Date  Speaker  Title/Abstract 

9/13/2019
CMSA G02 
Martin Lesourd (BHI)  Title: ShiTam’s Existence of Minimal Surfaces from QuasiLocal Mass
Abstract: Thorne’s hoop conjecture is an intuitive hypothesis intended to capture necessary and sufficient conditions for the existence of a black hole region. The first result in this direction was SchoenYau 83 and later Yau 01, which give sufficient conditions for the existence of an apparent horizon within a 3dimensional initial data set. ShiTam 07 have done something analogous in the Riemannian context and obtained the existence of minimal surfaces from conditions on quasilocal mass. We shall review the main ideas of their proof. 
9/20/2019  Martin Lesourd (BHI)  Title: Existence of minimal or marginally outer trapped surfaces from quasilocal mass
Abstract: I will describe work in progress with Aghil Alaee and ShingTung Yau in which sufficient conditions for the existence of minimal or marginally outer trapped surfaces inside a domain are expressed in terms of the quasilocal mass of the domain. 
10/4/2019  Cancelled  
10/11/2019  Graham Cox (Memorial University)  Title: Blowup solutions of Jang’s equation near a spacetime singularity
Abstract: Jang’s equation is a semilinear elliptic equation define on an initial data set. It was shown by Schoen and Yau that the (non)existence of global solutions is closely related to the existence of marginally outer trapped surfaces (MOTS), which are quasilocal analogues of black hole boundaries. As a result, Jang’s equation can be used to prove the existence of MOTS by imposing appropriate geometric conditions on the initial data set. These proofs proceed by contradiction: one assumes there is a global solution, then proves that its existence is not compatible with the given geometric assumptions. In this talk I will outline a constructive approach to proving the existence of MOTS. In particular, I will consider a distinguished family of spacelike hypersurfaces in the maximally extended Schwarzschild spacetime, and prove that Jang’s equation admits no global solutions once the hypersurfaces become sufficiently close to the r=0 singularity. This suggests a general strategy for relating spacetime singularities to the presence of MOTS. This is joint with Amir Aazami. 
10/18/2019  Jordan Keller (BHI)  Title: Angular Momentum and CenterofMass at Null Infinity
Abstract: We calculate the limits of the quasilocal angular momentum and centerofmass defined by ChenWangYau for a family of spacelike twospheres approaching future null infinity in an asymptotically flat spacetime admitting a BondiSachs expansion. Our result complements earlier work of ChenWangYau, where the authors calculate the quasilocal energy and linear momentum at null infinity. Working in the centerofmass frame, i.e. assuming vanishing of linear momentum at null infinity, we obtain explicit expressions for the angular momentum and centerofmass at future null infinity in terms of the observables appearing in the BondiSachs expansion of the spacetime metric. This is joint work with YeKai Wang and ShingTung Yau. 
10/25/2019  Cancelled  
11/1/2019  Peter Hintz (MIT)  Title: Linear stability of slowly rotating Kerr black holes
Abstract: I will describe joint work with Dietrich Häfner and András Vasy in which we study the asymptotic behavior of linearized gravitational perturbations of Schwarzschild and slowly rotating Kerr black hole spacetimes. We show that solutions of the linearized Einstein equation decay at an inverse polynomial rate to a stationary solution (given by an infinitesimal variation of the mass and angular momentum of the black hole), plus a pure gauge term. Our proof uses a detailed description of the low energy resolvent of an associated wave equation on symmetric 2tensors. 
11/8/2019  PeiKen Hung (MIT) 
Abstract: In this talk, I will discuss a wave equation for one forms in the Schwarzschild spacetime which is the linearization of a modified wave map gauge. The equation behaves like a damped wave equation and we obtain robust estimates. In particular, it allows us to show the stability of the modified wave map equation. This is ongoing joint work with S. Brendle.

