Seminars

Abstract: Consider a conformally covariant four-point function of identical scalar operators with a discrete spectrum, a twist gap, and compatible with the unitarity conditions. We give a mathematical proof confirming that the spectrum and OPE coefficients at large spin and fixed twist always become that of a generalized free field theory.

Abstract: I will explain a proposal for relating pure Einstein gravity with positive cosmological constant in 2+1d to a matrix model. The matrix model is capable of reproducing all the integrated cosmological correlators in the bulk as well as the exact de Sitter entropy. Based on 2501.01486 in collaboration with Scott Collier and Beatrix Mühlmann.

Abstract: In this seminar, we will explore the properties of the TTbar deformation --a distinctive integrable deformation built from components of the stress-energy tensor -- in the context of two-dimensional quantum field theories. While the bulk properties of TTbar-deformed theories are now well understood, thanks to their connections with coordinate transformations and topological gravity models, their behavior in the presence of boundaries and defects remains a largely uncharted territory. We will review recent analytical advancements, focusing on the exact g-function of TTbar deformed theories, derived using the Thermodynamic Bethe Ansatz (TBA) and verified through solutions to a Burgers-type flow equation. This comparison highlights a remarkable consistency between different approaches. The talk will conclude with a discussion of open problems and promising directions for future research in this evolving field.

Abstract: Holography has provided valuable insights into the time evolution of strongly coupled gauge theories in a fixed spacetime. However, this framework is insufficient if this spacetime is dynamical. We present a scheme to evolve a four-dimensional, strongly interacting gauge theory coupled to four-dimensional dynamical gravity in the semiclassical regime. We apply this framework to the description of the gravitational collapse and the subsequent formation of a black hole at the boundary. In the bulk, this corresponds to the formation of a black funnel. If time permits, we will also use holography to study the so-called BKL dynamics near the singularity behind the black hole horizon. 

Abstract:  I will discuss the late time behavior of thermal correlators in chaotic systems. Under certain assumptions, the thermal two point function exhibits exponential decay, with a discrete spectrum of complex frequencies. I will explain how these frequencies can be thought of as quasinormal modes of a black hole whose radial direction is identified with complexity. This perspective leads to an efficient method for computing the spectrum, and also a better understanding of the analytic structure. I will give various examples, mostly in the context of holography or the SYK model, where explicit computations can be done.

Abstract: In the first part of this talk I review recent progress clarifying some physical aspects of the extremal limit of black holes. I describe two puzzles that arise from a semiclassical treatment of near-extremal black hole thermodynamics. Both puzzles are resolved by realizing that quantum gravity effects become arbitrarily large at low temperatures. In the second part, I will describe some recent work clarifying the origin of these quantum effects from the gravitational path integral. Finally, I will describe ongoing work on identifying similar effects in certain cosmological setups.

Abstract:   Thin enough black strings are unstable to growing ripples along their length, eventually pinching and forming a naked singularity on the horizon. We investigate how string theory can resolve this singularity. First, we study the string-scale version of the static non-uniform black strings that branch off at the instability threshold: "string-ball strings", which are linearly extended, self-gravitating configurations of string balls obtained in the Horowitz-Polchinski (HP) approach to near-Hagedorn string states. We construct non-uniform HP strings  and show that, as the inhomogeneity increases, they approach localized HP balls. We also examine the thermodynamic properties of the different phases in the canonical and microcanonical ensembles. We find that, for a sufficiently small mass, the uniform HP string will be stable and not evolve into a non-uniform or localized configuration. Building on these results and independent evidence from the evolution of the black string instability with α′ corrections, we propose that, at least in d=4,5 spatial dimensions, string theory slows and eventually halts the pinching evolution at a classically stable stringy neck. The system then enters a slower phase in which the neck gradually evaporates into radiation. We discuss this scenario as a framework for understanding how string theory resolves the formation of naked singularities.

Abstract: Spindle and disc solutions have recently attracted substantial interest, though their physical interpretation remained somewhat elusive. In this talk, I will present a compelling interpretation of these solutions as arising from co-dimension two disorder defects positioned at the sphere’s poles. To support this perspective, I constructed explicit supergravity solutions and calculated corresponding holographic observables, offering robust evidence for our interpretation. I will illustrate this with various examples including the (2,0) theory compactified on a spindle as well as Gukov-Witten type defects in N=4 SYM. Additionally, I will introduce a bulk-to-defect inflow formalism, facilitating the extraction of diverse defect data.

