Topological censorship

The topological censorship theorem in the general theory of relativity claims that in the absence of exotic matter, the nontrivial space-time topology cannot be detected by an external observer, since any such regions collapse so quickly that the light does not have time to cross them. A more accurate formulation states that in a globally hyperbolic and asymptotically flat space-time , where the light energy conditions are satisfied , any causal curve from the light-like infinity of the past to the light-like infinity of the future is homotopy equivalent to a curve with the same end points in a topologically trivial neighborhood of infinity.