Acceleration field of a Universe modeled as a mixture of scalar and matter fields

TL;DR

This paper models the Universe as a mixture of scalar and matter fields, analyzing its acceleration phases through irreversible particle creation processes and scalar field dynamics with an exponential potential.

Contribution

It introduces a cosmological model combining scalar and matter fields with irreversible particle creation, capturing multiple acceleration and deceleration epochs of the Universe.

Findings

The model reproduces inflationary, decelerated, and current accelerated phases.

It shows transitions between scalar field and matter field dominance.

The scalar field's exponential potential influences the Universe's acceleration behavior.

Abstract

A model of the Universe as a mixture of a scalar (inflaton or rolling tachyon from the string theory) and a matter field (classical particles) is analyzed. The particles are created at the expense of the gravitational energy through an irreversible process whereas the scalar field is supposed to interact only with itself and to be minimally coupled with the gravitational field. The irreversible processes of particle creation are related to the non-equilibrium pressure within the framework of the extended (causal or second-order) thermodynamic theory. The scalar field (inflaton or tachyon) is described by an exponential potential density added by a parameter which represents its asymptotic value and can be interpreted as the vacuum energy. This model can simulate three phases of the acceleration field of the Universe, namely,(a) an inflationary epoch with a positive acceleration followed by a decrease of the acceleration field towards zero, (b) a past decelerated period where the acceleration field decreases to a maximum negative value followed by an increase towards zero, and (c) a present accelerated epoch. For the energy densities there exist also three distinct epochs which begin with a scalar field dominated period followed by a matter field dominated epoch and coming back to a scalar field dominated phase.