EFFECTS OF FRICTION ON COSMIC STRINGS

We study the evolution of cosmic strings taking into account the frict ional force due to the surrounding radiation. We consider small pertur bations on straight strings, oscillation of circular loops, and small perturbations on circular loops. For straight strings, friction expone ntially sup perturbations whose comoving scale crosses the horizon bef ore the cosmological time t approximately mu-2 (in Planck units), whe re mu is the string tension. Loops with a size much smaller than t wi ll be approximately circular at the time when they start the relativis tic collapse. We investigate the possibility that such loops will form black holes. We find that the number of black holes which are formed through this process is well below present observational limits, so th is does not give any lower or upper bounds on mu. We also consider the case of straight strings attached to walls and circular holes that ca n spontaneously nucleate on metastable domain walls.