Joint Belgian hep-th seminars: academic year 2024-2025
21 May 2025, Leuven
David Kubiznak (Charles U, Prague) | Regular black holes: from non-linear electrodynamics to pure gravity models
It is well known that (static) regular black hole spacetimes can be sourced by appropriately chosen theories of non-linear electrodynamics. More recently, it was shown that many such models can also be obtained as solutions of vacuum gravity equations, upon considering an infinite series of quasi-topological higher-curvature corrections. After reviewing both these approaches, I will show that the latter construction can be upgraded to yield regular black holes with vanishing inner horizon surface gravity — a necessary condition for the absence of classical instabilities associated with mass inflation on the inner horizon. I will also briefly discuss what happens when non-linear electrodynamics is combined with quasitopological gravity.
Oleg Evnin (Chulalongkorn U, Bangkok) | Krylov and Nielsen complexity
Recent years have seen a surge of interest in developing measures of complexity for the unitary evolution in quantum mechanics. One approach, known as Nielsen complexity, visualizes the evolution as a continuous “program” executed by the system and applies measures developed in computational complexity theory to judge whether the unitary evolution operators are simple or complex. Another approach, known as Krylov complexity, tracks how rapidly quantum states and operators spread over different independent directions as they evolve. One key question is whether these quantities are capable of distinguishing the evolution operators of solvable (simple) systems from generic (chaotic, complicated) ones. I will describe the challenges and achievements within this line of pursuit, as well as interrelations between the two apparently different approaches.
14 May 2025, Brussels
Pierluigi Niro (SISSA, Trieste) | Symmetry Breaking in 3d QED
QED in 2+1 dimensions is among the simplest and yet very rich examples of strongly interacting gauge theories, arising in many physical contexts. When the number of electrons is large, the theory is known to flow to a symmetry-preserving interacting CFT at low energies, but there is evidence that this scenario is excluded below some critical value of the number of electrons. Focusing on the case of two electrons, we argue that the theory must then spontaneously break its U(2) global symmetry to a U(1) subgroup via the condensation of monopole operators. This gives rise to a non-linear sigma model with target space a squashed three-sphere, equipped with a theta term required by anomaly matching. Time permitting, I will also discuss how this scenario is consistent with deformations of supersymmetric QED with eight supercharges.
Harold Erbin (IPHT, CEA-Saclay) | Building string field theory using machine learning
String field theory is a second-quantized formulation of string theory. While its general properties are well understood, its action is non-polynomial and requires the determination of complicated subspaces of the moduli spaces of Riemann surfaces (vertex regions) and some specific conformal maps. An elegant parametrization is provided by minimal area metrics built from Strebel differentials on n-punctured spheres, but it requires solving the notoriously difficult accessory parameter problem. In this talk, I will describe recent works where we construct the necessary data for the quartic interaction in terms of neural networks. As a consistency check, we recover the known quartic term in the closed string tachyon potential. I will also argue that the method generalizes to higher orders.
7 May 2025, Leuven
Antony Speranza (Amsterdam) | An intrinsic cosmological observer
There has been much recent interest in the necessity to include an observer degree of freedom in the description of local algebras in semiclassical gravity. In this talk, I will describe an example where the observer can be constructed intrinsically from the quantum fields. This construction involves the slow-roll inflation example recently analyzed by Chen and Penington, in which the gauge-invariant gravitational algebra arises from averaging over modular flow in a local patch. I will relate this procedure to the Connes-Takesaki theory of the flow of weights for type III von Neumann algebras, and further show that the resulting gravitational algebra can naturally be presented as a crossed product. This leads to a decomposition of the gravitational algebra into quantum field and observer degrees of freedom, with different choice of observer being related to changes in frame for the algebra. I will also connect this example to other constructions of type II algebras in semiclassical gravity, and argue they all share the feature of being the result of gauging modular flow.
