Vacuum Fluctuations of a massless spin-1/2 field around multiple cosmic   strings
  We study the interaction of a massless quantized spinor field with the gravitational filed of N parallel static cosmic strings by using a perturbative approach. We show that the presence of more than one cosmic string gives rise to an additional contribution to the energy density of vacuum fluctuations, thereby leading to a vacuum force attraction between two parallel cosmic strings. 
A.N. Aliev, M. Hortacsu and N. Ozdemir