Joint Belgian hep-th seminars: academic year 2024-2025

13 November 2024, Mons

Antoine Rignon-Bret (Marseille) | Dynamical observables on null hypersurfaces in general relativity
In this talk, I will review the application of Noether theorem to general relativity and the covariant phase space techniques to construct Noether charges and fluxes on null boundaries. In particular, I will detail the Wald-Zoupas procedure and focus on the physical interpretation. I will emphasize the necessity to construct charges and fluxes from a covariant symplectic structure and show that it leads to vanishing cocycles in the current algebra. Then, I will apply the procedure to null infinity assuming different fall-off conditions and different asymptotic symmetry groups. For gBMS, it will be impossible to find a covariant symplectic structure from which the charges arise. In addition, I will explain how different choices of covariant symplectic potentials can be thought of as different choices of thermodynamic potentials, and how this relates to recent proposals for the dynamical entropy of black holes and energy in asymptotically flat spacetime.
 Andrew Svesko (King’s College) | Quantum inequalities for quantum black holes
Spacetime inequalities play an important role in a variety of fields, ranging from mathematical relativity to holography. In this talk, I will formulate spacetime inequalities applicable to three-dimensional quantum-corrected black holes to all orders of backreaction in semiclassical gravity. Namely, I propose refined versions of the quantum Penrose and reverse isoperimetric inequalities, valid for all known three-dimensional asymptotically anti–de Sitter quantum black holes. Previous proposals of the quantum Penrose inequality apply in higher dimensions but fail when applied in three dimensions beyond the perturbative regime. The quantum Penrose inequality I propose, valid in three dimensions, holds at all orders of backreaction. This suggests cosmic censorship exists in nonperturbative semiclassical gravity. Further, the quantum reverse isoperimetric inequality implies a maximum entropy state for quantum black holes at fixed volume.
Stefan Prohazka (Vienna) | Quantizing carrollian field theories
We will first review Carroll symmetries and discuss some of their puzzling consequences, like particles with restricted mobility. We will then present evidence that (conformal) Carrollian field theories provide potential candidates for holographic duals to quantum gravity in asymptotically flat spacetimes. Finally, we will examine subtleties in the quantization of carrollian theories and suggest challenges for carrollian flat space holography.

6 November 2024, Brussels

Nicole Righi (King’s College) | Open strings and heterotic instantons
Motivated by closed string perturbation theory arguments, I consider nonperturbative effects in the string coupling in supersymmetric critical heterotic string theories that are stronger than field theory ones. I will argue that in 10D, such effects arise from heterotic “D-instantons,” i.e. heterotic disk diagrams, whose existence relies on a non-trivial interplay between worldsheet and spacetime degrees of freedom. Two general principles arise: The first is that the consistency of heterotic branes on which the fundamental string can end relies on an inflow mechanism for spacetime degrees of freedom. The second is that the perturbation theory argument, taken to its logical conclusion, implies that all closed-string theories must exhibit open strings as well. Finally, I will show how these new stringy effects can impact the vacuum energy of the 4D effective theory.
Sean Hartnoll (Cambridge)| The polarised IKKT model
The IKKT matrix model provides a holographic framework in which all spacetime dimensions are emergent. It remains poorly understood, not least because the ‘timeless’ nature of the model means that the standard renormalisation group interpretation of the mergent ‘radial’ dimension is not immediately applicable. I will discuss a supersymmetric deformation of the IKKT integral that gives a practical handle on the model. I will show haw well-established phenomena, including brane polarisation in the presence of background fluxes, arise in this context and thereby allow the rudiments of a holographic dictionary to be established.

30 October 2024, Ghent

Andrew McLeod (U. Edinburgh) | From Singularities to Scattering Amplitudes — the Landau Bootstrap
 
The singularity structure of scattering amplitudes has long been known to be constrained by physical principles such as causality and locality—however, the explicit form of these constraints has remained difficult to work out in practice. In this talk, I will present recent work that allows us to sidestep many of these difficulties, thereby allowing us to derive strong new constraints on the analytic structure of Feynman integrals and scattering amplitudes. I will then describe how these constraints can be fed into a bootstrap approach, via which the functional form of Feynman integrals can be determined from just knowledge of their singular behaviour.
Petr Kravchuk (King’s College) | Fusion of Conformal Defects 
Fusion of two conformal defects in conformal field theory can be understood as an RG flow whose IR fixed point is another conformal defect, with the running scale is set by the separation between the defects. When the separation is small, the system can be described by EFT techniques, in terms of an effective action on the IR defect. In this talk I will discuss the constraints of conformal symmetry on such effective actions and the implications of this picture for observables such as the cusp anomalous dimension. Joint work with Alexander Radcliffe and Ritam Sinha, arXiv:2406.04561.

16 October 2024, Leuven

Ruben Monten (CERN) | TBA

TBA

Francesco Bigazzi (INFN Florence) | Hagedorn temperature in confining gauge theories from holography

Quantum theories with a density of states which grows exponentially with the energy, display a thermal partition function which is well defined only below a certain temperature, called Hagedorn temperature. Examples include string theories but also ordinary gauge theories, as pure Yang-Mills in the confining phase. The computation of the Hagedorn temperature for generic confining theories is not an easy task. Combining the sigma-model string expansion with an effective approach, I will present a general formula for the Hagedorn temperature of a class of confining gauge theories with a holographic dual description. The result agrees with numerical quantum field theory estimates, available for some specific models, with remarkable precision.