Edoardo Lauria (LPENS, Paris) | The Ising model with 1/r^1.99 interaction
I will discuss the 1d Ising model with long-range interaction decaying as 1/r^{1+s}. At long distances, the critical model corresponds to a family of 1d conformal field theories whose data depend nontrivially on s in the range 1/2 ≤ s ≤ 1. The model is known to be described by a generalized free field with quartic interaction, which is weakly coupled near s = 1/2 but strongly coupled near the short-range crossover at s = 1. I will introduce a dual description which becomes weakly coupled at s = 1, where the model becomes an exactly solvable conformal boundary condition for the 2d free scalar. We perform a number of consistency checks of our proposal and calculate the perturbative CFT data around s = 1 analytically using both 1) our proposed field theory and 2) the analytic conformal bootstrap. Our results show complete agreement between the two methods. Based on ArXiv: 2412.12243, in collaboration with D. Benedetti, D. Mazac, P. van Vliet.
30 April 2025, Brussels
Nabil Iqbal (Durham) | Geometric deep learning and conformal symmetry
Recent years have seen computational methods based on deep learning and neural networks playing an increasingly important role in many areas of science (and society in general). One important organizing principle for building such neural networks is that of symmetry, i.e. that the symmetries of the problem should be encoded in the architecture of the network. I will provide an introduction to the resulting field of “geometric deep learning”. I will then discuss work in progress to build a neural network that is equivariant under the conformal group, i.e. the set of transformations that preserve angles. I will describe how our construction leverages some of the kinematics of the AdS/CFT correspondence of quantum gravity, with potential applications to problems in computer vision and critical phenomena. No previous machine learning background should be needed for the talk. Based on work in progress with Maksim Zhdanov, Erik Bekkers and Patrick Forré.
Niccolò Cribiori (Leuven) | Scale separation, how (not)?
The existence of a separation of scales between the four observed spacetime dimensions and the yet unobserved additional ones is a minimal requirement for (string) phenomenology. However, explaining the origin of this property remains an open problem. In this talk, I will explore both sides of this issue. First, I will present general bottom-up arguments suggesting that scale separation is challenging to achieve, if not impossible, in broad classes of models. Then, I will discuss top-down constructions offering potential avenues for scale separation. In either direction, this will demonstrate how, among all the hierarchies in nature, scale separation is one for which concrete progress may be within reach.
23 April 2025, Ghent
Tim Schuhmann (Ghent) | Krylov spread complexity as holographic complexity beyond JT gravity
One of the important open problems in quantum black hole physics is a dual interpretation of holographic complexity proposals. To date the only quantitative match is the equality between Krylov spread complexity in triple-scaled SYK and the complexity = volume proposal in classical JT gravity. In this talk, I will utilize the recent connection between double-scaled SYK and sine dilaton gravity to show that the quantitative relation between Krylov spread complexity and complexity = volume extends to finite temperatures and to full quantum regime on the gravity side at disk level. One key lesson in this work is that gravity demands to assign complexity also to Euclidean state preparation.
Marco Meineri (Turin) | No-go theorems for higher-spin charges in AdS_2
Higher-spin conserved currents and charges feature prominently in integrable 2d QFTs in flat space. Motivated by the question of integrable field theories in Anti de-Sitter space, we consider the consequences of higher-spin currents for QFTs in AdS2, and find that their effect is much more constraining than in flat space. Specifically, it is impossible to preserve: (a) any higher-spin charges when deforming a massive free field by interactions, or (b) any spin-4 charges when deforming a CFT by a Virasoro primary. Along the way, we explain how higher-spin charges lead to integer spacing in the spectrum of primaries and to constraints on correlation functions. We also comment on consequences for critical long-range statistical models.
