**Dates of the Seminar this Winter-Spring 2019**

**February 19, March 18, May 6**

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 Einstein-Maxwell-scalar field system in spherical symmetry and the linear wave equation in the black hole interior of Reissner-Nordström 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 quasi-normal spectrum. I will also discuss recent numerical computations of quasi-normal modes that suggest the failure of the SCCC in a near extremal regime of charged de Sitter black holes.

Abstract. Relying on the hyperboloidal foliation method, we establish the nonlinear stability of the ground state for the so-called U(1) standard model of electroweak interactions. This amounts to establishing a global-in-time theory for the initial value problem for a nonlinear wave-Klein-Gordon 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 log-growth. This is a joint work with P.G. LeFloch (Paris) and Z. Wyatt (Edinburgh).

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 non-linéaire 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.

Abstract. The Vlasov-Maxwell 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 time-like). 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.

Abstract.

Abstract.

**Dates of the Seminar this Fall 2018**

**September 24, October 29, November 19**

Abstract. We consider Dirac and Klein-Gordon 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 Klein-Gordon 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 Klein-Gordon 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.

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.

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 Lindblad-Rodnianski’s proof of global existence and in any attempt to apply other techniques to these equations. One such technique is the “p-weighted 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 p-weighted 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 small-data 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”.

Abstract. I will discuss the construction of global solutions from scattering data (at null infinity) for various semi-linear 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.

Abstract. We present a recently completed, non-polarized analogue of the asymptotic characterization of T2-symmetric 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 T2-symmetric 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 T2-symmetric solutions.

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 near-Schwarzschild-black 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 non-linear partial results in the near-Schwarzschild case, both backwards and forwards in time, with and without symmetries.

11 rue Pierre et Marie Curie, Paris

**Spyros Alexakis**(Univ. of Toronto)**Xinliang An**(Univ. Toronto)**Lars Andersson**(Einstein Inst., Potsdam)**Stefanos Aretakis**(Princeton)**Grigorios Fournodavlos**(Univ. of Cambridge)**Dejan Gajic**(Cambridge University)**Peter Hintz**(Univ. of California, Berkeley)**Gustav Holzegel**(Imperial College, London)**Cécile Huneau**(Ecole Polytechnique, Palaiseau)**Jérémie Joudioux**(Univ. of Vienna)**Jonathan Luk**(Stanford Univ.)**Sun-jin Oh**(Korea Inst. Advanced Study)**Andrzej Rostworowski**(Univ. Krakow)**Jan Sbierski**(Oxford Univ.)**Yakov Shlapentokh-Rothman**(Princeton)**Volker Schlue**(Sorbonne Univ.)**Jérémie Szeftel**(Sorbonne Univ.)

Schedule for MONDAY

- 9:00am coffee
- 9:30am
**L. Andersson** - 10:30am coffee break.
- 11:00am
**S. Alexakis** - 2:30pm
**P. Hintz** - 3:30pm coffee break
- 4:00pm
**V. Schlue**

Schedule for TUESDAY

- 9:00am: coffee
- 9:30am
**J Joudioux** - 10:30am coffee break
- 11:00am
**G. Fournodavlos** - 2:30pm
**Sun-jin Oh** - 3:30pm coffee break
- 4:00pm
**D Gajic**

Schedule for WEDNESDAY

- 9:00am coffee
- 9:30am
**G Holzegel** - 10:30am coffee break
- 11:00am
**Y. Shlapentokh-Rothman** - noon:
**C. Huneau** - 7:00pm
**Reception at Sorbonne Univ.**Central Tower, Jussieu campus, 4 Place Jussieu

Schedule for THURSDAY

- 9:00am coffee
- 9:30am
**S. Aretakis** - 10:30am coffee break
- 11:00am
**J. Sbierski** - 2:30pm
**A. Rostworowski** - 3:30pm coffee break
- 4:00pm
**X. An**

Schedule for FRIDAY

- 9:00am: coffee
- 9:30am
**J. Szeftel** - 10:30am coffee break
- 11:00am
**J Luk**

**Spyros Alexakis**(Univ. of Toronto): Singularity formation in black hole interiors: Polarized perturbations of SchwarzschildAbstract: We consider the stability of the Schwarzschild singularity in vacuum under polarized and axially symmetric perturbations. We find that the space-like singularities persist under such perturbations, but their dynamics exhibit a great richness, consistent with the asymptotically velocity term dominated behavior. The result relies crucially on a new approach for the Einstein equations in axial symmetry. Joint work with G. Fournodavlos.**Xinliang An**(Univ. Toronto): On apparent horizon formation.Abstract:Combining both hyperbolic and elliptic techniques, we study the formation of a marginally outer trapped tube (apparent horizon) in gravitational collapse. Analytic and geometric properties of this apparent horizon will also be discussed.

**Lars Andersson**(Einstein Inst., Potsdam): Linear stability for the Kerr spacetime.Abstract: The Teukolsky Master Equation governs the dynamics of linearized gravity on the Kerr rotating black hole spacetime. Recent work of Ma, and of Dafermos, Holzegel and Rodnianski provides energy, Morawetz, and pointwise decay estimates for solutions of the Teukolsky equation. In this talk I shall show how to derive improved decay estimates for the Teukolsky equation and explain how such results can be used to prove linearized stability for the Kerr spacetime, including energy, Morawetz, and pointwise estimates for the linearized metric. The proof relies on using a radiation gauge. This is based on ongoing joint work with Thomas Bäckdahl, Pieter Blue, and Siyuan Ma.

**Stefanos Aretakis**(Princeton): Asymptotics for the wave equation on black hole backgroundsAbstract: We will present asymptotic results for solutions to the wave equation for the full Reissner-Nostrom family of black holes. These spacetimes are spherically symmetric asymptotically flat solutions to the Einstein-Maxwell system. We will consider both the sub-extremal and the extremal cases. We will see that conservation laws on null hypersurfaces play an important role in the precise late-time asymptotics for solutions to the wave equation. For the extremal Reissner-Nordstrom the situation is more subtle given that there are two independent conservation laws (in contrast to the sub- extremal where this is only one such conservation law). We will also present a scattering theory in the extremal case which in particular allows us to construct exponentially decaying smooth solutions. This work is joint with Gajic (Cambridge) and Angelopoulos (UCLA).

**Grigorios Fournodavlos**(Univ. of Cambridge): On ‘hard stars’ in general relativityAbstract: After a brief review of the classical results on gravitational collapse in spherical symmetry, from the Oppenheimer-Snyder model (1939) to Christodoulou’s two-phase model (1995), I will discuss one possible end state in the latter model: hard stars. These are idealized models of neutron stars. I will present a variational characterization and discuss its relevance to the orbital stability problem in spherical symmetry. Various obstacles to a global in time result are outlined, in particular the absence of a dispersion mechanism, the trapped surface formation scenario due to reflecting boundary conditions (cf. AdS-scalar field) and the possibility of phase transitions within the two phase model to avoid Rayleigh-Taylor instabilities. This is a joint work with Volker Schlue.

**Dejan Gajic**(Cambridge University): Conservation laws and late-time tails of waves on Schwarzschild for all angular momenta

Abstract: In 1972, Price suggested that inverse polynomial tails should be present in the late-time behaviour of scalar fields on Schwarzschild black holes with fixed angular momentum and the decay rates should depend in a precise manner on the angular momentum. In the decades since, many features of these tails have been explored both numerically and heuristically. The presence of polynomial tails along event horizons has important implications for the nature of singularities inside dynamical black holes. In this talk I will discuss work done in collaboration with Y. Angelopoulos and S. Aretakis that establishes rigorously the existence of these polynomial late-time tails in Schwarzschild spacetimes. I will give a sketch of how the decay rates of Price can be derived using only physical space methods and how the coefficients in the late-time asymptotics of the scalar field are related to the existence of conservation laws.**Peter Hintz**(Univ. of California, Berkeley): Global stability problemsAbstract: I will discuss the problem of proving the stability of (families of) exact spacetimes (M,g) such as Minkowski space or the family of Kerr-de Sitter (KdS) black holes as solutions of Einstein’s vacuum equation, focussing on geometric aspects of this problem: how to compactify M for the purpose of analyzing the underlying nonlinear wave equation; how to choose a gauge to break the diffeomorphism invariance of Einstein’s equation; and the role of constraint damping.

