Thermodynamic equilibrium in the expanding universe

We show that a relativistic gas may be at "global" equilibrium in the expanding universe for any equation of state 0 < p ≤ ρ/3, provided that the gas particles move under the influence of a self-interacting, effective one-particle force in between elastic binary collisions. In the force-free limit we recover the equilibrium conditions for ultrarelativistic matter which imply the existence of a conformal timelike Killing vector