Topological and geometrical tools used in general relativity calculations. They are an outgrowth of the topological method used by R. Penrose to prove the singularity theorem. Hawking and Penrose proved in 1970 that the universe Eric Weisstein's World of Astronomy must have had a spacetime singularity at the Big Bang using global methods. In 1970, Hawking also used global methods to define the absolute horizon of a black hole.
