Mathematical GR and Hyperbolic PDE Seminar

On December 20th, 2024 we have

• Sifan Yu (National University of Singapore), Low-regularity Local Well-posedness of the Elastic Wave System

  In this talk, I will present a recent work on the elastic wave system in three spatial dimensions. For admissible harmonic elastic materials, we prove a low-regularity local well-posedness result for the corresponding elastic wave equations. For such materials, we can split the dynamics into the "divergence-part” and the "curl-part,” and each part satisfies a distinct coupled quasilinear wave system with respect to different acoustical metrics. Our main result is that the Sobolev norm H^{3+} of the "divergence-part” (the "faster-wave part”) and the H^{4+} of the "curl-part” (the "slower-wave part”) can be controlled in terms of initial data for short times. We note that the Sobolev norm assumption H^{3+} is optimal for the "divergence-part.” This is a joint work with Xinliang An and Haoyang Chen.

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Here is a list of our past speakers:

• Elena Giorgi (Princeton University), Electromagnetic-gravitational perturbations of Kerr-Newman spacetime

  The Kerr-Newman spacetime is the most general explicit black hole solution, and represents a stationary rotating charged black hole. Its stability to gravitational and electromagnetic perturbations has eluded a proof since the 80s in the black hole perturbation community, because of "the apparent indissolubility of the coupling between the spin-1 and spin-2 fields in the perturbed spacetime", as put by Chandrasekhar. We will present a derivation of the Teukolsky and Regge-Wheeler equations in Kerr-Newman in physical space and use it to obtain a quantitative proof of stability.

  
  
• John Anderson (Princeton University), Stability results for anisotropic systems of wave equations

  In this talk, I hope to show a global stability result for a nonlinear anisotropic system of wave equations. This is motivated by studying phenomena involving characteristics with multiple sheets. For the proof, I will describe a strategy for controlling the solution based on bilinear energy estimates. Through a duality argument, this will allow us to prove decay in physical space using decay estimates for the homogeneous wave equation as a black box. The final proof will also require us to exploit a certain null condition that is present when the anisotropic system of wave equations satisfies a structural property involving the light cones of the equations.

  
  
• Arick Shao (Queen Mary University of London), Extension of symmetries from conformal boundaries of vacuum asymptotically Anti-de Sitter spacetimes

  In this talk, we investigate whether correspondences exist between vacuum asymptotically Anti-de Sitter spacetimes and properties of their conformal boundaries. Here, we focus primarily on the question of whether a symmetry on such a conformal boundary must be inherited by the spacetime itself. In particular, we present the first positive results in this direction for non-stationary settings, and we discuss recent developments in unique continuation theory that lead to these results. The presentation involves joint works with Gustav Holzegel and Alex McGill (as well as work in progress with Athanasios Chatzikaleas).

  
  
• Maxime van de Moortel (Princeton University), The breakdown of weak null singularities inside black holes

  What singularities lie inside a black hole formed in gravitational collapse? Locally near time-like infinity, it is known for various models that a generic black hole has a weakly singular Cauchy horizon. The global structure of the black hole interior, however, has largely remained unexplored. I will present my recent proof that, in the spherical collapse of a charged scalar field, the weakly singular Cauchy horizon breaks down and gives way to either a “crushing type” singularity, or a locally naked singularity emanating from the center.

  
  
• Christoph Kehle (ETH), Diophantine Approximation as Cosmic Censor for AdS Black Holes

  The statement that general relativity is deterministic finds its mathematical formulation in the celebrated ‘Strong Cosmic Censorship Conjecture due to Roger Penrose. I will present my recent results on this conjecture in the case of negative cosmological constant and in the context of black holes. It turns out that this is intimately tied to Diophantine properties of a suitable ratio of mass and angular momentum of the black hole and that the validity of the conjecture depends in an unexpected way on the notion of genericity imposed.

  
  
• Rita Teixeira da Costa (University of Cambridge), Energy, spin-reversal, and other quirks of the Teukolsky equation on Kerr black holes

  The Teukolsky equation is one of the fundamental equations governing linear gravitational, electromagnetic and other perturbations of the Kerr black hole family. In this talk, we discuss the surprisingly rich structure concealed in this equation: spin-reversal through the Teukolsky--Starobinsky identities and the existence of a strange new conserved energy, among other characteristics. Though often neglected, these features are fundamental to understanding boundedness, decay and scattering properties of solutions to the Teukolsky equation in the full subextremal black hole parameter range. This talk includes joint work with Yakov Shlapentokh-Rothman (Princeton) and Marc Casals (CBPF / University College Dublin).

  
  
• Fred Alford (University of Cambridge), A Mathematical Discussion of Hawking Radiation

  In the first half of this talk, we will introduce the physical phenomenon of Hawking Radiation, discussing the basics of quantum field theory to arrive at the mathematical expression for the number of particles emitted by a black hole. In the second half of this talk, we will discuss this calculation in the case of Reissner-Nordstrom black holes. By evolving a solution to the linear wave equation backwards from future null infinity, we will outline how to perform this calculation in a rigorous manner. We will then discuss the result obtained, in a physical context.

