Seminars

Abstract:TBA

Abstract: What happens at spacetime singularities is poorly understood. The Penrose-Wall singularity theorem constrains possible scenarios, but until recently its key assumption--the generalized second law (GSL)--had only been proven perturbatively, severely limiting this application. We highlight that recent progress enables a proof of the GSL in holographic brane-world models, valid non-perturbatively at the species scale cG (with c the number of matter fields and G Newton's constant). This enables genuine constraints: an outer-trapped surface in the Einstein gravity regime implies geodesic incompleteness non-perturbatively at the species scale. Conversely, any genuine resolution must evade Penrose's criteria. We illustrate both possibilities with explicit examples: the classical BTZ black hole evolves to a more severe singularity, while a null singularity on the Rindler horizon is resolved, both by species-scale effects. Subject to the GSL, these constraints on singularity resolution apply beyond brane-worlds: namely, in any theory with a geometric UV scale--roughly, where the metric remains well-defined but classical Einstein gravity breaks down.

Abstract: In a spacetime with asymptotically anti-de-Sitter boundaries, localized bulk events produce characteristic signals at boundary locations that are lightlike from the event. I will describe (thought) experiments that use boundary wavepackets to measure the bulk geometry and bulk scattering amplitudes using these signals, and discuss some new signatures of bulk causality and local dynamics on boundary correlation functions.

Abstract: I will discuss our recent proposal for the Symmetry Topological Field Theory (SymTFT) for global, continuous spacetime symmetries (https://arxiv.org/abs/2509.07965 ). For a d-dimensional theory, it is given by a (d + 1)-dimensional BF-theory for the spacetime symmetry group, and whenever d is even, it includes also Chern-Simons couplings, that encode conformal and gravitational anomalies. This proposal is supported by the similarity to two-dimensional Jackiw-Teitelboim gravity in the d = 1 case and the topological limit of four-dimensional gravity in the d = 3 case. I will discuss how topological defects constructed from this SymTFT act by applying the associated spacetime symmetry transformation. Finally, I will discuss the relation to gravity and holography.

Abstract: I will present a method to compute four-point correlation functions with multiparticle operators in holography by using microstate geometries. Focussing on the simplest correlators built out of particles from the graviton multiplet, e.g. two single and two multi-particle insertions, I will present explicit results in N=4 SYM. The key point of the method is to use precision holography to build a coherent operator with the property that: it is analytic in its parameters and it admits a heavy regime dual to a smooth background geometry in AdS. Then, a two-point Witten diagram in this background computes the generating function of the multiparticle correlators of interest. This problem can be mapped to a connection problem for a Heun equation, and the resulting correlator can be expressed as a generalized BMN integral. Finally, the resulting correlator can be tested against OPE predictions in the CFT, providing new and non-trivial checks of the AdS/CFT correspondence.

Abstract:  "Entanglement islands” play a key role in recent progress understanding entropy and localization of information in quantum black holes. It has been suggested in the literature that a “massive graviton” is necessary for the existence of islands, based on (1) massive gravity arising in constructions requiring coupling to an external non-gravitating system and (2) challenges in defining gauge-invariant operators supported in a compact region such as an island. To address this concern, I will first clarify the precise statements of the island formula and associated operator reconstruction theorem, and the meaning of a “massive graviton”. I will then (1) present examples of islands with unambiguously massless gravity (including in asymptotically flat spacetimes), and (2) discuss the construction of compactly supported operators to all orders in perturbation theory.

Abstract: We investigate the vacuum structure of non-conformal, strongly coupled matter in de Sitter space using a class of bottom-up holographic models. We find that these models admit multiple coexisting de Sitter–symmetric states at fixed Hubble expansion rate. Although gravity is treated as non-dynamical, one can identify semiclassical de Sitter solutions self-consistent with the Friedmann equations. We suggest that these de Sitter–symmetric solutions can be reached through far-from-equilibrium dynamics. We argue that these states act as natural attractors for the late-time evolution of deeply overcooled matter that would otherwise support thermal inflation. In this way, they extend the phase of exponential acceleration beyond the regime of validity of hydrodynamics.

Abstract: In this talk I will explain how certain aspects of black hole physics can be understood using ideas from effective field theory. Many of the familiar notions from particle physics will play a role, including the renormalization group, universality and naturalness. In particular, I will explain why black holes naively seem like fine-tuned systems, but they are ultimately not. I will also explain how their effective description reveals a degree of universality that can be used to resum some features of the gravitational waveform for binary mergers observed by LIGO/Virgo/Kagra.

