Cosmic Strings Collision in Cosmological Backgrounds
  The collisions of cosmic strings loops and the dynamics of junctions
formations in expanding backgrounds are studied. The key parameter controlling
the dynamics of junctions formation, the cosmic strings zipping and unzipping
is the relative size of the loops compared to the Hubble expansion rate at the
time of collision. We study analytically and numerically these processes for
large super-horizon size loops, for small sub-horizon size loops as well as for
loops with the radii comparable to the Hubble expansion rate at the time of
collision.