  
  
• Sam Collingbourne (University of Cambridge), Conservation Laws in Double Null Gauge on Schwarzschild From Canonical Energy

  In this talk I will discuss conservation laws for linearised gravity on the Schwarzschild spacetime. In particular, I will discuss the connection of the canonical energy of Hollands and Wald (https://link.springer.com/article/10.1007/s00220-012-1638-1) with the energy conservation laws of Holzegel in double null gauge on the Schwarzschild background (https://iopscience.iop.org/article/10.1088/0264-9381/33/20/205004/pdf).

  In the paper of Hollands and Wald they define the notion of canonical energy as a symplectic form on the space of linear perturbations which is conserved. They establish a criterion for linear stability of general static or stationary, axisymmetric, vacuum black holes under (axisymmetric in the rotating, stationary, axisymmetric case) perturbations, based on the whether this canonical energy is non-negative. Due to its complexity, evaluation of the canonical energy directly is difficult; even for perturbations of Schwarzschild, the canonical energy has not yet been shown to produce non-negative energies on a foliation of spacelike slices connecting the event horizon and null infinity. A solution to this issue is to establish a connection to Holzegel’s work on linearised gravity on Schwarzschild. In Holzegel's paper, it is proven that the flux of the linearised shear on the event horizon and the weighted linearised shear on null infinity is bounded by initial data. Interpreting these fluxes as a measure of the total energy radiated to null infinity and the horizon, one may view boundedness of these fluxes as a weak form of stability. A key ingredient to producing these bounds is a conservation law for the linearised metric and connection coefficients. I will show that this conservation law of Holzegel is precisely the conservation law for the canonical energy.

  
  
• Cecile Huneau (Ecole Polytechnique), High frequency limit for Einstein equations with a U(1) symmetry

  Due to the nonlinear character of Einstein equations, a sequence of metrics, solutions in vacuum, which oscillate with higher and higher frequency may converge to a solution to Einstein equations coupled to some effective energy momentum tensor. This effect is called backreaction, and has been studied by physicists (Isaacson, Burnett, Green and Wald). It has been conjectured by Burnett, under some definition of the high frequency limit, that the only effective energy momentum tensor that could appear corresponds to a massless Vlasov field, and that reciprocally all solutions to Einstein equations coupled to a massless Vlasov field can be approached by a sequence of solutions to Einstein vacuum equations (with higher and higher frequencies oscillations). I will present a work in collaboration with Jonathan Luk around Burnett conjecture and its reverse.

  
  
• Federico Pasqualotto (UC Berkeley), The interaction of nonlinear waves from several localized sources

  In this talk I will describe a research program aimed at understanding the interaction of gravitational waves originating from multiple localized and distant sources. This draws motivation from the problem of interaction of several localized bodies in general relativity. I will focus on the simplified interaction problem of multiple nonlinear waves, restricting attention to a model problem. I will show how one can take advantage of the specific geometric configuration to control the interaction. I will then describe the relevant directions in the context of general relativity. This is joint work with John Anderson (Princeton).

  
  
• Jan Sbierski (University of Oxford), On the structure and strength of singularities: Inextendibility results for Lorentzian manifolds

  Given a solution of the Einstein equations a fundamental question is whether one can extend the solution or whether the solution is maximal. If the solution is inextendible in a certain regularity class due to the geometry becoming singular, a further question is whether the strength of the singularity is such that it terminates classical time-evolution.

  In this talk we give an overview of low-regularity inextendibility results for Lorentzian manifolds and then focus on the locally Lipschitz inextendibility of FLRW models with particle horizons and spherically symmetric weak null singularities. The latter in particular apply to the spherically symmetric spacetimes constructed by Luk and Oh, improving their C^2-formulation of strong cosmic censorship to a locally Lipschitz formulation.

  
  
• Sam Zbarsky (Princeton University), Exterior perturbations of self-similar wave maps blowup.

  One can construct various self-similar blowup solutions for supercritical wave maps equation. The question of whether such blowups are stable is open, though there are some results if we assume symmetric perturbations. However, assuming a suitable stability result in the past light cone of the singularity, one can obtain stability all the way to the future light cone for a large family of such solutions and general localized perturbations. I will be talking about this result.

  
  
• Leonardo Abbrescia (Vanderbilt University), The boundary of maximal hyperbolic development for compressible Euler shock solutions

  Christodoulou's celebrated 2007 monograph on shock formation for the irrotational and isentropic relativistic Euler equations set the stage for the dramatic advancements in the mathematical theory of compressible fluids seen in the past 15 years. Two important examples include: 1) the extension of Christodoulou's results to the compressible Euler equations for an open set of initial data with nontrivial vorticity by Luk-Speck; 2) Christodoulou's recent resolution of the restricted shock development problem. The restricted shock development problem is essentially a local well-posedness result for irrotational and isentropic weak solutions with a shock singularity. The term “restricted” means that Christodoulou considered only irrotational initial data, and he ignored the jump in entropy and vorticity across the shock hypersurface, so that the mathematical problem concerned only irrotational solutions. We stress that the “initial data" for the restricted shock development problem is provided by the state of the solution on the boundary of its maximal classical hyperbolic development (which was derived by Christodoulou in 2007 in the irrotational case, starting from smooth initial conditions).

