The no-defect conjecture in cosmic crystallography
  The topology of space is usually assumed simply connected, but could be multi-connected. We review in the latter case the possibility that topological defects arising at high energy phase transitions might still be present and find that either they are very unlikely to form at all, or space is effectively simply connected on scales up to the horizon size. 
Jean-Philippe Uzan and Patrick Peter (DARC - Observatoire de Meudon,   France)