The evolution of circular loops of a cosmic string with periodic tension
  In this paper the equation of circular loops of cosmic string with periodic tension is investigated in the Minkowski spacetime and Robertson-Walker universe respectively. We find that the cosmic string loops possessing this kind of time-varying tension will evolve to oscillate instead of collapsing to form a black hole if their initial radii are not small enough. 
Leilin Wang, Hongbo Cheng
hep-th