  Particle production processes in the expanding universe are described within
a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas
with variable particle number are investigated. We find that radiation and
nonrelativistic matter may be in equilibrium at the same temperature provided
the matter particles are created at a rate that is half the expansion rate.
Using the fact that the creation of particles is dynamically equivalent to a
nonvanishing bulk pressure we calculate the backreaction of this process on the
cosmological dynamics. It turns out that the `adiabatic' creation of massive
particles with an equilibrium distribution for the latter necessarily implies
power-law inflation. Exponential inflation in this context is shown to become
inconsistent with the second law of thermodynamics after a time interval of the
order of the Hubble time.