  In this talk, we will present our joint work with Speck on the construction of the maximal classical hyperbolic development launched by an open set of initial data with non-trivial vorticity and non-constant entropy. The maximal development we construct is necessary to initiate the true "unrestricted" shock development problem. The presence of vorticity and entropy necessitate elliptic estimates to prevent a derivative loss that was not present in Christodoulou's work. These estimates are known across constant-time hypersurfaces, but to the best of our knowledge, a sharp construction of the maximal development with vorticity and entropy cannot be achieved with these hypersurfaces. We will present a new quasilinear foliation of spacetime on which one can derive these elliptic estimates and provide a sharp construction of the maximal development.
  
  

• Haoyang Chen (National University of Singapore), Low regularity ill-posedness for elastic waves driven by shock formation

  In this talk, we generalize a classic result of Lindblad on the scalar quasilinear wave equation and we show that the Cauchy problem for 3D elastic waves, a physical quasilinear wave system with multiple wave-speeds, is ill-posed in $H^3(R^3)$. We further prove that the ill-posedness is caused by instantaneous shock formation, which is characterized by the vanishing of the inverse foliation density. The main difficulties arise from the multiple wave-speeds and its associated non-strict hyperbolicity. We design and combine a geometric approach and an algebraic approach to overcome these difficulties. This is based on joint work with Xinliang An and Silu Yin.

  
  
• Leonhard Kehrberger (University of Cambridge), The Case Against Smooth Null Infinity: Heuristics, Counter-Examples and Consequences

  In 2002, Christodoulou gave a heuristic argument stating that Penrose's proposal of smooth conformal compactification of spacetime (a.k.a. smooth null infinity) and the “peeling property” implied by it are incompatible with any physically realistic setup of $N$ infalling masses from the infinite past. Motivated by this argument, I will, in this talk, present various dynamical scattering constructions of spacetimes that model such setups and that do not have a smooth null infinity. These counter-examples will be in the context of the Einstein-Scalar field system under spherical symmetry. I will also show that these models motivate late-time asymptotics that are quite different from the usual and well-known Price's law asymptotics. In particular, they suggest that the non-smoothness of null infinity can, in principle, be measured. This talk will be based on the preprints: arXiv:2106.00035, arXiv:2105.08084, arXiv:2105.08079
  
  

• Hamed Masaood (University of Cambridge), A scattering theory for the linearised Einstein equations on the exterior of the Schwarzschild Black hole

  Scattering theory is fundamental to the study of black holes in general relativity, yet determining the extent to which the scattering problem can be said to be "well-posed" has seen little in the way of systematic study. In this talk I will describe my work towards an answer in the case of the linearised Einstein equations on the Schwarzschild exterior. I will first discuss preliminary results on scattering for the system of the Teukolsky equations of spin \pm 2 together with the Teukolsky--Starobinsky identities. I will then discuss ongoing work on using these results to obtain a scattering theory for the full system of linearised gravity.

  
  
• Renato Velozo (University of Cambridge), Stability of Schwarzschild for the spherically symmetric Einstein-massless Vlasov system

  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.
  
  

• Sanchit Chaturvedi (Stanford University), The Vlasov-Poisson-Landau system in the weakly collisional regime

  Consider the Vlasov-Poisson-Landau system in the weakly collisional regime. We show that solutions near global Maxwellians exhibit enhanced dissipation and Landau damping, resulting in a stronger stability threshold than previously known results. Our result is analogous to an earlier result of Bedrossian for the Vlasov-Poisson-Fokker-Planck system with the same threshold. However, unlike in the Fokker-Planck case, the linear operator cannot be inverted explicitly in the Landau case. For this reason, we develop a strategy based purely on energy methods, which combines Guo's energy method with the hypocoercive energy and the commuting vector field method. Joint work with Jonathan Luk and Toan Nguyen.

  
  
• Martin Taylor (Imperial College), The nonlinear stability of the Schwarzschild family of black holes

  I will present a theorem on the full finite codimension nonlinear asymptotic stability of the Schwarzschild family of black holes. The proof employs a double null gauge, is expressed entirely in physical space, and utilises the analysis of Dafermos--Holzegel--Rodnianski on the linear stability of the Schwarzschild family. This is joint work with M. Dafermos, G. Holzegel and I. Rodnianski.

  
  
• Sergiu Klainerman (Princeton University), Nonlinear stability of Kerr black holes for small angular momentum

  According to a well-known conjecture, initial data sets, for the Einstein vacuum equations, sufficiently close to a Kerr solution with parameters a, m, |a|/m <1, have maximal developments with complete future null infinity and with domain of outer communication (i.e complement of a future event horizon) which approaches (globally) a nearby Kerr solution. I will describe the main ideas in my recent joint work with Jeremie Szeftel concerning the resolution of the conjecture for small angular momentum, i.e. |a|/m sufficiently small. The work, ArXiv:2104.11857v1, also depends on forthcoming work on solutions of nonlinear wave equations in realistic perturbations of Kerr, with Szeftel and Elena Giorgi, which I will also describe.