**Gustav Holzegel**(Imperial College, London): Boundedness and Decay for Solutions to the Teukolsky Equation on slowly rotating Kerr spacetimesAbstract.Abstract: I will outline a proof (joint work with M. Dafermos and I. Rodnianski) of boundedness and polynomial decay statements for solutions of the spin ±2 Teukolsky equations on a Kerr exterior background with parameters satisfying |a| << M. The estimates are obtained through natural generalisations of the higher order quantities P and \underline{P} introduced in our previous work on the linear stability of the Schwarzschild metric.

**Cécile Huneau**(Ecole Polytechnique, Palaiseau): High frequency limit for Einstein equations with U(1) symmetry.Abstract: I will present the construction of a family of solutions to vacuum Einstein equations with U(1) symmetry which consist of an arbitrary number of high frequency waves travelling in different directions. In the high frequency limit, our family of solutions converges to a solution of Einstein equations coupled to null dusts. This construction is an illustration of the so called backreaction, studied by physicists (Isaacson, Burnet, Green, Wald…) : the small scale inhomogeneities have an effect on the large scale dynamics in the form of an energy impulsion tensor in the right-hand side of Einstein equations. This is a joint work with Jonathan Luk (Stanford).

**Jérémie Joudioux**(Univ. of Vienna): The vector-field method for the transport equation with application to the Einstein-Vlasov system.Abstract: The vector-field method, developed by Klainerman, was a key tool to understand the global existence of solutions to quasi-linear wave equations. In a series of work in collaboration with D. Fajman (Vienna), and J. Smulevici (Orsay), the vector- field method is extended to the relativistic transport equation where it is used to derive decay estimates for velocity averages for solutions to the relativistic Vlasov equation. An important application of this method is the proof of the stability of Minkowski space as a solution to the Einstein-Vlasov system. I will present in this talk this commutator technique for the transport equation, and describe the decay estimates for velocity averages, and sketch the key steps of the stability proof.Joint work with D. Fajman (Vienna) and J. Smulevici (Orsay).

**Jonathan Luk**(Stanford Univ.): The interior of extremal black holesAbstract: I will contrast the interior regions of subextremal and extremal black holes and present a recent result regarding the interior of dynamical extremal black holes for the Einstein-Maxwell-charged scalar field system in spherical symmetry. This is a joint work with Dejan Gajic.

**Sun-jin Oh**(Korea Inst. Advanced Study): Strong cosmic censorship and generic mass inflation for charged black holes in spherical symmetry.Abstract: I will first review a recent joint work with J. Luk, in which the C2-formulation of the strong cosmic censorship is proved for the Einstein-Maxwell-(real)-Scalar Field system in spherical symmetry for two-ended asymptotically flat data. More precisely, it was shown that a “generic” class of data for this model gives rise to maximal future developments which are future C2-inextendible. In the second part of the talk, I will present a new, complementary theorem (also joint with J. Luk) that for a further “generic” subclass of such data, the Hawking mass blows up identically along the Cauchy horizon. This result confirms, rigorously and unconditionally, the mass inflation scenario of Poisson-Israel and Dafermos for the model at hand.

**Andrzej Rostworowski**(Univ. Krakow): New insights into nonlinear perturbations of vacuum spacetimes.Abstract: I will present a systematic and robust approach to nonlinear gravitational perturbations of maximally symmetric black holes. In particular, I will show that at each order of perturbation expansion, the system of perturbative Einstein equations can be reduced to two (for each gravitational mode in 3+1 dimensions on which the study will be focused) scalar wave equations, and the metric perturbations can be explicitly obtained, once the solutions to these scalar wave equations are known. That is, this approach extends the field of gravitational

perturbations of black holes, initiated in the seminal Regge & Wheeler 57′ paper beyond linear order. The talk will be partially based on a recent work Phys. Rev. D96, 124026 (2017).**Jan Sbierski**(Oxford Univ.): On the unique evolution of solutions to wave equationsAbstract: The well-known theorem of Choquet-Bruhat and Geroch states that for given smooth initial data for the Einstein equations there exists a unique maximal globally hyperbolic development. In particular, the time-evolution of globally hyperbolic solutions is unique. This talk investigates whether the same results hold for quasilinear wave equations defined on a fixed background. We first present an example of a quasilinear wave equation for which unique evolution of smooth globally hyperbolic solutions in fact fails and contrast this case with the Einstein equations. We then proceed by presenting conditions which guarantee unique evolution. This talk is based on joint work with Felicity Eperon and Harvey Reall.

**Yakov Shlapentokh-Rothman**(Princeton Univ.): The asymptotically self-similar regime for the Einstein vacuum equations.Abstract: We will dynamically construct singular solutions to the Einstein vacuum equations which are asymptotically self-similar in that successive rescalings around the singularity converge to a self-similar solution. Connections both to Christodoulou’s bounded variation solutions of the spherically symmetric Einstein-scalar field system and to the ambient metric construction of Fefferman and Graham will be elaborated on. This is joint work with Igor Rodnianski.

**Volker Schlue**(Sorbonne Univ.): Expanding black hole space-times: Towards the stability of the cosmological regionAbstract: The Schwarzschild de Sitter space-time is the simplest model of a black hole in the expanding universe. I will discuss the challenges arising in the stability problem for this solution of the Einstein vacuum equations with positive cosmological constant, and focus in particular on the evolution in the expanding region beyond the cosmological horizon of the black hole. I will present a result for the decay of the conformal Weyl curvature, and discuss its relation to the existence of asymptotic degrees of freedom in this problem. Moreover, I will discuss the behavior of solutions to the Eikonal equation in de Sitter, and questions related to the definition of asymptotic quantities.

**Jérémie Szeftel**(Sorbonne Univ.): The nonlinear stability of SchwarzschildAbstract: I will discuss a joint work with Sergiu Klainerman on the stability of Schwarzschild as a solution to the Einstein vacuum equations with initial data subject to a certain symmetry class.

Philippe G. LeFloch (Paris), Jacques Smulevici (Orsay), Jérémie Szeftel (Paris)

**GEOWAKI**

*“The analysis of geometric non-linear 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**

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**Dates of the Seminar for this Academic year: **

**October 9, November 20, December 4, February 12, March 12**

**+ Conference from May 28 to June 1rst**

Abstract. We will discuss new developments in the analysis of asymptotic foliations by prescribed curvature in relativistic initial data sets with prescribed asymptotic decay, generalizing results by Huisken and Yau. We will relate these foliations to the definition of the center of mass of the initial data sets under consideration. The results presented are joint work with Cortier–Sakovich and with Nerz.

Abstract. Penrose’s Strong Cosmic Censorship Conjecture is one of the central problems of Mathematical General Relativity. Its proof for the Einstein-Maxwell-Uncharged-Scalar-Field (EMSF) model in spherical symmetry relies on the formation of a Cauchy horizon that is C0 regular but C2 singular for generic Cauchy data. EMSF model however only admits two-ended black holes, unlike its charged analogue that allow for one-ended black holes, relevant to the study of charged gravitational collapse in spherical symmetry. In this talk I will present my work about spherically symmetric charged and massive scalar fields on black holes. This includes a study of the black hole interior, that relates the behavior of fields on the event horizon to the formation of a C0 regular and C2 singular Cauchy horizon. I will also mention my more recent work on the black hole exterior stability, for weakly charged massless scalar fields.

Abstract. The idea that General Relativity (GR) may have something to say about the subatomic world is about as old as GR itself, but very few physicists have taken it seriously, and little is known rigorously about it. In this talk I use the problem of the “general- relativistic Dirac spectrum of Hydrogen” to convey the conceptual and technical issues one is up against, and survey recent results obtained in collaboration with my colleague Michael Kiessling and by some of our students and postdocs.

