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

Semester 1  

24/09/24 - Gabi Zafrir (Haifa U., Oranim)  -  *Note unusual day*  -  1:00 / 1:20,   54 / 7035 (7B) 

02/10/24 - Will Biggs (Cambridge)

09/10/24 - Tarek Anous (QMUL) - *Note unusual time & place* - 12:00-13:00 (main seminar), 13:00-13:20 (student session) - 54/8031 (8C)

16/10/24 - Ida Zadeh (Southampton)

23/10/24 - no seminar - STAG public lecture

30/10/24 - Gabriel Wong (Oxford)

06/11/24 - Olga Papadoulaki (Ecole Polytechnique, CPHT)

13/11/24 - Saikat Mondal (IIT Kanpur)

20/11/24 - Bob Knighton (Cambridge)

27/11/24 - no seminar - STAG colloquium

04/12/24 -  Akshay Yelleshpur Srikant (Oxford)

[Winter break: 11 Dec 2024 - 10 Jan 2025]

[Exam period: 13-24 Jan 2025]

Semester 2 

29/01/25 - Nikolay Gromov (KCL) - 54/10037 (10B)

05/02/25 -  Rishi Mouland (Imperial) - 54/10037 (10B)

12/02/25 - Cynthia Keeler (Arizona State) - 54/8033 (8B)

19/02/25 - Alexandre Belin (University of Milano-Bicocca) - 54/10037 (10B)

26/02/25 - Petar Tadić (Heriot-Watt University) - 54/10037 (10B)

05/03/25 - Diksha Jain (Cambridge) - 54/8033 (8B)

12/03/25 - Scott Melville (QMUL) - 54/10037 (10B)

19/03/25 - Felix Haehl  (Southampton U.) - 54/8033 (8B)

26/03/25 - Ana-Maria Raclariu (KCL) - 54/8033 (8B)

[Spring break: 31 March 2025 through 25 April 2025]

30/04/25 - Giorgio Frangi (U. Edinburgh) - 54/10037 (10B)

07/05/25 - Christopher Herzog (KCL) - 54/7035 (7B)

14/05/25 - Lorenzo Ruggeri  - 54/7035 (7B)

[Exam period 19 May 2025 - 6 June 2025]

Titles and Abstracts (reverse chronological order):

Ana-Maria Raclariu (KCL): An infrared on-shell action in asymptotically flat spacetimes

One of the main entries in the AdS/CFT dictionary is a relation between the bulk on-shell partition function with specified boundary conditions and the generating function of correlation functions of primary operators in the boundary CFT. In this talk I will show how to construct a similar relation for gravity in 4d asymptotically flat spacetimes. For simplicity, we will restrict to the leading infrared sector, where a careful treatment of soft modes and their canonical partners leads to a non-vanishing on-shell action. I will show that this action localizes to a codimension-2 surface and coincides with the generating function of 2d CFT correlators involving insertions of Kac-Moody currents. The latter were previously shown, using effective field theory methods, to reproduce the leading soft graviton theorems in 4d. I will conclude with comments on the implications of these results for the computation of soft charge fluctuations in the vacuum.

Felix Haehl (Southampton): Modular-invariant random matrix theory and AdS3 wormholes

We develop a non-perturbative definition of RMT2: a generalization of random matrix theory that is compatible with the symmetries of 2d CFT. Given any random matrix ensemble, its n-point spectral correlations admit a prescribed modular-invariant lift to RMT2, which moreover reduce to the original random matrix correlators in a near-extremal limit. Central to the prescription is a presentation of random matrix theory in Mellin space, which lifts to two dimensions via the SL(2, Z) spectral decomposition. As a demonstration we perform the explicit RMT2 lift of two-point correlations of the GUE Airy model. We use the formalism to propose an AdS3 pure gravity 3-boundary wormhole amplitude by "lifting" the corresponding expression from JT gravity.

Scott Melville (QMUL): Cosmological Scattering Amplitudes

Scattering amplitudes underpin much of our understanding of QFT on Minkowski and are essential for interpreting particle collider signals. This talk will describe some recent attempts to develop an analogous description of scattering in (quasi-)de Sitter spacetimes, such as the early inflationary Universe. These cosmological amplitudes open up new ways to interpret the "cosmological collider" signals in the Cosmic Microwave Background, and to improve our understanding of how fundamental properties such as unitarity and causality are realised in QFT on dS.

Diksha Jain (Cambridge): Dual Dressed Black Holes as the end point of the Charged Superradiant instability

In this talk, I will discuss charged superradiant instabilities suffered by black holes in asymptotically AdS_5 * S^5. Hairy black hole solutions (constructed within gauged supergravity) have previously been proposed as endpoints to this instability. In this work, we demonstrate that these hairy black holes are themselves unstable to the emission of large dual giant gravitons. We propose that the endpoint to this instability is given by Dual Dressed Black Holes (DDBH)s; configurations consisting of one, two, or three very large dual giant gravitons surrounding a core AdS black hole with one, two, or three SO(6) chemical potentials equal to unity. We conjecture that DDBHs dominate the phase diagram of N = 4 Yang-Mills over a range of energies around the BPS plane, and provide an explicit construction of this phase diagram, briefly discussing the interplay with supersymmetry. We also construct the 10-dimensional DDBH supergravity solutions.

