Lovelock theories are the most general diffeomorphism invariant theories of gravity in higher dimensions with second order equations of motion. Horndeski theories are the most general diffeomorphism invariant theories of gravity coupled to a scalar field in four dimensions with second order equations of motion. In this talk I will discuss well-posedness of the initial value problem for these theories. Previous work has shown that (generalised) harmonic gauge does not give a well-posed initial value problem. I will describe recent work with Aron Kovacs in which we introdued a modification of harmonic gauge that does give a well-posed initial value problem provided that the theory remains “weakly coupled”. Our modified harmonic gauge may also have applications in conventional GR.
I will describe recent and ongoing work with Y. Shlapentokh-Rothman on a construction of solutions to the Einstein vacuum equations corresponding to a naked singularity forming from a regular past. The talk will focus on a new geometric phenomenon, corresponding to twisting of null geodesics, new type of self-similarity, dynamical approach to the problem, and on the comparisons with the naked singularity solutions constructed by Christodoulou for the spherically symmetric Einstein-scalar field model.
Cauchy horizons are rather unique and peculiar objects that have been studied for decades. In this talk I will first review old and new breakthroughs on the subject by Isenberg and Moncrief, and by Petersen and Rácz, and then discuss joint recent work together with I. Bustamante where it is proved that non-degenerate Compact Cauchy horizons on smooth vacuum spacetimes (shortly CHs) have indeed constant non-zero temperature. It then follows by the work of Petersen and Petersen-Rácz that CHs are Killing horizons, and by the work of Beig-Chruściel-Schoen that such objects are non-generic on the initial data, (in agreement with the Cosmic Censorship conjecture). A null-orbital and topological classification of CHs will also be commented.
The study of static vacuum Riemannian metrics arises naturally in general relativity and differential geometry. A static vacuum metric produces a static spacetime by a warped product, and it is related to scalar curvature deformation and gluing. The well-known Uniqueness Theorem of Static Black Holes says that an asymptotically flat, static vacuum metric with black hole boundary must belong to the Schwarzschild family. In contrast to the rigidity phenomenon, R. Bartnik conjectured that there are asymptotically flat, static vacuum metric realizing certain arbitrarily specified boundary data. I will discuss recent progress toward this conjecture. It is based on joint work with Zhongshan An.
In the second part of the talk (if time permits), I will discuss related topics in the non-time-symmetric case. In many ways, a stationary vacuum initial data set is a generalization of a static vacuum metric. However, from the point of view of deforming the dominant energy condition, we discover that a “ground state” initial data set need not be stationary vacuum but can sit in a null dust spacetime with a global Killing vector field, such as the pp-waves. This part is based on joint work with Dan Lee.
We will discuss the large time behavior of solutions to Einstein's equations with a positive cosmological constant. This is a subject first addressed by Friedrich ('86) in vacuum, showing in particular the global stability of de Sitter space. Since then numerous works have investigated the asymptotic behavior of solutions with an accelerated expansion at infinity, in different contexts, and the effect of the expansion to various matter fields. We will give an account of known results and present a recent work that classifies the future dynamics of the FLRW solution for the massless-scalar field system.
I will review the notion of mass of asymptotically locally hyperbolic manifolds, and sketch the proof of positivity of the energy for manifolds with spherical conformal infinity.
Talk based on joint work with Erwann Delay in arXiv: 1901.05263 [math.DG].
The AdS instability conjecture provides an example of weak turbulence appearing in the dynamics of the Einstein equations in the presence of a negative cosmological constant. The conjecture claims the existence of arbitrarily small perturbations to the initial data of Anti-de Sitter spacetime which, under evolution by the vacuum Einstein equations with reflecting boundary conditions at conformal infinity, lead to the formation of black holes after sufficiently long time. In this talk, I will present a rigorous proof of the AdS instability conjecture in the setting of the spherically symmetric Einstein-scalar field system. The construction of the unstable initial data will require carefully designing a family of initial configurations of localized matter beams and estimating the exchange of energy taking place between interacting beams over long periods of time, as well as estimating the decoherence rate of those beams. I will also discuss possible paths for extending these ideas to the vacuum case.
Linear zero-rest-mass fields generically develop logarithmic singularities at the critical sets where spatial infinity meets null infinity. Friedrich's representation of spatial infinity is ideally suited to study this phenomenon. These logarithmic singularities are an obstruction to the smoothness of the zero-rest-mass field at null infinity and, in particular, to peeling. In the case of the spin-2 field it has been shown that these logarithmic singularities can be precluded if the initial data for the field satisfies a certain regularity condition involving the vanishing, at spatial infinity, of a certain spinor (the linearised Cotton spinor) and its totally symmetrised derivatives. In this article we investigate the relation between this regularity condition and the staticity of the spin-2 field. It is shown that while any static spin-2 field satisfies the regularity condition, not every solution satisfying the regularity condition is static.
This seminar is joint with CENTRA, and will take place on the Physics Department (seminar room, 2nd floor).
The singularity theorems of R. Penrose and S. Hawking from the 1960s show that a spacetime satisfying certain physically reasonable curvature and causality conditions cannot be causal geodesically complete. Despite their great success these classical theorems still have some drawbacks, one of them being that they require smoothness of the metric while in many physical models the metric is less regular. In my talk I will present work on singularity theorems based on distributional energy conditions for metrics that are merely continuously differentiable – a regularity where one still has existence but not uniqueness for solutions of the geodesic equation. We will see that an approximation-based approach to the low-regularity issue is closely linked to establishing singularity theorems under weakened energy conditions and while the improvements necessary are still entirely straightforward in the present case, attempting to lower the regularity further would require some new methods.
In this talk we introduce and solve the characteristic gluing problem for the Einstein vacuum equations. We prove that obstructions to characteristic gluing come from an infinite-dimensional space of conservation laws along null hypersurfaces for the linearized equations at Minkowski. We show that this obstruction space splits into an infinite-dimensional space of gauge-dependent charges and a 10-dimensional space of gauge-invariant charges. We identify the 10 gauge-invariant charges to be related to the energy, linear momentum, angular momentum and center-of-mass of the spacetime. Based on this identification, we explain how to characteristically glue a given spacetime to a suitably chosen Kerr black hole spacetime. As corollary we get an alternative proof of the Corvino-Schoen spacelike gluing to Kerr. Moreover, we apply our characteristic gluing method to localise characteristic initial data along null hypersurfaces. In particular, this yields a new proof of the Carlotto-Schoen spacelike localization where our method yields no loss of decay, thus resolving an open problem in this direction. We also outline further applications. This is joint work with S. Aretakis (Toronto) and I. Rodnianski (Princeton).