  
  
• Jacques Smulevici (Sorbonne Université), Nonlinear periodic waves on the Einstein cylinder

  Motivated by the study of small amplitudes non-linear waves in the Anti-de-Sitter spacetime and in particular the conjectured existence of periodic in time solutions, we construct families of arbitrary small time-periodic solutions to the conformal cubic wave equation and the spherically-symmetric Yang-Mills equations on the Einstein cylinder. For the conformal cubic wave equation, we consider both spherically-symmetric solutions and complexed-valued aspherical solutions with an ansatz relying on the Hopf fibration of the 3-sphere. Our proof relies on a theorem of Bambusi and Paleari, suitably modified in the Yang-Mills case, for which the main assumption is the existence of a non-degenerate zero for a non-linear operator associated with the resonant system. One easily check that any mode solutions of the associated linearized equations are zeroes of these operators and the main difficulty is the proof of the non-degeneracy conditions. As is classical with this type of approach, the smallness parameter does not belong to full interval, but to a Cantor-like set with zero as an accumulation point. This is a joint work with Athanasios Chatzikaleas.
  
  

• Grigorios Fournodavlos (Princeton University), Kasner-like singularities

  We will investigate the dynamics of Big Bang singularities exhibiting monotonic blow-up. These are called Kasner-like or sometimes asymptotically velocity term dominated (AVTD), asymptotically Kasner-like, of Kasner type etc, and the first related studies were inspired by the work of Kasner himself (1921), who discovered the first cosmological solutions in vacuum $Ric(g)=0$, $g_{Kasner} = -dt^2 + t^{2p_1}(dx^1)^2 + t^{2p_2}(dx^2)^2 +t^{2p_3}(dx^3)^2$ for $(t,x)\in (0,+\infty)\times\mathbb{T}^3$ with $\sum p_i = \sum p_i^2 = 1$. There is a regime, called subcritical in the physics literature, where Kasner-like singularities are expected to be stable. Outside the subcritical regime, instabilities are expected. Nonetheless, there is a stable submanifold in the moduli space of initial data, where Kasner-like behavior is exhibited. We will review recent results that provide robust frameworks in which one can study the various regimes of Kasner-like singularities, both by means of singularity construction and stability.
  
  

• Dejan Gajic (Radboud University), Late-time tails and instabilities of extremal black holes

  When Kerr black holes rotate at their maximally allowed angular velocity, they are said to be extremal. Extremal black holes display a variety of interesting phenomena that are not present in more slowly rotating black holes. I will introduce upcoming work on the existence of strong asymptotic instabilities of a non-axisymmetric nature for scalar waves propagating on extremal Kerr black hole backgrounds and I will discuss their connection with previous work on axisymmetric instabilities and the precise shape of late-time power law tails in the emitted radiation. If time permits, I will discuss the mechanism for deriving late-time tails and sharp asymptotics and compare it with previous work on late-time tails on subextremal black holes ("Price’s law").
  
  

• Olivier Graf (U of Munster), An L² curvature pinching result for the Euclidean 3-disk

  When studying the Cauchy problem of general relativity we typically obtain L² bounds on the (Ricci) curvature tensor of spacelike hypersurfaces and its derivatives. In many situations it is useful to deduce from these H^{k} bounds that there exists coordinates on the spacelike hypersurface with optimal H^{k+2} bounds on the components of the induced Riemannian metric. The general idea is that this can be achieved using harmonic coordinates -- in which the principal terms of the Ricci curvature tensor are the Laplace-Beltrami operators of the metric components -- and standard elliptic regularity results. In this talk, I will make this idea concrete in the case of Riemannian 3-manifolds with a 2-sphere boundary, with Ricci curvature in L² and second fundamental form of the boundary in H^{1/2} both close to their respective Euclidean unit 3-disk values. The crux of the proof is a refined Bochner identity with boundary for harmonic functions. The cherry on the cake is that this result does not require any topology assumption on the Riemannian 3-manifold (apart from its boundary), and that we obtain -- as a conclusion -- that it must be diffeomorphic to the 3-disk. This talk is based on a result that I obtained in [Global nonlinear stability of Minkowski space for spacelike-characteristic initial data, Appendix A] .
  
  

• Eric Ling (Rutgers U), On the discrete Dirac spectrum of a point electron in the zero-gravity Kerr-Newman spacetime

  In relativistic quantum mechanics, the discrete spectrum of the Dirac hamiltonian with a Coulomb potential famously agrees with Sommerfeld’s fine structure formula for the hydrogen atom. In the Coulomb approximation, the proton is assumed to only have a positive electric charge. However, the physical proton also appears to have a magnetic moment which yields a hyperfine structure of the hydrogen atom that’s normally computed perturbatively. Aiming towards a non-perturbative approach, Pekeris in 1987 proposed taking the Kerr-Newman spacetime with its ring singularity as a source for the proton’s electric charge and magnetic moment. Given the proton’s mass and electric charge, the resulting Kerr-Newman spacetime lies well within the naked singularity sector which possess closed timelike loops. In 2014 Tahvildar-Zadeh showed that the zero-gravity limit of the Kerr-Newman spacetime (zGKN) produces a flat but topologically nontrivial spacetime that’s no longer plagued by closed timelike loops. In 2015 Tahvildar-Zadeh and Kiessling studied the hydrogen problem with Dirac’s equation on the zGKN spacetime and found that the hamiltonian is essentially self-adjoint and contains a nonempty discrete spectrum. In this talk, we show how their ideas can be extended to classify the discrete spectrum completely and relate it back to the known hydrogenic Dirac spectrum but yielding hyperfine-like and Lamb shift-like effects.
  
