Gravitationally induced particle production: Thermodynamics and kinetic theory

A relativistic kinetic description for the irreversible thermodynamic process of gravitationally induced particle production is proposed in the context of an expanding Friedmann-Robertson-Walker geometry. We show that the covariant thermodynamic treatment referred to as ``adiabatic'' particle production provoked by the cosmic time-varying gravitational field has a consistent kinetic counterpart. The variation of the distribution function is associated to a noncollisional kinetic term of quantum-gravitational origin which is proportional to the ratio Γ/H, where Γ is the gravitational particle production rate and H is the Hubble parameter. For ΓH the process is negligible and as should be expected it also vanishes (regardless of the value of Γ) in the absence of gravitation. The resulting nonequilibrium distribution function has the same functional form of equilibrium with the evolution laws corrected by the particle production process. The macroscopic temperature evolution law is also kinetically derived for massive and massless particles. The present approach points to the possibility of an exact (semiclassical) quantum-gravitational kinetic treatment by incorporating backreaction effects in the cosmic background.