Petar Tadić (Heriot-Watt University): The five-point bootstrap

We discuss a new algorithm for the numerical evaluation of five-point conformal blocks in d-dimensions, which significantly improves the efficiency of their computation. To achieve this, we use an ansatz for the blocks as a series expansion in radial coordinates, derive a set of recursion relations for the unknown coefficients in the ansatz, and evaluate the series using a Pade approximant to accelerate its convergence. We then study several five-point correlators in the 3d critical Ising model by truncating the operator product expansion (OPE) and including only operators with conformal dimensions below a cutoff Δ ≤ Δcutoff ​ . We approximate the contributions of operators above the cutoff using the corresponding contributions from a suitable disconnected five-point correlators. Using this approach, we compute several currently unknown OPE coefficients that are not easily accessible via a four-point bootstrap.

Alexandre Belin (University of Milano-Bicocca): 3D gravity and a measure on the space of 2D CFTs

In recent years, low-dimensional models of gravity have been useful toy models to study deep puzzles of quantum gravity like the black hole information paradox. For two-dimensional JT gravity, a new duality was discovered: JT gravity is not dual to a definite quantum system with fixed Hamiltonian, but rather to an ensemble of Hamiltonians, i.e. to a matrix integral. Increasing evidence is pointing towards the fact that 3D gravity is also dual to an ensemble of quantum systems. However, by the virtue of AdS/CFT, these systems must be two-dimensional CFTs, which are much more constrained than Hamiltonians, since they must obey the bootstrap constraints. Very recently, there has been progress on understanding what the dual of pure 3D gravity could be, in terms of a random distribution of CFT data. In this talk, I will explore another possibility. I will consider the outcome of averaging over true CFTs in a window of central charge. This requires defining a normalizable measure on the space of CFTs, which I will provide. Along the way, I will discuss some interesting properties of the landscape of all 2d CFTs, for example the fact that the number of CFTs in an order one window of central charge grows doubly exponentially in the central charge at large $c$.

Cynthia Keeler (Arizona State): Classical Double Copy: Horizons, Penrose Limits, and Asymptotics

I'll briefly review the classical double copy, which maps exact solutions of classical gauge theories like electromagnetism, to solutions of general relativity.  We will discuss why a position-space map is feasible, and then relate several gravitational objects (including horizons, Penrose limits, and asymptotics) to their gauge theory analogues.

Rishi Mouland (Imperial): Giant Gravitons and Volume Minimisation

The partition function of a holographic CFT can be understood at least formally as a sum over bulk graviton states, supplemented by non-perturbative (i.e. brane) excitations. Conversely, in a regime of large charges, it is recovered by the free energy of a suitable black hole geometry. We study the relationship between these two pictures in the context of the superconformal index of 4d N=2 theories dual to AdS5 x SE5. We identify the contribution of a given giant graviton configuration with the volume of a specific metric deformation of SE5, corresponding to the near-horizon geometry of the full back-reacted solution. We recover the black hole free energy by extremising over such geometries, thus uncovering a map between giant graviton states and black hole microstates.

Nikolay Gromov (KCL): Long Range Asymptotic Baxter-Bethe Ansatz for N=4 BFKL

We start from considering some fishnet-based toy model for resuming diagrams dominating the Reggie limit of scattering amplitudes in a gauge theory leading to Balitsky-Fadin-Kuraev-Lipatov (BFKL) pomeron. Then we turn to N=4 SYM, where non-perturbative results can be obtained due to integrability.

We demonstrate that the BFKL regime of maximally supersymmetric Yang-Mills theory can be explicitly solved up to the L+1 order in weak coupling by uncovering a novel long-range asymptotic Baxter-Bethe ansatz for trajectories with L scalar fields. The set of equations we have found is reminiscent of the Beisert-Eden-Staudacher equations for local operators but instead applies to non-local operators corresponding to the horizontal Reggie trajectories. We also verify and give new predictions for the light-ray operator spectrum by resummation of the leading singularities in our result.

 Akshay Yelleshpur Srikant (Oxford): Carrollian Amplitudes from Holographic Correlators

Holography in flat space poses unique challenges due to its null boundary and the absence of a shared time direction between the bulk and the boundary. Carrollian Holography is the proposal that a special kind of theory living on the 3D boundary at null infinity. 

In this talk, I will first briefly review some basic aspects of Carrollian holography and the relationship between correlation functions in these theories and the bulk S-Matrix. Next, with the goal of obtaining an explicit exmaple, I will discuss how the Carrollian limit—where the speed of light approaches zero—is tied to the flat limit in the bulk. I will then demonstrate how the Carrollian limit can be used to obtain flat space supergravity amplitudes from ABJM correlators.

