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Monthly Seminar taking place at the
Laboratoire JacquesLouis Lions
Sorbonne Université, Paris
Organizers
Philippe G. LeFloch (Sorbonne, Paris)
Jacques Smulevici (Sorbonne, Paris)
Jérémie Szeftel (Sorbonne, Paris)
Lectures given during the Academic year 2021–2022
Wednesday December 8, 2021
lecture room 1516–309
14h Renato Velozo Ruiz (Cambridge, UK)
Stability of Schwarzschild for the spherically symmetric Einsteinmassless Vlasov system
Abstract. The Einstein–massless Vlasov system is a relevant model in the study of collisionless many particle systems in general relativity. In this talk, I will present a stability result for the exterior of Schwarzschild as a solution of this system assuming spherical symmetry. We exploit the hyperbolicity of the geodesic flow around the black hole to obtain decay of the energy momentum tensor, despite the presence of trapped null geodesics. The main result requires a precise understanding of radial derivatives of the energy momentum tensor, which we estimate using Jacobi fields on the tangent bundle in terms of the Sasaki metric.
15h30 Arthur Touati (Ecole Polytechnique, Palaiseau)
Construction of highfrequency spacetimes
Abstract. I will present a recent work on highfrequency solutions of Einstein’s vacuum equations. The motivation behind the study of such solutions comes from physical and mathematical questions. These solutions model the propagation of highfrequency gravitational waves, which enjoy some polarizationrelated properties. From a mathematical point of view, they partially answer Burnett’s conjecture in general relativity, which concerns the lack of compactness of a family of solutions to Einstein’s vacuum equations. I will start by reviewing the existing literature, and then discuss my results for a toy model. I will then sketch the proof of the local wellposedness in harmonic gauge for highfrequency solutions.
Wednesday November 10, 2021
lecture room 1525104
14h José Luis Jaramillo (Université de Bourgogne)
On the stability of black hole quasinormal modes: a pseudospectrum approach
Abstract. Black hole (BH) quasinormal modes (QNM) encode the resonant response of black holes under linear perturbations, their associated complex frequencies providing an invariant probe into the background spacetime geometry. In the late nineties, Nollert and Price found evidence of a BH QNM instability phenomenon, according to which perturbed QNMs of Schwarzschild spacetime migrate to new perturbed QNM branches of different qualitative behavior and asymptotics. Here we revisit this BH QNM instability issue by adopting a pseudospectrum approach. Specifically, rather than starting from the formulation of QNMs in scattering resonance theory, we cast the QNM problem as an eigenvalue problem for a nonselfadjoint operator by adopting a hyperboloidal formulation of spacetime. Nonselfadjoint (more generally nonnormal) operators suffer potentially of spectral instabilities, the notion of pseudospectrum providing a tool suitable for their study. We explore this problem in a numerical methodology based on pseudospectral methods. As a result, we find evidence that perturbed Nollert & Price BH QNM branches track the pseudospectrum contour lines, therefore probing the analytic structure of the resolvent. Specifically, we find strong support to claim: i) the stability of the slowest decaying (fundamental) mode, and ii) the instability of all QNM ‘overtones’. But numerical evidence is not a proof. Or goal in this talk is to boost the interaction between physicists and analysts to fully assess this BH QNM instability problem.
15h30 Allen Fang (Paris)
Nonlinear stability of Kerrde Sitter
Abstract. The nonlinear stability of the slowlyrotating Kerrde Sitter family was first proven by HIntzVasy in 2018 using microlocal techniques. I will present a novel proof of the nonlinear stability of Kerrde Sitter that avoids frequencyspace techniques outside of a neighborhood of the trapped set. Similar to the original work of HintzVasy, the critical step is to prove exponential decay for solutions of the linearized problem, which is done by using a highfrequency ILED estimate, and a mode stability result.
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International Conference
MATHEMATICAL GENERAL RELATIVITY
June 2nd to 5th, 2020
Unfortunately, we have to postpone this event,
and we will reschedule it in a few months.
Institut Henri Poincaré
11 rue Pierre et Marie Curie, Paris
Invited Speakers

Xinliang An (Singapore)

Mihalis Dafermos (Cambridge/Princeton)

David Fajman (Vienna)

Grigorios Fournodavlos (Sorbonne)

Elena Giorgi (Princeton)

Mahir Hadzic (London)

Gustav Holzegel (London)

Cécile Huneau (Palaiseau)

Sergiu Klainerman (Princeton)

Philippe G. LeFloch (Sorbonne)

Jonathan Luk (Stanford)

Siyuan Ma (Sorbonne)

Yue Ma (Xi’an)

Maxime Van De Moortel (Princeton)