Abstract. I shall discuss the linear stability of the Schwarzschild family of black holes as solutions to the Einstein vacuum equations when expressed in a generalised wave gauge, a result which complements the recent work of Dafermos, Holzegel and Rodnianski in a similar vein as the pioneering result of Lindblad and Rodnianski complemented the monumental achievement of Christodoulou and Klainerman in establishing the global nonlinear stability of the Minkowski space. The proof relies on classical insights regarding the linearised Einstein equations about the Schwarzschild family, in particular the decoupling of certain gauge-invariant scalars into the Regge—Wheeler and Zerilli equations, and recent advances for the linear wave equation on the Schwarzschild exterior, both of which shall be reviewed.

Abstract. I will consider both Maxwell field and linearized gravity on Kerr backgrounds, and present recent results in obtaining energy and Morawetz estimates for the extreme Newman-Penrose components.

Abstract. I will discuss a generalization of the mode stability result of Whiting (1989) for the Teukolsky equation for the case of real frequencies. The main result states that a separated solution of the Teukolsky equation governing massless test fields on the Kerr spacetime, which is purely outgoing at infinity, and purely ingoing at the horizon, must vanish.

Abstract. The Maxwell–Born–Infeld (MBI) theory is a hyperbolic system of PDEs which describes nonlinear electromagnetism. Due to its tensorial and quasilinear nature, this system can be seen as a nonlinear model problem to study the stability properties of solutions to the Einstein vacuum equations. In this talk, I will present a nonlinear stability result for the MBI system on a fixed Schwarzschild background, when the initial data are constrained to be small. A crucial element of the proof is the observation that some null components of the MBI field satisfy “good” Fackerell–Ipser equations, as in the linear Maxwell case. However, in the MBI case, these equations are coupled through cubic nonlinear right hand sides, which contain all components of the MBI field. In order to resolve the coupling, we prove high-order energy decay and, subsequently, pointwise decay for all the components of the MBI field. This is achieved through the application of many ideas developed in recent years, regarding the decay of linear fields.

Abstract. I will review the classical picture of gravitational collapse in spherical symmetry, from the Oppenheimer-Snyder model (1939) to Christodoulou’s two phase model (1995). I will then turn to the possible end states of gravitational collapse, in particular discuss non-trivial static solutions to the two-phase model, which are idealized models of neutron stars. The main results concern a variational characterization of hard stars, and I will outline their relevance for the orbital stability problem of neutron stars in spherical symmetry. I hope to conclude with a discussion of the various remaining problems that have to be overcome for a global in time result, in particular related to possible phase transitions in this model.

Abstract. A time function on a Lorentzian manifold is a continuous real valued function which is increasing along all future directed causal curves. A result of Hawking states that the existence of a time function is equivalent to stable causality, i.e. the impossibility of generating timelike loops even after small perturbations of the metric. We will discuss a construction of time functions which is quite different from Hawking’s construction, in the sense that it produces functions that still have interesting properties for non stably causal spacetimes (while Hawking’s time functions fail to be continuous without stable causality). Our approach is based on a notion of attracting sets in spacetimes, following the work of Conley on Lyapunov functions.

Abstract. We study massive matter fields evolving under their own gravitational field and we generalize results established by Christodoulou for the spherically symmetric evolution of massless scalar fields governed by the Einstein equations. We encompass both Einstein’s theory and the f(R)-theory of modified gravity defined from a generalized Hilbert-Einstein functional depending on a nonlinear function f(R) of the spacetime scalar curvature R. This is a joint work with P.G. LeFloch and F. Mena.

**Dates of the Seminar: **

**January 30, February 27, March 20, April 10, May 22, June 6, June 19, July 4**

Abstract. We will examine how certain geometric conditions on general asymptotically flat spacetimes (M,g) are related to stability or instability properties of solutions to the scalar wave equation on M. First, in the case when (M,g) possesses an event horizon with positive surface gravity and an ergo-region which is sufficiently small in terms of the near-horizon geometry, we will prove a logarithmic decay result for solutions to the wave equation, provided a uniform energy boundedness estimate holds. This result, applicable also in the absence of a horizon and an ergo-region, generalizes a result of Burq for the wave equation on the complement of an arbitrary compact obstacle in flat space. We will then apply the methods developed for the proof of this result in obtaining a rigorous proof of Friedman’s ergosphere instability for scalar waves in the case when (M,g) possesses an ergo-region and no event horizon.

Abstract. The ADM decomposition is the projection of the Einstein field equations on a spacelike foliation of the spacetime. It gives the constraint equations that must necessarily be satisfied by a riemannian metric and a 2-form to be the initial data of an Einstein spacetime. In this talk, we shall introduce some modified gravity theories: the scalar-tensor and Lovelock theories, and see how they behave under the ADM decomposition. We shall examine their constraint equations, and solve them in particular cases. This involves the study of whether a certain function of the elementary symmetric polynomials is concave or not.

Abstract. The Reissner-Nordstrøm-de Sitter spacetime models a spherically symmetric charged and non-rotating black hole in the presence of a positive cosmological constant. Depending on the parameters of the metric, this spacetime can have up tothree distinct event horizons. In the case of three horizons, we develop a scattering theory for Maxwell fields using the conformal geometric approach initiated by Penrose and Friedlander and referred to as conformal scattering. The idea is that a complete scattering theory is equivalent to the well-posedness of the Goursat problem (characteristic Cauchy problem) at the null boundary of the conformal manifold. Decay estimates obtained by geometric energy inequalities are essential tools for closing the estimates that allow the construction of the scattering operator : their role is to prove that energy cannot accumulate at timelike infinity, which can be understood as a weak form of Huygens’ principle.

Abstract. Let u be a solution to a quasi-linear cubic Klein-Gordon equation, with smooth, small Cauchy data. It is known that, under a suitable condition on the nonlinearity, the solution is global-in-time for compactly supported Cauchy data. We prove that the result holds even when data are not compactly supported but only decay like 1/r at infinity, combining the method of Klainerman vector fields with a semiclassical normal forms method introduced by Delort. Moreover, we get a one-term asymptotic expansion for the solutions and establish a modified scattering property.

Abstract. We consider solutions to the Klein-Gordon equation in the black hole exterior of Kerr-AdS spacetimes. It is known that, if the spacetime parameters satisfy the Hawking-Reall bound, solutions (with Dirichlet boundary conditions at infinity) decay logarithmically. We shall present our recent result of the existence of exponentially growing mode solutions in the parameter range where the Hawking-Reall bound is violated. We will discuss various boundary conditions at infinity.

Abstract. We will investigate blow-up properties for a class of initial data sets for the Einstein equations obtained from the conformal method in a scalar-field theory. In dimensions larger than 6, and when some stability conditions on the physics data are not satisfied, we will show that the conformal method produces blowing-up families of initial data sets. The proof of this result combines constructive variational methods with a priori asymptotic analysis blow-up techniques.

Abstract. The maximal analytic Schwarzschild spacetime is manifestly inextendible as a Lorentzian manifold with a twice continuously differentiable metric. In this talk I will describe how one proves the stronger statement that the maximal analytic Schwarzschild spacetime is inextendible as a Lorentzian manifold with a continuous metric. The investigation of low-regularity inextendibility criteria is motivated by the strong cosmic censorship conjecture in general relativity.

Abstract. We will discuss dynamical properties of the Schwarzschild interior, backwards and forwards (in time) with respect to the initial value problem for the Einstein vacuum equations.

Abstract. The late-time behaviour of solutions to the wave equation on a large class of asymptotically flat spacetimes does not conform to the strong Huygens principle. Instead, it is governed by polynomially decaying “tails”, as first discovered heuristically by Price. Their presence plays an important role in the study of singularities in black hole interiors. I will discuss a method for proving the precise leading-order asymptotics for the wave equation on these spacetimes and in the process I will introduce new energy decay estimates to obtain sharp decay rates that go beyond those obtained via traditional vector field methods. This talk is based on joint work with Yannis Angelopoulos and Stefanos Aretakis.

