Cosmic crystallography using short-lived objects - active galactic   nuclei
  Cosmic crystallography is based on the principle that peaks in the pair separation histogram (PSH) of objects in a catalogue should be induced by the high number of topologically lensed pairs that are separated by Clifford translations, in excess to ``random'' pairs of objects. Here we present modifications of this method that successively improve the signal-to-noise ratio by removing a large part of the noise and then false signals induced by selection effects. Given the transient nature of the most readily available tracer objects, active galactic nuclei (AGNs), the former is possible because a natural filter for removing many of the noise pairs is available: when counting pairs of objects in order to create PSHs, only those with nearly identical redshifts need to be counted. This redshift filter (a maximum value of $\Delta z/z = 0.005$) was applied to a compilation of AGN catalogues. Further noise was removed by applying a second filter, a maximum angle $\Delta \theta =0.075$ rad, and a minimum number of pairs $\protect\npairs=3$ to find each ``bunch of pairs'' (BoP) where the {\em vectors} (in Euclidean comoving space) defined by pairs are required to be nearly equal, whereas in the PSH only the {\em lengths} must be nearly equal. These filters reveal significant signals, which, however, are due to selection effects. A third filter, a minimum length $\Lselec=150$ {\hMpc} between the (parallel) vectors in a BoP, is found to effectively remove these selection effect pairs. After application of these successive filters, no significant topological signal was found. 
Andrzej Marecki (1), Boudewijn F. Roukema (1), Stanislaw Bajtlik (2)   ((1) Torun Centre for Astronomy, N. Copernicus Univer...