Cosmic rate equation and massive particle--antiparticle pair production

Abstract: In a Friedmann universe with a time-varying cosmological term, the production of massive particle--antiparticle pairs and their exchange of energy with the spacetime vacuum are described by a cosmic rate equation, $$\dotρ<em>M+3Hρ_M=Γ_M(ρ_M^{H-ρ_M)-Γ_M^{\rm} de}ρ_M$$. Together with the Friedmann equations, an energy-balance equation for the time-varying cosmological term, and a reheating equation for the radiation, it forms a closed set of four independent background equations. We derive this set from a complex scalar field with a global U(1) symmetry and a non-minimal coupling to curvature. In the adiabatic case, the occupation of non-relativistic modes gives $ρ_M<sup>H=2χm<sup>2H<sup>2$. A kinetic equation, based on the pairwise coupling $ξR|Φ|<sup>2$ to fluctuations of the spacetime vacuum, gives the relaxation rate $Γ_M$, and a Yukawa coupling gives the decay rate $Γ_M^{\rm de}$. In the fast-oscillating case, the Hubble function oscillates at twice the pair mass and locks in phase with the pairs, so that it exchanges no net energy with them. The energy of the created pairs is supplied instead by the decrease of the cosmological term in the time evolution, $$-\dotρ</em>Λ=[(3-2ε) H+Γ<em>M^{\rm de}]ρ_M<sup>H$$. Averaging over the fast time gives the cosmic rate equation in the detailed-balance regime $Γ_M\gg H$. In this regime the closed set has an exact power-law solution with $$ε=-\dot H/H^{2=χm^2/m}</em>{\rm pl}<sup>2$$. We discuss the application of the closed set to inflation, reheating and standard cosmology.