Abstract. It has been observed by physicists (Isaacson, Burnett, Green-Wald) that metric perturbations of a background solution, which are small amplitude but with high frequency yield at the limit to a non trivial contribution which corresponds to the presence of an energy impulsion tensor in the equation for the background metric. This non trivial contribution is of due to the nonlinearities in Einstein equations, which involve products of derivatives of the metric. It has been conjectured by Burnett that the only tensors which can be obtained this way are massless Vlasov, and it has been proved by Green and Wald that the limit tensor must be traceless and satisfy the dominant energy condition. The known exemples of this phenomena are constructed under symmetry reductions which involve two Killing fields and lead to an energy impulsion tensor which consists in at most two dust propagating in null directions. In this talk, I will explain our construction, under a symmetry reduction involving one Killing field, which lead to an energy impulsion tensor consisting in N dust fields propagating in arbitrary null directions. This is a joint work with Jonathan Luk (Stanford).

Abstract. The rigidity problem in General Relativity consists in showing that an (electro)vacuum, asymptotically flat and stationary spacetime is isometric to Kerr(-Newman). The problem was solved for analytic manifolds by Hawking in the so called “no-hair theorem”. We overview the known results related to the rigidity problem for Ricci flat smooth manifolds. In the non-analytic case, Ionescu-Klainerman extended the Hawking Killing field along the horizon to the outer domain of dependence. This was done through a unique continuation procedure, relying on Carleman estimates. We generalize the result to the case of Einstein equation coupled with Maxwell equations. Finally, we summarize what is known in the case of degenerate horizons, which corresponds to the extremal Kerr.

Abstract. The hidden symmetry of the Kerr spacetime, encoded in its pair of conformal Killing-Yano tensors, implies hidden symmetries for various test fields on such a background. Starting from certain natural operator identities we derive two such symmetries of the linearized Einstein operator. The first one is of differential order four and the relation to the classical theory of Debye potentials as well as to the Chandrasekhar transformation will be explained. The second one is of differential order six and related to the separability of an integrability condition to the linearized Einstein equations — the Teukolsky equation. Advanced symbolic computer algebra tools for xAct were developed for this purpose and if time permits, I will give an overview on the current status.

Abstract. In theoretical physics, it is often conjectured that a correspondence exists between the gravitational dynamics of asymptotically Anti-de Sitter (AdS) spacetimes and a conformal field theory of their boundaries. In the context of classical relativity, one can attempt to rigorously formulate such a correspondence statement as a unique continuation problem for PDEs: Is an asymptotically AdS solution of the Einstein equations uniquely determined by its data on its conformal boundary at infinity? In this presentation, we establish a key step: we prove such a unique continuation result for wave equations on fixed asymptotically AdS spacetimes. In particular, we highlight the analytic and geometric features of AdS spacetime which enable this uniqueness result, as well as obstacles preventing such a result from holding in other cases. If time permits, we will also discuss some applications of this result toward symmetry extension and rigidity theorems.

This Fall: **October 10, November 21, and December 12, 2016**

**Abstract. **In joint work with András Vasy, we recently established the stability of the Kerr-de Sitter family of black holes as solutions of the initial value problem for the Einstein vacuum equations with positive cosmological constant, for small angular momenta but without any symmetry assumptions on the initial data. I will explain the general framework which enables us to deal systematically with the diffeomorphism invariance of Einstein’s equations, and thus how our solution scheme finds a suitable (wave map type) gauge within a carefully chosen finite-dimensional family of gauges; I will also address the issue of finding the mass and the angular momentum of the final black hole.

**Abstract. **In this talk we present a new extension procedure for the maximal constraint equations of general relativity, motivated by applications to the Cauchy problem. Given a small solution on the unit ball, we can extend it to an asymptotically flat global solution. The main features are that our extension procedure does not need a gluing region, preserves regularity and works in weak regularity. For the proof, we use new methods to solve the prescribed divergence equation for the second fundamental form and the prescribed scalar curvature equation for the metric. We use the under-determinedness of the constraint equations to conserve regularity.

**Abstract. **The presentation will be divided into two parts. First, I will introduce the conformal equations and present recent results for these equations as well as questions arising naturally. In a second part, I will talk about the “half-continuity method” and explain how to use this method for giving answers to the questions posed in the first part.

**Abstract. **It is widely believed that at low energies, quantum gravity should yield an effective theory described by Einstein equations with a stress-energy tensor made of averaged fluctuations of quantum fields. The construction of that stress-energy tensor is however very problematic and its intricate dependence on spacetime geometry results in highly non-linear equations that possess no qualitative theory to date. In this talk I will review this problem as a motivation for improving the construction of linear Klein-Gordon quantum fields, and discuss recent progress that allows for a better control of the dependence on the spacetime metric (partly based on joint work with Christian Gérard).

**Abstract. **I will first give a brief presentation of the Schwarzschild-Anti-de Sitter spacetime and of some of its geometrical properties that will concern us. Then I will present the massive Dirac equation in this space-time and first study the Cauchy problem which is not completely obvious since our spacetime is not globally hyperbolic. I will then give a result concerning the asymptotic completeness for these fields. By means of a Mourre estimate, it is possible to obtain that the minimal velocity for these fields is 1. I will then show that our dynamics behaves in asymptotic regions like a transport at unit speed in the direction of the black hole. In a third part, I will study the local energy decay for these fields. First, using the existence of exponentially accurate quasi-modes, I will show a logarithmic lower bound on the local energy decay which is in accordance with the results of G. Holzegel and J. Smulevici in the Kerr-Anti-de Sitter spacetime for the Klein-Gordon fields. In order to obtain an upper bound, I will prove the existence of resonances and give some tools in order to localize them.

]]>

**Benjamin Boutin **Numerical boundary layers for linear hyperbolic IBVP and semigroup estimate

**Christophe Chalons **On the computation of non conservative products and cell averages in finite volume methods

**Makridakis Charalambos** ** **Energy/entropy consistent computational methods

**Sergey Gavrilyuk** Shock-droplet interaction via a new hyperbolic phase field model

**Pierangelo Marcati** Splash singularities for incompressible viscoelatic fluids

**Siddhartha Mishra **Statistical solutions of systems of conservation laws

**Carlos Pares** Entropy stable schemes for degenerate convection-diffusion equations

**Nils Risebro** Numerical methods for scalar conservation laws with a stochastically driven flux

**Giovanni Russo** Shock capturing schemes for all Mach number flow in gas dynamics

**Lev Truskinovsky** Solitary waves in the FPU lattice: from quasi-continuum to anti-continuum limit

**Tuesday afternoon**

2pm-2:45pm: C. Makridakis

2:45-3:30pm: C. Pares

3:30pm: coffee break

4pm-4:45pm G. Russo

**Wednesday morning**

10am-10:45am: S. Gavrilyuk

10:45am: coffee break

11:15am: C. Chalons

Noon: lunch buffet

**Wednesday afternoon**

2pm-2:45pm R. Abgrall

2:45pm-3:30pm S. Mishra

3:30am coffee break

4pm L. Truskinovsky

**Thursday morning**

9:30am-10:15am N. Risebro

10:15am coffee break

10:45am B. Boutin

11:30am P. Marcati

12:15 lunch buffet (end of the workshop)

- Remi Abgrall (Zurich) remi.abgrall at math.uzh.ch
- Benjamin Boutin (Rennes) chalons at math.jussieu.fr
- Christophe Chalons (Versailles) chalons at math.jussieu.fr
- Yangyang Cao (Paris) caoyangyang0721 at 163.com
- Marco De Lorenzo (Paris) marco.de-lorenzo at edf.fr
- Christian Dickopp (Aachen) dickopp at web.de
- Alain Forestier (Saclay) alain.forestier at cea.fr
- Sergey Gavrilyuk (Marseille) sergey.gavrilyuk at univ-amu.fr
- Maren Hantke (Magdeburg) maren.hantke at ovgu.de
- David Iampetro (Marseille) david.iampietro at edf.fr
- Philippe G. LeFloch (Paris) contact philippelefloch.org
- Charalambos Makridakis (Brighton) C.Makridakis at sussex.ac.uk
- Pierangelo Marcati (L’Aquila) pierangelo.marcati at gssi.infn.it
- Christoph Matern (Magdeburg) christoph.matern at ovgu.de
- Siddhartha Mishra (Zurich) siddhartha.mishra at sam.math.ethz.ch
- Roberto Molina (Paris) rmolina.sep at gmail.com
- Hieu Nguyen (Aachen) nguyen at instmath.rwth-aachen.de
- Carlos Pares (Malaga) pares at anamat.cie.uma.es
- Yohan Penel (Paris) penel at ann.jussieu.fr
- Chaoyu Quan (Paris) quanchaoyu at gmail.com
- Pierre-Arnaud Raviart (Paris) pa at raviart.com
- Nils Risebro (Oslo) nilshr at math.uio.no
- Giovanni Russo (Catania) russo at dmi.unict.it
- Lev Truskinovsky (Palaiseau) trusk at lms.polytechnique.fr
- Shugo Yasuda (Hyogo) yasuda at sim.u-hyogo.ac.jp
- Dimitrios Zacharenakis (Stuttgart) zachards at mathematik.uni-stuttgart.de

The workshop will take place in the main lecture room 309 of the Laboratoire Jacques-Louis Lions, Université Pierre et Marie Curie, which is located in the building 15-16.

