  A common feature in the thermodynamic analysis of homogeneous and isotropic
world models is the assumption that the temperature of the fluids inside the
cosmic horizon (including dark energy) coincides with the temperature of the
latter, whether it be either the event or the apparent horizon. We examine up
to what extent this assumption may be justified, given that these temperatures
evolve under different time-temperature laws. We argue that while radiation
cannot reach thermal equilibrium with the horizon, non-relativistic matter may,
and dark energy might though only approximately.
