Scientific programme

In this talk, our object of study is the Einstein-Vlasov system with a massless Vlasov matter field. Our main contribution is the provision of a novel technique for controlling the Vlasov matter in a double null gauge, relying purely on vector field commutation and bypassing the need for introducing Jacobi fields, which has so far been the only existing technique outside of symmetry in this gauge. To obtain it, we have to overcome stringent regularity issues that are existent along this path. Because it relies solely on vector field commutation, our technique is fundamentally simple and flexible enough to be applicable in any data regime. We thus anticipate this to be a helpful tool for many subsequent problems regarding the Einstein-Vlasov system, including, for instance, simplified approaches to the proof of nonlinear stability of Minkowski spacetime with massless Vlasov matter, scattering problems for data close to black holes etc. We further introduce and work in an optimal regularity framework, using this new technique, to obtain a large data semi-global existence theorem and a dynamical trapped surface formation statement for our system. Joint work with Puskar Mondal and Shing-Tung Yau.

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Robinson-Trautman waves are a class of exact solutions to Einstein's equation that describe a system that settles down to boosted Schwarzschild black holes through the emission of gravitational radiation. By analyzing them through the lens of physics on null infinity, we are able to explicitly work out their displacement and non-linear memory as well as the thermodynamics of boosted Schwarzschild black holes, which correspond to the equilibrium manifold of this dissipative system.

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Dynamics on Lie groups are ubiquitous in physics, with examples ranging from rigid body motion to spin chains or perfect fluids. In this talk, I will present recent works about the quantization and semiclassical analysis of these systems. More precisely, starting from the Hilbert space of square integrable functions on a Lie group, I will derive the path integral quantization of the system. Crucially, this construction requires defining a 'momentum space' for functions on a group, related to position space via a noncommutative Fourier transform. Based on 2512.19840 and 2607.16029 in collaboration with Blagoje Oblak and Marios Petropoulos.

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We review how electric and magnetic Carrollian actions for bosons of arbitrary spin can be defined as inequivalent c → 0 limits of the relativistic Fronsdal actions. We then discuss how electric and magnetic Carrollian actions for fermions of arbitrary spin can be defined along similar lines. 

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In this talk, we study Carrollian fluids through an intrinsic kinetic derivation. It relies on a statistical analysis of interacting space-filling branes on a flat background, thereby providing the closest Carrollian analogue to the Galilean gas of interacting particles that underpins Boltzmann’s collision theory. By adapting Boltzmann’s statistical approach within this framework, we are able to provide a first-principles microscopic derivation Carrollian fluid equations. Transforming covariantly under the Carroll group, they are nonetheless inequivalent to the vanishing-speed-of-light expansion of relativistic fluids. This discrepancy is understood thanks to a symmetry group analysis, and a Carrollian physical system is exhibited to have the properties of the later expansion.

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In this talk, we will discuss the initial boundary value problem in general relativity from a mathematical point of view. We will first make the analogy to the classical Cauchy problem for the Einstein equations, first resolved by Yvonne Choquet-Bruhat in 1952, and highlight some desired properties that a potential treatment of timelike boundaries should have. Then, we will go over the various technical difficulties that arise in such an endeavor. The main result that we will present in the end concerns the local well-posedness of the problem for homogeneous Neumann conditions, in geometric terms, when the boundary is umbilic. This is joint work with Jacques Smulevici.

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I will introduce the notion of three-dimensional asymptotically-FLRW spacetimes, and study in detail the case of a coupling to scalar matter. This will illustrate in particular important analogies and lessons for four-dimensional asymptotically-flat spacetimes.

In this talk, I will discuss recent findings concerning the Euclidean four-sphere (S⁴) path integral of de Sitter (dS) higher-spin gravity, with special emphasis on the microscopic completion of the theory. I will explain how, based on a one-loop analysis, we are led to a gluing formula expressing the full S⁴ path integral in terms of an underlying S³ path integral. The S³ is viewed as a boundary hypersurface splitting S⁴ into two halves. For a higher-spin spectrum containing even spins only, the theory living on the S³ boundary is the Sp(N)-invariant sector of N anti-commuting, conformally coupled free scalars, with conformal higher-spin sources mediating the gluing. I will then shift focus to the supersymmetric case, where the gluing formula is instead built from an N=2 superconformal boundary field theory coupled to U(N) invariant superconformal sources. Remarkably, the leading contribution to the S⁴ partition function is 2^N, aligning well with a "count of states" interpretation of the path integral. These results are based on joint work with D. Anninos, C. Baracco and B. Mühlmann (arxiv:2602.19812).

I will also share some thoughts concerning the possibility that the microscopic degrees of freedom are encoded in the cosmological horizon, a natural habitat for Carrollian theories

Asymptotically flat spacetimes provide a framework for describing isolated gravitational systems at astrophysical scales, with well-defined energy and angular momentum, which can emit gravitational waves. At cosmological scales, however, the Universe undergoes expansion and the approximation of a flat background is no longer adequate and needs to be refined. Recently, it has been shown that various techniques developed to study asymptotically flat spacetimes can be extended to Friedmann–Lemaître–Robertson–Walker (FLRW) spacetimes. In this talk, I will focus on the generalisation of the notion of Bondi mass and its interplay with radiation by discussing flux-balance laws in an appropriate gauge.

