Presence Of Bulk Viscosity Research Articles

Bulk Viscosity Analysis with New Holographic Dark Energy in f(R,T) theory

This paper represents the viscous circumstances to imitate new holographic dark energy behaviour in modified f(R, T ) gravity theory with a flat Friedmann-Robertson-Walker model with bulk viscosity. Here we assume bulk viscosity to be directly proportional to Hubble's parameter (ζ ∝ H), i.e., ζ = ζ0H, where ζ is assumed to be bulk viscosity coefficient and ζ0 is positive constant and H is Hubble’s parameter. In this particular model, we consider f(R, T ) = R + λT , where R represents the Ricci scalar, λ as a constant and T as trace of the energy-momentum tensor. The focus of this article majorly revolves around the concept that the negative pressure induced by bulk viscosity could potentially act as the driving force behind dark energy, contributing to the expansion of the universe. We precisely determine the scale factor’s primary solution and examine all possible conditions: deceleration as well as acceleration which contributes to the universe’s evolution. Depending on the values of the viscous terms, it has been observed that the model shows the decelerated period as well as the accelerated period. Later, we examine the diagnostic parameters, statefinder {r, s} to distinguish our model from the other existing dark energy models. Thus, a modified theory of gravity can explain the dark energy phenomenon in the presence of bulk viscosity.

Late time cosmological solutions in f(R,T) gravity in a viscous universe

Considering the condition on conservation of energy–momentum tensor (EMT), we study late time cosmological solutions in the context of [Formula: see text] gravity (where [Formula: see text] and n are constants) in a flat Friedmann–Lemaître–Robertson–Walker (FLRW) spacetime. The present model discusses the case of a barotropic perfect fluid as the matter content of the Universe, along with the case when dissipative effects are taken into account. Briefly, assuming a single perfect fluid, we find that the mentioned model is not capable of presenting an observationally consistent picture of the late time accelerated expansion of the Universe; nevertheless, the model leads to admissible solutions when the bulk viscosity is included. In this regard, a consistent setting is found considering the Eckart, Truncated and Full Israel–Stewart theories which determine the behavior of bulk viscosity. In the presence of bulk viscosity, the behavior of the deceleration parameter (DP) shows that the underlying model can describe an acceptable evolution even if the isotropic pressure of matter content of the Universe is negligible.

Viscous effect in the late time evolution of phantom universe

We investigate the cosmological implications of a phantom dark energy model with bulk viscosity. We explore this model as a possible way to resolve the big rip singularity problem that plagues the phantom models. We use the latest type Ia supernova and Hubble parameter data to constrain the model parameters and find that the data favor a significant bulk viscosity over a non-constant potential term for the phantom field. We perform a dynamical analysis of the model and show that the only stable and physical attractor corresponds to a phantom-dominated era with a total equation of state that can be greater than -1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-1$$\\end{document} due to the viscosity. We also study the general effect of viscosity on the phantom field and the late time evolution of the universe. We apply the statefinder diagnostic to the model and find that it approaches a nearby fixed point asymptotically, indicating that the universe can escape the big rip singularity with the presence of bulk viscosity. We conclude that bulk viscosity can play an important role in affecting the late-time behavior as well as alleviating the singularity problem of the phantom universe.

Realization of Bounce in a Modified Gravity Framework and Information Theoretic Approach to the Bouncing Point

In this work, we report a study on bouncing cosmology with modified generalized Chaplygin Gas (mgCG) in a bulk viscosity framework. Reconstruction schemes were demonstrated in Einstein and modified f(T) gravity framework under the purview of viscous cosmological settings. We also took non-viscous cases into account. We studied the equation of state (EoS) parameter under various circumstances and judged the stability of the models through the sign of the squared speed of sound. We observed the mgCG behaving like avoidance of Big Rip in the presence of bulk viscosity at the turnaround point and in non-viscous cases, a phantom-like behavior appears. The turnaround point equation of state parameter crosses the phantom boundary, violating NEC. The role of the mgCG’s model parameters was also investigated before and after the bounce. A Hubble flow dynamics was carried out and, it was revealed that mgCG is capable of realizing an inflationary phase as well as an exit from inflation. An f(T) gravitational paradigm was also considered, where the mgCG density was reconstructed in the presence of bulk viscosity. The role of the parameters associated with the bouncing scale factor, describing how fast the bounce takes place, was also studied in this framework. Finally, the reconstructed mgCG turned out to be stable against small perturbations irrespective of the presence of bulk viscosity and modified gravity scenario. Finally, the reconstruction scheme was assessed using statistical analysis, Shannon entropy.

