Hausdorff separability of the boundaries for spacetimes and sequential spaces

There are several ideal boundaries and completions in General Relativity sharing the topological property of being sequential, i.e., determined by the convergence of its sequences and, so, by some limit operator $L$. As emphasized in a classical article by Geroch, Liang and Wald, some of them have the property, commonly regarded as a drawback, that there are points of the spacetime $M$ non $T_1$-separated from points of the boundary $\partial M$. Here we show that this problem can be solved from a general topological viewpoint. In particular, there is a canonical minimum refinement of the topology in the completion $\overline{M}$ which $T_2$-separates the spacetime $M$ and its boundary $\partial M$ ---no matter the type of completion one chooses. Moreover, we analyze the case of sequential spaces and show how the refined $T_2$-separating topology can be constructed from a modification $L^*$ of the original limit operator $L$. Finally, we particularize this procedure to the case of the causal boundary and show how the separability of $M$ and $\partial M$ can be introduced as an abstract axiom in its definition.

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