 

9 October 2024, Brussels

Carlo Heissenberg (Queen Mary U. of London) | An Eikonal Approach to Gravitational scattering and waveforms

In this talk, I will discuss recent developments in the calculation of the gravitational waveform emitted during a scattering of two compact objects, considering two complementary regimes. The first is the post-Minkowskian (PM) approximation, where one focuses on widely separated objects, i.e. scatterings at large impact parameters. In this setup, interactions are weak and can be treated perturbatively. A particularly natural approach to attack this problem is to exploit the connection with scattering amplitudes, for which the eikonal framework offers a systematic way to describe the classical limit. I will discuss in particular how the next-to-leading PM waveform can be extracted from a one-loop 2->3 amplitude. The second approximation consists in focusing on low-frequency emissions, which are governed by universal soft theorems. These are simple relations that dictate in particular the structure of leading log-enhanced pieces of the type $\omega^{n-1}(\log\omega)^n$ for $n=0,1,2…$ in the low-frequency expansion, as $\omega\to0$. I will present a recent proposal for a resummation of all such terms and discuss their contribution to the energy emission spectrum.

Shan-Ming Ruan (VUB) | Non-extremal Island in de Sitter Gravity

We investigate the challenges and resolutions in computing the entanglement entropy for the quantum field theory coupled to de Sitter (dS) gravity along a timelike boundary. The conventional island formula, originally designed to calculate the fine-grained entropy for a non-gravitational system coupled to anti-de Sitter (AdS) gravity, encounters difficulties in de Sitter gravitational spacetime, failing to provide a physically plausible extremal island. To overcome these problems, we introduce a doubly holographic model by embedding a dS2 braneworld in an AdS3 bulk spacetime. This approach facilitates the computation of entanglement entropy through holographic correlation functions, effectively circumventing the constraints of the island formula. We demonstrate that the correct recipe for calculating entanglement entropy with dS gravity involves the non-extremal island, whose boundary is instead defined at the edge of the dS gravitational region. Our findings indicate that, during the island phase, the entanglement wedge of the non-gravitational bath includes the entire dS gravitational space. Using the second variation formula, we further show that the existence of a locally minimal surface anchored on the gravitational brane is intrinsically linked to the extrinsic curvature of the brane.

 

2 October 2024, Leuven

Francesco Galvagno (Queen Mary U. of London) | Brane scattering from N=4 integrated correlators

Integrated correlators in N=4 SYM represent a powerful tool to obtain exact results in the coupling constant, and can be used as constraints for dual scattering amplitudes in AdS. In this talk we study special classes of integrated correlators, dual to scattering processes in presence of extended branes in the bulk. First, we consider 4pt correlators with determinant operators, which in the planar limit are heavy operators realizing a giant graviton D3-brane in the dual space. Secondly, we discuss correlators with line defects such as Wilson/’t Hooft loops, dual to extended (p,q)-strings in the bulk. We compute their integrated correlator via supersymmetric localization exactly in the ‘t Hooft coupling, interpreting such results as worldsheet integrated amplitudes in presence of boundaries.

Shai Chester (Imperial College London) | Bootstrapping string and M-theory

We combine supersymmetric localization with the numerical conformal bootstrap to non-perturbatively study 4d N=4 super-Yang-Mills (SYM) and 3d ABJM theory for all N and coupling, which is dual to string theory and M-theory, respectively. For N=4 SYM, our bound on the lowest dimension operator interpolates between weak coupling results for the Konishi operator, and strong coupling results for the lowest double trace operator, including the first few stringy corrections. For ABJM, our bounds match the protected strong coupling results from M-theory, and give the first prediction for the first unprotected correction D^8R^4. In both cases, our results suggest that bootstrap + localization is sufficient to numerically solve holographic theories non-perturbatively, opening a new window on strongly coupled quantum gravity.

 

25 September 2024, Brussels

Akshay Srikant  (Oxford) | Carrollian Amplitudes from Holographic Correlators

Carrollian amplitudes are flat space amplitudes written in position space at null infinity which can be re-interpreted as correlators in a putative dual Carrollian CFT. We argue that these amplitudes are the natural objects obtained in the flat space limit of AdS Lorentzian boundary correlators. The flat limit is taken entirely in position space by introducing Bondi coordinates in the bulk. From the bulk perspective, this procedure makes it manifest that the flat limit of any Witten diagram is the corresponding flat space Feynman diagram. It also makes explicit the fact that the flat limit in the bulk is implemented by a Carrollian limit at the boundary. We systematically analyse tree-level two, three and four-point correlators. Familiar features such as the distributional nature of Carrollian amplitudes and the presence of a bulk point singularity arise naturally as a consequence of requiring a finite and non-trivial Carrollian limit.

Nejc Ceplak (Trinity College Dublin) | Black Hole Singularity from OPE

Eternal asymptotically AdS black holes are dual to thermofield double states in the boundary CFT. It has long been known that black hole singularities have certain signatures in boundary thermal two-point functions related to null geodesics bouncing off the singularities (bouncing geodesics). In this talk I will discuss the manifestations of black hole singularities in the dual CFT. By decomposing the boundary CFT correlator of scalar operators using the Operator Product Expansion (OPE) and focusing on the contributions from the identity, the stress tensor, and its products, I will show that this part of the correlator develops singularities precisely at the points that are connected by bulk bouncing geodesics. Black hole singularities are thus encoded in the analytic behavior of the boundary correlators determined by multiple stress tensor exchanges. Furthermore, I will show that in the limit where the conformal dimension of the operators is large, the sum of multi-stress-tensor contributions develops a branch point singularity as predicted by the geodesic analysis. I will then argue that the appearance of complexified geodesics, which play an important role in computing the full correlator, is related to the contributions of the double-trace operators in the boundary CFT.