The interaction of extended Bose–Einstein condensate dark matter with viscous f(T,B) gravity

In this paper, we study the viscous $f(T, B)$ gravity model as a source of dark energy, and the Extended Bose-Einstein Condensate (EBEC) as a source of dark matter, in a flat-FRW metric. In the presence of bulk viscosity, we obtain Friedmann equations and write two continuity equations of dark energy and dark matter by interacting them. Using the generalized Gross-Pitaeveskii equation, we earn Equation of State (EoS) of dark matter by EBEC regime as $p_m = \alpha \rho_{m} + \beta \rho_{m}^2$ in which the both of terms are respectively introduced as normal dark matter and dark matter halo. The innovation of the work is that we can simultaneously describe the nature of the dark parts of the universe with the viscous $f(T, B)$ gravity and the EBEC regime, which leads to a deep understanding of the different epochs of the universe from early to late times. In what follows, the energy density and the pressure of dark energy are reconstructed in terms of the redshift parameter, and then we fit the obtained results with 53 supernova data from the Hubble data constraints. Next, we plot the cosmological parameters in terms of the redshift parameter and conclude that the current universe is in an accelerated phase. Finally, we analyze the stability and instability of the current model with the sound speed parameter as well as we draw the density parameter values for dark energy in terms of the redshift parameter.

Hybrid expansion law in viscous braneworld gravity with Gauss–Bonnet terms

The cosmological model executing hybrid expansion law (HEL) of scale factor in Randall–Sundrum type II (RS) braneworld gravity with Gauss–Bonnet terms in the presence of bulk viscosity described by Eckart theory, Truncated Israel–Stewart (TIS) theory, Full Israel–Stewart (FIS) theory and nonlinear Israel–Stewart (nIS) theory is studied. In the hybrid expansion law, the scale factor of the universe is described by the product of power-law and exponential expansions. In the HEL model, the early inflation and its transition from deceleration phase to present accelerated phase of expansion can be explored. The constraints of hybrid expansion law model parameters are determined using the recent observational data. Thereafter, the estimated parameters are considered to explore the present value of deceleration parameter, jerk parameter and transition epoch from early deceleration to the present accelerating phase.

Viscous generalised Chaplygin gas under the purview of f(T) gravity and the model assessment through probabilistic information theory

In this work we have reported a study on the generalized Chaplygin gas (GCG) in the presence of bulk viscosity. Reconstruction schemes have been demonstrated in Einstein and modified f(T) gravity framework 4(T is the torsion scalar) in interacting scenario and under the purview of viscous cosmological settings. Non-viscous cases have also been taken into account. The equation of state (EoS) parameter has been studied under the various circumstances and the stability of the models has been judged through the sign of squared speed of sound. The GCG interacting with pressure-less dark matter has been found to be behaving like quintom in presence of bulk viscosity and in non-viscous case a quintessence like behaviour has been reported. Finally, the reconstructed GCG comes out to be stable against small perturbations irrespective of the presence of bulk viscosity. Finally, the reconstruction scheme has been assessed using statistical analysis, Shannon entropy and Gaussian Mixture Model.

Bulk Viscous Bianchi Type-V Cosmological Model in f(R, T) Theory of Gravity

This paper deals with the bulk viscous Bianchi type-V cosmological model with an exponential scale factor in Lyra geometry based on f(R, T) gravity, by considering a time dependent displacement field. To determine the nature and physical properties of the model, we considered Harko et al. (Harko et al., Phys. Rev. D, 2011, 84, 024020) [proposed the linear form f(R, T) = f1(R) + f2(T)], in which the barotropic equation of state for pressure, density, and bulk viscous pressure is proportional to energy density. The kinematical properties of the model are also discussed in the presence of bulk viscosity. Evolution of energy conditions is also studied and examined the behaviour of that in examined in order to explain the late-time cosmic acceleration.

