COLLISIONAL EQUILIBRIUM, PARTICLE-PRODUCTION, AND THE INFLATIONARY UNIVERSE

Particle production processes in the expanding universe are described within a simple kinetic model. The equilibrium conditions for a Maxwel l-Boltzmann gas with variable particle number are investigated. We fin d that radiation and nonrelativistic matter may be in equilibrium at t he same temperature provided the matter particles are created at a rat e that is half the expansion rate. Using the fact that the creation of particles is dynamically equivalent to a nonvanishing bulk pressure w e calculate the back reaction of this process on the cosmological dyna mics. It turns out that the 'adiabatic' creation of massive particles with an equilibrium distribution for the latter necessarily implies po wer-law inflation. Exponential inflation in this context is shown to b ecome inconsistent with the second law of thermodynamics after a time interval of the order of the Hubble time.