Georgios Moschidis (Princeton)

Hans Ringström (Stockholm)

Jared Speck (Cambridge, USA)

Shiwu Yang (Beijing)

Pin Yu (Beijing)
Schedule of the conference: TBA
Schedule for TUESDAY
Schedule for WEDNESDAY
Schedule for THURSDAY
Schedule for FRIDAY
Titles and abstracts of the lectures: TBA
Organizers
Philippe G. LeFloch (Sorbonne), Jacques Smulevici (Sorbonne), Jérémie Szeftel (Sorbonne)
Funding
 GEOWAKI
“The analysis of geometric nonlinear wave and kinetic equations”
Principal investigator: Jacques Smulevici
ERC Starting Grant 2016
 EPGR
“The Evolution Problem in General Relativity”
Principal investigator: Jérémie Szeftel
ERC Consolidator Grant 2016
 Hotel de la Tour, 19 boulevard Edgar Quinet www.hoteldelatourparis.fr
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 Hotel des 3 Collèges, 16 rue Cujas www.3colleges.com
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Seminar at the
Laboratoire JacquesLouis Lions
Sorbonne Université, Paris
Organizers
Philippe G. LeFloch (Sorbonne, Paris)
Jacques Smulevici (Sorbonne, Paris)
Jérémie Szeftel (Sorbonne, Paris)
Seminar organized during the WinterSpring 2019
Tuesday February 19, 2019
lecture room 15/16309
14h João Costa (Lisbon)
Strong cosmic censorship, linear waves, and quasinormal modes
Abstract. I will present some recent results concerning the Strong Cosmic Censorship Conjecture (SCCC) in the presence of a positive cosmological constant. I will start by reviewing some of the progress made in the context of the EinsteinMaxwellscalar field system in spherical symmetry and the linear wave equation in the black hole interior of ReissnerNordström de Sitter. These results show that the validity of the SCCC hinges on the precise decay rates of perturbations along the event horizon, which are known to be determined by the black hole’s quasinormal spectrum. I will also discuss recent numerical computations of quasinormal modes that suggest the failure of the SCCC in a near extremal regime of charged de Sitter black holes.
15h30 Shijie Dong (Paris)
Evolution of the U(1) Higgs Boson: global nonlinear stability with energy bounds
Abstract. Relying on the hyperboloidal foliation method, we establish the nonlinear stability of the ground state for the socalled U(1) standard model of electroweak interactions. This amounts to establishing a globalintime theory for the initial value problem for a nonlinear waveKleinGordon system coupling massive (Dirac, scalar, gauge) equations together. In particular, we need to investigate here for the Dirac operator and the properties of energy functionals associated with the hyperboloidal foliation of Minkowski spacetime. We also provide a unified decay result for the Dirac equation when the mass coefficient can be arbitrarily small. Our energy bounds are uniform with respect to our (hyperboloidal) time variable, except for a mild loggrowth. This is a joint work with P.G. LeFloch (Paris) and Z. Wyatt (Edinburgh).
Monday March 18, 2019
lecture room 15/16101
14h MariaCaterina Valcu (Lyon)
Des équations de contrainte en relativité générale
Abstract. On s’intéresse à la caractérisation des données initiales en relativité générale, c’est à dire aux solutions des équations de contrainte. On s’appuie sur une version modifiée de la méthode conforme, introduite cette fois par David Maxwell, qui semble mieux adaptée à l’étude du système dans le cas où la courbure moyenne n’est pas constante. Par contre, le système devient bien plus compliqué du point de vue analytique. On travaille sous des conditions de petitesse sur nos paramètres, en petite dimension (n=3,4,5) et en présence d’un champ scalaire avec potentiel positif, ce qui mène à un terme nonlinéaire dominant focalisant. L’analyse est assez fine et implique une série d’outils différents, dont des résultats de compacité et un théorème du point fixe.
15h30 Léo Bigorgne (Orsay)
Sharp asymptotic behavior of solutions of the 3d VlasovMaxwell system with small data
Abstract. The VlasovMaxwell system is a classical model in plasma physics. Glassey and Strauss proved global existence for the small data solutions of this system under a compact support assumption on the initial data. They used in particular that under these hypotheses, the velocity support of the particle density remains compact. This allows a clean separation between the characteristics of the wave equations (which are null geodesics) and those of the transport equation (which are timelike). We will see how vector field methods can be applied to revisit this problem. In particular, it allows to remove all compact support assumptions on the initial data and obtain sharp asymptotics on the solutions and their derivatives. We will also study the null structure of the system, which constitutes a crucial element of the proof and allows us to deal with high velocities.
Monday May 6, 2019
lecture room 15/16309
14h Erwann Delay (Avignon)
Le théorème d’énergie positive hyperbolique
