How Efficient Is The Langacker-Pi Mechanism of Monopole Annihilation?
  We investigate the dynamics of monopole annihilation by the Langacker-Pi mechanism. We find taht considerations of causality, flux-tube energetics and the friction from Aharonov-Bohm scatteering suggest that the monopole annihilation is most efficient if electromagnetism is spontaneously broken at the lowest temperature ($T_{em} \approx 10^6 GeV$) consistent with not having the monopoles dominate the energy density of the universe. 
R. Holman, T.W.B. Kibble and Soo-Jong Rey