Scalar particle production in a simple Horndeski theory

Abstract

The scalar particle production through a scalar field nonminimally coupled with geometry is investigated in the context of a spatially homogeneous and isotropic Universe. In this paper, to study the evolution of particle production over time in the case of analytical solutions, we focus on a simple Horndeski theory. We first suppose that the Universe is dominated by a scalar field and derive the energy conservation condition. Then, from the thermodynamic point of view, the macroscopic nonconservation of the scalar field energy-momentum tensor can be explained as an irreversible production of the scalar particles. Based on the explanation, we obtain a scalar particle-production rate and the corresponding entropy. Finally, since the Universe, in general, could be regarded as a closed system satisfying the laws of thermodynamics, we naturally impose some thermodynamic constraints on it. The thermodynamic properties of the Universe can provide additional constraints on the simple Horndeski theory.