  

• Georgios Mavrogiannis (University of Cambridge), Linear and quasilinear wave equations on Schwarzschild de Sitter

  We discuss a new physical space approach to proving exponential decay for the wave equation on Schwarzschild de Sitter. In this approach, exponential decay follows from a novel "relatively non-degenerate" estimate on fixed time slabs. We then apply our physical space "relatively non-degenerate" estimates of the linear theory to prove stability for solutions of the quasilinear wave equation on Schwarzschild de Sitter.
  
  

• Allen Fang (Sorbonne University), A new proof for the nonlinear stability of slowly-rotating Kerr-de Sitter

  The nonlinear stability of the slowly-rotating Kerr-de Sitter family was first proven by Hintz and Vasy in 2016 using microlocal techniques. In my talk, I will present a novel proof of the nonlinear stability of slowly-rotating Kerr-de Sitter spacetimes that avoids frequency-space techniques outside of a neighborhood of the trapped set. The proof uses vectorfield techniques to uncover a spectral gap corresponding to exponential decay at the level of the linearized equation. The exponential decay of solutions to the linearized problem is then used in a bootstrap proof to conclude nonlinear stability.
  
  

• André Lisibach (Bern University of Applied Sciences), Shock Reflection and Interaction in Plane Symmetry

  The reflection of a shock on a wall and the interaction of two oncoming shocks in plane symmetry are classical problems in gas dynamics. In my talk, after presenting the physical picture and reviewing the governing equations, I will give a proof of local existence of a solution to the reflection problem. The proof is based on reformulating the problem in terms of characteristic coordinates and using twice differentiable functions.
  
  

• Arthur Touati (École Polytechnique), Construction of high-frequency spacetimes

  In this talk, I will present recent work on high-frequency solutions to the Einstein vacuum equations. From a physical point of view, these solutions model high-frequency gravitational waves and describe how waves travel on a fixed background metric. There are also interesting when studying the Burnett conjecture, which adresses the lack of compactness of the family of vacuum spacetimes. These high-frequency spacetimes are singular and require to work under the regime of well-posedness for the Einstein vacuum equations. I will review the literature on the subject and then show how one can construct them in generalised wave gauge by defining high-frequency ansatz.
  
  

• Jan Sbierski (University of Edinburgh), Instability of the Kerr Cauchy horizon under linearised gravity

  One can paraphrase Penrose’s strong cosmic censorship conjecture as stating that general relativity is generically a deterministic theory. While the full conjecture remains wide open there has long been evidence pointing towards its validity at least for small perturbations of exact rotating Kerr black holes. In this talk I will give a brief historical account of this evidence, discuss the analytic as well as the geometric aspects of this conjecture, and conclude by presenting recent work on the linear instability of the Kerr Cauchy horizon.
  
  

• Dawei Shen (Sorbonne University), General Covariant Modulated (GCM) Procedure

  In this talk, I will start by introducing some of the main idea of the proof of "Kerr stability for small angular momentum" by Klainerman and Szeftel, as the motivation to introduce the GCM procedure. Then, I will talk about the paper "Constructions of GCM spheres in perturbations of Kerr" of Klainerman and Szeftel concerning the construction of GCM spheres. Finally, applying the GCM spheres, I will explain the main result of "Constructions of GCM hypersurfaces in perturbations of Kerr".
  
  

• John Anderson (Stanford University), Global stability for wave equations satisfying a generalized null condition

  In this talk, I describe a proof joint with Sam Zbarsky of global stability for nonlinear wave equations satisfying the null condition. The proof will in fact allow for null forms which do not have constant coefficients. Pointwise estimates are shown using bilinear energy estimates and a duality argument. However, the fact that the coefficients of the null forms may not be constant means that we may have rather weak pointwise decay away from the light cone. To compensate for this, we use the $r^p$ weighted energy estimates of Dafermos--Rodnianski to show that the solution decays faster on average.
  
  

• Mario Apetroaie (University of Toronto), Linear instability of gravitational and electromagnetic perturbations of Extreme Reissner–Nordström

  In a series of papers, Giorgi proved linear stability to gravitational and electromagnetic perturbations for the full subextremal range Q < M of Reissner-Nordström spacetimes as solutions to the Einstein-Maxwell equations. We address the aforementioned problem for the extremal Q=M Reissner-Nordström spacetime, and contrary to the subextremal case we see that instability results hold for a gauge invariant hierarchy along the event horizon. In particular, depending on the number of translation invariant derivatives of derived gauge-invariant quantities, we show decay, non-decay and polynomial blow up estimates asymptotically along the horizon. As a consequence, for generic initial data, solutions to the generalized Teukolsky system of positive and negative spin satisfy analogous estimates. It is worth mentioning that more unstable behavior is realized for the negative spin solutions, with the $L^2$-norm of the extreme curvature component $ \underline{\alpha} $ not decaying asymptotically along the event horizon.
  
