Gravitational, dilatonic and axionic radiative damping of cosmic strings

We study the radiation reaction on cosmic strings due to the emission of dilatonic, gravitational and axionic waves. After discussing the renormalization of the string tension induced by time-symmetric self-interactions (thereby correcting errors in the literature concerning the effect of the dilaton field), we concentrate on the finite radiation damping force associated with the halfretarded minus half-advanced “reactive” fields. We revisit a recent proposal of using a “local back reaction approximation” for the reactive fields. We find, contrary to previous claims, that this proposal leads to antidamping in the case of the axionic field, and to zero (integrated) damping in the case of the gravitational field. One gets normal positive damping only in the case of the dilatonic field. We propose to use a suitably modified version of the local dilatonic radiation reaction as a substitute for the exact (non local) gravitational radiation reaction. The incorporation of such a local approximation to gravitational radiation reaction should allow one to complete, in a computationally non intensive way, string network simulations and to give better estimates of the amount and spectrum of gravitational radiation emitted by a cosmologically evolving network of massive strings.