COSMIC INFLATION IN BIANCHI TYPE IX SPACE WITH BULK VISCOSITY

In present paper we have constructed the Bianchitype-IX inflationary universe under framework of the effect of bulk viscosity and flat potential. To developed inflationary model we have consider the supplementary criteria that shear coefficient is directly proportional to expansion scalar which provides an appropriate relation with coefficients of metric $b=a^n$ where n is non-negative constant other than one.We conclude that rate of Higgs field decreases with time and proper volume V is increasing function of time which indicates that universe is expanded continuously. To find solutions of fields equations, we assume $ \xi \theta = \alpha$ (constant) as given by Brevik et al. where $\xi $is coefficient of bulk viscosity and $\theta$ expansion in model. The presence of bulk viscosity provides inflationary solution in current model. The model isotropize in special case. The cosmological parameters of model are also studied.

Observational constraints and stability in viscous gravity

In this paper, we study the [Formula: see text] gravity model in the presence of bulk viscosity by the flat Friedmann–Robertson–Walker metric. The field equation is obtained by teleparallel gravity with a tetrad field. The universe components are considered matter and dark energy, with the dark energy component associated with viscous [Formula: see text] gravity. After calculating the Friedmann equations, we obtain the energy density, pressure, and equation of state of dark energy in terms of the redshift parameter. Afterward, we plot the corresponding cosmological parameters versus the redshift parameter and examine the accelerated expansion of the universe. In the end, we explore the system stability using a function called the speed sound parameter.

Modified holographic energy density-driven inflation and some cosmological outcomes

Motivated by the work of Nojiri et al. [S. Nojiri, S. D. Odintsov and E. N. Saridakis, Holographic inflation, Phys. Lett. B 797 (2019) 134829], this study reports a model of inflation under the consideration that the inflationary regime is originated by a type of holographic energy density. The infrared cutoff has been selected based on the modified holographic model that is a particular case of Nojiri–Odintsov holographic dark energy [S. Nojiri and S. D. Odintsov, Unifying phantom inflation with late-time acceleration: Scalar phantom–non-phantom transition model and generalized holographic dark energy, Gen. Relativ. Gravit. 38 (2006) 1285] that unifies phantom inflation with the acceleration of the universe on late time. On getting an analytical solution for Hubble parameter we considered the presence of bulk viscosity and the effective equation of state parameter appeared to be consistent with inflationary scenario with some constraints. It has also being observed that in the inflationary scenario the contribution of bulk viscosity is not of much significance and its influence is increasing with the evolution of the universe. Inflationary observables have been computed for the model and the slow-roll parameters have been computed. Finally, it has been observed that the trajectories in [Formula: see text] are compatible with the observational bound found by Planck. It has been concluded that the tensor to scalar ratio for this model can explain the primordial fluctuation in the early universe as well.

Cosmology of a generalised version of holographic dark energy in presence of bulk viscosity and its inflationary dynamics through slow roll parameters

This study reports a reconstruction scheme of a Dark Energy (DE) model with higher-order derivative of Hubble parameter, which is a particular case of Nojiri–Odintsov holographic DE (HDE) [S. Nojiri and S. D. Odintsov, Gen. Relativ. Gravit. 38 (2006) 1285.] that unifies phantom inflation with the acceleration of the universe on late-time. The reconstruction has been carried out in the presence of bulk viscosity, where the bulk-viscous pressure has been taken as a function of Hubble parameter. Ranges of cosmic time [Formula: see text] have been derived for quintessence, cosmological constant and phantom behaviour of the equation-of-state (EoS) parameter. In the viscous scenario, the reconstruction has been carried out in an interacting and noninteracting situations and in both the cases stability against small perturbations has been observed. Finally, the slow roll parameters have been studied and a scope of exit from inflation has been observed. Also, the availability of quasi-exponential expansion has been demonstrated for interacting viscous scenario and a study through tensor-to-scalar ratio has ensured consistency of the model with the observational bound by Planck. Alongwith primordial fluctuations, the interacting scenario has been found to generate strong dissipative regime.