Abstract:  Since the first direct detection of gravitational waves, the interest in perturbative approaches to gravitational dynamics has been steadily growing within the hep-th community. I will give a brief overview on how the tools developed for theoretical particle physics, such as Feynman integrals and Seiberg-Witten theory, are being applied to post-Minkowskian (PM) gravitational dynamics.

Abstract: Tree-level string theory extends Einstein gravity by an infinite set of massive higher spin particles. From a purely spacetime perspective (if we didn't know about the worldsheet picture) the consistency of string amplitudes would appear truly miraculous. This prompts the question: is string theory the unique framework for a higher spin extension of gravity?  We investigate this question by bootstrap methods, focussing on maximal 10D supergravity. We parametrize theory space by the first few EFT coefficients and by the on-shell coupling of the lightest massive state, and impose on these data causality and positivity constraints. While Type II string theory lives strictly inside the allowed region, we uncover a novel extremal solution of the bootstrap problem, which appears to contain a single linear Regge trajectory. We repeat a similar analysis for supergluon scattering.

Abstract: Black hole perturbation theory captures a few important effects in the dynamics of binary mergers, such as tidal deformations and the decay of ringdown modes, as well as the physics of the photon ring. However, deriving qualitative results that lead to robust predictions in this theory remains a challenging problem, despite a rich scientific history.

Abstract: The big bang does not describe anymore the start of time and everything else in modern cosmology. It will be argued that quantum mechanics plays a crucial role in the period of the universe before the big bang. Possible present and future experiments and observations will be discussed, that may allow to probe the physics of the pre-big bang era. The role of gravitational waves is central in such experiments. 

Abstract:  In this talk I will consider a QCD-like theory with a flavor condensing at a large scale, with distinct confining and chiral symmetry breaking transitions. The theory, admitting a holographic dual, can have a rich cosmological history, with a number of defects such as strings, domain walls, vortons, etc. I will show how the holographic description allows to derived their physics beyond the effective field theory approach.

Abstract: I will describe the dynamical evolution of a universe containing a single black hole. If the black hole has sufficiently large initial charge, it will be driven very close to extremality by the emission of neutral Hawking radiation, while charged particle emission is exponentially suppressed. At low enough temperatures, quantum gravity becomes important and Hawking-style quantum field theory in curved spacetime calculations give completely incorrect answers, even for simply questions like the energy spectrum of emitted radiation. This leads to interesting new physics, e.g. in certain regimes the dominant radiation channel becomes entangled pairs of photons, as in the "forbidden'' 2s->1s hydrogen atom transition. By careful analysis of the relevant metric fluctuations, we can calculate the quantum gravity effects in a controlled manner and tell the complete story of the black hole evaporation in both a universe with a matter content similar to ours and in a supersymmetric universe described by supergravity.

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Abstract: We summarize the famous tensions between various observational datasets and theoretical predictions of the Standard Model of Cosmology, such as the H0 and S8 tensions, that could be a sign that we are approaching New Physics. Then we provide possible solutions, arising from modifications /extensions of the standard lore, focusing on (late-time) modified gravity solutions, and in particular on geometric theories such as torsional and non-metricity gravities.

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Abstract: General properties of the renormalisation group (RG) are of immense theoretical interest, as they have implications for the evolution of physical systems from high to low energies. In a perturbative setting, RG flows are determined by a vector field, the beta function, that can be computed in a loop expansion. In this talk we will discuss the gradient property of the RG up to six loops in multi-scalar models in d=4 and d=4-ε dimensions. After elucidating a variety of subtleties, we will derive and discuss highly nontrivial constraints that need to be satisfied for the RG flow to be gradient.

Abstract: Critical phenomena in the presence of interfaces provide a much richer arena than their more studied cousin of boundary critical phenomena. In this talk, I'll review certain observables that are new and unique in interface conformal field theories and then demonstrate some novel bounds we can derive on these quantities using techniques from holography and quantum information.

Abstract: 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. 

