Semiclassical backreaction around a nearly spinning cosmic string

This paper investigates semiclassical backreaction of a conformally coupled massless scalar field on the geometrical background of a nearly spinning cosmic string - the spin density is smaller than, but arbitrarily close to, the dislocation parameter. As the spin density approaches the dislocation parameter, it is shown that an "ergoregion" spreads indefinitely around the cosmic string, boosting along the string axis the once static observers. Considering that the geometrical background contains closed timelike curves when the spin density exceeds the dislocation parameter, it is argued that the appearance of the "ergoregion" may be part of a chronology protection mechanism that takes place in related non stationary geometries.Comment: 6 pages, no figures, REVTeX4 file. This version provides a clearer discussion stressing that the ergoregion is a coordinate independent effect. Misleading sentences regarding this matter are amende