On the radius of injectivity of null hypersurfaces
  The paper is concerned with regularity properties of boundaries of causal pasts of points in a 3+1-dimensional Einstein-vacuum spacetime. In a Lorentzian manifold such boundaries play crucial role in propagation of linear and nonlinear waves. We prove a uniform lower bound on the radius of injectivity of these null boundaries in terms of the Riemann curvature flux along them and some additional quantities arising specifically in a problem of a large data breakdown criterion in General Relativity 
S. Klainerman and I. Rodnianski