New holographic dark energy model with bulk viscosity in f(R, T) gravity

In this paper, we present the bulk viscous phenomena to mimic new holographic dark energy behavior in modified f(R, T) gravity. We assume the bulk viscosity coefficient, $$\zeta$$ in the form of $$\zeta = \zeta_{0} + \zeta_{1} H$$ , where $$\zeta_{0} , \zeta_{1}$$ are positive constants and H is the Hubble parameter. We obtain the exact solution of the scale factor and classify all the possible scenarios (deceleration, acceleration and their transition) of the evolution of the Universe. It is observed that the model transits from the decelerated phase to accelerated phase in early or late time depending on the values of viscous terms. For large values of viscous terms, the model always accelerates throughout the evolution. Furthermore, we also analyze two diagnostic parameters, statefinder $$\left\{ {r,s} \right\}$$ and Om to discriminate our model with the other existing dark energy models. The evolutions of these diagnostics are shown in $$r - s,r - q$$ and Om − z planes. It is found that the behavior of trajectories in different planes depends on the bulk viscous coefficients. A small combination of $$\zeta_{0}$$ and $$\zeta_{1}$$ gives the quintessence-like behavior, whereas a large combination gives Chaplygin gas-like model. However, the model approaches the ΛCDM model in the late time evolution of the Universe. The value of the effective equation of state parameter comes to be − 0.9745 which is very close to the observational data. The entropy and generalized second law of thermodynamics are valid under certain constraints. The analysis shows that the dark energy phenomena may be explained in the presence of bulk viscosity in the modified theory of gravity.

A holographic reconstruction scheme for f(R) gravity and the study of stability and thermodynamic consequences

The present paper reports a study of the reconstruction of f(R) gravity from holographic Ricci dark energy, a specific case of Nojiri–Odintsov holographic DE (ref Nojiri and Odinstov, 2005a), in the presence of bulk viscosity. The reconstructed equation of state (EoS) parameter wR was found to have a transition from quintessence to phantom in the absence of the bulk viscosity. Thus, the behaviour of wR for this reconstruction scheme was identified as “quintom” in the absence of the bulk viscosity. However, in the case of the bulk viscous scenario wR behaves like “quintessence”. Furthermore, the generalized second law of thermodynamics was tested in the case of the reconstructed f(R) gravity. Finally, the stability of this model against small perturbations was demonstrated with the aid of squared speed of sound.

A study of the bulk viscous pressure in scalar fields and holographic Ricci dark energy considered in the modified gravity framework

The work presented in this paper reports a rigorous study of the reconstruction of the modified gravity in the framework of the scalar field models of dark energy and holographic Ricci dark energy, a generalized version of the holographic dark energy presented in S. Nojiri and S.D. Odintsov. Gen. Relativ. Gravitation, 38, 1285 (2006). The tachyon and quintessence scalar fields have been considered and the cosmology associated with the presence of bulk viscosity has been studied. In the first part of our study, we have demonstrated the behaviour of the bulk viscosity coefficient in the framework of the reconstructed tachyon scalar field model of dark energy. The scale factor is chosen in the form a(t) = a0tβ, where β > 0. Two scalar field models, namely, tachyon and quintessence, have been considered in the framework of the modified field equations through incorporation of the bulk viscous pressure. The reconstructed density and pressure of the scalar field models have been explored for the cosmological consequences in the presence of bulk viscosity. The behaviour of the effective equation of state parameters has been investigated. Finally, we have reconstructed f(T) gravity in the presence of holographic Ricci dark energy and a transition of the effective equation of state parameter from quintessence to phantom has been observed.

Bulk viscous Bianchi-V cosmological model within the formalism of $f(R,T)=f_{1}(R)+f_{2}(R)f_{3}(T) $ gravity

