We show that a relativistic gas may be at ``global'' equilibrium in the
expanding universe for any equation of state 0<pβ€Ο/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.Comment: 10 pages, Latex, to appear in GR