Address: 4 Place Jussieu, 75258 Paris. Subway station: Jussieu.

List of hotels in the vicinity of the university

]]>*____________________________*

**Main organizer**

**Philippe G. LeFloch (Paris)**

**Co-organizers**

**Dietmar Kroener (Freiburg)**

**Frédéric Coquel (Palaiseau)**

14h30 -15h **Robert Eymard** ** (**Marne-La-Vallée)

15h – 15h30 **Jan Giesselman** (Stuttgart)

15h30 – 16h *Coffee break*

16h-16h30 **Philippe Helluy** (Strasbourg)

16h30-17h **Mirko Kraenkel** (Freiburg)

10h-10h30 **Christophe Berthon** (Nantes)

10h30-11h **Johannes Daube** (Freiburg)

11h-11h30 *Coffee break *

11h30-12h **Christophe Chalons** (Versailles)

12h-12h30 **Dietmar Kroener** (Freiburg)

12h30-14h30 *Lunch break*

14h30-15h **Bruno Després** (Paris)

15h-15h30 **Arnold Reusken** (Aachen)

15h30-16h *Coffee break*

16h-16h30 **Nina Aguillon** (Paris)

16h30-17h **Rüdiger Müller** (Berlin)

10h-10h30 **Carlos Pares** (Malaga)

10h30-11h **Florence Drui** (Châtenay-Malabry)

11h-11h30 Coffee break

11h30-12h **Christian Dickopp** (Aachen)

12h-12h30 **Frédéric Coquel** (Palaiseau)

TITLES and ABSTRACTS

Abstract. We present an approach to approximate numerically the solution of coupled hyperbolic conservation laws. The coupling is achieved through a fixed interface, in which interface conditions are linking the traces on both sides. The main interest of the scheme is that it does not use the knowledge of the solution to the Riemann problem, and hence it is quite flexible and easy to implement. The idea of the scheme is to balance the effects ot the waves that enter the interface, in order to numerically mimic the structure of the Riemann problem. The scheme is well balanced with respect to all the piecewise equilibria associated with the interface conditions. If one of the interface condition is the conservation of a conserved variable, the scheme maintains it exactly. We will present a detailed analysis in the classical case, and a variety of test cases assessing the quality of the method. This is a joint work with Raul Borsche (Technische Universität Kaiserslautern)

**Christophe Berthon** (Nantes) __Numerical convergence rate for a diffusive limit of hyperbolic systems: p-system with damping__

*Abstract.* The Navier–Stokes–Korteweg model, an extension of the compressible Navier–Stokes equations, is a diffuse interface model for liquid-vapour flows which allows for phase transitions. In the model, a small parameter represents the thickness of an interfacial area, where phase transitions occur. Its static version was studied by Hermsdoerfer, Kraus and Kroener and the corresponding interface conditions were obtained. Assuming convergence of an associated energy functional to a suitable surface measure, we will perform the sharp interface limit in the dynamic case. More precisely, by means of compactness, we will ensure that solutions to the diffusive Navier-Stokes-Korteweg equations converge to solutions of an appropriate sharp interface model as the interface thickness tends to zero. This is joint work with H. Abels (Regensburg), C. Kraus (Wuerzburg-Schweinfurt) and D. Kroener (Freiburg).

Abstract: The modeling of uncertainties is fundamental in industry and in CFD. For nonlinear equations, it questions the compatibility of L1-BV techniques (for conservation laws) with L2 approaches (for the uncertainties). I will review recent progresses on the modeling at the kinetic level (with B. Perthame), and present recent ideas which show connection of the so-called kinetic polynomials with optimal control (with E. Treat).

Abstract. As a model problem to investigate cavitation damaging the collapse of a single gas bubble collapsing near to an elastic or elastic-plastic solid wall is simulated numerically. This transient three-phase system is modeled by the compressible Euler equations completed by a stiffened gas law for both fluids, where the liquid and the gas phase are distinguished by a level set approach, and either the pure elastodynamical equations for a linear-elastic solid or an extension to describe plastic effects. A weak coupling strategy connects the alternating calculations of the fluid solver and the solid solver using transient boundary conditions that are updated by the other solver.

Abstract. Compressible two-fluid models offer a potential solution for simulating separated two-phase flows configurations. On the other hand, a specific family of such models has been developed for the regime of dispersed gas bubbles and show good agreement with experiments in the case of small acoustic perturbations. On the way to connect both types of flows, we propose here a hierarchy of homogeneous two-fluid models. Starting with Hamilton’s variational principle and adding thermodynamically consistent dissipative structures, we built a new connected hierarchy, each level of which being mathematically well-posed. Every new relaxation small parameter is physically identified through acoustic linearization and analysis of the systems dispersion relations. Furthermore, numerical methods based on finite volume schemes are developed so as to preserve the properties of the models at the continuous level and to asymptotically handle the transition from each subsystem to another. Finally, simulations of simple academic configurations are performed and show the expected properties of the first models of the hierarchy, the numerical methods and dynamically adaptive mesh techniques with the potential for massively parallel simulations.

*Abstract.* In this talk we consider a one dimensional model for isothermal two-phase flows using Lagrangian coordinates. The model is of diffuse interface type with a non-monotone pressure law. We will present a priori error analysis of a semi-discrete discontinuous Galerkin method, which satisfies a discrete version of the energy inequality which is valid on the continuous level. It also satisfies a relative energy type stability theory. Combining this stability framework with suitable projection operators for the exact solution allows us to derive optimal order error estimates. We will also present numerical results obtained using a fully-discrete version of the scheme, which validate our theoretical results.

Abstract. We present an implicit discontinuous Galerkin solver for the transport equation. Due to the upwind nature of the numerical flux, the linear system in the implicit step is block triangular. The scheme is thus well adapted to a task-based implementation. We present such an implementation using the StarPU library and we discuss applications to fluid dynamics.

Abstract. The Lippmann equation is considered as universal relationship between interfacial tension, double layer charge, and cell potential. Recently we have derived a general thermodynamically consistent model for electrochemical interfaces, which shows a remarkable agreement to single crystal experimental data. In this talk, we apply the model to a curved liquid metal electrode in contact with an electrolyte. By matched asymptotic analysis we obtain the Lippmann equation whenever the Debye length is small compared to electrode curvature radius. The interfacial tension of the Lippmann equation is composed of the surface tension of our general model, and additional contributions arising from the adjacent space charge layers. Comparison with experimental data of several mercury-electrolyte interfaces confirms the theoretical results. This is a joint work with Wolfgang Dreyer, Clemens Guhlke, and Manuel Landstorfer.

