingredients. We argue that M-theory on a hyperbolic manifold with small closed geodesics supporting Casimir energy, along with 7-form flux, contains a 3-term structure for volume stabilization at positive potential energy. Field deformations are highly constrained by hyperbolic rigidity and warping effects, leading to calculations giving strong indications of a positive Hessian. We test this via explicit backreacted solutions and perturbations in patches, and initiate a neural network study of further aspects of the internal fields. As a relatively simple de Sitter uplift of the large-N M2-brane theory, the construction provides an explicit setup for de Sitter holography, and introduces new connections between mathematics and the physics of string/M theory compactifications.
Based on SciPost Phys. 12 (2022) 083.

Wednesday, November 1
Suzanne Bintanja, U. Amsterdam
Title: A toolbox for holographic CFTs
Abstract: Holographic CFTs are CFTs whose gravitational dual have a low energy effective description of semiclassical general relativity minimally coupled to (possibly interacting) matter. Such CFTs must satisfy stringent constraints. In this talk I will review these constraints and discuss how to find holographic CFTs using conformal manifolds. The conformal manifold can be explored using exactly marginal deformations, which come in many different flavours. I will discuss the effects of the different types of marginal deformations and in doing so we will see how this toolbox can be used to explore the space of holographic CFTs.

Wednesday, November 8
Nima Lashkari, Perdue University
Title: Chaos in Observable Algebras of Quantum Gravity
Abstract: The Hawking-Page phase transition in holography is associated with the emergence of new symmetries and dynamical properties of correlators. Above the transition point: (1) Correlators cluster in time (Maldacena’s information loss) (2) There exists a coarse-grained entropy that grows monotonically in time (Second law) (3) A large class of correlators decay exponentially (Quasi-normal modes) (4) There is an emergent approximate Lie group  (near-horizon symmetries).

I discuss the properties above from the point of view of the modular flow of the observable algebra of quantum gravity on the boundary. I prove that property (1) implies that the observable algebra is a type III_1 von Neumann factor. I point out that there is a class of quantum ergodic systems (K-systems) characterized by the existence of future and past subalgebras that satisfy all the four properties above. In other words, modular K-systems are maximally chaotic. I comment on the implications of the results above for the emergence of spacetime in holography, the generalizations of these results beyond modular flow, and the quantum ergodic hierarchy.

Wednesday, November 15
Hofie Hannesdottir, Princeton
Title: What can be measured asymptotically?
Abstract: We consider asymptotic observables in quantum field theories in which the S-matrix makes sense. We argue that in addition to scattering amplitudes, a whole compendium of inclusive observables exists where the time ordering is relaxed. These include expectation values of electromagnetic or gravitational radiation fields as well as out-of-time-order amplitudes. We explain how to calculate them in different ways: by relating them to amplitudes and products of amplitudes, and by using generalizations of the LSZ reduction formula. We show how the generalized reduction formulas lead to constraints on discontinuities and cuts of asymptotic observables. Finally, we discuss how to relate asymptotic observables to one another through new versions of crossing symmetry. As an application, we discuss computations of gravitational waveforms emitted in black-hole scattering, emphasizing the role of classical cut contributions and highlighting the infrared physics of in-in observables.

*Monday, November 20 (Virtual speaker) at 1:00 PM
Carlo Maccaferri, Turin University
Title: On open-closed duality in string field theory 
Abstract:  I approach the problem of open-closed duality through the  perspective of string field theory (SFT) where I provide a  description of the backreaction of a large N stack of D-branes as a new closed string background, possibly without D-branes anymore. To achieve this, I first of all give a new convenient formulation of open-closed SFT based on a single open-closed nilpotent structure which captures the consistency of the theory. Then I perform the ‘t Hooft large N limit, obtaining at the leading order a classical closed SFT plus a quantum but planar open SFT. I then discuss  the integration-out the open string sector applying the so-called homotopy transfer to this new nilpotent structure, ending up with a purely classical closed SFT. The obtained closed string theory have tadpoles, whose strength is controlled by the ‘t Hooft coupling. To get rid of the tadpoles I finally perform a vacuum shift, which describes closed strings in the backreacted new background, without D-branes. I discuss this construction in the context of minimal string theory where, following well-known results by Gaiotto and Rastelli, I show how we can move in the space of $(2,p)$ closed string backgrounds by computing the backreaction of a large number of FZZT branes in the $(2,1)$ background. This talk is based on  2305.02843, 2305.02844 and work in progress.