Abstract: Charged and Rotating Black Holes in $AdS_5 \times S^5$ are the dual description of  phases of N=4 Yang Mills theory (as a function of energy, angular momentum and charge). Familiar Kerr-Resiner Nordstorm AdS black holes dominate at high energies, but turn out to be unstable in a band of energies around extremality. We construct new black hole solutions that yield new phases that are the end point of this instability. These new phases dominate at low energies, and in particular in the BPS limit. We use our construction to provide a new conjecture for the supersymmetric cohomology of N=4 Yang Mills at large $N$, and discuss the interplay of our conjecture with field theory results for the superconformal Index.

Abstract: In this talk, I will discuss some interesting features of heavy–heavy–light–light correlators in N=4 supersymmetric Yang–Mills theory, where the light operators belong to the stress-tensor multiplet and the heavy ones correspond to giant gravitons, realised holographically as D3-branes. I will focus in particular on the associated Integrated Correlators, for which exact expressions can be obtained despite few results are known for the correlators themselves. I will highlight several interesting properties of these integrated correlators, especially the emergence of universal structures in the strong-coupling regime.

Abstract: We study gravitational form factors for baryons using the holographic description of QCD based on a D4-D8 brane system in type IIA string theory.  In holographic QCD, the baryons are represented as solitons in a 5-dimensional gauge theory. We obtain the soliton solution by solving the equations of motion numerically. Using this result, the gravitational form factors and related quantities are calculated.

Abstract: We argue that collider observables such as hadron number flux can be matched onto a linear combination of detectors/light-ray operators in perturbative QCD. The spectrum of detectors in QCD is subtle, due to recombination between the DGLAP and BFKL trajectories. We explain how to define and renormalize these trajectories at one-loop, systematically incorporating their recombination. The leading and subleading soft gluon theorems play an important role, and our analysis suggests the presence of an infinite series of further subleading soft theorems for squared-amplitudes/form factors. Combined with our light-ray matching hypothesis, the anomalous dimensions of recombined DGLAP/BFKL detectors yield a prediction for the energy dependence of the number of particles in a jet, as well as other predictions for more general energy-weighted hadron measurements. We compare these predictions to Monte-Carlo simulations, finding good agreement.

Abstract: Almost 30 years ago Dima Kharzeev argued that, contrary to naive intuition, the flow of baryon number in highly inelastic high energy processes is not traced by the valence quarks but by a gluonic structure, the string junction, postulated many years earlier by Giancarlo Rossi and myself and, independently from QCD, by Xavier Artru. I will discuss recent experimental evidence in favour of this claim by using the 50+ year old Feynman-Wilson analog model adapted to the topological expansion of QCD.

Abstract: We consider planar codimension-one defects and interfaces in N = 4 super-symmetric Yang–Mills (SYM) theory, realized by the D3/D5-brane intersection. Working in the probe limit, where the number of D5-branes is small compared to the number of D3-branes, we obtain analytic results for the holographic entanglement entropy of a ball-shaped region centered on the defect. A defect renormalization group flow is triggered by giving the defect hypermultiplets a mass, which corresponds to separating the D3- and D5-branes. Along this flow the entanglement C-function decreases monotonically. We also allow the D5-branes to carry worldvolume flux corresponding to dissolved D3-branes, in which case the setup describes an interface between two copies of N = 4 SYM theory with different gauge groups, where an RG flow is triggered by a mass term for vector multiplets. Here we again find monotonic behavior of the entanglement C-function, although its interpretation as a measure of effective degrees of freedom is problematic. We investigate possible alternative measures of degrees of freedom.

Abstract: Many string compactifications have moduli fields that can take different possible values at infinity. We address the question: given two compactifications that differ by the asymptotic values of the moduli, should we regard them as different theories or different states of the same theory? We fix a precise criterion that distinguishes between these two possibilities and show that the answer depends on whether the non-compact part of the space-time is asymptotically flat or asymptotically AdS. In the former case different values of asymptotic moduli give different states of the same theory while in the latter case they correspond to different theories.

Abstract: Numerical investigation of the non-linear fate of the Aretakis instability has pointed towards the existence of a dynamical black hole solution which remains extremal and features a persistent instability and associated horizon hair forever. I will discuss how one can realize this solution analytically within JT gravity.