Cosmological particle production, causal thermodynamics, and inflationary expansion

Abstract

Combining the equivalence between cosmological particle creation and an effective viscous fluid pressure with the fact that the latter represents a dynamical degree of freedom within the secondorder Israel-Stewart theory for imperfect fluids, we reconsider the possibility of accelerated expansion in fluid cosmology. We find an inherent self-limitation for the magnitude of an effective bulk pressure which is due to adiabatic (isentropic) particle production. For a production rate which depends quadratically on the Hubble rate we confirm the existence of solutions which describe a smooth transition from inflationary to noninflationary behavior and discuss their interpretation within the model of a decaying vacuum energy density. An alternative formulation of the effective imperfect fluid dynamics in terms of a minimally coupled scalar field is given. The corresponding potential is discussed and an entropy equivalent for the scalar field is found.

Particle production and universal thermodynamics

“…In other words, the cosmic substratum is not a conventional dissipative fluid, rather a perfect fluid with varying particle number. This statement can clearly be understood for the simple case of adiabatic (i.e., isentropic) particle production as follows [34,35]Starting from the Gibb's relation [i.e., from Eq. (2)]…”

“…Now using the Einstein field equations (6), the condition for adiabatic process [i.e., Eq. (10)] can be stated as [34,35] …”

“…However, due to increase of perfect fluid particles, the phase space of the system is enlarged and as a result, there is entropy production. Also, the condition for isentropic process gives a simple relation between the bulk viscous pressure and the particle creation rate and hence the particle production rate is no longer a parameter, it becomes a dynamical variable [33,34].…”

Irreversible processes in inflationary cosmological models

“…In this work we have analyzed the irreversible processes within the framework of ordinary thermodynamics instead of extended thermodynamics (see for example [3,5,6,7,8,9]). The reason is that -apart from causality -there exist some thermodynamic systems in which ordinary and extended thermodynamics lead to the same results, like the problems concerning the propagation of low frequency waves in fluids and those related to the flow and the heat transfer in fluids in the continuum regime (see for example [24]).…”

“…Cosmological models are among the most important formulations that can be put under analysis when we combine the thermodynamics of irreversible processes with Einstein's general relativity [1,2,3,4,5,6,7,8,9,10]. Using these theories the different eras of our universe can be modeled; the subject received great attention: the starting point was to consider the hypothesis of homogeneity and isotropy in the form of the Robertson-Walker (RW) metric.…”