  One of the central difficulties of settling the $L^2$-bounded curvature
conjecture for the Einstein -Vacuum equations is to be able to control the
causal structure of spacetimes with such limited regularity.
  In this paper we show how to circumvent this difficulty by showing that the
geometry of null hypersurfaces of Enstein-Vacuum spacetimes can be controlled
in terms of initial data and the total curvature flux through the hypersurface.