11/13/2019
Wednesday 1:00pm CMSA G10 
Eric Woolgar (University of Alberta) 
Abstract: Curvaturedimension inequalities are modifications of a Ricci curvature bound or, in the language of relativity, an energy condition. They have proved useful in applications of Fourier analysis to diffusion processes. As tools to prove theorems in Riemannian geometry and general relativity, they are often as powerful as the usual Ricci curvature bounds and can yield new results. Applications include static Einstein metrics, nearextremalhorizon geometry, and scalartensor gravity. I will discuss an application of a Riemannian curvaturedimension bound to horizon topology, and use Lorentzian curvaturedimension bounds to prove some singularity theorems and splitting theorems. Parts of the talk are based on joint work with Marcus Khuri, Will Wylie, and Greg Galloway.

11/22/2019  Sahar Hadar (BHI)  Title: Universal signatures of a black hole’s photon ring
Abstract: The Event Horizon Telescope image of the supermassive black hole in the galaxy M87 is dominated by a bright, unresolved ring. General relativity predicts that embedded within this image lies a thin “photon ring,” which is composed of an infinite sequence of selfsimilar subrings that are indexed by the number of photon orbits around the black hole. The subrings approach the edge of the black hole “shadow,” becoming exponentially narrower but weaker with increasing orbit number, with seemingly negligible contributions from high order subrings. In the talk, I will discuss the structure of the photon ring, starting with nonrotating black holes, and then proceeding to the complex patterns that emerge when rotation is taken into account. Subsequently I will argue that the subrings produce strong and universal signatures on long interferometric baselines. These signatures offer the possibility of precise measurements of black hole mass and spin, as well as tests of general relativity, using only a sparse interferometric array. 
11/29/2019  Thanksgiving Holiday  
12/6/2019  
12/13/2019 
Date  Speaker  Title/abstract 
9/7/2018  Christos Mantoulidis (MIT)  Title: Capacity and quasilocal mass
Abstract. This talk is based on work with P. Miao and L.F. Tam. We derive new inequalities between the boundary capacity of an asymptotically flat 3manifold with nonnegative scalar curvature and boundary quantities that relate to quasilocal mass; one relates to BrownYork mass and the other is new. Among other things, our work yields new variational characterizations of Riemannian Schwarzschild manifolds and new comparison results for surfaces in them. 
9/12/2018  Aghil Alaee (CMSA)  Title: Massangular momentum inequality for black holes
Abstract: In this talk, I will review the results of massangular momentum inequality for fourdimensional axisymmetric black holes. Then I will establish versions of this inequality for fivedimensional black holes and in particular black ring, which is the most intriguing asymptotically flat solution of vacuum Einstein equations. Moreover, I will show these inequalities are sharp if and only if the initial data sets are isometric to the canonical slices of known extreme stationary solutions. These results are joint work with Marcus Khuri and Hari Kunduri. 
9/19/2018  PeiKen Hung (MIT)  Title: The linear stability of the Schwarzschild spacetime in the harmonic gauge: odd part
Abstract: We study the odd solution of the linearlized Einstein equation on the Schwarzschild background and in the harmonic gauge. With the aid of ReggeWheeler quantities, we are able to estimate the odd part of Lichnerowicz d’Alembertian equation. In particular, we prove the solution decays at rate $\tau^{1+\delta}$ to a linearlized Kerr solution. 
9/26/2018  Jordan Keller (BHI)  Title: Quasilocal Angular Momentum and CenterofMass at Future Null Infinity
Abstract: We calculate the limits of the quasilocal angular momentum and centerofmass defined by ChenWangYau [3] for a family of spacelike twospheres approaching future null infinity in an asymptotically flat spacetime admitting a BondiSachs expansion. Our result complements earlier work of ChenWangYau [2], where the authors calculate the quasilocal energy and linear momentum at null infinity. Finiteness of the quasilocal centerofmass requires that the spacetime be in the socalled centerofmass frame, which amounts to a mild assumption on the mass aspect function corresponding to vanishing of the quasilocal linear momentum calculated in [2]. With this condition and the assumption that the mass aspect function is nontrivial, we obtain explicit expressions for the quasilocal angular momentum and centerofmass at future null infinity in terms of the observables appearing in the BondiSachs expansion of the spacetime metric. This is joint work with YeKai Wang and ShingTung Yau. 