Abstract. Le théorème d’énergie positive hyperbolique affirme que toute variété riemannienne complète, asymptotique à l’espace hyperbolique réel, et dont la courbure scalaire est minorée par celle du modèle, possède un vecteur énergieimpulsion de genre temps dirigé vers le futur, ce vecteur étant nul seulement pour le modèle. Nous verrons une preuve de ce résultat en toutes dimensions et sans condition spin. Il s’agit d’un travail en collaboration avec Piotr Chrusciel.
15h30 Olivier Graf (Sorbonne)
The spacelikecharacteristic Cauchy problem with L2 bounded curvature
Abstract. The bounded L2 curvature theorem by Klainerman, Rodnianski, and Szeftel states that the time of existence of a solution to Einstein’s vacuum equations is controled by the L2 norm of its curvature on spacelike Cauchy hypersurfaces. I will present a version of this result where the curvature is bounded in L2 on null hypersurfaces. This provides a first breakdown criterion on characteristic hypersurfaces at this level of regularity. The proof relies on an extension procedure, as well as on the existence and control at low regularity of a new parabolic foliation of null hypersurfaces. This is a joint work with Stefan Czimek (Toronto).
Monday June 24, 2018
lecture room 15/16309
14h Oscar J. C. CamposDias (Southampton)
Strong cosmic censorship (in de Sitter backgrounds)
Abstract. Generically, strong cosmic censorship (SCC) is the statement that physics within general relativity should be predicted from initial data prescribed on a Cauchy hypersurface. In this talk I will review how finetuned versions of SCC have been formulated and evolved along the last decades up to the point where we believe that Christodoulou’s version is true in asymptotically flat spacetimes. However, I will also describe that in the last 2 years it was found that this is no longer necessarily true for some other backgrounds, namely in de Sitter (with a positive cosmological) spacetimes.
15h30 Shiyuan Ma (Sorbonne)
Linear stability for the Kerr spacetime
Abstract. The Teukolsky Master Equation governs the dynamics of linearized gravity on the Kerr rotating black hole spacetime. In this talk, based on recent works on basic energy and Morawetz estimates for solutions of the Teukolsky equation, I shall show how to derive improved decay estimates for the Teukolsky equation and explain how such results can be used to prove linear stability for the Kerr spacetime. The proof relies on using a radiation gauge. This is joint work with Lars Andersson, Thomas Bäckdahl, and Pieter Blue.
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TwoDay Meeting
“Modeling and Computation of Shocks and Interfaces”
Laboratoire JacquesLouis Lions
Sorbonne Université, Paris
March 20 and 21, 2019
Location: lecture room 1516–309
Organizer: Philippe G. LeFloch (Paris)
Supported by the ModCompShock ITN project
Speakers
Stavros Avgerinos (Catania)
Benjamin Boutin (Rennes)
Frédéric Coquel (Ecole Polytechnique)
Charalambos Makridakis (Brighton)
Carlos Pares (Malaga)
Giovanni Russo (Catania)
Titles of the Lecture and Schedule
Wednesday March 20
11am–noon: Benjamin Boutin Finite difference convergence results for linear hyperbolic initialboundary value problems — FIRST TALK CANCELLED — We will start at 2pm
2pm3pm Carlos Pares Wellbalance highorder finite volume methods for systems of balance laws
3:30pm4:30pm: Giovanni Russo Semiimplicit schemes for allspeed flows in gas dynamics and shallow water equations
Thursday March 21rst
10am11am Stavros Avgerinos A semiimplicit scheme for the Exner model
11:30am Charalambos Makridakis Approximate Young measures, kinetic models and measure valued solutions of hyperbolic problems.
2:30pm: Frédéric Coquel Jin and Xin’s relaxation schemes with defect measure corrections for nonlinear systems of conservation laws
Practical informations
The talks will take place in the main lecture room 309 of the Laboratoire JacquesLouis Lions, Sorbonne Université, which is located in the building 1516.
Address: 4 Place Jussieu, 75258 Paris. Subway station: Jussieu.
List of hotels in the vicinity of the university
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Seminar at the
Laboratoire JacquesLouis Lions
Sorbonne Université, Paris
Organizers
Philippe G. LeFloch (Sorbonne, Paris)
Jacques Smulevici (Sorbonne, Paris)
Jérémie Szeftel (Sorbonne, Paris)
Dates of the Seminar this Fall 2018
September 24, October 29, November 19
Monday September 24, 2018
room 15/16309
14h Dietrich Häfner (Grenoble)
Scattering for Dirac and KleinGordon fields on the (De Sitter) Kerr metric and the Hawking effect
Abstract. We consider Dirac and KleinGordon fields on the (De Sitter) Kerr metric which describes rotating black holes. Whereas there exists a conserved L2 norm for the Dirac field, no positive conserved quantity exists for the KleinGordon field, which makes the analysis more difficult for the latter. We obtain asymptotic completeness results for the Dirac field on the Kerr and for the KleinGordon field on the De Sitter Kerr metric. We then present a rigorous result about the Hawking effect for fermions in the setting of a collapse of a rotating charged star. This effect predicts the creation of particles by black holes.