  

• Max Weissenbacher (Imperial College), (Non-)decay for the massless Vlasov equation on subextremal and extremal Reissner–Nordström

  We study the massless Vlasov equation on the exterior of the subextremal and extremal Reissner-Nordström spacetimes. We prove that moments of solutions decay at an exponential rate in the subextremal case and obtain a sharp inverse polynomial decay rate along the event horizon for the extremal case. Furthermore in the extremal case we derive a condition under which transversal derivatives of certain components of the energy--momentum tensor do not decay along the horizon. Our results may be viewed as analogous to the Aretakis instability. Our proof does not rely on the existence of a conservation law.
  
  

• Toan Nguyen (Penn State University), (no) Landau damping for the relativistic Vlasov-Poisson on the whole space

  The talk is to present a joint work with E. Grenier and I. Rodnianski that addresses the nonlinear Landau damping for the relativistic Vlasov-Poisson with finite regularity initial data near general radially monotone equilibria on the whole space. Due to the presence of far away interactions, the large time dynamics is driven by the electric field which is oscillatory in time and dispersed in space through the dispersion of a Schrodinger-type equation, classically known as plasma oscillations or Langmuir waves, which are now proven to be dominant globally in time for the relativistic Vlasov-Poisson and for the non-relativistic Vlasov-Poisson with bounded velocities. Unlike near vacuum, particles scatter without modification.
  
  

• Xinliang An (National University of Singapore), Anisotropic Apparent Horizons and Recent Progress in Gravitational Collapse

  In this talk, I will report some recent progress on anisotropic apparent horizons and cosmic censorship within and without spherical symmetry.
  
  

• Dietrich Häfner (Université Grenoble), On linear stability of Kerr black holes

  I will explain a result obtained in collaboration with P. Hintz and A. Vasy on the linear stability of slowly rotating Kerr black holes as solutions of the Einstein vacuum equations: linearized perturbations of a Kerr metric decay at an inverse polynomial rate to a linearized Kerr metric plus a pure gauge term. We work in a natural generalized wave map/DeTurck gauge and show that the pure gauge term can be taken to lie in a fixed finite dimensional space with a simple geometric interpretation. Our proof rests on a robust general framework, based on recent advances in microlocal analysis and non-elliptic Fredholm theory. The restriction to small angular momentum mainly comes from the analysis of mode solutions and I will explain at the end of the talk how this analysis can be carried out also in the case of large angular momentum of the black hole (this last part is based on joint work with L. Andersson and B. Whiting).
  
  

• Jaydeep Singh (Princeton University), A dynamical construction of spherical naked singularities

  As a step towards better understanding the weak cosmic censorship conjecture, we provide a new construction of naked singularities for the spherically symmetric, Einstein-scalar field model. The resulting spacetimes are globally close to the k-self-similar solutions introduced by Christodoulou, and feature an asymptotically self-similar structure near the singular point. After a discussion of Christodoulou’s solutions in an appropriate double-null gauge, we will describe how a stability argument with fine-tuned initial data perturbations may be used to construct new naked singularity spacetimes. Key components are a backwards stability argument in the interior region, and a study of the full forwards problem for small self-similar data in the exterior region.
  
  

• Serban Cicortas (Princeton University), Examples of self-similar vacuum spacetimes with null singularities, spacelike singularities, and Cauchy horizons of Taub--NUT type

  Self-similarity represents a suitable regime for constructing singular spacetimes in general relativity. In this talk, we discuss the Lorentzian Hawking--Page solutions, which represent (4+1)-dimensional cone interior self-similar vacuum solutions defined in the chronological past of the scaling origin. We extend these spacetimes to the cone exterior region in the class of (4+1)-dimensional self-similar vacuum solutions with an $SO(3)\times U(1)$ isometry. Our main result is that each Lorentzian Hawking--Page solution has extensions with a null curvature singularity, extensions with a spacelike curvature singularity, and extensions with a null Cauchy horizon of Taub--NUT type.
  
  

• Federico Pasqualotto (University of California Berkeley), The asymptotics of massive fields on stationary spherically symmetric black holes for all angular momenta

  In this talk, I will describe a proof of the asymptotics for the Klein--Gordon equation on stationary and spherically symmetric black holes (including Schwarzschild and Reissner--Nordström in the full subextremal range). We show that fixed angular mode solutions admit a precise asymptotic tail of the form $t^{-5/6} f(t)$ as t tends to infinity, for a fixed value of r, where $f(t)$ is an explicit oscillating profile. This is joint work with Yakov Shlapentokh-Rothman and Maxime Van de Moortel.
  
  

• Gabriele Benomio (Princeton University), A new gauge for gravitational perturbations of Kerr spacetimes

  I will present a new geometric framework to address the stability of the Kerr solution to gravitational perturbations in the full sub-extremal range. Central to the framework is a new formulation of nonlinear gravitational perturbations of Kerr in a geometric gauge tailored to the outgoing principal null geodesics of Kerr. The main features of the framework will be illustrated in the context of the linearised theory, which serves as a fundamental building block in nonlinear applications.
  
