UNIVERSALITY OF THE FUTURE CHRONOLOGICAL BOUNDARY

The purpose of this note is to establish, in a categorical manner, the universality of the Geroch-Kronheimer-Penrose (GKP) causal boundary w hen considering the types of causal structures that may profitably be put on any sort of boundary for a space-time. Actually, this can only be done for the future causal boundary (or the past causal boundary! s eparately; furthermore, only the chronology relation, not the causalit y relation, is considered, and the GKP topology is eschewed. The final result is that there is a unique map, with the proper causal properti es, from the future causal boundary of a space-time onto any ''reasona ble'' boundary which supports some sort of chronological structure and which purports to consist of a future completion of the space-time. F urthermore, the future causal boundary construction is categorically u nique in this regard. (C) 1998 American Institute of Physics. [S0022-2 488(98)01910-0].