10/3/2018  Christos Mantoulidis (MIT)  Title: The Bartnik mass of apparent horizons
Abstract: We will discuss a spectral characterization of apparent horizons in threedimensional timesymmetric initial data sets. Then, for a dense class of nondegenerate apparent horizons, we will construct sharp asymptotically flat extensions to conclude that their Bartnik mass equals their Hawking mass. This is joint work with R. Schoen. 
10/10/2018  Salem Al Mosleh (CMSA)  Title: Thin elastic shells and isometric embedding of surfaces in threedimensional Euclidean space
Abstract: We will first discuss the reduction of theories describing elastic bodies in threedimensions to effective descriptions defined on embedded surfaces. Then, we describe the isometric deformations of surfaces and the key role of played by asymptotic curves, curves with zero normal curvature, in determining the local mechanical behavior of thin shells. This was joint work with C. Santangelo. 
10/17/2018  Sébastien Picard (Harvard)  Title: The Anomaly flow over Riemann surfaces
Abstract: The Anomaly flow is a geometric flow on CalabiYau threefolds which is motivated by string theory. We will study the flow on certain fibrations where it reduces to a scalar evolution equation on a Riemann surface. This is joint work with T. Fei and Z. Huang. 
10/31/2018  Alex Lupsasca (Harvard)  Title: Polarization Whorls from M87 at the Event Horizon Telescope
Abstract: The Event Horizon Telescope (EHT) is expected to soon produce polarimetric images of the supermassive black hole at the center of the neighboring galaxy M87. This black hole is believed to be very rapidly spinning, within 2% of extremality. General relativity predicts that such a highspin black hole has an emergent conformal symmetry near its event horizon. In this talk, I will briefly review this symmetry and use it to derive an analytic prediction for the polarized nearhorizon emissions to be seen at the EHT. The resulting pattern is very distinctive and consists of whorls aligned with the spin. 
11/7/2018  Jordan Keller
(BHI) 
Title: Linear Stability of Higher Dimensional Schwarzschild Black Holes
Abstract: The SchwarzschildTangherlini black holes are higherdimensional generalizations of the Schwarzschild spacetimes, comprising a static, spherically symmetric family of black hole solutions to higherdimensional vacuum gravity. The physical relevance of such solutions is intimately related to their stability under gravitational perturbations. This talk will address results on the linear stability of the SchwarzschildTangherlini black holes, part of ongoing joint work with PeiKen Hung and MuTao Wang. 
11/14/2018  Niky Kamran
(McGill) 
Title: Lorentzian Einstein metrics with prescribed conformal infinity
Abstract: We prove a local wellposedness theorem for the $(n+1)$dimensional Einstein equations in Lorentzian signature, with initial data whose asymptotic geometry at infinity is similar to that antide Sitter (AdS) space and compatible boundary data prescribed at the timelike conformal boundary of spacetime. This extends the fundamental result of Friedrich on the existence of antide Sitter spacetimes in 3+1 dimensions to arbitrary spacetime dimensions, by a different approach that allows for generic smoothness and polyhomogeneity assumptions on the initial data. This is joint work with Alberto Enciso (ICMAT, Madrid). 
12/05/2018
*room G02* 
Pengzi Miao (University of Miami)  Title: Localization of the Penrose inequality and variation of quasilocal mass
Abstract: In the study of manifolds with nonnegative scalar curvature, a fundamental result is the Riemannian Positive mass theorem. If the manifold has horizon boundary, one has the Riemannian Penrose inequality. Given a compact region with boundary in these manifolds, one wants to understand how much mass or energy is localized in such a region. This question is usually referred to as the quasilocal mass problem. In this talk, we discuss an inequality on a compact manifold with nonnegative scalar curvature, which can be thought as a body surrounding horizons. Our discussion of the rigidity case of this inequality reveals an intriguing relation between two of the most important notions of quasilocal mass, the Bartnik mass and the WangYau mass. The talk is based on joint work with Siyuan Lu. 