  

• Warren Li (Princeton University), Kasner-like description of spacelike singularities in spherical symmetry

  Following the investigations of Christodoulou, the spherically symmetric Einstein-(Maxwell)-scalar field equations have been popularised as a toy model for the Einstein equations, particularly regarding singularity formation and the cosmic censorship conjectures. Focusing on the space-like part of the singular boundary, we present new results describing the precise leading order blow-up rates of the scalar field, the Hawking mass, and the Kretschmann scalar near the singularity. We further view these spacetimes in the perspective of the BKL heuristics in relativistic cosmology; near the singular boundary we obtain a BKL-like expansion including a description of Kasner exponents. In particular we signify the importance of cosmological phenomena including subcritical regimes and Kasner bounces in the gravitational collapse setting. Part of this talk is based on joint work with Maxime van de Moortel (Rutgers).
  
  

• Istvan Kadar (Cambridge University), Stability for nonlinear wave equations: The role of asymptotics

  In their pioneering works, Christodoulou and Klainerman proved the sufficiency of the null condition for stability of the trivial solution for systems of wave equations in Minkowski space. Subsequently, during their study of the Einstein equations in harmonic gauge, Lindblad and Rodnianski introduced the weak null condition, sparking extensive research on stability problems. However, the sufficiency of this condition remained a puzzle. In this talk, I will to tell you a tale about the late time behaviour of waves in asymptotically flat backgrounds and how this relates to stability for systems of wave equations. In particular, I'll present a system satisfying the weak null condition, but which has unstable zero solution.
  
  

• Arick Shao (Queen Mary University of London), Bulk-boundary correspondence for vacuum asymptotically Anti-de Sitter spacetimes

  The AdS/CFT conjecture in physics posits the existence of a correspondence between gravitational theories in asymptotically Anti-de Sitter (aAdS) spacetimes and field theories on their conformal boundary. In this presentation, we prove a rigorous mathematical statement toward this conjecture in the classical relativistic setting. In particular, we show there is a one-to-one correspondence between aAdS solutions of the Einstein-vacuum equations and a suitable space of data on the conformal boundary (consisting of the boundary metric and the boundary stress-energy tensor), provided the boundary satisfies a geometric condition.
  
  

• Zoe Wyatt (Cambridge University), Stabilizing Relativistic Fluids on Slowly Expanding Cosmological Spacetimes

  The relativistic Euler equations are known to admit unstable homogeneous solutions with finite-time shock formation on a fixed Minkowski geometry. By contrast, such shock formation can be suppressed on cosmological spacetimes whose spatial slices expand at an accelerated rate. In this talk, I will present some recent joint work concerning the Einstein--Euler equations for geometries expanding at a linear rate.
  
  

• Yannis Angelopoulos (California Institute of Technology), Global constructions of impulsive gravitational waves

  In this talk, I will review some of the results in the area of impulsive gravitational wave spacetimes, and will describe the construction of a globally geodesically complete spacetime that contains the interaction of two impulsive gravitational waves (upcoming joint work with Jonathan Luk).
  
  

• Dawei Shen (Sorbonne Universite), Global stability of Minkowski spacetime with minimal decay

  The global stability of Minkowski spacetime has been proven in the celebrated work of Christodoulou-Klainerman in 1993. In 2007, Bieri has extended the result of Christodoulou-Klainerman under lower decay and regularity assumptions on the initial data. In this talk, I will introduce a recent work, which extends the result of Bieri to minimal decay assumptions.
  
  

• Jaydeep Singh (Princeton University), Quantifying the strength of the blue-shift on naked singularity interiors

  Central to Christodoulou’s proof of weak cosmic censorship in spherical symmetry is the identification of a blue-shift instability along the past light-cone of naked singularities. In this talk we revisit the blue-shift in the setting of $k$-self-similar spacetimes, and quantify its strength by studying the asymptotics of linear waves. For waves supported in the causal past of the singularity, we find a surprising connection between the regularity of initial data, and the strength of the instability. Ultimately we will connect this discussion to the prospect of proving a weak cosmic censorship result in high regularity.
  
  

• Sifan Yu (National University of Singapore), Characteristic Initial Value Problem for the 3D Compressible Euler Equations

  We present the first result for the characteristic initial value problem of the compressible Euler equations in three space dimensions without any symmetry assumption. We allow presence of vorticity and consider any equation of state. Compared to the standard Cauchy problem, where initial data can be freely prescribed on a constant-time hypersurface, we formulate the problem by distinguishing between the "free-component" and the "constrained-component" of the initial data. The latter is to be solved by the "free-component" utilizing the properties of the compressible Euler equations on the initial null hypersurfaces. Then, we establish a priori estimates, followed by a local well-posedness and a continuation criterion argument. Moreover, we prove a regularity theory in Sobolev norms. Our analysis critically relies on the vectorfield method. This is a joint work with Jared Speck.
  
