In-person, Wednesdays at 1:00pm (student session), 1:20pm (main seminar), unless otherwise stated.
 

Semester 1  in 2/1083 (L/T B)

05/10/22-  Felix Haehl (Southampton)

12/10/22 - Harry Goodhew (Cambridge)

19/10/22 - Praxitelis Ntokos (Edinburgh)

26/10/22 - Mark Mezei (Oxford)

01/11/22 - Jianrong Li **Tuesday 12:00, 54/5A**

02/11/22 - Andrea Puhm (Ecole Polytechnique)

09/11/22 - **Postponed** due to train strike disruption [Alejandra Castro]

16/11/22 - Pavel Kovtun (Victoria)

22/11/22 - Yossi Nir (Weizmann) **Tuesday 12:00, 54/5A**

23/11/22 - Hadleigh Frost

30/11/22 - Lionel Mason

07/12/22 - Blaise Gouteraux (Ecole Polytechnique)

[Winter break: 12 Dec 2022 - 13 Jan 2023]

[Exam period: 16-27 Jan 2023]

Semester 2 in 54/8033 (8B) unless otherwise stated

01/02/23 - Marija Tomasevic (Ecole Polytechnique), 44/1057 (L/T B)

03/02/23 - Akash Jain (Amsterdam) **Friday 1pm, 54/7033**

08/02/23 - Itamar Yaakov (Southampton)

15/02/23 - Pascal Anastasopoulos (Uni Wien), 54/8033 (8B)

17/02/23 - Kedar Kolekar (IIT Kanpur), **Friday 1pm, 54/8033 (8B)**

22/02/23 - **Cancelled** Sameer Murthy (Kings College), 54/8033 (8B) 

23/02/23 - Christoph Uhlemann (Oxford) Thursday 1 pm, 58/1023 

28/02/23 - Adam Bzowski (Warsaw) **Tuesday 12pm, 54/8033 (8B)** 

01/03/23 - Alejandra Castro (Cambridge), 54/8033 (8B)

02/03/23 - Paul McFadden (Newcastle) **Thursday 1pm, 02/1085 (L/T C)** 

08/03/23 - Hynek Paul (IPHT,  Saclay), 54/8033 (8B)

15/03/23 - Gustav Holzegel (Imperial), 54/8033 (8B)

22/03/23 - Sean Hartnoll (Cambridge), 54/8033 (8B)

[Spring break: 26 March 2022 through 23 April 2022]

26/04/23 - No talk due to Euro Strings 2023, 54/8033 (8B)

03/05/23 - Silvia Nagy (Durham), 54/8033 (8B)

09/05/23 - Federico Capone (Jena) **Tuesday 1pm, 100/5017, followed by Journal Club**

10/05/23 -  Manus Visser (Cambridge)  , 54/8033 (8B)

17/05/23 -  Dominik Neuenfeld (Amsterdam) , 54/8033 (8B)

Titles and Abstracts (reverse chronological order):

Dominik Neuenfeld (Amsterdam) Comments on the Causal Structure of Double Holography

Doubly-holographic models allow for superluminal signalling in the brane perspective, which raises questions about their usefulness, e.g., in discussing the information paradox. We will argue that such causality violations are not observable at length scales for which the brane description is valid, implying that superluminal signalling is a UV effect which can be understood within the framework of effective field theory. Moreover, we explain how short-distance non-localities in Anti-de Sitter space can generally give rise to apparent causality violations.

However, the causal structure in the brane description of double holography is in fact not simply determined by the induced metric on the brane. We introduce the notion of a "Unitary Domain of Dependence" (UDoD), which defines a subregion where the density matrix can be computed from an initial state using an effective local and unitary description. The UDoD can be bounded using entropic inequalities. For a certain class of examples, the UDoD is strictly contained within the classical domain of dependence, but outside the region where effective field theory is naively expected to hold.

