The geometry of the space of null geodesics

The topology and geometry of the space of null geodesics N of a space‐time M are used to study the causal structure of the space‐time itself. In particular, the question of whether the topology of N is Hausdorff or admits a compatible manifold structure carries information on the global structure of M, and the transversality properties of the intersections of skies of points tell whether the points are conjugate points on a null geodesic.