Abstract: We will begin with a review of the classical BKL heuristics regarding solutions to the Einstein equations containing a Big Bang singularity. We will discuss the conjectural picture of the very rich dynamics of Big Bang singularities and comment on the existing works in the literature. Then, we will present some of the state of the art results concerning solutions that exhibit Kasner-like behavior.

Abstract: Topological recursion is a recursive algorithm discovered around 2007. In this talk, our goal is threefold: introduce the algorithm, provide recent results and their motivations, and present the current interests of the field. We will begin by introducing the initial data of topological recursion, which includes a Riemann surface, along with a differential and a bidifferential defined on the surface. Its output is correlation functions that encode the information of interest. Then, we will discuss the example of intersection numbers and new results on this direction. We will conclude with a general overview of the field.

Abstract: Energy correlations characterize the energy flux through detectors at infinity produced in a collision event. In this talk I will describe work on these observables in the context of holographic CFTs, and the relation to high-energy scattering in the bulk gravity dual. Using known properties and unitarization in this regime, we explore the leading quantum-gravity correction and find an enhancement in the number of colors of the gauge theory log N_c. The corrections are sensitive to the full bulk geometry providing a refined probe of the emergent bulk geometry. Moreover I will comment on work in progress on bootstrapping energy correlations in planar N=4 SYM at finite 't Hooft coupling, extending beyond previous known weak and strong coupling results.

Abstract: Conformal field theories that exhibit spontaneous breaking of conformal symmetry (a moduli space of vacua) are expectedly rare in the landscape of CFTs. Intuitively, this is because a delicate conspiracy is needed to fine-tune to zero the potential of the dilaton. Yet, it is difficult to phrase this intuition in abstract CFT language, and we do not know a sharp criterion that establishes which CFTs, as abstractly defined by their conformal bootstrap data, admit moduli spaces of vacua. In this talk, I will discuss some results and limitations of two different approaches to address this issue: a bootstrap equation for two-point functions in the broken vacuum, and the large charge expansion. 

Abstract: Viscosity represents one of the most fundamental properties of liquid dynamics when measuring the resistance of a fluid to shearing motion. For strongly interacting systems that allow gravity duals, holographic computations using the linear response theory (Kubo formula) have led to a viscosity-bound conjecture, i.e. the Kovtun-Son-Starinets (KSS) bound. Nevertheless, when the deformation becomes large, the viscosity becomes a nonlinear function of the deformation, producing a plethora of interesting and ubiquitous phenomena.  I will introduce our recent work on the viscosity in anisotropic fluids using the holographic approach, including the holographic p-wave superfluid near equilibrium, and the real-time dissipative dynamics of several holographic models under large shear deformations.

Abstract: Dark matter is one of the most puzzling open problems of modern physics. Although its effects are imprinted on the formation of structures, its detection remains elusive, despite searches at different scales in the Universe. In this talk, I will explain how we can probe a big class of dark matter models known as fuzzy dark matter, through its effect on the dynamics of stars in our galaxy. 

Abstract: 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, consisting 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. 

Abstract:  If you throw your diary into a black hole, are the words lost forever? This question is central to the black hole information paradox: an observer falling in with the diary believes the information is locked behind the horizon, while an exterior observer eventually finds it scrambled into Hawking radiation. Insights from holography allow us to construct a "holographic map" that encodes the interior description within the exterior one, though such a map must annihilate many states seen by the interior observer. In this talk, I will present the "backwards-forwards map," a candidate holographic map that incorporates both interior and exterior dynamics from qubit toy models of a black hole.

Abstract: Analytic continuations of holographic entanglement entropy in which the boundary subregion extends along a timelike direction have brought the promise of a novel, time-centric probe of the emergence of spacetime. In this talk, I will propose that the bulk carriers of this holographic timelike entanglement entropy are boundary-anchored extremal surfaces probing the analytic continuation of holographic spacetimes into complex coordinates. This proposal not only provides a geometric interpretation of all the known cases obtained by direct analytic continuation of closed-form expressions of holographic entanglement entropy of a strip subregion but crucially also opens a window to study holographic timelike entanglement entropy in full generality. As a novel application of this proposal, I will focus on higher-dimensional anti-de Sitter black branes, finding multiple complex extremal surfaces and discussing possible principles to single out the physical contribution.