Manus Visser (Cambridge): Partition function for a volume of space

In their seminal 1977 paper, Gibbons and Hawking applied concepts of quantum statistical mechanics to ensembles containing black holes, finding that a semiclassical saddle point approximation to the partition function recovers the laws of black hole thermodynamics. We will generalise the Gibbons-Hawking method by defining a partition function of a ball of space at fixed proper volume. In the zero-loop approximation the result is the exponential of the Bekenstein-Hawking entropy of the boundary of the ball, indicating the holographic nature of nonperturbative quantum gravity in generic finite volumes of space. Based on the recent work 2212.10607 with Ted Jacobson.

Silvia Nagy (Durham): Self-dual gravity and color/kinematics duality in AdS4

Abstract: I will show that self-dual gravity in four-dimensional Anti-de Sitter space (AdS4​) can be described by a minimally coupled scalar field with a cubic interaction written in terms of a deformed Poisson bracket, providing a remarkably simple generalisation of the Plebanski action for self-dual gravity in flat space. This implies a novel symmetry algebra in self-dual gravity, notably an AdS4​ version of the so-called kinematic algebra. This provides a concrete starting point for defining the double copy for Einstein gravity in AdS4​ by expanding around the self-dual sector. Moreover, I will show that the new kinematic Lie algebra can be lifted to a deformed version of the w1+∞​ algebra, which plays a prominent role in celestial holography.

Sean Hartnoll (Cambridge): Entanglement and Emergent Space from Large Matrices

Abstract: The locality in space of interactions between elementary particles is a key property of our universe. This locality is hardwired into quantum field theoretic descriptions of nature. However, locality and indeed space itself are likely not fundamental concepts. In holographic duality, local interactions on a dynamical spacetime emerge from "large N" matrices where no locality is manifest in the microscopic Hamiltonian. The emergence of locality from matrix theories is well-established but not well-understood. In recent years it has been appreciated that locality is closely tied up with so-called “area law” entanglement of the microscopic degrees of freedom. I will discuss a particularly robust notion of entanglement in matrix theories that is rooted in an underlying Gauss law constraint and show how simple models of matrix, or ‘fuzzy' geometry contain area law entanglement.

Gustav Holzegel (Imperial College): Dynamics and Unique Continuation in asymptotically anti-de Sitter spacetimes

Abstract: I will give an overview of results on asymptotically anti-de Sitter (aAdS) spacetimes in the mathematical community. I will discuss the issue of (in)stability and existence of time periodic solutions and then focus on recent joint work with Arick Shao (arXiv:2207.14217) addressing the question in what form the metric and the stress tensor on the conformal boundary determine (locally) the infilling aAdS spacetime.

Hynek Paul (IPhT, Saclay):  Integrated Correlators in N=4 SYM at Large Charge

I will describe an infinite tower of <22pp> integrated four-point correlation functions of half-BPS operators in N=4 SYM with gauge group SU(N). Using the framework of the SL(2,Z) spectral decomposition we find explicit and remarkably simple results for these observables, *exactly* as a function of N, the charge p and the complexified gauge coupling \tau. Thanks to this exact solution, we can access various physically interesting regimes: the usual large N ('t Hooft) limit, the large charge limit at finite N, as well as different double-scaling limits. Based on [https://arxiv.org/abs/2209.06639] and upcoming work with Eric Perlmutter and Himanshu Raj.

Paul McFadden  (NewCastle):  Shift operators from the simplex representation in momentum-space CFT

This talk will discuss parametric integral representations for the general n-point function of scalar operators in momentum-space conformal field theory.  Recently, the n-point function was expressed as a generalised Feynman integral with the topology of a simplex, where the numerator contains an arbitrary function of momentum-space cross ratios. Here, we show the corresponding graph polynomials can be expressed in terms of the first and second minors of the Laplacian matrix for the simplex. Computing the effective resistance between nodes of the corresponding electrical network, an inverse parametrisation is found in terms of the determinant and first minors of the Cayley-Menger matrix. These parametrisations reveal new families of weight-shifting operators, expressible as determinants, that connect n-point functions in spacetime dimensions differing by two.

