Properties of the Null Distance and Spacetime Convergence

The null distance for Lorentzian manifolds was recently introduced by Sormani and Vega. Under mild assumptions on the time function of the spacetime, the null distance gives rise to an intrinsic, conformally invariant metric that induces the manifold topology. We show when warped products of low regularity and globally hyperbolic spacetimes endowed with the null distance are (local) integral current spaces. This metric and integral current structure sets the stage for investigating convergence analogous to Riemannian geometry. Our main theorem is a general convergence result for warped product spacetimes relating uniform, Gromov–Hausdorff, and Sormani–Wenger intrinsic flat convergence of the corresponding null distances. In addition, we show that nonuniform convergence of warping functions in general leads to distinct limiting behavior, such as limits that disagree.

The pointed intrinsic flat distance between locally integral current spaces

In this note, we define a distance between two pointed locally integral current spaces. We prove that a sequence of pointed locally integral current spaces converges with respect to this distance if and only if it converges in the sense of Lang–Wenger. This enables us to state the compactness theorem by Lang–Wenger for pointed locally integral current spaces in terms of a distance function.

Convergence of manifolds and metric spaces with boundary

We study sequences of oriented Riemannian manifolds with boundary and, more generally, integral current spaces and metric spaces with boundary. We prove theorems demonstrating when the Gromov–Hausdorff (GH) and Sormani–Wenger Intrinsic Flat (SWIF) limits of sequences of such metric spaces agree. Thus in particular the limit spaces are countably [Formula: see text] rectifiable spaces. From these theorems we derive compactness theorems for sequences of Riemannian manifolds with boundary where both the GH and SWIF limits agree. For sequences of Riemannian manifolds with boundary we only require non-negative Ricci curvature, upper bounds on volume, noncollapsing conditions on the interior of the manifold and diameter controls on the level sets near the boundary.

The Sormani–Wenger intrinsic flat convergence of Alexandrov spaces

We study sequences of integral current spaces [Formula: see text] such that the integral current structure [Formula: see text] has weight [Formula: see text] and no boundary and, all [Formula: see text] are closed Alexandrov spaces with curvature uniformly bounded from below and diameter uniformly bounded from above. We prove that for such sequences either their limits collapse or the Gromov–Hausdorff and Sormani–Wenger Intrinsic Flat limits agree. The latter is done showing that the lower [Formula: see text]-dimensional density of the mass measure at any regular point of the Gromov–Hausdorff limit space is positive by passing to a filling volume estimate. In an appendix, we show that the filling volume of the standard [Formula: see text]-dimensional integral current space coming from an [Formula: see text]-dimensional sphere of radius [Formula: see text] in Euclidean space equals [Formula: see text] times the filling volume of the [Formula: see text]-dimensional integral current space coming from the [Formula: see text]-dimensional sphere of radius [Formula: see text].

Intrinsic flat stability of the positive mass theorem for graphical hypersurfaces of Euclidean space

The rigidity of the Positive Mass Theorem states that the only complete asymptotically flat manifold of nonnegative scalar curvature and