Bulk Viscous Model Research Articles

Reconsidering bulk viscosity in Brans–Dicke theory

In this paper, we reconsider the concept of bulk viscosity in Brans–Dicke theory to study accelerated expansion of the universe in a flat Friedmann–Robertson–Walker metric. In recent works on bulk viscosity in Brans–Dicke theory, the power-law form of Brans–Dicke scalar field has been taken to explain recent phase transition of the universe, however, power-law form confronts constant deceleration parameter problem. Therefore, we consider recently proposed well-motivated logarithmic form of Brans–Dicke scalar field to discuss bulk viscous model in Brans–Dicke theory. We obtain the values of deceleration parameter and equation of state parameter for the model, and plot their graphs against the redshift parameter z. The plots show that the model is able to explain the recent phase transition of the universe and equation of state parameter shows the behavior of dark energy. Further, we apply two important analysis, namely, [Formula: see text] analysis and thermodynamic analysis to examine our model. The [Formula: see text] analysis shows that our model belong to thawing region and shows similarities with quintessence model. All the trajectories start from the point [Formula: see text] with different negative values of w having quintessence behavior. The thermodynamic analysis shows that the model satisfies generalized second law of thermodynamics.

Viscous Fluid Cosmology in Symmetric Teleparallel Gravity

AbstractIn this article, the viscous fluid cosmological model is analyzed in the framework of recently proposed gravity by assuming three different forms of bulk viscosity coefficients, specifically, , , and and a linear model, particularly, where is free model parameter. The bulk viscosity coefficients and the model parameter values using the combined H(z)+Pantheon+BAO data set are estimated. The asymptotic behavior of our cosmological bulk viscous model is studied by utilizing the phase space method. It is found that corresponding to all three cases, our model depicts the evolution of the universe from matter dominated decelerated epoch (a past attractor) to a stable de‐sitter accelerated epoch (a future attractor). Furthermore, the physical behavior of effective pressure, effective equation of state (EoS), and the statefinder parameters are studied. It is also found that the pressure component in the presence of bulk viscosity shows negative behavior and the effective EoS parameter predicts the accelerated expansion phase of the universe for all three cases. Moreover, the trajectories of the model lie in the quintessence region and it converges to the ΛCDM fixed point in the far future. It is found that the accelerated de‐Sitter like phase comes purely from the case without any geometrical modification to GR. Moreover, the late‐time behavior of all three cases of viscosity coefficients are identical. Furthermore, a non‐linear model is considered, specifically, , and then analyzed the behavior of model using dynamical approach. It is also found that the late‐time behavior of the considered non‐linear model with is similar to the linear case, whereas for the case results are quite different.

Contrasting the bulk viscous model with the standard $\Lambda$CDM using Bayesian statistics

The causal dissipative model is a plausible choice to explain the late accelerated epoch of the universe. At the same time, \(\Lambda\)CDM is considered the standard model to explain the cosmological data corresponding to the late evolution of the universe. We consider a bulk viscous model in which dissipation is driven by the bulk viscosity \(\zeta =\alpha \rho ^{1/2}\), described using the full causal Israel-Stewart theory. We computed the model parameters using the latest observational data (Pantheon). We contrasted this model with \(\Lambda\)CDM model for the late accelerated phase using the Bayesian inference method. The Bayes factor was obtained by calculating the likelihood for Pantheon data. Suitable prior values were assumed for model parameters to calculate the likelihood. It shows that the evidence for \(\Lambda\)CDM against this viscous model is very strong according to the Jeffreys scale.