  

• Tianwen Luo (South China Normal University), On multi-dimensional rarefaction waves

  We study the two-dimensional acoustical rarefaction waves under the irrotational assumptions. We provide a new energy estimates without loss of derivatives. We also give a detailed geometric description of the rarefaction wave fronts. As an application, we show that the Riemann problem is structurally stable in the regime of two families of rarefaction waves. This is a joint work with Prof. Pin Yu in Tsinghua Univerisity.
  
  

• Arthur Touati (IHES), Initial data for stability theorems in general relativity

  In this talk I will present two recent results on the construction of initial data solving the constraint equations and which suits the decay requirements of either Minkowski stability or Kerr stability. The Minkowski result is the most technical since KIDS are handle by Kerr asymptotics, the control of which requires some work. The Kerr result benefits from the presence of the event horizon and the freedom in choosing the boundary condition for the conformal method. In particular, we are able to completely avoid the KIDS. This is joint work with Allen Juntao Fang and Jérémie Szeftel.
  
  

• Leonardo Abbrescia (Vanderbilt/Georgia Tech), Cauchy horizons of shock singularities and the shock development problem

  Weak solutions of hyperbolic PDEs were introduced to study those that do not have sufficient regularity to be considered classical pointwise solutions. For many equations of physical interest, such as the compressible Euler equations of gas dynamics, there is a breakdown of the classical theory due to a phenomenon called shock formation. Upon this breakdown, one is forced to enter the regime of weak solutions. Describing the transition from classical-to-weak solutions is called the shock development problem, and for the full 3D Euler system, remains an outstanding open problem. Setting up the PDE analysis for the shock development problem requires a full understanding of the boundary of the (classical) maximal globally hyperbolic development (MGHD). We have recently understood a localized portion of the MGHD called the singular boundary B, where the characteristics of the hyperbolic PDEs collapse. We will describe an upcoming result on the remaining part of the MGHD, called the Cauchy horizon, where the solution remains regular. If time permits, we will describe how our upcoming work improves on the methods in the construction of B that could, in principle, be used to study the full shock development problem.
  
  

• Ely Sandine (UC Berkeley), Existence of more self-similar implosion profiles for the gravitational Euler-Poisson system

  I will discuss an implosion scenario for the equations describing the evolution of an isentropic gas which is compressible, isothermal and self-gravitating. Under the hypotheses of radial symmetry and self-similarity, the equations reduce to a system of ODEs which has been studied by the astrophysical community using numerical methods. One solution to this system, discovered by Larson and Penston in 1969, was rigorously proved to exist by Guo, Hadžić and Jang. In this talk, I will discuss recent work establishing the existence of a subset of the discrete family of self-similar solutions found numerically by Hunter in 1977. We use methods developed by Collot, Raphael and Szeftel in the context of energy-supercritical nonlinear heat equations.
  
  

• Jingbo Wan (Columbia University), Axisymmetric Teukolsky System in Slowly Rotating, Strongly Charged Sub-Extremal Kerr-Newman Spacetime

  We establish boundedness and decay results for the Teukolsky system in the exterior spacetime of slowly rotating and strongly charged sub-extremal Kerr-Newman black holes, with a focus on axially symmetric solutions. The key step is deriving a physical-space Morawetz estimate for the associated generalized Regge-Wheeler system, without relying on spherical harmonic decomposition. This is based on a joint work with Elena Giorgi.
  
  

• Sebastian Gurriaran (Sorbonne Universite), Oscillatory blow-up of the spin +2 Teukolsky field at the Kerr Cauchy horizon

  I will describe a recent work on the linear instability of the Kerr Cauchy horizon, first proven by Sbierski. I will present the precise asymptotics of the spin +2 Teukolsky scalar in the interior region of the black hole, which prove its exponential blow-up on the Cauchy horizon, as well as an oscillatory behavior caused by the non-zero angular momentum of the black hole. Together with a recent work by Sbierski, the result supports a Lipschitz version of Strong Cosmic Censorship in Kerr black holes. I will give a sketch of the proof, done purely in physical space.
  
  

• Olivier Graf (Universite de Grenoble), The linear stability of Schwarzschild-anti-de Sitter spacetimes

  Schwarzschild-adS spacetimes are stationary and spherically symmetric solutions to the Einstein equations with negative cosmological constant. They contain a black hole region and a conformal anti-de Sitter timelike boundary at infinity. In this talk I will present a result of linear stability for these spacetimes under gravitational perturbations preserving the anti-de Sitter boundary condition. As in the Schwarzschild case, the linearisation of the Einstein equations is governed by two Regge-Wheeler wave equations. I will show that the boundary conditions inherited by the Regge-Wheeler quantities can be decoupled into two boundary conditions: a Dirichlet boundary condition and a higher order Robin-type boundary condition. I will show that these boundary conditions are conservative and yield to the decay of a coercive energy quantity. By red-shift and Carleman estimates for each spherical mode, one can obtain a 1/log(t) decay for the Regge-Wheeler quantities, which can further be infered for the full system of gravitational perturbations. I will also show how to construct quasimode solutions for the system of gravitational perturbations which prove that the the above mentioned bounds are optimal. This is joint work with Gustav Holzegel.

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The organizers,

Xinliang An, John Anderson, Dejan Gajic, Elena Giorgi, Shadi Tahvildar-Zadeh