15h30 JeanPhilippe Nicolas (Brest)
Peeling for scalar fields on the Kerr metric
Abstract. The peeling is an asymptotic behavior of massless fields along outgoing null geodesics in asymptotically flat spacetimes, initially observed by Sachs at the beginning of the 1960’s, then reformulated in very simple terms by Penrose in 1965 using conformal geometry. The question of its genericity, especially when talking about the peeling of the Weyl tensor of an Einstein spacetime, was controversial for several decades after Penrose’s paper. For Einstein’s equations, the question is now essentially settled, but given an Einstein spacetime, it is not clear whether there is a large class of Cauchy data giving rise to solutions with a good peeling. Lionel Mason and the speaker answered the question for fields of spin 0, 1/2 and 1 on Schwarzschild’s spacetime in 2009 and 2012. We extended recently the results to linear and non linear scalar fields on the Kerr geometry in a joint work with Pham Truong Xuan. We shall recall the history of the subject, describe the principles of the approach developed with Lionel Mason and talk about the specific features of our work for Kerr metrics.
Monday October 29, 2018
room 15/16101
14h Joe Keir (Cambridge)
The weak null condition and the pweighted energy method
Abstract. The Einstein equations in wave coordinates are an example of a system which does not obey Klainerman’s “null condition”. Their failure to satisfy this condition leads to many difficulties, both in LindbladRodnianski’s proof of global existence and in any attempt to apply other techniques to these equations. One such technique is the “pweighted energy method” of Dafermos Rodnianski, which is a very powerful and robust method that can easily be adapted to understand the behavior of waves in many interesting situations, including black holes. In this talk I will explain how to modify this method to systems which only obey the “weak null condition”, including the Einstein equations. This involves adapting the pweighted energy method, and combining it with the many of the geometric methods used by Christodoulou and Klainerman. Among other things, this allows us to enlarge the class of wave equations which are known to admit smalldata global solutions, and it also yields a detailed description of null infinity. In particular, in some situations we can understand the geometric origin of the slow decay towards null infinity exhibited by these systems: it is due to the formation of “shocks at infinity”.
15h30 Volker Schlue (Paris)
Scattering from infinity for semilinear wave equations
Abstract. I will discuss the construction of global solutions from scattering data (at null infinity) for various semilinear wave equations on Minkowski space satisfying the (weak) null condition. I will elaborate on the proof which relies, i) on a fractional Morawetz estimate, and (ii) on the construction of suitable approximate solutions from the scattering data. Finally I will outline the application of these results to Einstein’s equations in harmonic coordinates. This is joint work with Hans Lindblad.
Monday November 19, 2018
room 15/16309
14h Adam Layne (Stockholm)
Stability within T2symmetric expanding spacetimes
Abstract. We present a recently completed, nonpolarized analogue of the asymptotic characterization of T2symmetric Einstein flow solutions by P. LeFloch and J. Smulevici. We impose a far weaker condition, but obtain similar rates of decay for the normalized energy and associated quantities. Critical to this work have been novel numerical simulations which indicate that there is locally attractive behavior for those T2symmetric solutions not subject to this weakened condition. This local attractor is distinct from the local attractor in our main theorem, thereby indicating that the polarized asymptotics are on one hand stable within a larger class than merely polarized solutions, but unstable within all T2symmetric solutions.
15h30 Grigorios Fournodavlos (Sorbonne)
Dynamics of the Einstein vacuum equations about the Schwarzschild black hole interior
Abstract. We will talk about the dynamical behavior of the Schwarzschild black hole singularity, in the context of the Einstein equations in vacuum, from the point of view of the Cauchy problem in general relativity. As it is well known, the Schwarzschild singularity is highly unstable under arbitrarily small perturbations, which makes the study of its dynamics in full generality a difficult problem. We will begin by giving an overview of the current status of the nearSchwarzschildblack hole interior problem and we will compare it to the dynamics observed near other singularity models, in black hole interiors or Big Bangs. Then we will discuss linear and nonlinear partial results in the nearSchwarzschild case, both backwards and forwards in time, with and without symmetries.