Friction forces in cosmological models

TL;DR

This paper studies how friction forces affect particle motion in different cosmological models, revealing significant impacts in closed universes and minimal effects in open or flat universes.

Contribution

It introduces a Poynting-Robertson-like friction force into cosmological particle dynamics and analyzes its effects across various universe geometries and equations of state.

Findings

Friction causes particles in closed universes to slow from relativistic speeds to co-moving speeds.

In open or flat universes, friction has minimal impact on particle velocities.

Friction effects are significant mainly in closed FRW models, especially near recollapse.

Abstract

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.