  We investigate the dynamics of test particles undergoing friction forces in a
Friedmann-Robertson-Walker (FRW) spacetime. The interaction with the background
fluid is modeled by introducing a Poynting-Robertson-like friction force in the
equations of motion, leading to measurable (at least in principle) deviations
of the particle trajectories from geodesic motion. The effect on the peculiar
velocities of the particles is investigated for various equations of state of
the background fluid and different standard cosmological models. The friction
force is found to have major effects on particle motion in closed FRW
universes, where it turns the time-asymptotic value (approaching the
recollapse) of the peculiar particle velocity from ultra-relativistic (close to
light speed) to a co-moving one, i.e., zero peculiar speed. On the other hand,
for open or flat universes the effect of the friction is not so significant,
because the time-asymptotic peculiar particle speed is largely non-relativistic
also in the geodesic case.
