Black Hole Horizons and How They Begin

Abstract Properties of black holes are examined that follow from the definition of the horizon as the boundary of the past of null infinity. The beginning of the black hole is defined as the set of spacetime events at which the null geodesics that generate the horizon enter the horizon. This set is spacelike and appears as a crease in the horizon because at least two generators cross at each of its points. The relation between the crease set and future null infinity is that between object and image produced by the gravitational lensing of the collapsing mass-energy. The crease set is not a manifold but has the structure of a tree. Near a vertex the horizon is a cone over a polyhedron.