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

TL;DR

This paper explores variations of stable causality in spacetime, introducing the concept of compact stable causality, and analyzes the topological properties of the space of metrics related to causality conditions.

Contribution

It introduces the notion of compact stable causality, relates it to antisymmetry of a new causal relation, and examines the topological structure of the metric space affecting causality.

Findings

Compact stable causality can be characterized by a new antisymmetric causal relation.

The Geroch's interval topology on metrics is not Frechet-Urysohn, affecting causality stability.

Stable causality with the C^0 fine topology aligns with the traditional definition.

Abstract

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 compact arbitrarily large 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 of M is not Frechet-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 C^0 fine topology on metrics leads to the usual notion of stable causality.