Average Thermal Evolution of the Universe
  We sketch the average behaviour of the temperatures and densities of the main
components of the $\Lambda$CDM universe after inflation. It is modelled as a
perfect fluid with dark energy associated with the macroscopic effect of
conformal variations of the metric. The main events of the thermal evolution
are studied, such as the effect of particle annihilations and decoupling, and
the transitions between the eras dominated by different entities. Estimates of
the average present epoch temperature of baryonic matter and dark matter
composed of neutralinos are given. We study the eventual presence of a sterile
neutrino component and find that the sterile neutrino density at the epoch of
primordial nucleosynthesis is in agreement with expectations when their
evolution starts, at the end of inflation, in temperature equilibrium with the
rest of the universe.
