
Speaker: Suman KunduTitle: ‘Grey Galaxy’ as the endpoint of the KerrAdS super radiant blackholeVenue: Jefferson 453General Relativity Seminar Speaker: Suman Kundu (Weizmann Institute) Title: ‘Grey Galaxy’ as the endpoint of the KerrAdS super radiant blackhole Abstract: Kerr AdS$_{d+1}$ black holes for $d\geq 3$ suffer from classical superradiant instabilities over a range of masses near extremality. We conjecture that these instabilities settle down into Grey Galaxies (GG)s – a new class of solutions to Einstein’s equations which we construct for $d=3$. Grey Galaxies consist of an $\omega=1$ black hole in the `centre’ of $AdS$, surrounded by a uniformly thick and very large disk of thermal bulk matter that revolves around the centre of AdS at the speed of light. The parametrically low energy density and parametrically large radius of the gas disk are inversely related;… 

Speaker: Aghil AlaeeTitle: Positivity of Static quasilocal Mass in general relativityVenue: virtualGeneral Relativity Seminar Speaker: Aghil Alaee, Clark University Title: Positivity of Static quasilocal Mass in general relativity Abstract: In this talk, we review results on the PMT of quasilocal masses and prove the positivity of static quasilocal masses with respect to the AdS and AdS Schwarzschild spacetimes. 

Speaker: Vitor CardosoTitle: Testing GR with GWsVenue: CMSA Room G10General Relativity Seminar Speaker: Vitor Cardoso, IST, Lisbon and The Niels Bohr Institute, Copenhagen Title: Testing GR with GWs Abstract: One of the most remarkable possibilities of General Relativity concerns gravitational collapse to black holes, leaving behind a geometry with light rings, ergoregions and horizons. These peculiarities are responsible for uniqueness properties and energy extraction mechanisms that turn black holes into ideal laboratories of strong gravity, of particle physics (yes!) and of possible quantumgravity effects. I will discuss some of the latest progress in tests of General Relativity with black holes. 

Speaker: Philippe G. LeFlochTitle: The localized seedtosolution method for the Einstein constraintsVenue: virtualGeneral Relativity Seminar Speaker: Philippe G. LeFloch, Sorbonne University and CNRS Title: The localized seedtosolution method for the Einstein constraints Abstract: I will discuss advances on asymptotically Euclidian initial data sets and the variational method introduced by J. Corvino and R. Schoen. This talk is based on joint papers with TheCang Nguyen (Montpellier) and Bruno Le Floch (Sorbonne Univ. and CNRS). In the vicinity of any given reference data set, we define a “localized seedtosolution” map, which allows us to parametrize the initial data sets satisfying the Einstein constraints (possibly with matter fields). The parametrization is defined over classes of data sets understood modulo the image of the dual linearized constraints. Our main contribution concerns the sharp behavior… 

Speaker: Tin Yau TsangTitle: Recent advances in scalar curvature and positive mass theoremsVenue: CMSA Room G10General Relativity Seminar Speaker: Tin Yau Tsang, University of California Irvine Title: Recent advances in scalar curvature and positive mass theorems Abstract: First, we have a review of classical tools for studying scalar curvature and positive mass theorem. Then we are going to discuss some advances and new perspectives on these tools which lead to a deeper understanding of geometry and initial data sets. 

Speaker: Shahar HadarTitle: Resolving the photon ringVenue: CMSA Room G10General Relativity Seminar Speaker: Shahar Hadar (University of Haifa) Title: Resolving the photon ring Abstract: In the past few years, the Event Horizon Telescope has released the first closeup interferometric images of two supermassive black holes, M87* and SgrA*. It is believed that within these images is embedded a fine, yetunresolved brightness enhancement called the photon ring. The ring is a universal consequence of strong lensing by the black hole and thereby conveys information on its spacetime geometry, potentially providing a new independent avenue for tests of general relativity in the strongfield regime. In the talk I will briefly review the theory of the photon ring and its corresponding spacetime region, the photon shell, which governs the universal lensing structure. I will… 

Speaker: Sifan YuTitle: Rough solutions of the relativistic Euler equationsVenue: CMSA Room G10General Relativity Seminar Speaker: Sifan Yu, Vanderbilt University Title: Rough solutions of the relativistic Euler equations Abstract: I will discuss recent works on the relativistic Euler equations with dynamic vorticity and entropy. We use a new formulation of the equations, which has geoanalytic structures. In this geometric formulation, we decompose the flow into geometric “soundwave part” and “transportdivcurl part”. This allows us to derive sharp results about the dynamics, including the existence of lowregularity solutions. Then, I will discuss the results of rough solutions of the relativistic Euler equations and the role that nonlinear geometric optics plays in the framework. Our main result is that the Sobolev norm $H^{2+}$ of the variables in the “wavepart” and the H\”older… 

