Effect of dynamical friction on the motion of cosmic strings

As cosmic strings move through a fluid, their motion is damped by dynamical friction caused by their gravitational interaction with the surrounding matter. Both the center-of-mass motion and the oscillations of loops of string can be affected by dynamical friction. We have made a detailed estimate of this effect, and have used this estimate to derive a covariant equation of motion for a cos- mic string in an arbitrary radiation-dominated or matter-dominated fluid spacetime. The equation is valid even for string segments that move relativistically through the fluid. When applied to the very early Universe, as described by a Robertson-Walker spacetime, we find that, for loops whose scale is larger than the horizon, dynamical friction is a negligible effect compared to the conformal stretching caused by the universal expansion; and for loops that are small compared to the horizon, dynamical friction, in addition to being small compared to the conformal stretching, is also small compared to the dissipative effects of gravitational radiation emission.