Conference

We will discuss the notion of categorical Lie algebra actions, as introduced by Rouquier and Khovanov-Lauda. In particular, we will give examples of categorical Lie algebra actions on derived categories of coherent sheaves. We will show that such categorical Lie algebra actions lead to actions of braid groups.

There appear to be only two essentially distinct ways to understand intersection numbers on moduli spaces of curves --- via Hurwitz numbers or symplectic volumes. In this talk, we will consider polynomials defined by Norbury which bridge the gap between these two pictures. They appear in the enumeration of lattice points in moduli spaces of curves and it appears that their coefficients store interesting information. We will also describe a connection between these polynomials and the topological recursion defined by Eynard and Orantin.

The Petrov classification of the Weyl tensor is an important tool in the study of exact solutions of the Einstein equation in 4d. For example, the Kerr solution was discovered in a study of spacetimes with algebraically special Weyl tensors. Algebraic classification of the Weyl tensor has been extended to higher dimensions. I shall review this classification and describe known families of algebraically special solutions. Recent progress towards obtaining a higher dimensional generalization of the Goldberg-Sachs theorem will be described.

I will discuss recent joint work with A. Ionescu and S. Klainerman on the black hole uniqueness problem. A classical result of Hawking (building on earlier work of Carter and Robinson) asserts that any vacuum, stationary black hole exterior region must be isometric to the Kerr exterior, under the restrictive assumption that the space-time metric shouldbe analytic in the entire exterior region. We prove that Hawking's theorem remains valid without the assumption of analyticity, for black hole exteriors which are apriori assumed to be ''close'' to the Kerr exterior solution in a very precise sense. Our method of proof relies on certain geometric Carleman-type estimates for the wave operator. Time permitting, some more recent developments will also be surveyed.

The statistics of strong lensing by galaxy clusters are sensitive both to cosmology and the detailed physics that determines the structure of halos. To exploit these sensitivites requires large and well defined samples of lenses on these mass scales. I will report on efforts to provide such samples - we finally now have uniformly selected samples of several hundred lenses to work with.

Modified gravity theories under consideration typically reduce to a scalar-tensor form in the appropriate limits. I will discuss in what sense a universal scalar coupling is stable against quantum corrections, when the scalar equivalence principle is violated, how to look for such violations, and the connection with cosmic acceleration.

Instead of adding another dark component to the energy budget of the Universe, one can ask whether the observed accelerated expansion might in fact be due to the behavior of gravity itself on the largest scales. In this talk I will focus on two popular modified gravity theories which realize this scenario: f(R) gravity and the DGP model. While these models yield an accelerated expansion, they also affect the formation of structure on much smaller scales. We have studied this with cosmological N-body simulations which consistently solve for the modified gravitational force. I will discuss the effects of modified gravity on dark matter halo properties as well as cosmological observables. For f(R) gravity, our first simulation-calibrated constraints from the observed abundance of massive clusters improve on previous constraints from the CMB and ISW by a factor of ~1000. This exemplifies the sensitivity of cosmological observables in the non-linear regime as probes of gravity.