Eur. Phys. J. C (2013) 73: 2334
https://doi.org/10.1140/epjc/s10052-013-2334-9

Regular Article - Theoretical Physics

Friction forces in cosmological models

¹ Istituto per le Applicazioni del Calcolo “M. Picone,” CNR, 00185, Rome, Italy
² ICRA, University of Rome “La Sapienza”, 00185, Rome, Italy
³ INFN, Sezione di Firenze, 00185, Sesto Fiorentino (FI), Italy
⁴ Astronomical Observatory of Torino, INAF, 10025, Pino Torinese (TO), Italy
⁵ Physics Department, University of Rome “La Sapienza”, 00185, Rome, Italy
⁶ Physics Department, University of Stockholm, 106 91, Stockholm, Sweden
⁷ Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Am Mühlenberg 1, 14476, Potsdam, Germany

^* e-mail: binid@icra.it

Received: 12 December 2012
Revised: 30 January 2013
Published online: 26 February 2013

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.