Bianchi Type V Universe and Bulk Viscous Models with Time Dependent Gravitational Constant and Cosmological Constant in General Relativity

This paper deals with Bulk Viscous Models and Bianchi type- universe in the standard general relativity theory by assuming where constants, is the density. Einstein field equations ( ) have been solved in the presence of a variable gravitational constant as well as a variable cosmological constant together with a bulk viscous fluid. In order to find a deterministic solution of EFEs, a simple form of Hubble parameter being constant when where has been considered here that shows the signature flip from early deceleration to present acceleration. Moreover, the bulk viscous coefficient is allowed to vary with the density as . The physical and geometrical behavior are described in detail for the obtained model. The model obtained here is in good agreement with the present cosmological observations.

Challenging bulk viscous unified scenarios with cosmological observations

In a spatially flat Friedmann-Lema\^{i}tre-Robertson-Walker universe, we investigate a unified cosmic fluid scenario endowed with bulk viscosity in which the coefficient of the bulk viscosity has a power law evolution. The power law in the bulk viscous coefficient is a general case in this study which naturally includes several choices as special cases. Considering such a general bulk viscous scenario, in the present work we have extracted the observational constraints using the latest cosmological datasets and examine their behaviour at the level of both background and perturbations. From the observational analyses, we find that a non-zero bulk viscous coefficient is always favored and some of the models in this series are able to weaken the current tension on $H_0$ for some dataset. However, from the Bayesian evidence analysis, $\Lambda$CDM is favored over the bulk viscous model.

Bulk viscous fluid cosmological models in f(R, T) gravity

In the literature, a number of modified theories have been discussed to explain early and late time expansion of the universe. In this context, f(R) gravity (R being the Ricci scalar) is the simplest and most popular modification of general relativity. Other modified theories like f(T) gravity, where T is the torsion has been proposed to overcome many problems in cosmology. Kaluza–Klein cosmological models in f(R, T) theory of gravity are investigated when the source of gravitation is the bulk viscous fluid. In this work, it is considered that the function f(R, T) as f(R,T)=R+2f(T) or f(R,T)=f1(R)+f2(T). Such a choice of the functional f(R, T) leads to an evolving effective cosmological constant Λ, which depends on the stress energy tensor. f1(R) and f2(T) are arbitrary functions of R and T, respectively and f(R, T) = R f(T) may be considered to solve the field equations. We however do not consider the general case, but restrict ourselves to the following form f(R, T) = R + 2f(T), where f(T) is an arbitrary function of the trace of energy-momentum tensor of matter. The term 2f(T) in the gravitational action modified the gravitational interaction between matter and curvature. It is observed that the first case of the model be reduced to effective stiff fluid model of the universe while the second case gives general bulk viscous model of the universe. The exact solutions of the field equations are obtained. Keeping an eye on the accelerating nature of the universe in the present epoch, the dynamics and physical behaviour of the models are discussed.

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

We have constructed Locally Rotationally Symmetric Bianchi type I (LRSBI) cosmological models in the f(R,T) theory of gravity when the source of gravitation is the bulk viscous fluid. The models are constructed for f(R,T)=R+2f(T) and f(R,T)=f1(R)+f2(T). We found that in the first case the model degenerates into effective stiff fluid model of the universe. In the second case we obtained degenerate effective stiff fluid model as well as general bulk viscous models of the universe. Some physical and kinematical properties of the models are also discussed.

Evolution of the universe in entropic cosmologies via different formulations

We study two types of entropic-force models in a homogeneous, isotropic, spatially flat, matter-dominated universe. The first type is a `$\Lambda(t)$ type' similar to $\Lambda(t)$CDM (varying-lambda cold dark matter) models in which both the Friedmann equation and the acceleration equation include an extra driving term. The second type is a `BV type' similar to bulk viscous models in which the acceleration equation includes an extra driving term whereas the Friedmann equation does not. In order to examine the two types systematically, we consider an extended entropic-force model that includes a Hubble parameter ($H$) term and a constant term in entropic-force terms. The $H$ term is derived from a volume entropy whereas the constant term is derived from an entropy proportional to the square of an area entropy. Based on the extended entropic-force model, we examine four models obtained from combining the $H$ and constant terms with the $\Lambda(t)$ and BV types. The four models agree well with the observed supernova data and describe the background evolution of the late universe properly. However, the evolution of first-order density perturbations is different in each of the four models, especially for low redshift, assuming that an effective sound speed is negligible. The $\Lambda(t)$ type is found to be consistent with the observed growth rate of clustering, in contrast with the BV type examined in this study. A unified formulation is proposed as well, in order to examine density perturbations of the two types systematically.