In this paper, we have studied transitioning Bianchi-V cosmological universe with bulk viscous matter within formalism of $f(R,T) $ gravity. The negative pressure which is generated by bulk viscosity may act as dark energy component and can drive accelerated expansion of universe. This is the main reason for taking into account the bulk viscosity in the present model. To describe the simplest coupling between matter and geometry using specific model $f(R,T)=f_{1}(R)+f _{2}(R)f_{3}(T) $ in Bianchi-V space-time; where $f_{1}(R)=f_{2}(R)=R $ and $f_{3}(T)=\gamma T $ , $\gamma $ is a constant. The dynamic bulk viscosity coefficient $\xi $ expected to enhance the rate of expansion is taken as $\xi =\frac{p^{(\mathit{eff})}-\bar{p}^{(\mathit{eff})}}{3 H} $ , where $\bar{p}^{(\mathit{eff})} $ is effective bulk viscous pressure and $p^{(\mathit{eff})} $ is normal fluid pressure. The bulk viscosity increases, which in turn leads to rise in negative pressure, that shows accelerated expansion. The equation of state is taken as $p^{(\mathit{eff})}=\zeta \rho ^{(\mathit{eff})} $ , where $\zeta $ is a constant whose value lies between 0 and 1. We assume hybrid scale factor $a= \{t^{m}\exp (\lambda t) \} ^{\frac{1}{n}} $ (where $m, \lambda $ and $n $ are constants) to obtained deterministic solutions of Einstein’s field equations. It has been observed that in the presence of bulk viscosity, the solutions are more stable and explains well the physical behavior of the phase transition of universe and also confirms the stability of the model through validation of energy conditions.

Bulk viscous Bianchi-I embedded cosmological model in f(R,T) = f1(R) + f2(R)f3(T) gravity

In this paper, we have searched the existence of bulk viscous Bianchi-I embedded cosmological model in [Formula: see text] gravity by taking into account the simplest coupling between matter and geometry such that [Formula: see text] and [Formula: see text] with [Formula: see text] a constant. To obtain the deterministic solution, we assume the scale factor in hybrid form — product of power law and exponential type of function that describes a model of transitioning universe and explain the physical behavior of early universe as well as present universe. It has been found that in the presence of bulk viscosity, the solution is stable and in favor of realization of the energy conditions (ECs) which turn into imply the derived model agrees with the cosmological viability. Some physical aspects of the model have been also discussed.

Bulk viscous cosmological model in f(R,T) theory of gravity

In this paper, we have presented bulk viscous cosmological model of the universe in the modified gravity theory in which the Lagragian of the gravitational action contains a general function [Formula: see text], where [Formula: see text] and [Formula: see text] denote the curvature scalar and the trace of the energy–momentum tensor, respectively, in the framework of a flat Friedmann–Robertson–Walker model with isotropic fluid. We obtain cosmological solution in [Formula: see text] theory of gravity, specially of particular choice [Formula: see text], where [Formula: see text] is a constant, in the presence of bulk viscosity that are permitted in Eckart theory, Truncated Israel Stewart (TIS) theory and Full Israel Stewart (FIS) theory. The physical and geometrical properties of the models in Eckart, TIS theory and FIS theory are studied in detail. The analysis of the variation of bulk viscous pressure, energy density, scale factor, Hubble parameter and deceleration parameter with cosmic evolution is done in the respective theories. The models are analyzed by comparison with recent observational data. The cosmological models are compatible with observations.

Emergent universe model with dissipative effects

Emergent universe model is presented in general theory of relativity with isotropic fluid in addition to viscosity. We obtain cosmological solutions that permit emergent universe scenario in the presence of bulk viscosity that are described by either Eckart theory or Truncated Israel Stewart (TIS) theory. The stability of the solutions are also studied. In this case, the emergent universe (EU) model is analyzed with observational data. In the presence of viscosity, one obtains emergent universe scenario, which however is not permitted in the absence of viscosity. The EU model is compatible with cosmological observations.

A study on the bouncing behavior of modified Chaplygin gas in presence of bulk viscosity and its consequences in the modified gravity framework

The present paper reports a study on the bouncing behavior of the viscous modified Chaplygin gas (MCG) in Einstein as well as modified gravity framework. For a bouncing scale factor proposed by Cai et al., Class. Quantum Grav. 28 (2011) 215011, we have studied the cosmology of MCG in presence of bulk viscosity. In Einstein gravity framework, we have studied the equation of state parameter and it has been found to cross [Formula: see text] indicating the end of the early accelerated expansion and it has also been observed that for flat FRW universe the presence of bulk viscosity induces the crossing of phantom boundary. Role of the model parameters of the MCG has also been investigated before and after the bounce. A Hubble flow dynamics has been carried out and, it was revealed that MCG is capable of realizing inflationary phase as well as an exit from inflation. A [Formula: see text] gravitational paradigm has also been considered, where the MCG density has been reconstructed in presence of bulk viscosity. Role of [Formula: see text] of the bouncing scale factor, describing how fast the bounce takes place, has also been studied in this framework.