2 April 2025, Brussels
Sebastian Pogel (PSI Villigen) | Geometry of Feynman Integrals
Multi-loop Feynman integrals are crucial for phenomenology predictions in particle physics, and exhibit a rich mathematical structure. I will discuss the central role that geometry plays in computing such integrals. Significant classes of geometries appearing in Feynman integral are elliptic curves and their natural generalizations, namely Calabi-Yau manifolds and higher genus curves. On the example of a simple class of Calabi-Yau integrals, the so-called Banana integrals, I will demonstrate how geometric information can be leveraged to evaluate Feynman integrals.
Andrei Parnachev (Trinity College Dublin) | OPE and Thermal Holography
Holographic thermal two-point functions can be expanded using the operator product expansion (OPE). There are two types of terms — stress tensor and its products (the analog of the Virasoro block in 2d) and double-trace contributions. The former can be computed by analyzing the equation of motion in the AdS-Schwarzschild background and has an asymptotic behavior related to light rays bouncing off the black hole singularity. We discuss the approximation of thermal correlators by this asymptotic expansion and the computation of the remaining double-trace part of the correlators.
26 March 2025, Leuven
Marc Geiller (ENS, Lyon) | Subleading structure of asymptotically-flat spacetimes, and some generalizations
Asymptotic symmetries provide a powerful insight into the structure of gravitational radiation and memory effects in asymptotically flat spacetimes. In this talk I will review the recent developments aiming at enlarging the available asymptotic symmetries. This will be explained via a dictionary between the Bondi-Sachs formalism and the Newman-Penrose formalism which enables to study the fine subleading structure of asymptotically-flat spacetimes and argue for the emergence of hidden symmetry structures. I will also discuss potential generalizations to other types of boundary conditions beyond standard asymptotically-flat spacetimes, including e.g. violations of peeling, a cosmological constant, and twisting geometries.
Konstantin Zarembo (Nordita) | ’t Hooft loops: holography, integrability and localization
According to AdS/CFT, ’t Hooft loops are dual to D1-branes in AdS(5)xS(5). With the crucial insights from integrability, localization and S-duality one can get a glimpse on how holography works at the non-perturbative level for this class of observables.
19 March 2025, Brussels
Arthur Hebecker (Heidelberg) | From Stringy de Sitter to Early Cosmology and the Measure Problem
Superstring theory has an enormous set of solutions, but finding de Sitter space in this landscape has proven difficult. I will try to explain the origins of the problem and review some of the most recent issues in this ongoing debate. While it is far from clear what this may imply for the beginning of our universe, it is nevertheless interesting to reconsider the predictivity of the landscape with a de-Sitter-skeptical Swampland perspective in mind. In this spirit, I will recall the measure problem, focusing on a proposal motivated by the Cosmological Central Dogma and discuss its difficulties as well as potential implications.
Bernardo Zan (Genoa) | Quantum Groups as Global Symmetries
Quantum groups are algebras which have been known to describe the symmetry of certain lattice systems. In the continuum, instead, they seem to play a different and more subtle role, appearing in two-dimensional CFTs in a more indirect manner; in the literature, they are sometimes referred to as “hidden symmetries”. I’ll elucidate the role of quantum groups as global symmetries in the continuum by considering CFTs which have the quantum group as a bona fide global symmetry; I will discuss this question in the case of the simplest quantum group Uq(sl2), giving both the general picture and studying a specific example arising from the continuum limit of a lattice model. I will discuss the consequences of this symmetry, and how this is related to some of those ordinary theories (e.g. minimal models) where quantum groups are known to appear indirectly.
12 March 2025, Mons
Dario Francia (INFN, Rome) | Double-copy supertranslations
In the framework of the convolutional double copy, we investigate the asymptotic symmetries of the gravitational multiplet stemming from the residual symmetries of its single-copy constituents at null infinity. We show that the asymptotic symmetries of Maxwell fields in D = 4 imply “double-copy supertranslations”, i.e. BMS supertranslations and two-form asymptotic symmetries, together with the existence of infinitely many conserved charges involving the double-copy scalar.