Spherical collapse for unified dark matter models

We study the nonlinear spherical ``top hat'' collapse for Chaplygin and viscous unified cosmologies. The term unified refers to models where dark energy and dark matter are replaced by one single component. For the generalized Chaplygin gas (GCG), we extend previous results of R. A. A. Fernandes et al., [Phys. Rev. D 85, 083501 (2012)]. We discuss the differences at nonlinear level between the GCG with $\ensuremath{\alpha}=0$ and the $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model. We show that both are indeed different. The bulk viscous model which differs from the GCG due to the existence of nonadiabatic perturbations is also studied. In this case, the clustering process is in general suppressed, and the viable parameter space of the viscous model that accelerates the background expansion does not lead to collapsed structures. This result challenges the viability of unified viscous models.

VISCOUS DARK FLUID UNIVERSE: A UNIFIED MODEL OF THE DARK SECTOR?

The Universe is modeled as consisting of pressureless baryonic matter and a bulk viscous fluid which is supposed to represent a unified description of the dark sector. In the homogeneous and isotropic background the total energy density of this mixture behaves as a generalized Chaplygin gas. The perturbations of this energy density are intrinsically nonadiabatic and source relative entropy perturbations. The resulting baryonic matter power spectrum is shown to be compatible with the 2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also Hubble-function and supernovae Ia data, shows that, different from other studies, there exists a maximum in the probability distribution for a negative present value of the deceleration parameter. Moreover, the unified model presented here favors a matter content that is of the order of the baryonic matter abundance suggested by big-bang nucleosynthesis. A problem of simple bulk viscous models, however, is the behavior of the gravitational potential and the reproduction of the CMB power spectrum.

Viscous cosmologies and the second law of thermodynamics

In the framework of viscous cosmologies, it is possible to smoothly connect a phase where the dominant energy condition (DEC) is violated to a phase where the DEC is preserved. It is well known that, depending upon the sign of the bulk viscosity coefficient, the entropy of the sources driving the geometry can very well decrease. It was recently argued that such behavior is not acceptable. We show that such an assessment might be not justified since it only takes into account the entropy of the sources but not the entropy of the geometry itself. The entropic properties of bulk viscous models violating the DEC are examined in light of previous results on the subject. We discuss the physical ambiguities of the generalized second law of thermodynamics with particular attention to the case of backgrounds with shrinking event horizons.

Cosmological Models with Variable Cosmological and Gravitational “Constants” and Bulk Viscous Models

A viscous model with variable gravitational and cosmological constant has been considered. Several solutions are presented and some are shown to be equivalent to Berman, Kalligas et al. and Brans-Dicke solutions.

Cosmological models with a variable cosmological term and bulk viscous models.

Recently, flat variable-$\ensuremath{\Lambda}$ cosmological models have been discussed by Berman. It is shown that these models are equivalent to perfect fluid models with bulk viscosity which have been presented previously. The model of Berman, which is free of particle creation and which is claimed to solve the entropy problem, is shown to be the usual Einstein-de Sitter model. The nonflat variable-$\ensuremath{\Lambda}$ solutions are obtained and their relationship to bulk viscous solutions is elucidated. Exponentially expanding solutions are also derived. The question of the stability of the models is briefly addressed.

Anisotropic, spatially homogeneous, bulk viscous cosmological model

An anisotropic, spatially homogeneous, bulk viscous model of the universe without shear viscosity is investigated. The exact solutions of the model are derived and some physical consequences of the model are discussed.