Alejandra Castro (Cambridge): Designing Gravitational Theories via Symmetric Product Orbifolds

I will discuss the large-N limit of two-dimensional symmetric product orbifolds. The goal is to single out which symmetric product orbifold theory could lead to a strongly coupled point in their moduli space, whose dual could be a semi-classical theory of AdS_3 gravity. To this end, we consider the symmetric product orbifold of N=(2,2) SCFT_2, and classify them according to two criteria.  The first criterion is the existence of a single-trace twisted exactly marginal operator in their moduli space.  The second criterion is a sparseness condition on the growth of light states in the elliptic genera. In this context, we encounter a strange variety: theories that obey the first criterion but the second criterion falls into a Hagedorn-like growth. I will explain why this may be counter-intuitive and discuss how it might be accounted for in conformal perturbation theory. I will also present a new infinite class of theories that obey both criteria, which are necessary conditions for their moduli spaces to contain a supergravity points.

Adam Bzowski (Warsaw University): Wormholes, geons, and the illusion of the tensor product

I will argue that the Hilbert space of states of a holographic, traversable wormhole does not factorize into the tensor

product of the boundary Hilbert spaces. I will analyze the scalar sector of the BTZ geon as an example. I will show that the number of peculiarities of the wormhole physics, such as null states and null operators, highly entangled vacuum states and the cross-boundary interactions, are all emerging as avatars of non-factorization. Finally, I will present a simple model where the non-factorization of the Hilbert space can be interpreted in terms of non-perturbative interactions.

Christoph Uhlemann (Oxford):  Double holography and Page curves in Type IIB

In recent progress on the black hole information paradox, Page curves consistent with unitarity have been obtained in 2d models and in bottom-up braneworld models using the notion of double holography. In this talk we discuss top-down models realizing 4d black holes coupled to a bath in Type IIB string theory and obtain Page curves. We make the ideas behind double holography precise in these models and address causality puzzles which have arisen in the bottom-up models, leading to a refinement of their interpretation.

Kedar Kolekar (IIT Kanpur):  Carrollian origins of Bjorken flow

Bjorken flow describes the motion of the highly energetic state of matter produced in heavy ion collisions as an ultra-relativistic fluid. It is among the simplest phenomenological models of fluids moving at speeds very close to the speed of light. We know that fluids moving at the speed of light are restricted to move on null surfaces. On the other hand, Carroll symmetries arise on generic null surfaces and Carroll fluids describe fluids moving on a null manifold. Thus, it seems very natural that the Bjorken flow and the associated phenomenological approximations should be completely captured by Carroll fluids. In this talk, we will show that this is indeed the case, and thereby demonstrate that Carrollian hydrodynamics is quite ubiquitous, and provides a concrete framework for fluids moving at speeds close to the speed of light.

Pascal Anastasopoulos (Uni Wien): Anomalous and axial Z' contributions to g-2

In this talk, we will study the effects of an anomalous Z' boson on the anomalous magnetic moment of the muon (g-2), and especially the impact of its axial coupling. We mainly evaluate the negative contribution to (g-2) of such couplings at one-loop and look at the anomalous couplings generated at two loops. We find areas of the parameter space, where the anomalous contribution becomes comparable and even dominant compared to the one-loop contribution. We show that in such cases, the cutoff of the theory is sufficiently low, so that new charged fermions can be found in the next round of collider experiments. 

Itamar Yaakov (Southampton): Supersymmetric gauge theory, defects, and TQFT

Recent work on generalized symmetries has rekindled the community’s interest in finding a mathematically rigorous construction for QFT. Unfortunately, this dream does not seem to be close at hand. A more modest goal, however, does seem to be within reach: to clarify the abstract symmetry structure of a generic QFT using tools from TQFT. In this talk, I will give a brief introduction to TQFT, topological defects, and their connection with generalized symmetries. I will then review some of my past and ongoing work on constructing defect operators in gauge theories, with and without supersymmetry, and highlight their interaction with the abstract symmetry structure. Specifically, I will emphasize the way in which non-topological defects, especially charged objects, provide a dual viewpoint on symmetries and phases of gauge theories. The student session will feature a lightning review of supersymmetric localization and its connection to TQFT.

