Friedmann model with viscous cosmology in modified $$f(R,T)$$ f ( R , T ) gravity theory

Abstract

In this paper, we introduce the bulk viscosity in the formalism of modified gravity theory in which the gravitational action contains a general function f (R, T ), where R and T denote the curvature scalar and the trace of the energy-momentum tensor, respectively, within the framework of a flat Friedmann-Robertson-Walker model. As an equation of state for a prefect fluid, we take p = (γ − 1)ρ, where 0 ≤ γ ≤ 2 and a viscous term as a bulk viscosity due to the isotropic model, of the form ζ = ζ 0 + ζ 1 H , where ζ 0 and ζ 1 are constants, and H is the Hubble parameter. The exact non-singular solutions to the corresponding field equations are obtained with non-viscous and viscous fluids, respectively, by assuming a simplest particular model of the form of f (R, T ) = R + 2 f (T ), where f (T ) = αT (α is a constant). A big-rip singularity is also observed for γ < 0 at a finite value of cosmic time under certain constraints. We study all possible scenarios with the possible positive and negative ranges of α to analyze the expansion history of the universe. It is observed that the universe accelerates or exhibits a transition from a decelerated phase to an accelerated phase under certain constraints of ζ 0 and ζ 1 . We compare the viscous models with the non-viscous one through the graph plotted between the scale factor and cosmic time and find that the bulk viscosity plays a major role in the expansion of the universe. A similar graph is plotted for the deceleration parameter with non-viscous and viscous fluids and we find a transition from decelerated to accelerated phase with some form of bulk viscosity.

Configurational entropy in $$f\,(R,T\,)$$ f ( R , T ) brane models

“…We emphasize that the f (R, T ) modified theory of gravity has been analyzed in the literature in several scenarios. For instance, we can find studies in cosmological models [6][7][8][9][10]49], in quantum chromodynamics [50], in a Bianchi Type-II string with magnetic field [51], and in thermodynamics [52][53][54][55].…”

“…Note that f (R) and f (R, T ) cosmological models [2][3][4][5][6][7][8][9][10], with R and T being, respectively, the Ricci scalar and the trace of the energy-momentum tensor, are able to describe the cosmic acceleration our universe is undergoing [11,12] with no need of invoking a cosmological constant. On the other hand, the hierarchy problem can be solved by assuming that our observable universe is a sub-manifold which is embedded in an anti-de Sitter five-dimensional space (AdS 5 ), called the bulk, as in the Randall-Sundrum braneworld model [13].…”

Synthesis and characterization of conductive composite films of polyisoprene/PA12@ PANI

“…In these media, conductivity is because of hopping of charge carriers between localized states . Similar behavior has been observed in various conducting polymers and their composites, and in other conducting composites . The conductivity is suitably described by Jonscher's universal power law: …”

“…This behavior is clearly noticed on Figure ‐a showing the relationship between dc conductivity and weight concentrations of PA12@PANI. The evaluated values of dc conductivity for all examined samples were extracted from the frequency independent plateau by extrapolating data to the static frequency . An increase of the blend films conductivity is observed with the increase of the PA12@PANI weight percentage.…”

Cosmological viable mimetic f(R) and f(R, T) theories via Noether symmetry

“…Because of simplicity and beauty form, this theory attracted several activities in literature [18]- [33]. Symmetry is an important issue to be addressed in any physical system under study.…”