Organisation des séminaires et calendrier

Extremal black holes in flat space feature the attractor mechanism, meaning that moduli are pulled towards certain values at the horizon in such a way that bigger black holes make scalar fields travel bigger distances. Using the distance conjecture, or looking at top-down examples, this means that near the horizon of big black holes physics from the ultra violet completion is at play. This seems to contradict effective field theory expectations. In this talk I will explain how this is a consequence of having moduli and that in phenomenological compactifications we should get the opposite behavior: the bigger the black hole the less scalar fields shift positions and less of the UV modes are relevant. Yet, if UV physics is needed to solve certain black hole paradoxes then this implies that compactifications consistent with quantum gravity must have light fields. We verify some of these ideas in actual top down examples, but first discuss bottom up models. Based on recent work with Matilda Delgado and Sebastien Reymond.

I will describe a variety of applications of non-invertible symmetries and dualities. One use is to extend Kramers-Wannier duality to a large class of models, explaining exact degeneracies between non-(conventional) symmetry-related ground states as well as in the low-energy spectrum. For critical models, the universal behaviour under Dehn twists gives exact results for scaling dimensions, while gluing a topological defect to a boundary allows universal ratios of the boundary g-factor to be computed exactly on the lattice. Time permitting I will also describe analogous 2+1d models that provide good candidates for realising chiral topological order on the lattice.

We will discuss some aspects of two dynamics, motivated from cosmology but which originally appeared

in hydrodynamics, statistical physics and condensed matter, the Burgers and the Gross-Pitaevskii equations.

In the first part, we will present a simple solvable case in 1D with Poisson initial conditions.

In the second part, we will discuss the formation of dark matter vortices (as in superfluids).

Nous étudions la percolation FK où les paramètres des arêtes sont choisis comme des variables aléatoires indépendantes dans le régime proche-critique. Nous montrons que si ces paramètres satisfont une condition naturelle de centrage autour du point critique, alors le modèle quenched présente typiquement un comportement critique à des échelles bien plus grandes que la longueur caractéristique déterministe. Plus précisément, dans une boîte de taille N, si le modèle homogène avec un paramètre d’arête déterministe p a un comportement critique dans le régime |p − pc| ≤ W, alors le modèle quenched avec des paramètres d’arête aléatoires p qui satisfont typiquement |p − pc| ≤ W^{1/3} a encore un comportement critique. Nous traitons également le cas particulier de la percolation de Bernoulli, où nous montrons que si l’on commence par échantillonner des paramètres d’arêtes aléatoires indépendants non dégénérés centrés autour de 1/2, puis une configuration de percolation sur ces arêtes, le modèle quenched  est presque sûrement critique à grande échelle. Basé sur un travail avec Emile Avérous, disponible sur arXiv 2509.08938.

We use stellar dynamics as a testbed for statistical closure theory. We focus on the process of « Vector Resonant Relaxation, » a long-range, non-linear, and correlated relaxation mechanism that drives the reorientation of stellar orbital planes around a supermassive black hole. This process provides a natural setting to evaluate the predictive power of generic statistical closure schemes for dynamical correlation functions, in the fully non-linear and non-perturbative regime. We develop a numerical scheme that explicitly implements the seminal « Martin-Siggia-Rose » formalism at one-loop order via an iterative fixed-point approach, thereby improving upon the bare order from the « Direct Interaction Approximation. » Using this framework, we quantitatively validate the ability of the formalism to predict (i) the two-point two-time correlation function; (ii) the renormalised three-point interaction vertex; (iii) the three-point three-time correlation function. These predictions are compared to direct measurements from numerical simulations. We conclude by discussing the limitations of this approach and presenting possible future venues.

TBA

Abstract: 

(i) We study the statistics of stretched two-dimensional random walks (Brownian bridges) in the vicinity of an impermeable disc using the optimal–fluctuation approach. We show that the transverse span of the walks away from the boundary scales with the Kardar–Parisi–Zhang (KPZ) exponent $1/3$. (ii) Using the analogy between the optimal fluctuation in this setting and that in the one-dimensional Balagurov–Vaks trapping problem, we propose a connection between KPZ-like statistics and Lifshitz tails arising in a deterministic large-deviation landscape. (iii) By interpreting the radial component of the random walk above the disc as diffusion in a conformally invariant $1/r^{2}$ potential, we recover the Efimov–BKT behavior of the associated renormalization-group flow. From a large-deviation perspective, we argue that the typical paths responsible for the BKT-like behavior belong to a sub-ensemble of stretched Brownian bridges driven into the large-deviation regime.

TBA