Wednesday, November 29
Yin-Chen He, Perimeter University
Title: Fuzzy sphere regularization of 3D CFTs
Abstract: Conformal Field Theory (CFT) represents a class of quantum field theories that has profound applications across various physics domains, from critical phenomena in statistical mechanics to quantum matter, quantum gravity, and string theory. In this talk, I will introduce our recently proposed ‘fuzzy (non-commutative) sphere regularization’ scheme, a method that addresses and offers a solution to the longstanding need for a non-perturbative approach to 3D CFTs. I will first elucidate its fundamental concepts and then diving into illustrative examples, particularly, the 3D Ising transition. Specifically, we showcase that this scheme is not only potent—revealing a wealth of universal data on 3D CFTs otherwise inaccessible through existing methods—but also efficient, as the necessary computations can be performed on a laptop within an hour. Our innovative scheme not only heralds a new era for the study of CFTs but also hints at a profound interplay between non-commutative geometry and both CFTs and QFTs at large.

*Friday, December 8 (note different day)
Muyang Liu, University of Southern Denmark
Title: Families of T-dual Little String Theories from Heterotic Strings on ALE spaces
Abstract: 6D Little string theories (LSTs) are UV-complete non-local theories that are decoupled from gravity with an intrinsic string scale. In this talk, I will revisit the properties of 6D (1,0) LSTs obtained from heterotic string compactifications on ALE spaces. So far, instantonic heterotic E8 × E8 theories are rarely visited due to the lack of the Lagrangian description, with the exception of a few cases explored by Aspinwall and Morrison as well as Blum and Intriligator. I will present the construction of novel heterotic E8 × E8 ALE instantonic theories using the 6D conformal matter approach. All LSTs have a continuous 2-group symmetry structure; the relevant 2-group data, combined with Coulomb and tensor branch data, provide matching criteria for those novel theories to be predicted as T-dual partners with the known Spin(32)/Z2 ALE instantonic LSTs from the theoretical perspective. These predictions are proven by utilizing duality with the geometric engineering in F-theory, where the T-dual system is realized as the inequivalent elliptic fibration structure of the same geometry in the toric construction. I will also explore the geometric engineering limit for the Heterotic string on an ALE singularity. In the end, the Heterotic/F-theory duality shows that the Calabi-Yau threefold in our consideration also admits a nested elliptic K3 fibration structure. The underlying K3 fiber determines the flavor group and the corresponding global forms, and inequivalent elliptic fibration structure read from the K3 has the potential of building much more enriched families of T-dual 

Wednesday, December 13
Seminar cancelled

Wednesday, September 14
Zhenbin Yang, Stanford (Zoom)
Title: Firewalls from wormholes
Abstract: Spacetime wormholes can lead to surprises in black hole physics. We show that a very old black hole can tunnel to a white hole/firewall by emitting a large baby universe. We study the process for a perturbed thermofield double black hole in Jackiw-Teitelboim (JT) gravity, using the lowest order (genus one) spacetime wormhole that corresponds to single baby-universe emission. The probability for tunneling to a white hole is proportional to t2e−2S where t is the age of the black hole and S is the entropy of one black hole.

