
Speaker: Andras VasyTitle: The Feynman propagator and selfadjointnessVenue: virtualGeneral Relativity Seminar Speaker: Andras Vasy (Stanford) Title: The Feynman propagator and selfadjointness Abstract: In this talk I will discuss the Feynman and antiFeynman inverses for wave operators on certain Lorentzian manifolds; these are two inverses which from a microlocal analysis perspective are more natural than the standard causal (advanced/retarded) ones. For instance, for the spectral family of the wave operator, these are the natural inverses when the spectral parameter is nonreal. Indeed, I will explain that these connect to the selfadjointness of the wave operator, and the positivity properties that follow. 

Speaker: David KubiznakTitle: Remarkable symmetries of rotating black holesVenue: CMSA, 20 Garden St, G10General Relativity Seminar Speaker: David Kubiznak (Charles University) Title: Remarkable symmetries of rotating black holes Abstract: It is well known that the Kerr geometry admits a nontrivial Killing tensor and its ‘square root’ known as the KillingYano tensor. These two objects stand behind Carter’s constant of geodesic motion as well as allow for separability of test field equations in this background. The situation is even more remarkable in higher dimensions, where a single object — the principal KillingYano tensor — generates a tower of explicit and hidden symmetries responsible for integrability of geodesics and separability of test fields around higherdimensional rotating black holes. Interestingly, similar yet different structure is already present for the slowly rotating black holes described by… 

Speaker: Daniel KolbTitle: QuasiLocal Mass in a Binary Black Hole MergerVenue: virtualGeneral Relativity Seminar Speaker: Daniel Kolb (Max Planck Institute) Title: QuasiLocal Mass in a Binary Black Hole Merger Abstract: One of the major open problems in classical general relativity is how one should define the mass of a finite region of space. In this talk, we will investigate a promising definition proposed by Wang and Yau in 2009. A closed 2surface bounding the region of interest is embedded isometrically into Minkowski space. The mass is then calculated by comparing the extrinsic geometries. The WangYau mass has many desirable properties, but it has previously not been calculated for surfaces in dynamical spacetimes. To remedy this, we will discuss how the WangYau mass can be computed in practice and extend… 

Speaker: David FajmanTitle: Fluid stabilization in slowly expanding cosmological spacetimeVenue: Hybrid G10General Relativity Seminar Speaker: David Fajman (Vienna) Title: Fluid stabilization in slowly expanding cosmological spacetime Abstract: Relativistic fluids are known to form shocks during their evolution from nearhomogeneous initial data. In expanding spacetimes, shock formation is suppressed, if the expansion is sufficiently strong. We refer to this effect as fluid stabilization. The occurrence of this phenomenon depends on features of the fluid and has implications for our understanding of structure formation and cosmological evolution. While the effect is well studied in the regime of accelerated expansion, in recent years it has been shown that fluid stabilization occurs as well in spacetimes with slower expansion rates. In this talk we present different recent results on fluid stabilization in slowly expanding spacetimes… 

Speaker: Leo SteinTitle: Resolving memory in numerical relativity, and fixing BMS frames for modelingVenue: CMSA, 20 Garden St, G10General Relativity Seminar Speaker: Leo Stein (Mississippi) Title: Resolving memory in numerical relativity, and fixing BMS frames for modeling Abstract: Numerical relativity waveforms serve as ground truth for detection and parameter estimation of binary black hole mergers. Most NR waveforms to date miss memory effects, as they were extracted from simulations using an approximation called extrapolation. I will report on the SXS collaboration’s capacity to resolve memory effects in production NR simulations using Cauchycharacteristic evolution (CCE), and in the future with Cauchycharacteristic matching (CCM). I will further report on how BH perturbation and postNewtonian theory furnish natural BMS frames. With these BMS frames, we can extract welldefined remnant quantities, perform precision ringdown modeling, and build complete surrogate waveform models that… 

Speaker: Maria RodriguezTitle: Tidal Squeezing of Black HolesVenue: CMSA, 20 Garden St, G10General Relativity Seminar Speaker: Maria Rodriguez (Utah) Title: Tidal Squeezing of Black Holes Abstract: Recent developments indicate that Kerr black holes do not deform when perturbed by a static external gravitational field. Relying on hidden symmetries, compelling progress has been achieved to explain that Love numbers for Kerr black holes vanish. How does the phenomenon of tidal squeezing manifest in broader contexts? An elementary presentation of dynamical tidal squeezing of Kerr black holes will be given. 

Speaker: Chethan KrishnanTitle: A Smooth Horizon without a Smooth HorizonVenue: CMSA, 20 Garden St, G10General Relativity Seminar Speaker: Chethan Krishnan (IISc Bangalore) Title: A Smooth Horizon without a Smooth Horizon Abstract: I will talk about some work that is about to appear, where we note one precise way in which the stretched horizon can simulate a smooth horizon. I will also make an effort to put things in some perspective (brickwalls, fuzzballs, Type I algebras,…) 

Speaker: Lionel MasonTitle: Geometry at Strong coupling for amplitudes/Wilson loopsVenue: CMSA, 20 Garden St, G10General Relativity Seminar Speaker: Lionel Mason (Oxford) Title: Geometry at Strong coupling for amplitudes/Wilson loops Abstract: The amplitude/Wilson loop correspondence identifies planar N=4 superYangMills amplitudes with certain null polygonal Wilson loops at all the values of the coupling. At strong coupling this equates the amplitude/Wilson loop computed by Alday & Maldacena in terms of the area of a minimal surface in AdS_5. To do so they developed a `Ysystem’ for computing the amplitude. This talk reinterprets their construction as providing the underlying twistor space for a hyperKahler structure on the corresponding space of kinematic data. In particular, the area is given by a Kahler scalar for the pseudohyperkahler structure and satisfies a version of the Plebanski equations,… 

Speaker: Rodrigo SilvaTitle: Quantization of causal diamonds in 2+1 dimensional gravityVenue: CMSA, 20 Garden St, G10General Relativity Seminar Speaker: Rodrigo Silva, University of Maryland Title: Quantization of causal diamonds in 2+1 dimensional gravity Abstract: We develop the reduced phase space quantization of causal diamonds in $2+1$ dimensional gravity with a nonpositive cosmological constant. The system is defined as the domain of dependence of a spacelike topological disk with a fixed boundary metric. By solving the constraints in a constantmeancurvature time gauge and removing all the spatial gauge redundancy, we find that the phase space is the cotangent bundle of $Diff^+(S^1)/PSL(2, \mathbb{R})$, i.e., the group of orientationpreserving diffeomorphisms of the circle modulo the projective special linear subgroup. Classically, the states correspond to causal diamonds embedded in $AdS_3$ (or $Mink_3$ if $\Lambda = 0$),… 

Speaker: Diandian WangTitle: Pole skipping, quasinormal modes, shockwaves and their connection to chaosVenue: CMSA, 20 Garden St, G10General Relativity Seminar Speaker: Diandian Wang(Harvard University) Title: Pole skipping, quasinormal modes, shockwaves and their connection to chaos Abstract: A chaotic quantum system can be studied using the outoftimeorder correlator (OTOC). I will tell you about pole skipping — a recently discovered feature of the retarded Green’s function — that seems to also know things: things like the Lyapunov exponent and the butterfly velocity, which are important quantifiers of the OTOC. Then I will talk about a systematic way of deriving poleskipping conditions for general holographic CFTs dual to classical bulk theories and how to use this framework to derive a few interesting statements including: (1) theories with higher spins generally violate the chaos bound; (2) the butterfly velocity calculated using pole… 

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… 