1/31/2019
SC 232 34pm 
Shahar Hadar (Harvard University)  Title: Latetime behavior of nearextremal black holes from symmetry
Abstract: Linear perturbations of extremal black holes exhibit the Aretakis instability, in which higher derivatives of the fields grow polynomially with time along the event horizon. Nearextremal black holes display similar behavior for some time, and eventually decay exponentially through quasinormal modes. In the talk I will show that the above behaviors are dictated by the conformal symmetry of the nearhorizon region of such black holes. I will then discuss the significance of backreaction in the problem, and show how it can be simply accounted for within the nearhorizon picture. 
2/7/2019
SC 411 34pm 
PeiKen Hung (MIT)  Title: The linear stability of the Schwarzschild spacetime in the harmonic gauge: even part
Abstract: We study the even solution of the linearlized Einstein equation on the Schwarzschild background and in the harmonic gauge. With the aid of the Zerilli equation, we estimate the even part of Lichnerowicz d’Alembertian equation. In particular, we show that up to a one dimensional stationary mode, the solution decays to a linearlized Kerr solution. This is ongoing joint work with S. Brendle. 
2/14/2019
SC 411 34pm 
Charles Marteau (Ecole Polytechnique)  Title: Null hypersurfaces and ultrarelativistic physics in gravity
Abstract: I will explain how the induced geometry on a null hypersurface gives rise to a particular type of structure called Carrollian geometry. The latter emerges when taking the ultrarelativistic limit of the usual pseudoRiemannian metric. This property has strong consequences on the gravitational dynamics satisfied by the extrinsic geometry of the null hypersurface and on its symmetry group. We will see how the first one can be interpreted as ultrarelativistic conservation laws while the second corresponds to the isometries of the induced Carrollian geometry. These are very general statements for any null hypersurface but I will focus all along on a physically interesting case: the null infinity of an asymptotically flat spacetime. 
2/21/2019  HsinYu Chen (Black Hole Initiative)  Title: Measuring the Hubble Constant with Gravitational Waves
Abstract: The first detection of binary neutron star merger by Advanced LIGOVirgo and the discovery of the optical counterpart allowed for the first independent measurement of Hubble constant with gravitational waves. In this talk, I will summarize latest cosmological measurements with gravitational waves, and discuss the future aspects of them. I will then talk about the potential challenges and how we improve the measurements. 
3/7/2019  Laura Donnay (Harvard)  Title: Carrollian physics at the blackhole horizon
Abstract: In this talk, I will show that the nearhorizon geometry of a black hole can be described as a Carrollian geometry emerging from an ultrarelativistic limit. The laws governing the dynamic of a black hole horizon, the null Raychaudhuri and Damour equations, are shown to be Carrollian conservation laws obtained by taking the ultrarelativistic limit of the conservation of an energymomentum tensor. Vector fields preserving the Carrollian geometry of the horizon, dubbed Carrollian Killings, include BMSlike supertranslations and superrotations, and have nontrivial associated conserved charges. If time allows, I will discuss their relation with the infinitedimensional horizon charges of the covariant phase space formalism. 
3/14/2019
3:30pm 
Peter Hintz (MIT)  Title: Stability of Minkowski space and polyhomogeneity of the metric
Abstract: I will explain a new proof of the nonlinear stability of the Minkowski spacetime as a solution of the Einstein vacuum equation. The proof relies on an iteration scheme at each step of which one solves a linear wavetype equation globally. The analysis takes place on a compactification of R^4 to a manifold with corners whose boundary hypersurfaces correspond to spacelike, null, and timelike infinity. I will describe how the asymptotic behavior of the metric can be deduced from the structure of simple model operators at these boundaries. Joint work with András Vasy. 