Boris Andreianov boris.andreianov at lmpt.univ-tours.fr

Robert Eymard robert.eymard at univ-mlv.fr

Jan Giesselmann jan.giesselmann@mathematik.uni-stuttgart.de

Philippe Helluy helluy at math.u-strasbg.fr

Mirko Keaenkel kraenkel at mathematik.uni-freiburg.de

Bruno Despres despres at ann.jussieu.fr

Christophe Berthon christophe.berthon at math.univ-nantes.fr

Philippe LeFloch contact at philippelefloch.org

Ruediger Mueller mueller at wias-berlin.de

Arnold Reusken reusken at igpm.rwth-aachen.de

Carlos Parés pares at anamat.cie.uma.es

Johannes Daube hannes at mathematik.uni-freiburg.de

Chalons Christophe christophe.chalons@uvsq.fr

Nina Aguillon aguillon at ljll.math.upmc.fr

Christian Dickopp dickopp at web.de

Florence Drui florence.drui at centralesupelec.fr

Emmanuel Audusse eaudusse at yahoo.fr

Gautier Dakin gautier.dakin at gmail.com

Roland Duclous roland.duclous at gmail.com

Mehdi Khalloufi mehdi.khalloufi at mines-paristech.fr

Pierre-Arnaud Raviart pa at raviart.com

Frederic Coquel frederic.coquel at cmap.polytechnique.fr

Dietmar Kroener dietmar at mathematik.uni-freiburg.de

**PRACTICAL INFORMATIONS **

How to come to the Laboratoire Jacques-Louis Lions

Hotels near the University Pierre et Marie Curie

**EARLIER WORKSHOPS “Micro-Macro Modeling and Simulation of Liquid-Vapor Flows”**

Tenth Workshop, Freiburg, February 2015

Ninth Workshop, Paris, February 2014

Eight Workshop, Berlin, February 2013

Seventh Workshop, Paris, February 2012

Sixth Workshop, Stuttgart, January 2011

Fifth Workshop, Strasbourg, April 2010

Fourth Workshop, Aachen, February 2009

Third Workshop, Strasbourg, January 2008

Second Workshop, Bordeaux, November 2007

Opening Workshop, Kirchzarten, November 2005

]]>**MAIN THEMES OF THE PROGRAM**

Einstein’s field equation of general relativity is one of the most important geometric partial differential equations. Over the past decade, the mathematical research on Einstein equation has made spectacular progress on many fronts (Cauchy problem, cosmic censorship, asymptotic behavior). These developments have brought into focus the deep connections between the Einstein equation and other important geometric PDE’s, including the wave map equation, Yang-Mills equation, Yamabe problem, as well as Hamilton’s Ricci flow. The field is of growing interest for mathematicians and of intense current activity, as is illustrated by major recent breakthrough, concerning the uniqueness and stability of the Kerr black hole model, the formation of trapped surfaces, and the bounded L2 curvature problem. Specifically, the themes of mathematical interest that will be developed in the present Program and are currently most active include:

- The initial value problem for Einstein equation and the causal geometry of spacetimes with low regularity, formation of trapped surfaces

- Techniques of Lorentzian geometry: injectivity radius estimates, geometry of null cones; construction of parametrix

- Geometry of black hole spacetimes: uniqueness theorems, censorship principles

- Coupling of Einstein equation for self-gravitating matter models, weakly regular spacetimes, nonlinear stability of Minkowski space with matter

General schedule for the Trimester

**SCIENTIFIC ACTIVITIES during the Trimester**

**Workshops and Conferences**(see below)**On the Mathematical Theory of Black Holes,**Course- Begins on October 13 from 2pm to 4:30pm, and the following Tuesdays.
- Video of the lectures

**An Introduction to Self-Gravitating Matter**__,__Course by P.G. LeFloch.- Begins on October 9 from 2pm to 4:30pm, and the following Fridays.
- LECTURE NOTES for this course
- Video 1: An introduction to self-gravitating matter
- Video 2: Modified gravity and weakly regular spacetimes
- Video 3: Weak solutions to the Einstein equations
- Video 4: Weakly regular Cauchy developments
- Video 5: Self-gravitating fluids with bounded variation
- Video 6: The geometry of weakly regular spacetimes
- Video 7: Nonlinear stability of Minkowski space for massive fields

**Weekly Seminar on Mathematical General Relativity**- organized by L. Andersson, jointly with S. Klainerman, P.G. LeFloch, J. Szeftel (Paris), and A. Zeghib (Lyon).
- Begins on October 7 from 2pm to 4:30pm, and the following Wednesdays.
- Speakers in the Seminar :
- Bruno Premoselli (Cergy-Pontoise), Martin Taylor (Cambridge, UK)
- Steffen Aksteiner (Potsdam), Lars Andersson (Potsdam)
- Jesus Oliver (San Diego)
- Anne Franzen (Utrecht), Jared Wunsch (Evanston)
- Sung-Jin Oh (Berkeley), Anna Sakovich (Vienna)
- Hans Lindblad (Baltimore), Florian Beyer (Dunedin)

- VIDEOS available (courses, main conference)
**Tea break**every day 3pm-3:30pm

**WORKSHOPS AND CONFERENCES**

**Sept. 14 to 18, 2015** **Summer School – INTRODUCTION TO MATHEMATICAL GENERAL RELATIVITY**

**List of speakers**

Greg Galloway (Miami)

Gerhard Huisken (Tuebingen)

Hans Ringstrom (Stockholm)

**Sept. 23 to 25, 2015 ** **Workshop – RECENT ADVANCES IN MATHEMATICAL GENERAL RELATIVITY**

**List of speakers**

Spyros Alexakis (Toronto)

Piotr Chrusciel (Vienna)

Joao Costa (Lisbon)

Semyon Dyatlov (Cambridge, USA)

Stefan Hollands (Cardiff)

Alexandru Ionescu (Princeton)

Lionel Mason (Oxford)

Vincent Moncrief (Yale)

Jean-Philippe Nicolas (Brest)

Harvey Reall (Cambridge, UK)

Hans Ringstrom (Stockholm)

Mu-Tao Wang (New York)

**Sept. 28 to Oct. 1, 2015 ** ** Workshop – GEOMETRIC ASPECTS OF MATHEMATICAL RELATIVITY** (Hold in Montpellier and organized by Marc Herzlich and Erwann Delay)

**List of speakers**

Piotr Chrusciel (Vienna)

Michael Eichmair (Zürich)

Mu-Tao Wang (New York)

**Oct. 26 to 29, 2015 Workshop – DYNAMICS OF SELF-GRAVITATING MATTER**

**List of speakers**

Hakan Andreasson (Gothenburg)

Thierry Barbot (Avignon)

Robert Beig (Vienna)

David Fajman (Vienna)

Marc Mars (Salamanca)

David Maxwell (Fairbanks)

Todd Oliynyk (Monash)

Volker Schlue (Toronto)

Bernd Schmidt (Potsdam)

Jared Speck (Cambridge, USA)

Shadi Tahvildar-Zadeh (Rutgers)

Eric Woolgar (Alberta)

**Nov. 16 to 20, 2015 ** **International Conference – GENERAL RELATIVITY – A Celebration of the 100th Anniversary**

**List of speakers Schedule and title**

Jean-Pierre Bourguignon (Bures-sur-Yvette)

Demetrios Christodoulou (Zürich & Athens)

Mihalis Dafermos (Princeton)

Thibault Damour (Bures-sur-Yvette)

Georges Ellis (Cape Town)

Richard Hamilton (New York)

Gustav Holzegel (London)

Jonathan Luk (Cambridge, UK)

Roger Penrose (Oxford)

Richard Schoen (Stanford & Irvine)

Jacques Smulevici (Orsay)* *

Jérémie Szeftel (Paris)

Robert Wald (Chicago)

Qian Wang (Oxford)

**Dec. 14 to 16, 2015 ** ** International Conference- RELATIVITY AND GEOMETRY – IN MEMORY OF A. LICHNEROWICZ ** (Organized by Giuseppe Dito, Jean-Pierre Francoise, Paul Gauduchon, Richard Kerner, Yvette Kosmann-Schwarzbach et Daniel Sternheimer)

**List of speakers**

Olivier Biquard (Paris 6)

Robert Bryant (Durham)

Pierre Cartier (Gif-Sur-Yvette)

Thibault Damour (Gif-Sur-Yvette)

Nathalie Deruelle (Paris 7)

Simon Donaldson (Stony Brook & London)

Michel Dubois-Violette (Paris 11)

Charles Francès (Strasbourg)

Edward Frenkel (Berkeley)

Christian Fronsdal (Los Angeles)

Simone Gutt (Bruxelles)

James Isenberg (Eugene)

Sergiu Klainerman (Princeton)

Maxim Kontsevich (Gif-Sur-Yvette)

Alan Weinstein (Berkeley)