String spectra comprise infinitely many physical states of arbitrarily high mass and spin, the presence of all of which is crucial to the good UV behaviour of string scattering amplitudes. Yet what do all these states look like? In this talk, we will discuss an efficient new technology of constructing entire string trajectories deep in the string spectrum and their scattering amplitudes. We will also briefly refer to upcoming work on the null string and its Carroll-Weyl symmetries. The aim of this research programme is to attempt to shed light on the underlying nature of string theory per se.

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Null boundaries provide a natural setting for exploring soft degrees of freedom and infinite-dimensional symmetries. I will discuss the Hamiltonian structure of electromagnetism in three and four spacetime dimensions, considering two representative types of null boundaries: null infinity in flat spacetime and black hole or cosmological horizons. Besides the standard charges associated with gauge transformations, the null foliation reveals additional edge observables generated by an intrinsic symmetry of the boundary. Together, these charges form infinite-dimensional algebras with nontrivial central extensions, whose detailed structure depends on the spacetime dimension and the geometry of the boundary. These results suggest that additional soft sectors are a general feature of field theories formulated on null hypersurfaces.

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In this talk I will present two protocols---one for asymptotically AdS spacetimes and one for asymptotically flat spacetimes---for how to detect small excitations in the bulk of the spacetime, from measurements performed in a small time-band at the AdS boundary in the asymptotically AdS case and  from measurements in a small cut at the past of the future null infinity, in the asymptotically flat case, respectively. The protocols are possible due to the failure of the split property for quantum gravitational theories and hint to the holographic nature of quantum gravity. Moreover, I will emphasise the various assumptions and limitations and I will connect these results with recent works on the type of the algebras of observables in the two cases.

In this talk, I will provide a first-principles definition of Cotton tensors on conformal Carroll geometries by gauging the conformal Carroll algebra. On the mathematical side, this analysis yields a conformally covariant object designed to quantify deviations from conformal flatness in Carrollian geometries. On the physical side, it offers a powerful geometric tool to describe gravitational radiative degrees of freedom at the conformal boundary of four-dimensional asymptotically flat spacetimes. Unlike the Bondi news, whose fully covariant definition requires amendment by supplementary data that we identify with the Geroch tensor, the Carroll-Cotton tensors offer the natural geometric objects to quantify deviations from stationarity due to the emission of gravitational waves. Based on [2608.28741].

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In this talk, I will present how techniques from quantum information theory and quantum thermodynamics can be used to derive the second law of thermodynamics for quantum fields. I will begin with the case of a finite quantum system undergoing a Markovian evolution driven by an infinite bath, whose dynamics are governed by the Lindblad equation and whose thermodynamic properties are well understood. Building on this framework, I will show how a natural generalization allows us to extend these methods to the thermodynamics of quantum fields defined on causal horizons. Unlike finite quantum systems, quantum field theory admits many unitarily inequivalent Hilbert spaces, each constructed from a distinct choice of vacuum state. I will argue that these different vacua can be interpreted as corresponding to different reservoirs driving the dynamics. In particular, I will demonstrate how Wall’s proof of the generalized second law fits naturally within this framework, and how analogous thermodynamic laws describing black hole thermodynamics emerge for asymptotic observers. These laws correspond to different thermodynamic potentials associated with distinct vacuum states, such as the Boulware, Hartle–Hawking, and Unruh vacua.

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NS5-branes play a pivotal role in the interplay among geometry, CFTs, and string theory. For parallel NS5-branes located at a point or distributed uniformly on a circle, exactly marginal deformations resolve them geometrically in their transverse space. Integrable (non-marginal) deformations of point-like NS5-brane configurations have been established in recent years as special cases of integrable λ-deformations. This talk addresses the corresponding problem for branes distributed uniformly along a circle.

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The fine asymptotic properties of dispersive gauge theories, the archetypal examples of which are the Maxwell-Klein-Gordon and the Yang-Mills-Higgs equations, depend crucially on the choice of gauge as well as the scale and regularity of the initial data. In physics, these asymptotic properties are expected to play a role as some of the conditions for a good definition of a unitary S-matrix and an analytic understanding of infrared divergences. Motivated by these questions, in this talk I will discuss the analytic properties of the Cauchy and Goursat problems for the Maxwell-Klein-Gordon equations, the appearance of null and weak null structures in the equations depending on gauge, and discuss recent work (with J.-P. Nicolas) in which we prove that certain dynamical charged scalar fields possess scattering states on null infinity. Time permitting, I will finish by discussing ongoing work which develops a novel time-asymmetric ``radiation gauge’’ adapted to the Goursat problem from null infinity, and touch on the question of time-asymmetric peeling going back to Penrose, Damour, and Christodoulou.

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A given metric solution space with its asymptotic symmetry algebra can be described in terms of tetrad variables, but this adds local Lorentz freedom. Imposing supplementary tetrad gauge conditions forces the metric asymptotic symmetry generators to carry compensating Lorentz transformations, and leaves a residual Lorentz subgroup H. A naive lift of the metric symmetry algebra then closes only up to a cocycle valued in the Lie algebra of H. I show that the data of a reference tetrad satisfying the gauge conditions gives a canonical homomorphic lift, for any H, so that the tetrad algebra is the semi-direct sum of the metric algebra with the ideal of pure residual Lorentz transformations. I illustrate this on the BMSW-u solution space, with H=U(1).

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