Widening the light cones on subsets of spacetime: some variations to stable causality

By definition a spacetime is stably causal if it is possible to widen the light cones all over the spacetime without spoiling causality. We prove that if the spacetime is at least non-total imprisoning then it is stably causal provided the light cones can be widened outside any arbitrarily large compact set, i.e. in a neighborhood of infinity, without spoiling causality. Furthermore, we prove that the new causality level ‘compact stable causality’ can be obtained as the antisymmetry condition of a new causal relation which we identify, but it cannot be obtained as a causal stability condition with respect to a topology on metrics. The difference between stable causality and compact stable causality is shown to follow from the fact that Geroch’s interval topology on the space of conformal metrics ofMis not Fr´ echet–Urysohn (in fact it is not even T sequential). In particular we prove that (compact) stably causal metrics are those in the (sequential) interior of the set of chronological metrics. Finally, contrary to previous claims it is shown that stable causality with respect to the C0 fine topology on metrics leads to the usual notion of stable causality.