Tassos Petkou (Aristotle U., Thessaloniki) | On the correspondence between thermal one-point functions and conformal Feynman graphs
I will describe a recently uncovered correspondence between thermal one-point functions of operators in massive free scalar theories, and conformal 4pt fynctions given by Feynman ladder graphs in all even dimensions and loop order. The corresponding web of the graphs is built by successive applications of a set of integrodifferential operators on the partition function of a twisted harmonic oscillator, Along the way one uncovers novel relations for single-valued polylogarithms, and an inviting connection between the spin of an operator and the loop order of conformal ladder graphs.
Yannick Herfray (IDP, Tours) | BMS particles
In the last years, it has been understood that infrared (IR) divergences in quantum field theory generically arise because conventional Poincaré particle states are not suited for conservation of Bondi-Metzner-Sachs (BMS) charges. This motivates extending the usual notion of particles, defined as unitary irreducible representations (UIRs) of Poincaré, to BMS particles, i.e. UIR of BMS. In a recent article with X. Bekaert and L. Donnay we constructed wavefunctions for BMS particles: As I will show, they all turn ou to describe quantum superpositions of (Poincaré) particles propagating on inequivalent gravity vacua. This realisation follows from reconsidering McCarthy’s classification of BMS UIRs through a unique, Lorentz-invariant but non-linear, decomposition of supermomenta into hard and soft pieces.
26 February 2025, Brussels
Panos Betzios (Ghent) | An Inflationary Cosmology from (AdS) Wormholes
In the context of inflationary cosmology, a natural question arises on what precedes inflation itself. We review aspects of the Hartle-Hawking (no boundary) and Vilenkin (tunneling) proposals. These raise some paradoxes that are complementary in nature and in clash with observations. On the other hand, theories of quantum gravity are better defined in the presence of a negative cosmological constant. I will propose a new type of wavefunction of the universe with asymptotically AdS boundary conditions in the far (Euclidean) past. In the semiclassical limit, it describes a Euclidean (half)-wormhole geometry with properties that result in an expanding universe upon analytic continuation to Lorentzian signature. Finally I will show how this idea can be realized in the standard model coupled to general relativity, the inflaton being identified with the Higgs boson.
Tarek Anous (Queen Mary U. London) | What makes a de Sitter invariant QFT?
QFT calculations on de Sitter are much thornier than their flat space counterparts. I will begin by reviewing several unresolved issues with de Sitter QFT that require our attention. Following this, I will propose a candidate quantum mechanical system toy model whose large-N limit may help us gain a handle on the problem.
19 February 2025, Leuven
Johanna Erdmenger (Wurzburg) | Non-Locality induces isometry and factorisation in holography
In holography, two manifestations of the black hole information paradox are given by the non-isometric nature of the bulk-boundary map and by the factorisation puzzle. By considering time-shifted microstates of the eternal black hole, we demonstrate that both these puzzles may be simultaneously resolved by taking into account non-local quantum corrections that correspond to wormholes arising from state averaging. This is achieved by showing, using a resolvent technique, that the resulting Hilbert space for an eternal black hole in Anti-de Sitter space is finite-dimensional with a discrete energy spectrum. The latter gives rise to a transition to a type I von Neumann algebra. Talk based on 2411.09616 [hep-th].
Adolfo Guarino (Oviedo) | On Type II orientifold flux vacua in 3D
Combining supergravity and algebraic geometry techniques, I will introduce a systematic approach to the study of type II orientifold flux vacua in three dimensions. As a landscape appetizer, I will present the first examples of type IIB AdS3 flux vacua which feature parametrically-controlled scale separation and come along with integer-valued conformal dimensions of the would-be dual CFT2 operators. I will also comment on the implications of our results in light of the Swampland program.