Wednesday, September 21
Clifford Johnson, USC
Title: Random Matrix Spectroscopy and Quantum Gravity
Abstract: It is argued that in order to fully interpret the meaning of random matrix model formulations of JT gravity and related 2D gravity models, it is essential to have a fully non-perturbative formulation. Rather than supply an ensemble dual, as is so often stated (with puzzling consequences), the RMM is instead a natural Wignerian toolbox for understanding the properties of the 1D holographic dual, in particular the discrete black hole spectrum implied by the Bekenstein-Hawking entropy. Within this framework, black hole thermodynamics, as well as the emergence of wormholes, are simply understood. New analytic results for the properties of the ground state distribution function are also presented. It is a gravitational analogue of the Tracy-Widom distribution.

Wednesday, September 28
 Ying-Hsuan Lin, Harvard
Title: Words to describe a black hole
Abstract: We revamp the constructive enumeration of 1/16-BPS states in the maximally supersymmetric Yang-Mills in four-dimensions, and search for ones that are not of multi-graviton form.  A handful of such states are found for gauge group SU(2) at relatively high energies, resolving a decade-old enigma.  Along the way, we clarify various subtleties in the literature, and prove a non-renormalization theorem about the exactness of the cohomological enumeration in perturbation theory.  We point out a giant-graviton-like feature in our results, and envision that a deep analysis of our data will elucidate fundamental properties of black hole microstates.

Wednesday, October 5
Indranil Halder, Harvard University
Title: Counting stringy microstates of blackholes and beyond
Abstract: In the first part of the talk we will consider bosonic string theory on AdS₃ supported by Kalb-Ramond flux. It is well known that the α′ exact worldsheet theory is described by the SL(2,R) WZW model. We’ll present an α′ exact dual description involving a winding condensate on a free theory background. We give the explicit map of vertex operators for normalizable states on both sides of the duality and demonstrate the equivalence of their two and three point functions by direct computation. The duality is of strong-weak nature in α′ making the winding description to be very “close” to the spacetime CFT – we hope that this woulds shed lights on ER=EPR when the modular invariance of the spacetime CFT is taken into account. In the second part of the talk, we will look into flat space M-theory on quintic and discuss counting super-symmetric blackhole and blackring microstates using the help of topological strings. In particular we will see an interpretation of topological stings Ward identities in terms of unitarity of the superstring scattering amplitudes.

Wednesday, October 12 (Zoom)
Marc Henneaux, Université libre de Bruxelles
Title: Description of the BMS symmetry at Spatial infinity
Abstract: The asymptotic structure of gravity in the asymptotically flat case will be described in four and higher spacetime dimensions by making central use of the Hamiltonian formalism. The crucial distinction between “proper” and “improper” gauge transformations will be stressed. How the relevant infinite-dimensional asymptotic symmetry group (BMS group) emerges at spatial infinity will be explained. Non-linear structures which appear in five (and higher) spacetime dimensions will also be mentioned.

Wednesday, October 19
Elena Giorgi, Columbia University
Title: Black Hole Stability Problems in GR
Abstract:  In this talk, I will give an overview of the stability problems for black hole solutions, starting with the mode stability results in black hole perturbation theory in the 80’s to more recent mathematical proofs, as the linear and the fully non-linear stability of black hole solutions require new mathematical techniques. Finally, I will present some aspects of our recent proof with Klainerman and Szeftel of the non-linear stability of the slowly rotating Kerr black hole. 

Wednesday, October 26
Brianna Grado-White, Brandeis University
Title:  Minimax Surfaces and Holographic Entropy Inequalities
Abstract: Entanglement entropy plays a key role in our understanding of bulk emergence in AdS/CFT. In this talk, I will review a recent reformulation from Headrick and Hubeny of the Hubeny-Rangamani-Takayanagi formula for covariant holographic entanglement entropy. This minimax formula involves finding certain maximal area surfaces living on a timelike hypersurface, then minimizing over the set of all such hypersurfaces. Here, I will discuss various new properties for minimax surfaces, including a new result for covariant holographic entanglement entropy: namely, that any entanglement entropy inequality satisfied by static spacetimes will be satisfied by dynamical spacetimes as well.

