2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/104431This paper has been withdrawn because the new one gr-qc/0512095 includes all its results (as well as those in gr-qc/0507018), in a clearer way.This paper has been withdrawn; the new paper gr-qc/0512095 replaces both, the present one and gr-qc/0507018 (including all the results in a clearer way)General Relativity and Quantum Cosmologytext