Causal continuity in degenerate spacetimes

Abstract

A change of spatial topology in a causal, compact spacetime cannot occur when the metric is globally Lorentzian. On any cobordism manifold, however, one can construct from a Morse function f and an auxiliary Riemannian metric hµicons/Journals/Common/nu" ALT="nu" ALIGN="TOP"/> a causal metric gµicons/Journals/Common/nu" ALT="nu" ALIGN="TOP"/> which is Lorentzian almost everywhere except that it degenerates to zero at each critical point of f. We investigate causal structure in the neighbourhood of such a degeneracy, when the auxiliary Riemannian metric is taken to be Cartesian flat in appropriate coordinates. For these geometries, we verify the conjecture that causal discontinuity occurs if and only if the Morse index is 1 or n - 1.