Wednesday, November 2
Severin Lüst, Harvard
Title: Holography and the KKLT Scenario
Abstract: The KKLT scenario, one of the few ideas to realize dS vacua in string theory, consists of two steps: the first involves the construction of a supersymmetric AdS vacuum with a small negative cosmological constant, and the second involves breaking supersymmetry and uplifting the energy to achieve dS. In this talk I discuss conventional holography to argue why it is not possible to complete the first step. This is obtained this by putting a bound on the central charge of the dual theory which involves branes wrapping special Lagrangian cycles in CY 4-folds. 

Wednesday, November 9
Miguel Montero, Harvard
Title: The Dark Dimension
Abstract: We will see how the Emergent String Conjecture, together with general assumptions about the asymptotic structure of scalar potentials in string theory and experimental observations suggest that, if our universe is to sit close to a weakly coupled corner in the string landscape, there should be a single large extra dimension of roughly micrometer size. I will describe some possible experimental signatures of this scenario that might be testable in the near future. A failure to detect them would suggest that, if our Universe is described by a string theory vacuum, it is strongly coupled.

*Friday, November 18 at 10am
Miranda Cheng, University of Amsterdam
Title: State Counting and Topology with Quantum Modular Forms
Abstract: Quantum modular forms are functions with delicate modular properties that generalize mock modular forms.  The q-series 3-manifold invariants provide new insights and computational tools in 3-manifold topology, 3d SQFT, and M-theory compactifications. In this talk I will survey the relation between these q-series  invariants and quantum modular forms.
*Note different day and time for this week only

Wednesday, November 30
Luca Iliesiu, Stanford University
Title: Black hole microstate counting from the gravitational path integral
Abstract: Reproducing the integer count of black hole micro-states from the gravitational path integral is an important problem in quantum gravity. In this paper, we show that, by using supersymmetric localization, the gravitational path integral for 1/16-BPS black holes in supergravity reproduces the index obtained in the string theory construction of such black holes, including all non-perturbatively suppressed geometries. A more refined argument then shows that, not only the black hole index but also the total number of black hole microstates within an energy window above extremality that is polynomially suppressed in the charges also matches this string theory index. To achieve such a match, we compute the one-loop determinant arising in the localization calculation for all N=2 supergravity supermultiplets in the N=8 gravity supermultiplet. Furthermore, we carefully account for the contribution of boundary zero-modes, which can be seen as arising from the zero-temperature limit of the N=4 super-Schwarzian, and show that performing the exact path integral over such modes provides a critical contribution needed for the match to be achieved.

December 7
No seminar this week

December 14
Timo Weigand, DESY (Zoom)
Title: Quantum Gravity Bounds on 4d EFTs with Minimal Supersymmetry
Abstract: According to the Swampland idea, quantum gravity effects put severe constraints on effective field theories beyond the usual consistency conditions from quantum field theory such as absence of gauge or gravitational anomalies. In this talk, we propose such constraints for four-dimensional N=1 supergravities based on consistency of certain axionic, or EFT, strings. These are certain strings which are magnetically charged under the axionic components  of the chiral N=1 superfields; their existence follows from the Completeness Conjecture in quantum gravity. The key observation is that anomaly inflow from the four-dimensional theory to the string worldsheet induces anomalies on the string which must be cancelled by local anomalies on the string. This results in various quantisation conditions as well as  bounds on the signs of axionic couplings in the supergravity, including the sign of the Gauss-Bonnet term, and also bounds on the rank of the gauge group in terms of these Gauss-Bonnet couplings. These constraints can rule out supergravity theories which  otherwise look perfectly healthy as effective field theories of quantum gravity. We also test the proposed constraints by comparing them with explicit string models, in particular with F-theory compactifications to four dimensions.

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
Adam Levine, Institute for Advanced Study
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