Cosmic Crystallography
  We assume that the Universe has a non trivial topology whose compact spatial sections have a volume significantly smaller than the horizon volume. By a topological lens effect, such a "folded" space configuration generates multiple images of cosmic sources, e.g. clusters of galaxies. We present a simple and powerful method to unveil non ambiguous observational effects, independently of the sign of the curvature and of the topological type. 
R. Lehoucq (1), M. Lachieze-Rey (1), J.P.Luminet (2) ((1)   CE-Saclay/Service d'Astrophysique, (2) Observatoire de Meudon/DARC)