Bob Knighton (Cambridge): Deriving the long-string CFT in AdS3

The AdS/CFT correspondence relates string theory in anti-de Sitter space to a lower-dimensional conformal field theory. Despite great success in validating the correspondence for various special cases, there is still no known method for determining the CFT dual of a given bulk string theory. In this talk, I will discuss progress toward a resolution of this problem in the context of bosonic strings in three-dimensions. I will show that a specific subsector of the worldsheet theory (the 'long-string' sector) can be solved exactly, and its path integral organizes itself precisely into the path integral of a certain 2D CFT on the boundary, allowing one to almost 'read off' the dual CFT directly from the bulk theory.

Saikat Mondal (IIT Kanpur): C for Carroll

 Physics beyond relativistic invariance and without Lorentz (or Poincaré) symmetry and the geometry underlying these non-Lorentzian structures have become very fashionable of late. This is primarily due to the discovery of uses of non-Lorentzian structures in various branches of physics, including condensed matter physics, classical and quantum gravity, fluid dynamics, cosmology, etc. In this talk, I will be talking about one such theory - Carrollian theory, where the Carroll group replaces the Poincare group as the symmetry group of interest. 

Interestingly, any null hypersurface is a Carroll manifold and the Killing vectors on the null manifold generate Carroll algebra. Historically, Carroll group was first obtained from the Poincaré group via a contraction by taking the speed of light going to zero limit as a “degenerate cousin of the Poincaré group”.  I will shed some light on Carrollian fermions, i.e. fermions defined on generic null surfaces. Due to the degenerate nature of the Carroll manifold, there exist two distinct Carroll Clifford algebras and, correspondingly, two different Carroll fermionic theories. I will discuss them in detail. Then, I will show some examples; when the dispersion relation becomes trivial, i.e. energy bands flatten out, there can be a possibility of the emergence of Carroll symmetry.

Olga Papadoulaki (CPHT): An inflationary cosmology from Anti-de-Sitter wormholes

There are various proposals as to what initial state can give rise to an inflationary cosmology. The two most popular ones are the no boundary (Hartle-Hawking) and the tunneling (Vilenkin) proposals. Both of them explain only part of the observations and lead to some paradoxes. In this talk, I will review these proposals and I will propose a novel initial state (wavefunction) of the universe which in the far past has asymptotically AdS boundary conditions. In the semiclassical limit it is a Euclidean wormhole solution that can give rise to an expanding universe upon analytic continuation to Lorentzian signature. This proposal evades some of the issues that plagued the no boundary and the tunneling proposals. Moreover, the asymptotic AdS conditions in the Euclidean past could in principle allow for the description of inflationary cosmologies and their perturbations within the context of holography, leading to microscopic models. 

 Gabriel Wong (Oxford): 3d gravity as a random ensemble

One of the major insights gained from holographic duality is the relation between the physics of black holes and quantum chaotic systems. This relation is made precise in the duality between two dimensional JT gravity and random matrix theory. In this work, we generalize this to a duality between AdS3 gravity and a random ensemble of approximate CFT's. The latter is described by a combined tensor and matrix model, describing the OPE coefficients and spectrum of a theory that approximately satisfies the bootstrap constraints. We will explain how the Feynman diagrams of the random ensemble produce a sum over 3 manifolds that agrees with the partition function of 3d gravity. Our model makes explicit the intriguing relation between the sum over topologies and the implementation of the bootstrap equations.  Time permitting, we will discuss some first steps in generalizing this story to de Sitter space.

Ida Zadeh (Southampton): Heterotic islands

In this talk I will discuss asymmetric orbifolds and will focus on their application to toroidal compactifications of heterotic string theory. I will consider theories in 6 and 4 dimensions with 16 supercharges and reduced rank. I will present a novel formalism, based on the Leech lattice, to construct ‘islands’ without vector multiplets.

Tarek Anous (QMUL): Integrability in de Sitter

Perturbative calculations in de Sitter are particularly thorny: They are riddled with IR ambiguities and, at late times, signal the breakdown of the standard EFT paradigm. I will start with a light review of these issues and will motivate why exactly solvable models can be a useful tool for organizing our thoughts. To motivate these claims, I will work through a particularly simple example of such an exactly solvable model: QED in two dimensions on a fixed de Sitter background.  

Will Biggs (Cambridge): Static Charged Black Hole Binaries in AdS

Very few stationary multi-black hole solutions are known to exist. In this talk we will discuss the first examples of static binary black hole solutions in anti-de Sitter space. The attractive force between the black holes is balanced by the addition of a background electric field, sourced at the conformal boundary. I'll briefly review how these solutions can be obtained numerically before addressing some of their properties, including how they satisfy a first law relationship. We'll see that there is a continuous family of bulk solutions for a given boundary profile and temperature, suggesting there is continuous non-uniqueness of the black hole solutions. We'll discuss how this non-uniqueness is particularly surprising and confusing in the light of the AdS/CFT correspondence.

Gabi Zafrir (Haifa U., Oranim):  Dimensionally Reducing Generalized Symmetries from (3+1)-Dimensions

Recently there has been a renewed interest in the subject of novel types of symmetries, now known as generalized symmetries. An interesting question is what happens to these more general symmetry structures upon compactification to lower dimensions. In this talk, we shall explore this in the context of the compactification of 4d N=1 SCFTs to 2d on spheres.