Program coordinated by the Centre Emile Borel at IHP. Financial support provided by the Institut Henri Poincaré and the ANR Project *“Mathematical General Relativity. Analysis and geometry of spacetimes with low regularity”.*

]]>

Einstein’s field equation of general relativity is one of the most important geometric partial differential equations. Over the past decade, the mathematical research on Einstein equation has made spectacular progress on many fronts, including the Cauchy problem, cosmic censorship, and asymptotic behavior. These developments have brought into focus the deep connections between the Einstein equation and other important geometric partial differential equations, including the wave map equation, Yang-Mills equation, Yamabe equation, as well as Hamilton’s Ricci flow. The field is of growing interest for mathematicians and of intense current activity, as is illustrated by major recent breakthroughs concerning the uniqueness and stability of black hole models, the formation of trapped surfaces, and the bounded L2 curvature problem. The themes of mathematical interest that will be particularly developed in the present Program include the formation of trapped surfaces and the nonlinear interaction of gravitational waves. The new results are based on a vast extension of the earlier technique by Christodoulou and Klainerman establishing the nonlinear stability of the Minkowski space. This Program will be an excellent place in order to present the recent breakthrough on the bounded L2 curvature problem for the Einstein equation, which currently provides the lower regularity theory for the initial value problem, as well as the recently developed theory of weakly regular Einstein spacetimes with distributional curvature.

**Long-term participants**

Michael Anderson (Stony Brook)

Piotr Chrusciel (Vienna)

Mihalis Dafermos (Princeton)

Cécile Huneau (Paris)

Alexandru D. Ionescu (Princeton)

James Isenberg (Eugene)

Sergiu Klainerman (Princeton)

Philippe G. LeFloch (Paris)

Jared Speck (Cambridge, USA)

Jinhua Wang (Hangzhou)

Mu-Tao Wang (New York)

Qian Wang (Oxford)

Willie Wong (Lausanne)

**Speakers during the Workshop**

- Tuesday January 20
- Sung-Ji Oh (Berkeley) Linear instability of the Cauchy horizon in subextremal Reissner-Nordström spacetime under scalar perturbations
- Volker Schlue (Toronto) Stationarity of time-periodic vacuum spacetimes
- Alexandru D. Ionescu (Princeton) The Euler–Maxwell system for electrons: global solutions in 2D
- Joachim Krieger (Lausanne) Concentration-compactness for the critical Maxwell-Klein-Gordon equation

- Wednesday January 21
- Xianliang An (Piscataway) Two results on formation of trapped surfaces
- Tahvildar-Zadeh (Piscataway) The Dirac electron and the Kerr-Newman spacetime
- Mihalis Dafermos (Princeton)
- Jim Isenberg (Eugene) Asymptotically hyperbolic shear-free solutions of the Einstein constraint equations

- Thursday January 22
- Cécile Huneau (Paris) Stability in exponential time of Minkowski
- Jacques Smulevici (Orsay) Vector field methods for transport equations with applications to the Vlasov-Poisson system
- Mu-Tao Wang (New York) Quasi-local angular momentum and the limit at infinity
- Spyros Alexakis (Toronto) The Penrose inequality for perturbations of the Schwarzschild exterior

- Friday January 23
- Mihai Tohaneanu (Statesboro) Pointwise decay for the Maxwell system on black holes
- Qian Wang (Oxford)
- Peter Blue (Edinburgh) Revisiting decay of fields outside a Schwarzschild black hole
- Philippe G. LeFloch (Paris) Weak solutions to the Einstein equations in spherical or T2 symmetry

**11h Emmanuel Hebey (Cergy-Pontoise) **Systèmes de Kirchhoff critiques stationnaires sur des variétés compactes

**14h Lydia Bieri (Ann Arbor) **Gravitational radiation and two types of memory

**Abstract. **We are believed to live on the verge of detection of gravitational waves, which are predicted by General Relativity. In order to understand gravitational radiation, we have to investigate analytic and geometric properties of corresponding solutions to the Einstein equations. Gravitational waves leave a footprint in the spacetime regions they pass, changing the manifold – and therefore displacing test masses – permanently. This is known as the memory effect. It has been believed that for the Einstein equations, being nonlinear, there exists one such effect with a small `linear’ and a large `nonlinear’ part. In this talk, I present some of my joint work with D. Garfinkle showing that these are in fact two different effects.

**14h Thierry Barbot (Avignon) **Surfaces polygonales fuchsiennes et espace de Teichmüller décoré

**Abstract. **Dans l’article “Fuchsian polyhedra in Lorentzian space-forms, Mathematische Annalen 350, 2, pp. 417-453, 2011″, F. Fillastre a montré que toute métrique euclidienne avec singularités coniques d’angles > 2 pi sur une surface compacte se réalise de manière unique comme une surface de Cauchy polygonale dans un espace-temps globalement hyperbolique localement plat radial (i.e. dont le groupe d’holonomie fixe un point de l’espace de Minkowski). Dans cet exposé, j’évoquerai le travail de L. Brunswic dans son travail de thèse sous ma direction, qui vise à reprouver ce résultat et à l’étendre au cas des surfaces polygonales dans un espace-temps localement plat mais admettant des particules massives. Le but est de montrer qu’il y a encore existence et unicité une fois prescrit la masse des particules massives (le cas régulier montré par Fillastre correspondant au cas où l’angle singulier des particules massives est 2pi). Je montrerai aussi que la situation étudiée par R. Penner dans l’article “The Decorated Teichmϋller Space of Punctured Surfaces, Commun. Math. Phys. 113, 299-339 (1987)” est un cas limite de la situation étudiée par Brunswic, et correspond au cas où les particules sont d’angle conique nul. Je montrerai aussi comment répondre positivement à la question dans le cas où il n’y a qu’une singularité.

**15h30 Andrea Seppi (Pavia) **Convex surfaces in (2+1)-dimensional Minkowski space

**Abstract. **It is known that the hyperbolic plane admits an isometric embedding into Minkowski space; in 1983 Hanu and Nomizu first observed the existence of non-equivalent isometric embeddings, thus showing a relevant difference with the Euclidean case. In this talk, I will introduce some natural properties of a convex surface in Minkowski space, concerning causality and asymptotic behavior. I will then explain some new results (jointly with Francesco Bonsante) on the classification of constant curvature surfaces with bounded principal curvatures and on the solvability of Minkowski problem in (2+1)-dimensional Minkowski space. If time permits, I will give the main ideas of the proof and especially the relation to some type of Monge-Ampere equations.

**14h Shiwu Yang (Cambridge) **Decay properties of solutions of Maxwell Klein-Gordon equations

**Abstract. **I will present some recent progress on the asymptotic behavior of global solutions to Maxwell-Klein-Gordon equations. I will show that the integrated local energy and the energy flux through the outgoing null hypersurfaces decays polynomially in the retarded time in Minkowski space with data merely bounded in some gauge invariant weighted Sobolev space. This in particular includes the case with large charge. One novelty of this work is that these decay estimates precisely capture the asymptotic properties for the non-linear fields with arbitrarily large data. If in addition that the initial data for the scalar field is sufficiently small, then we show the pointwise decay of the solutions. This result improves the previous result of Lindblad and Sterbenz in which smallness is required for both the scalar field and the Maxwell field.

**15h30 Gustav Holzegel (London)** Local and global dynamics in asymptotically anti de Sitter spacetimes

**Abstract. **Asymptotically anti de Sitter (aAdS) spacetimes play a prominent role in theoretical physics and mathematics. Due to the presence of a timelike hypersurface at infinity these spacetimes are not globally hyperbolic, a fact that leads to intricate initial boundary value problems when studying global dynamics of hyperbolic equations on these backgrounds. In this talk, I will present several local and global results for the massive wave equation on aAdS spacetimes (including black hole spacetimes) with emphasis on how different boundary conditions (Dirichlet, Neumann or dissipative) influence the global dynamics. In particular, I will outline a recent proof (obtained in collaboration with J. Luk, J. Smulevici and C. Warnick) of linear stability and decay for gravitational perturbations on anti de Sitter space under dissipative boundary conditions. The proof unravels an interesting trapping phenomenon near the conformal boundary which necessarily leads to a degeneration in the decay estimates. Time permitting some future applications will also be discussed.