Speaker: Alejandra CastroTitle: Gravitational perturbations near to extreme KerrVenue: CMSA Room G10General Relativity Seminar Speaker: Alejandra Castro (University of Cambridge) Title: Gravitational perturbations near to extreme Kerr Abstract: Gravitational perturbations of a black hole illustrate the invaluable synergy between theory, experiment, and numerical simulations in general relativity. A recent development in the theory side has been the identification of the relevant degrees of freedom describing the low energy physics driving a black hole away from extremality. For simple cases, this low energy sector determines important aspects of the gravitational backreaction, and several properties that are key to our microscopic (quantum) understanding of black hole physics. In this talk I will discuss these developments in the context of the (near)extreme Kerr black hole. In particular, I will revisit the spectrum of linear… 

Speaker: Prahar MitraTitle: New Phases of N=4 SYMVenue: CMSA Room G10General Relativity Seminar Speaker: Prahar Mitra (University of Cambridge) Title: New Phases of N=4 SYM Abstract: We construct new static solutions to gauged supergravity that, via the AdS/CFT correspondence, are dual to thermal phases in N=4 SYM at finite chemical potential. These solutions dominate the microcanonical ensemble and are required to ultimately reproduce the microscopic entropy of AdS black holes. These are constructed in two distinct truncations of gauged supergravity and can be uplifted to solutions of type IIB supergravity. Together with the known phases of the truncation with three equal charges, our findings permit a good understanding of the full phase space of SYM thermal states with three arbitrary chemical potentials. We will also discuss the status of hairy supersymmetric… 

Speaker: Jose Luis JaramilloTitle: Pseudospectrum and black hole quasinormal mode instability: An ultraviolet universality conjectureVenue: CMSA Room G10General Relativity Seminar Speaker: Jose Luis Jaramillo (Bourgogne U.) Title: Pseudospectrum and black hole quasinormal mode instability: an ultraviolet universality conjecture Abstract: Can we measure the ‘effective regularity’ of spacetime from the perturbation of quasinormal mode (QNM) overtones? Black hole (BH) QNMs encode the resonant response of black holes under linear perturbations, their associated complex frequencies providing an invariant probe into the background spacetime geometry. In the late nineties, Nollert and Price found evidence of a BH QNM instability phenomenon, according to which perturbed QNMs of Schwarzschild spacetime migrate to new perturbed branches of different qualitative behaviour and asymptotics. Here we revisit this BH QNM instability issue by adopting a pseudospectrum approach. Specifically, we cast the QNM… 

Speaker: Nikolaos AthanasiouTitle: Formation of trapped surfaces in the EinsteinYangMills systemVenue: virtualGeneral Relativity Seminar Speaker: Nikolaos Athanasiou (University of Crete, Greece) Title: Formation of trapped surfaces in the EinsteinYangMills system Abstract: The purpose of this talk is to give an overview of a semiglobal existence result and a trapped surface formation results in the context of the EinsteinYangMills system. Adopting a “signature for decay rates” approach first introduced by An, we develop a novel gauge (and scale) invariant hierarchy of nonlinear estimates for the YangMills curvature which, together with the estimates for the gravitational degrees of freedom, yield the desired semiglobal existence result. Once semiglobal existence has been established, we will explain how the formation of a trapped surface follows from a standard ODE argument. This is joint… 

Speaker: Kwinten FransenTitle: Quasinormal Modes from Penrose LimitsVenue: CMSA Room G10General Relativity Seminar Speaker: Kwinten Fransen (UC Santa Barbara) Title: Quasinormal Modes from Penrose Limits Abstract: In this talk, I will explain how to describe quasinormal modes with large real frequencies using Penrose limits. To do so, I first recall relevant aspects of the Penrose limit, and its resulting plane wave spacetimes, as well as quasinormal modes to subsequently tie these together. Having established the main principle, I will illustrate the usefulness of this point of view with the geometric realization of the emergent symmetry algebra underlying the quasinormal modes in the large real frequency limit and present its application to the astrophysically important example of Kerr black holes. Based on arXiv:2301.06999. 

Speaker: Maciej ZworskiTitle: Quasinormal modes and Ruelle resonances: mathematician’s perspectiveVenue: CMSA Room G10General Relativity Seminar Speaker: Maciej Zworski, UC Berkeley Title: Quasinormal modes and Ruelle resonances: mathematician’s perspective Abstract: Quasinormal modes of gravitational waves and Ruelle resonances in hyperbolic classical dynamics share many general properties and can be considered “scattering resonances”: they appear in expansions of correlations, as poles of Green functions and are associated to trapping of trajectories (and are both notoriously hard to observe in nature, unlike, say, quantum resonances in chemistry or scattering poles in acoustical scattering). I will present a mathematical perspective that also includes zeros of the Riemann zeta function (scattering resonances for the Hamiltonian given by the Laplacian on the modular surface) and stresses the importance of different kinds of trapping phenomena, resulting,… 

Speaker: Chiara ToldoTitle: Near extremal de Sitter black holes and JT gravityVenue: virtualGeneral Relativity Seminar Speaker: Chiara Toldo (Harvard) Title: Near extremal de Sitter black holes and JT gravity Abstract: In this talk I will explore the thermodynamic response near extremality of charged black holes in fourdimensional EinsteinMaxwell theory with a positive cosmological constant. The latter exhibit three different extremal limits, dubbed cold, Nariai and ultracold configurations, with different nearhorizon geometries. For each of these three cases I will analyze small deformations away from extremality, and construct the effective twodimensional theory, obtained by dimensional reduction, that captures these features. The ultracold case in particular shows an interesting interplay between the entropy variation and charge variation, realizing a different symmetry breaking with respect to the other two nearextremal limits. 