3/28/2019  TBA  TBA 
4/4/2019
CMSA G02 
Marcus Khuri (Stony Brook)  Title: Stationary Vacuum Black Holes in Higher Dimensions
Abstract: A result of Galloway and Schoen asserts that horizon crosssections must be of positive Yamabe invariant. In this talk we discuss results on a converse problem. That is, which manifolds of positive Yamabe invariant arise as horizon crosssections in a stationary vacuum spacetime. 
4/11/2019  Amir Babak Aazami (Clarks)  Title: Kähler metrics via Lorentzian geometry in dimension 4
Abstract: Given a Lorentzian manifold with two distinguished vector fields satisfying properties determined by their shear, twist and various Lie bracket relations, a family of Kähler metrics is constructed on . Under certain conditions and share various properties, such as a Killing vector field or a vector field with geodesic flow. The Ricci and scalar curvatures of are computed in some cases in terms of data associated to ; in certain cases the Kähler manifold will be complete and Einstein. Many classical spacetimes fit into this construction: warped products, for instance de Sitter spacetime, as well as gravitational plane waves and metrics of Petrov type , such as Kerr and NUT metrics. This work is joint with Gideon Maschler. 
4/19/2019
CMSA G02 Friday @ 9:30am 
Lydia Bieri (University of Michigan)  Title: Logarithmic or Not Logarithmic
Abstract: In General Relativity, we describe isolated gravitating systems by asymptotically flat solutions of the Einstein equations. For various classes of initial data corresponding classes of solutions have been constructed in the nonlinear stability proofs when slightly moving away from Minkowski spacetime. Many of the null asymptotic results still hold when one replaces the small initial data by large initial data. Therefore, these solutions have become an interesting and important source to understand gravitational waves and memory as observed at null infinity. Lately, discussions have flared up whether logarithmic terms are present at highest order in crucial components of the Riemannian curvature and shear of the spacetime and whether such terms would give a tail effect for ordinary memory. There is a large literature (older and newer) on terms of this sort. In this talk, I will address some of my recent work that proves that for asymptotically flat solutions of the Einstein equations the crucial curvature and shear components do not have logarithmic terms at highest order, but logarithmic terms naturally show up at lower order. From thisit follows that there is no divergent memory caused by logarithmic terms. However, in my earlier work, considering spacetimes with very slow decay, the ordinary memory diverges (though not logarithmically but faster) and null memory is always finite. Last but not least, these logarithmic terms do show up at leading order of certain other curvature and geometric components for specific decay of the initial data. These mathematical results are in accordance with a physical argument that I will present as well. 
4/25/2019
10:30pm CMSA G02 
Pengyu Le (University of Michigan)  Title: Perturbations of Null Hypersurfaces and Null Penrose Inequality
Abstract: The Penrose inequality in general relativity is a conjectured inequality between the area of the horizon and the mass of a blackhole spacetime. The null Penrose inequality is the case where it concerns the area of the horizon and the Bondi mass at null infinity on a null hypersurface. An effective method to prove Penrosetype inequalities is to exploit the monotonicity of the Hawking mass along certain foliations. The constant mass aspect function foliation is such a desired foliation, but the behavior of the foliation at past null infinity is an obstacle for the proof. An idea to overcome this difficulty is to vary the null hypersurface to achieve the desired behavior of the foliation at null infinity, leading to a spacetime version of the Penrose inequality. To formalise this idea, one need to study perturbations of null hypersurfaces. I will talk about my work on the study of perturbations of null hypersurfaces and its application to the null Penrose inequality. 
4/26/2019
2:30pm CMSA G02 
Armando Cabrera Pacheco (Universität Tübingen)  Title: Asymptotically flat extensions with charge
Abstract: Inspired by the Mantoulidis and Schoen construction, we obtain timesymmetric black hole initial data sets for the Einstein–Maxwell equations satisfying the dominant energy condition, such that their horizon boundary geometry is prescribed, and their total masses and total charges are controlled. We also formulate a notion of boundary Bartnik mass in this context and compute its value for minimal Bartnik data. This talk is based on a joint work with A. Alaee and C. Cederbaum. 