The influence of spin in a system of classical particles on the propagation
of gravitational waves is analyzed in the cosmological context of primordial
thermal equilibrium. On a flat Friedmann-Robertson-Walker metric, when the
precession is neglected, there is no contribution due to the spin to the
distribution function of the particles. Adding a small tensor perturbation to
the background metric, we study if a coupling between gravitational waves and
spin exists that can modify the evolution of the distribution function, leading
to new terms in the anisotropic stress, and then to a new source for
gravitational waves. In the chosen gauge, the final result is that, in the
absence of other kind of perturbations, there is no coupling between spin and
gravitational waves.Comment: 4 pages, to appear in Proceedings of the II Stueckelberg Workshop -
Int. J. Mod. Phys.