Akash Jain (Amsterdam): Higher-form symmetries and topological phase transitions

Symmetries and their breaking patterns are a valuable tool in physics for classifying and describing phases of matter. In recent years, a new generalised notion of symmetries has emerged, dubbed "higher-form symmetries", where the associated conserved charges are carried by higher-dimensional objects like strings and surfaces, instead of the usual point-like particles. In this talk, I will give a brief overview of higher-form symmetries and their spontaneous and explicit breaking patterns. In particular, the explicit breaking of higher-form symmetries is associated with the emergence of topological defects in many-body systems. We will outline a new hydrodynamic framework for systems with (approximate) higher-form symmetries and use this to study transitions between topological phases of matter. This includes the melting phase transition in smectic crystals, the plasma phase transition from polarised gases to magnetohydrodynamics, the superfluid to neutral fluid phase transition, and the Meissner effect in superconductors, among many others. The talk will be based on: https://arxiv.org/abs/2301.09628.

Marija Tomasevic (Ecole Polytechnique): Holographic duals to evaporating black holes

We describe the dynamical evaporation of a black hole as the classical evolution in time of a black hole in an Anti-de Sitter braneworld. A bulk black hole whose horizon intersects the brane yields the classical bulk dual of a black hole coupled to quantum conformal fields. The evaporation of this black hole happens when the bulk horizon slides off the brane, making the horizon on the brane shrink. We use a large-D effective theory of the bulk Einstein equations to solve the time evolution of these systems. With this method, we study the dual evaporation of a variety of black holes interacting with colder radiation baths. We also obtain the dual of the collapse of holographic radiation to form a black hole on the brane.

Blaise Gouteraux: Emergent higher-form symmetries in holographic matter

In this talk, I will explain how higher-form symmetries emerge in the infrared of certain phases of holographic matter, with an emphasis on Einstein-Maxwell-Dilaton theories and probe brane theories. To put this into context, I will start by reviewing how superfluid hydrodynamics in d+1 dimensions can be recast as the conservation of a U(1) current which has a mixed 't Hooft anomaly with an emergent (d-1)-form symmetry, which corresponds to the conservation of the number of winding hyperplanes. This symmetry is explicitly broken by condensed vortices, leading to relaxation of the supercurrent. We will see how similar features apply to the low-energy, low-temperature description of the holographic phases. I'll conclude with some comments on the boundary nature of the degrees of freedom of charged horizons in the bulk and the relation to Luttinger theorem.

Lionel Mason: Twistors and Lw_{1+infinity} symmetries in celestial formulations of 4d  gravity. 

This talk has two parts.  The first explains how the gravitational phase space for 4d gravity can be re-expressed, using old ideas of Newman and Penrose on null infinity and asymptotic twistor spaces,  so as to geometrically realize Strominger and coworkers celestial  Lw-infinity symmetry.  It will be explained how this geometrical realization  becomes particularly sharp in split signature.  

The second part introduces an open chiral sigma model with target the asymptotic twistor space. The full gravity tree-level S-matrix can then be obtained from tree correlators of this model.  The action of the Lw_{1+\infty} symmetry on the tree-level S-matrix of full gravity beyond the self-dual sector is  then realized by the action of certain vertex operators and acts on the full amplitude.  This was unclear in the original derivation from soft limits in the self-dual sector. 

Hadleigh Frost:  Braid matrices and \alpha' expansions

There is a family of representations of the Braid Lie algebra naturally associated to Veneziano-like integrals on $\mathcal{M}_{0,n}$. I present new explicit formulas for these representations, and explain how they are related to a very nice algebraic structure relating functions on the moduli space. Similar matrices arise for genus $g=1$ string integrals. I will explain how the commutators of these matrices control the computation of the $\alpha'$ expansion of string amplitudes.

