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Published online by Cambridge University Press: 07 February 2017
Singularities in space-time can be broadly divided into three classes: past-spacelike (in white holes or the big bang), timelike (naked singularities) and future-spacelike (in black holes or the final recollapse). In a closed Universe, if a simple restriction is made to eliminate timelike singularities, the inference may be drawn that the topology of the Universe is unchanging with time. Thermodynamical considerations lead one to infer that the final singularity of recollapse must differ markedly in structure from the initial big bang. This may plausibly be related to the existence of black holes and the presumed non-existence of white holes.