L6

We study the nonlinear wave equations on a class of asymptotically flat Lorentzian manifolds $(\mathbb{R}^{3+1}, g)$ with time dependent inhomogeneous metric g. Based on a new approach for proving the decay of solutions of linear wave equations, we give several applications to nonlinear problems. In particular, we show the small data global existence result for quasilinear wave equations satisfying the null condition on a class of time dependent inhomogeneous backgrounds which do not settle to any particular stationary metric.

Calculus of Variations for $L^{\infty}$ functionals has a successful history of 50 years, but until recently was restricted to the scalar case. Motivated by these developments, we have recently initiated the vector-valued case. In order to handle the complicated non-divergence PDE systems which arise as the analogue of the Euler-Lagrange equations, we have introduced a theory of "weak solutions" for general fully nonlinear PDE systems. This theory extends Viscosity Solutions of Crandall-Ishii-Lions to the general vector case. A central ingredient is the discovery of a vectorial notion of extremum for maps which is a vectorial substitute of the "Maximum Principle Calculus" and allows to "pass derivatives to test maps" in a duality-free fashion. In this talk we will discuss some rudimentary aspects of these recent developments.

Penrose’s Weyl Curvature Hypothesis, which dates from the late 70s, is a hypothesis, motivated by observation, about the nature of the Big Bang as a singularity of the space-time manifold. His Conformal Cyclic Cosmology is a remarkable suggestion, made a few years ago and still being explored, about the nature of the universe, in the light of the current consensus among cosmologists that there is a positive cosmological constant.  I shall review both sets of ideas within the framework of general relativity, and emphasise how the second set solves a problem posed by the first.

The vanishing mean curvature flow in Minkowski space is the

natural evolutionary generalisation of the minimal surface equation,

and has applications in cosmology as a model equation for cosmic

strings and membranes. The equation clearly admits initial data which

leads to singularity formation in finite time; Nguyen and Tian have

even shown stability of the singularity formation in low dimension. On

the other hand, Brendle and Lindblad separately have shown that all

"nearly flat" initial data leads to global existence of solutions. In

this talk, I describe an intermediate regime where global existence

of solutions can be proven on a codimension 1 set of initial data; and

where the codimension 1 condition is optimal --- The

catenoid, being a minimal surface in R^3, is a static solution to the

vanishing mean curvature flow. Its variational instability as a

minimal surface leads to a linear instability under the flow. By

appropriately "modding out" this unstable mode we can show the

existence of a stable manifold of initial data that gives rise to

solutions which scatters toward to the

catenoid. This is joint work with Roland Donninger, Joachim Krieger,

and Jeremy Szeftel. The preprint is available at http://arxiv.org/abs/1310.5606v1

We show that the positive mass theorem holds for

asymptotically flat, $n$-dimensional Riemannian manifolds with a metric

that is continuous, lies in the Sobolev space $W^{2, n/2}_{loc}$, and

has non-negative scalar curvature in the distributional sense. Our

approach requires an analysis of smooth approximations to the metric,

and a careful control of elliptic estimates for a related conformal

transformation problem. If the metric lies in $W^{2, p}_{loc}$ for

$p>n/2$, then we show that our metrics may be approximated locally

uniformly by smooth metrics with non-negative scalar curvature.

This talk is based on joint work with N. Tassotti and conversations with

J.J. Bevan.

The description of the behaviour of local minima or evolution problems for families of energies cannot in general be deduced from their Gamma-limit, which is a concept designed to treat static global minimum problems. Nevertheless this can be taken as a starting point. Various issues that have been addressed are:

Find criteria that ensure the convergence of local minimizers and critical points. In case this does not occur then modify the Gamma-limit in order to match this requirement. We note that in this way we `correct' some limit theories, finding (or `validating') other ones present in the literature;

Modify the concept of local minimizer, so that it may be more `compatible' with the process of Gamma-limit;

Treat evolution problems for energies with many local minima obtained by a time-discrete scheme introducing the notion of `minimizing movements along a sequence of functionals'. In this case the minimizing movement of the Gamma-limit can always be obtained by a choice of the space- and time-scale, but more interesting behaviors can be obtained at a critical ratio between them. In many cases a `critical scale' can be computed and an effective motion, from which all other minimizing movements are obtained by scaling.

Relate minimizing movements to general variational evolution results, in particular recent theories of quasistatic motion and gradient flow in metric spaces.

I will illustrate some of these points.