Abstract: This talk examines fluid mechanics linked to near-extremal black branes in Anti de Sitter (AdS) spacetime, focusing on an unconventional regime. In this setting, fluid temperature is significantly lower than the spatial and temporal variation rates of the fluid parameters, in contrast with the standard regime. Interestingly, within this low-temperature regime, the Einstein-Maxwell equations still permit a systematic perturbative expansion. This scenario brings new features to fluid dynamics, such as non-localities in constitutive relations beyond the first derivative level. I will introduce a simplified model that illustrates the main elements of the near-extremal fluid-gravity correspondence and discuss key insights for the fluid system that serves as a dual to the gravitational theory. Additionally, I will address aspects of near-extremal black holes and, time permitting, some recent related findings.

Abstract: Quantum Chromo Dynamics (QCD) the theory of strong interactions is an asymptotically free gauge theory.  Perturbation theory works well at large energies. At low energy it is well described by a non-linear sigma model describing the interactions of pions. Although at very low energies the non-linear sigma model is weakly coupled, the coupling grows fast with energy. In the energy region from ~500MeV to ~2GeV both descriptions are strongly coupled and currently only lattice simulations produce reliable results. 

Abstract: Interacting quantum field theories typically thermalize, leading to the emergence of hydrodynamics at late times. I will talk about (1+1)d QFTs at high and low temperatures, where the proximity to a CFT results in parametrically slow thermalization, with much of the associated dynamics tractable. I will first explain how the UV effective theory – conformal perturbation theory – breaks down universally at late times due to the unsuppressed exchange of stress tensors, giving room for hydrodynamics to emerge. Specialising to the case of large central charge, I will then argue that the IR effective theory – hydrodynamics -- has universal transport coefficients and use this to show that it breaks down at early times due to the existence of thermal CFT excitations. The timescales at which the two effective theories break down agree.

Abstract: In this talk, I will discuss recent developments in the calculation of the gravitational waveform sourced by 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 natural approach to attack this problem is offered by the connection with scattering amplitudes, whose eikonal exponentiation captures the classical limit. I will discuss in particular how the next-to-leading PM waveform can be extracted from a one-loop $2\to3$ amplitude. The second approximation consists in focusing on low-frequency emissions, which are governed by universal soft theorems. These are simple relations that dictate the structure of log-enhanced terms 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 such terms and discuss their contribution to the energy emission spectrum. 

Abstract: I’ll first review the black hole information problem (unitarity crisis) and some proposed resolutions.  Many of these involve new physics near the would-be horizon.  I’ll discuss the question of possible sensitivity to new near-horizon effects in gravitational wave observations.

Abstract: Modular forms play a pivotal role in the counting of black hole microstates. The underlying modular symmetry of counting formulae was key in the precise match between the Bekenstein-Hawking entropy of supersymmetric black holes and Cardy's formula for the asymptotic growth of states. The goal of this talk is to revisit the connection between modular forms and black hole entropy, and tie it with other consistency conditions of AdS/CFT. We will focus our attention on weak Jacobi forms. I will quantify how constraints on polar states affect the asymptotic growth of non-polar states in weak Jacobi forms. The constraints I'll consider are sparseness conditions on the Fourier coefficients of these forms, which are necessary to interpret them as gravitational path integrals. In short, the constraints will leave an imprint on the subleading corrections to the asymptotic growth of heavy states.  With this we will revisit the UV/IR connection that relates black hole microstate counting to modular forms. In particular, I’ll provide a microscopic interpretation of the logarithmic corrections to the entropy of supersymmetric black holes and tie it to consistency conditions in AdS_3/CFT_2.

Abstract:  The study of non-abelian gauge theories in compact or non-flat spaces can be useful to gather insights and new perspectives on the confinement problem. We consider Yang-Mills theory on four dimensional Anti-de Sitter space and wonder how signals of confinement in the bulk can be detected from boundary observables. The Dirichlet boundary condition cannot exist at arbitrarily large radius because it would give rise to colored asymptotic states in flat space and hence a deconfinement-confinement transition has to occur as the radius is increased. By perturbative computations we provide evidence for the scenario of merger and annihilation. Namely, the theory with Dirichlet boundary condition stops existing because it merges and annihilates with another theory.  We also derive a general result for the leading-order anomalous dimension of the so called displacement operator for a generic perturbation in Anti-de Sitter, showing that it is related to the beta function of bulk couplings.