Yossi Nir : The jewels in the crown of the LHC

Pavel Kovtun:  Relativistic dissipation: from hydrodynamics to effective field theory

What is the effective description of macroscopic states in a relativistic theory? Going one step beyond thermodynamics, the simplest classical effective description is given by hydrodynamics. It is well known that relativistic dissipative fluid-dynamical equations found in classic textbooks (Weinberg, Landau & Lifshitz) predict violations of causality and non-existence of equilibrium. In this talk, I will discuss how one can make sense of dissipative relativistic hydrodynamics. Time permitting, I will also discuss why some of the long-distance, late-time predictions of classical hydrodynamics are not universal.

Andrea Puhm (Ecole Polytechnique, CPHT): Celestial holography on non-trivial backgrounds

The holographic principle is a powerful tool in gaining insight into quantum gravity for spacetimes with a negative cosmological constant, but it is an outstanding question how generally it applies. I will review recent advances in celestial holography which conjectures that quantum gravity in spacetimes with flat asymptotics is dual to a co-dimension two conformal field theory on the celestial sphere at null infinity. Recasting the S-matrix, which is the basic observable in quantum gravity, in a basis of boost (rather than energy) eigenstates indeed reveals similarities with CFT correlation functions. I will highlight several intriguing and some puzzling features and discuss celestial holography for general spacetime dimensions and for non-trivial asymptotically flat backgrounds of Kerr-Schild type which include the Coulomb field of a static and spinning charge, the Schwarzschild and Kerr geometry as well as electromagnetic and gravitational shockwaves. 

Jianrong Li (Vienna): Quantum affine algebras and Grassmannian cluster algebras.

In this talk, I will explain how to use representations of quantum affine algebras to obtain dual canonical bases of Grassmannian cluster algebras. I will also talk about how to compute the dual canonical bases of Grassmannian cluster algebras. This is joint work with Wen Chang, Bing Duan, and Chris Fraser. 

Mark Mezei (Oxford): Semiclassics for Large Quantum Numbers  

According to the correspondence principle, classical physics emerges in the limit of large quantum numbers. We examine three examples of the semiclassical description of conformal field theory data: large charge boundary operators in the O(2) model, large spin impurities in the free triplet scalar field theory and large charge Wilson lines in QED. By simultaneously taking the coupling to zero and quantum numbers to infinity, we can connect the microscopic to the emergent classical description smoothly.

Praxitelis Ntokos (Edinburgh):  Black hole superpotential as a unifying entropy function and BPS thermodynamics

In the last few years, there has been enormous progress on the statistical description of the entropy of BPS black holes in AdS_D for 𝐷>3 in terms of states in the dual field theory. The success of such developments relies on the existence of an extremisation principle in the bulk which maps to the evaluation of the partition function in the field theory in the large charge limit. In this talk, I will describe an ''off-shell'' approach to the study of black hole thermodynamics in AdS_5 based on an effective superpotential. This approach offers a powerful tool to analyse the thermodynamics without resorting to explicit solutions and can be in principle implemented even to non-supersymmetric configurations. For BPS black holes, it provides the framework where the aforementioned (Hosseini-Hristov-Zaffaroni) extremisation principle emerges naturally in the bulk while it is also directly related to Sen’s entropy function for extremal black holes.

Harry Goodhew (Cambridge): Cutting Cosmological Correlators

The initial conditions of our universe appear to us in the form of a classical probability distribution that we probe with cosmological observations. In the current leading paradigm, this probability distribution arises from a quantum mechanical wavefunction of the universe. In this talk I will discuss how quantum mechanics imprints on the late time observables, in particular showing that the requirement of unitary time evolution, colloquially the conservation of probabilities, fixes the analytic structure of the wavefunction and of all the cosmological correlators it encodes. 

Felix Haehl (Southampton): Quantum Chaos and Effective Field Theory

After reviewing aspects of thermalization and chaos in holographic systems, I will argue that universal aspects can be captured using an effective field theory framework that shares similarities with hydrodynamics. Focusing on the quantum butterfly effect, I will demonstrate how to develop a simple effective theory of the 'scramblon'. After discussing maximally chaotic examples, I will turn to a particular system that displays sub-maximal chaos and discuss its effective description. Finally, I will mention progress on the spatial propagation of more fine-grained chaos characteristics, in particular spectral correlations.