12 February 2025, Brussels
David Andriot (Annecy) | Comments on quintessence from string theory
Recent cosmological observations suggest that dark energy, responsible for the current accelerated expansion of our universe, could be dynamical, instead of a cosmological constant. In this talk, we will discuss realizations of this scenario in the form of quintessence, namely, scalar fields evolving in a scalar potential. We will first discuss the constraints for obtaining such a model from string theory. We will then discuss the characteristics of realistic and stringy quintessence cosmological solutions. Finally, we will compare those to the latest observational constraints and discuss related challenges.
Felipe Figueroa (Annecy) | Bootstrapping Dual Model amplitudes with Regge Theory
Dual model amplitudes are meromorphic amplitudes involving the exchange of infinitely-many higher spin states. They are the staple of tree-level string theory, with the Veneziano and Virasoro-Shapiro amplitudes being their most famous representatives. They also appear in large N gauge theories as QCD, which becomes a weakly coupled theory of baryons and glueballs in the limit where the number of colors goes to infinity whose interactions are described by this class of objects. Amplitudes of this class must satisfy very constraining consistency conditions, which makes them very good candidates for bootstrap approaches. Yet, only recently they are receiving the attention of the S-matrix bootstrap community, partly because many standard S-matrix bootstrap techniques are not well suited for their study. In this talk I will show some recent progress in the analysis of dual model amplitudes from a bootstrap perspective using techniques from Regge theory of complex angular momentum. After a short motivation/introduction to dual model amplitudes and presenting the main ideas of Regge theory, I will explain how these techniques can be used to study them and showcase some recent results: the proof that dual model amplitudes require infinitely many trajectories to be consistent, and a numerical method to build consistent dual amplitudes with customizable spectrum and high energy behavior.
5 February 2025, Leuven
Grégoire Mathys (EPFL Lausanne) | Averaged null energy and the renormalisation group
The averaged null energy operator (ANEC) is a light-ray integral of the null energy, which is known to have far-reaching consequences in CFT, such as the Lorentzian Inversion Formula. It is also closely connected to modular Hamiltonian in QFT. In this talk, I will discuss a new connection between the ANEC operator and monotonicity of the renormalization group. In particular, I will show how the 2d c-theorem and 4d a-theorem can be derived using the ANEC. This derivation relies on contact terms appearing in specific ANEC correlators. I will also review a new infinite set of constraints that can be derived from the ANEC in 2d QFT. This program hints at a more general role for light-ray operators in QFT, which I will argue for.
Alan Rios Fukelman (King’s College) | Exact results for de Sitter QFTs
In this talk I will discuss some recent developments in the study of quantum fields on a fixed de Sitter background. I will discuss the two-dimensional Schwinger model of a massless charged fermion coupled to an Abelian gauge field. The theory admits an exact solution that can be analyzed efficiently using Euclidean methods. I will discuss the fully non-perturbative, all loop correlation function of the electric field as well as the fermion field and demonstrate many features endemic of quantum field theory in de Sitter space, including the appearance of late-time logarithm, their resummation and the role of non-perturbative phenomena.
29 January 2025, Brussels
Felix Haehl (Southampton) | RMT2: random matrix universality, modular invariance, and wormholes
Examples of chaotic 2d CFTs are elusive despite their expected importance in holography. I will suggest that random matrix universality should be considered as an essential property characterizing the statistics of the operator spectrum of such theories. This begs the question how to marry the stringent constraints of random matrix universality with those imposed by modular invariance of a CFT. I will describe a formalism referred to as RMT2, which allows us to “lift” any standard random matrix theory to two dimensions in a modular invariant fashion. Applications include: the AdS3 pure gravity wormhole with two torus boundaries, a prediction for multi-boundary wormhole amplitudes, and the modular invariant topological expansion of various instances of RMT2.
Agnese Bissi (Trieste & Uppsala) | Multi-trace operators in CFTs
In this talk I will discuss how to deal with multi-trace operators, in particular in the context of N=4 Super Yang Mills. I will review their relevance in computing holographic correlators and discuss recent developments on how to treat them.
11 December 2024, Brussels
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. Finally, we will discuss some first steps in generalizing this story to de Sitter space.
