Cosmic crystallography looks at the 3-dimensional observed distribution of high redshift sources (e.g. galaxy clusters, quasars) in order to discover repeating patterns in their distribution, much like the repeating patterns of atoms observed in crystals. They showed that "pair separation histograms" are in most cases able to detect a multi- connected topology of space, in the form of spikes clearly standing out above the noise distribution as expected in the simply-connected case. The researchers have particularly studied small universe models, which explain the billions of visible galaxies are repeating images of a smaller number of actual galaxies.