Speaker: Prashant KocherlakotaTitle: Testing spacetime geometry with images of supermassive compact objects: Current status and the futureVenue: virtualGeneral Relativity Seminar Speaker: Prashant Kocherlakota (BHI) Title: Testing spacetime geometry with images of supermassive compact objects: Current status and the future Abstract: Astrophysical black holes (BHs) are expected to be described by the Kerr solution of the Einstein equations. Several frameworks have recently been developed to parametrically deform the Kerr metric in significantly different ways, to enable formulations of tests of the nohair theorems. Testing the viability status of alternative models – such as nonKerr BHs from general relativity, BHs from alternative theories, wormholes, and other exotic objects – as descriptors of astrophysical objects has been of longstanding interest. The Event Horizon Telescope (EHT) recently imaged Sagittarius A* (Sgr A*), the supermassive compact object at the… 

Speaker: Allen FangTitle: A new proof for the nonlinear stability of slowlyrotating Kerrde SitterVenue: virtualGeneral Relativity Seminar Speaker: Allen Fang (Princeton) Title: A new proof for the nonlinear stability of slowlyrotating Kerrde Sitter Abstract: The nonlinear stability of the slowlyrotating Kerrde 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 slowlyrotating Kerrde Sitter spacetimes that avoids frequencyspace techniques outside of a neighborhood of the trapped set. The proof uses vector field 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. 

Speaker: Semyon DyatlovTitle: Ringdown and geometry of trapping for black holesVenue: HybridGeneral Relativity Seminar Speaker: Semyon Dyatlov (MIT) Title: Ringdown and geometry of trapping for black holes Abstract: Quasinormal modes are complex exponential frequencies appearing in long time expansions of solutions to linear wave equations on black hole backgrounds. They appear in particular during the ringdown phase of a black hole merger when the dynamics is expected to be driven by linear effects. In this talk I give an overview of various results in pure mathematics which relate asymptotic behavior of quasinormal modes at high frequency to the geometry of the set of trapped null geodesics, such as the photon sphere in Schwarzschild (de Sitter). These trapped geodesics have two kinds of behavior: the geodesic flow is hyperbolic in directions normal… 

Speaker: Pierre HeidmannTitle: Schwarzschildlike Topological Solitons in GravityVenue: virtualGeneral Relativity Seminar Speaker: Pierre Heidmann (Johns Hopkins) Title: Schwarzschildlike Topological Solitons in Gravity Abstract: We present large classes of nonextremal solitons in gravity that are asymptotic to fourdimensional Minkowski spacetime plus extra compact dimensions. They correspond to smooth horizonless geometries induced by topology in spacetime and supported by electromagnetic flux, which characterize coherent states of quantum gravity. We discuss a new approach to deal with EinsteinMaxwell equations in more than four dimensions, such that they decompose into a set of Ernst equations. We generate the solitons by applying different techniques associated with the Ernst formalism. We focus on solitons with zero net charge yet supported by flux, and compare them to Schwarzschild black holes. These are also ultracompact geometries with very high redshift but… 

Speaker: Pengyu LeTitle: Asymptotic geometry of null hypersurface in Schwarzschild spacetime and null Penrose inequalityVenue: virtualGeneral Relativity Seminar Speaker: Pengyu Le (BIMSA) Title: Asymptotic geometry of null hypersurface in Schwarzschild spacetime and null Penrose inequality Abstract: Null Penrose inequality is an important case of the wellknown Penrose inequality on a null hypersurface. It conjectures the relation between the area of the outmost marginally trapped surface and the Bondi mass at null infinity. Following the proposal of Christodoulou and Sauter, we employ the perturbation method to study the asymptotic geometry of null hypersurfaces at null infinity in a perturbed vacuum Schwarzshild spacetime. We explain how to apply this perturbation theory to prove null Penrose inequality on a nearly spherically symmetric null hypersurface in a perturbed vacuum Schwarzschild spacetime. 

Speaker: Naqing XieTitle: Gravitational Wave, Angular Momentum, and Supertranslation AmbiguityVenue: virtualGeneral Relativity Seminar Speaker: Naqing Xie (Fudan University) Title: Gravitational Wave, Angular Momentum, and Supertranslation Ambiguity Abstract: The supertranslation ambiguity of angular momentum is a longstanding and conceptually important issue in general relativity. Recently, there appeared the first definition of angular momentum at null infinity that is supertranslation invariant. However, in the compact binary coalescence community, supertranslation ambiguity is often ignored. We have shown that, in the linearised theory of gravitational wave, the new angular momentum coincides with the classical definition at the quadrupole level. This talk is based on a recent joint work with Xiaokai He and Xiaoning Wu. 