**14h François Fillastre (Cergy-Pontoise**) Minkowski problem in Minkowski space

**Abstract. **T. Barbot, F. Beguin and A. Zeghib solved a smooth Lorentzian version of the Minkowski problem in dimension (2+1). More precisely they proved that if M is a flat 3-dimensional maximal globally hyperbolic spatially compact spacetime, then there exists a unique strictly convex smooth space-like surface in M with a prescribed smooth positive Gauss curvature. We will look at this problem for any dimensions. The existence part is solved in a generalized way (a measure is prescribed rather than a function). Concerning the regularity of the solution, the 2+1 case is specific. The arguments are based on tools from the geometry of convex sets. Joint work with Francesco Bonsante (Pisa).

**14h Bruno Premoselli** (Cergy-Pontoise) Robustness of the conformal constraints in a scalar-field setting

Abstract. The constraint equations arise in the initial-value formulation of the Einstein equations. The conformal method allows one to rewrite the constraint equations into a determined system of nonlinear, supercritical, elliptic PDE’s. In this talk, we will investigate some stability properties for this elliptic system. The notion of stability under consideration, defined as the continuous dependence of the set of solutions of the conformal constraint system with respect to its coefficients, is reformulated for the conformal method. The analysis of these stability properties involves blow-up techniques concerning defects of compactness for supercritical nonlinear elliptic equations. This is a joint work with Olivier Druet.

**15h30 Christophe Bavard** (Bordeaux)** **Points conjugués des tores lorentziens

Abstract. Les points conjugués jouent un rôle important dans l’étude des variétés riemanniennes et lorentziennes, en particulier pour l’étude du rayon d’injectivité. Dans le cadre riemannien, l’absence de points conjugués impose des contraintes assez fortes sur la topologie de la variété, et parfois même sur sa géométrie. Ainsi, un résultat de Hopf (1948), généralisé par Burago et Ivanov (1994), affirme qu’un tore riemannien sans points conjugués est nécessairement plat. Dans cet exposé, je montrerai l’existence de métriques sans points conjugués dans toute composante connexe de l’espace des métriques lorentziennes sur le tore de dimension 2 ; cela prouve en particulier l’existence de tores lorentziens sans points conjugués et non plats. Il s’agit d’un travail conjoint avec Pierre Mounoud.

]]>**14h Qian Wang** (Oxford) A geometric approach to the sharp local well-posedness theory for quasilinear wave equations

Abstract. The commuting vector fields approach, devised for Strichartz estimates by Klainerman, was employed for proving the local well-posedness in the Sobolev spaces Hs with s>2+(2-\sqrt 3)/2 for general quasilinear wave equation in (1+3) spacetime by him and Rodnianski. Via this approach they obtained the local well-posedness with s>2 for (1+3) vacuum Einstein equations. A proof of the sharp H2+ local well-posedness result for general quasilinear wave equation was provided by Smith and Tataru by constructing a parametrix using wave packets. The difficulty of the problem is that one has to face the major hurdle caused by the Ricci tensor of the metric for the quasilinear wave equations. I will present my recent work, which proves the sharp local well-posedness of general quasilinear wave equation in (1+3) spacetime by a vector field approach, based on geometric normalization and new observations on the mass aspect functions.

**15h30 Jonathan Luk** (Cambridge, UK)** **Stability of the Kerr Cauchy horizon

Abstract. The celebrated strong cosmic censorship conjecture in general relativity in particular suggests that the Cauchy horizon in the interior of the Kerr black hole is unstable and small perturbations would give rise to singularities. We present a recent result proving that the Cauchy horizon is stable in the sense that spacetime arising from data close to that of Kerr has a continuous metric up to the Cauchy horizon. We discuss its implications on the nature of the potential singularity in the interior of the black hole. This is joint work with Mihalis Dafermos.

]]>**14h Arick Shao (Imperial College) **Unique continuation from infinity for linear waves

Abstract. We prove various unique continuation results from infinity for linear waves on asymptotically flat space-times. Assuming vanishing of the solution to infinite order on suitable parts of future and past null infinities, we derive that the solution must vanish in an open set in the interior. The parts of infinity where we must impose a vanishing condition depend strongly on the background geometry; in particular, for backgrounds with positive mass (such as Schwarzschild or Kerr), the required assumptions are much weaker than in Minkowski spacetime. These results rely on a new family of geometrically robust Carleman estimates near null cones and on an adaptation of the standard conformal inversion of Minkowski spacetime. Also, the results are nearly optimal in many respects. This is joint work with Spyros Alexakis and Volker Schlue.

**15h30 Claude Warnick (Warwick) ** Dynamics in anti-de Sitter spacetimes

Abstract. When solving Einstein’s equations with negative cosmological constant, the natural setting is that of an initial-boundary value problem. Data is specified on the timelike conformal boundary as well as on some initial spacelike hypersurface. Questions of local well-posedness and global stability are sensitive to the choices of boundary conditions. I will present recent work exploring the effects of non-trivial boundary data for the asymptotically AdS initial-boundary value problem, including a recent result in collaboration with Holzegel. I will also outline some interesting open problems in the area.

]]>**14h Jan Sbierski (Cambridge, UK)** A Zorn-free proof of the existence of a maximal Cauchy development for the Einstein equations

Abstract. In 1969, Choquet-Bruhat and Geroch showed that there exists a unique maximal Cauchy development of given initial data for the Einstein equations. Their proof, however, has the unsatisfactory feature that it relies crucially on the axiom of choice in the form of Zorn’s lemma. In particular, their proof ensures the existence of the maximal development without actually constructing it. In this talk, we present a proof of the existence of a maximal Cauchy development that avoids the use of Zorn’s lemma and, moreover, provides an explicit construction of the maximal development.

**15h15 Sergiu Klainerman (Princeton) ** Remarks on the stability of the Kerr solution in axial symmetry

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**14h Erwann Delay (Avignon)** A study of some curvature operators near the Euclidian metric

Abstract. We will show that some curvature operators of Ricci (or Einstein) type are locally invertible, in some weighted Sobolev spaces on Rn, near the euclidian metric. In the smooth case, we then deduce that the image of some Riemann-Christoffel type operators are smooth submanifolds in the neighborhood of the Euclidian metric.

**15h30 Mahir Hadzic (London)** Stability problem in the dust-Einstein system with a positive cosmological constant

Abstract. The dust-Einstein system models the evolution of a spacetime containing a pressureless fluid, i.e. dust. We will show nonlinear stability of the well-known Friedman-Lemaitre-Robertson-Walker (FLRW) family of solutions to the dust-Einstein system with positive cosmological constant. FLRW solutions represent initially a quiet fluid evolving in a spacetime undergoing accelerated expansion. We work in a harmonic-type coordinate system, inspired by prior works of Rodnianski and Speck on Euler-Einstein system, and Ringstrom’s work on the Einstein-scalar-field system. The main new mathematical difficulty is the additional loss of one degree of differentiability of the dust matter. To deal with this degeneracy, we commute the equations with a well-chosen differential operator and derive a family of elliptic estimates to complement the high-order energy estimates. This is joint work with Jared Speck.

Abstract. We consider the dynamics of Bianchi A scalar field models which undergo inflation. The main question is under which conditions does inflation come to an end and is succeeded by a decelerated epoch. This so-called ‘graceful exit’ from inflation is an important ingredient in the standard model of cosmology, but is, at this stage, only understood for restricted classes of solutions. We present new results obtained by a combination of analytical and numerical techniques.

Abstract. In the presence of a space-like translational Killing field, vacuum Einstein equations in 3+1 dimensions reduces to 2+1 Einstein equations with a scalar field. Minkowski space-time is a trivial solution of vacuum Einstein equation with a translational Killing field. A natural question is therefore the nonlinear stability of Minkowski solution in this setting. A first step in addressing this problem is the study of the constraint equations. The main difficulty in that case is due to the delicate inversion of the Laplacian. In particular, we have to work in the non constant mean curvature setting, which enforces us to consider the intricate coupling of the Einstein constraint equations.

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