Simon Ekhammar (King’s College) | Strongly Coupled N=4 SYM via Integrability
N=4 Super Yang-Mills (SYM) is a highly symmetric theory which appears to be solvable in the planar limit. In particular, the full non-perturbative spectrum is encapsulated in the integrability-based Quantum Spectral Curve (QSC). However, while the QSC can be readily solved at weak coupling, strong coupling has proved much more challenging. In this talk, I will describe the QSC, discuss recent progress at strong coupling and present new results for the spectrum of strongly coupled N=4 SYM. (Based on 2406.02698 with Nikolay Gromov and Paul Ryan)
4 December 2024, Leuven
Igal Arav (Leuven) | Superconformal Monodromy Defects in Holography
I will discuss the properties of codimension two superconformal monodromy defects in 3d and 4d SCFTs, associated with imposing non-trivial monodromies for global U(1) symmetries around the defect. I will study them in the context of holographic supergravity solutions dual to SCFTs corresponding to UV and IR fixed points of certain RG flows, and demonstrate some novel properties of such defects: I will show how defects of these UV and IR theories are connected by a line of “non-localized” conformal defects with spatially dependent mass terms, as well as by defect-bulk RG flow, and discuss the behaviour of the central charge associated with them under such a defect-bulk RG flow.
Vijay Balasubramanian (UPenn) | Spread complexity and the saturation of wormhole size
Recent proposals equate the size of Einstein-Rosen bridges in JT gravity to spread complexity of a dual, double-scaled SYK theory (DSSYK). I will show that the auxiliary “chord basis” of these proposals is an extrapolation from a sub-exponential part of the finite-dimensional physical Krylov basis of a spreading thermofield double state. The physical tridiagonal Hamiltonian coincides with the DSSYK approximation on the initial Krylov basis, but deviates markedly over an exponentially large part of the state space. I will non-perturbatively extend the identification of ER bridge size and spread complexity to the complete Hilbert space, and show that it saturates at late times. Along the way, I will use methods for tridiagonalizing random Hamiltonians to study all universality classes to which large N SYK theories and JT gravities can belong. The saturation dynamics depends on the universality class, and displays “white hole” physics at late times where the ER bridge shrinks from maximum size to a plateau. I will also describe extensions of our results to higher dimensions.
27 November 2024, Brussels
Nele Callebaut (Cologne U.) | Radial canonical AdS_3 gravity and TTbar theory
In this talk, I will employ an ADM deparametrization strategy to discuss the radial canonical formalism for asymptotically AdS_3 gravity. It leads to the identification of a radial ‘time’ before quantization, namely the volume time, which is canonically conjugate to York time. Holographically, this allows to interpret the semi-classical partition function of TTbar theory as a Schrodinger wavefunctional satisfying a Schrodinger evolution equation in volume time. The canonical perspective can be used to construct from the Hamilton-Jacobi equation the BTZ solution, and corresponding semi-classical Wheeler-DeWitt states. Based on 2406.02508 with Matthew J. Blacker, Blanca Hergueta and Sirui Ning.
Robin Karlsson (CERN) | Conformal collider at finite coupling
Energy correlators measure the flux of energy in a collision event. In QCD, the coupling runs with the angle between the detectors, and as it varies, there is a transition from a perturbative regime described by quarks and gluons to free hadron propagation. In N=4 SYM, there is a similar transition when the coupling is varied from weak to strong. I will describe work in progress on applying conformal bootstrap in this setting to obtain rigorous narrow bootstrap bounds on the energy-energy correlator at any finite value of the coupling. This provides the first application of numerical conformal bootstrap to a collider observable. Moreover, I will describe how to obtain new subleading analytical predictions at strong coupling by considering stringy contributions. Time permitting, I’ll comment on how the bulk geometry is encoded in the energy-energy correlator in the context of AdS/CFT and its relation to high-energy gravitational scattering in the bulk.
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.