Anisotropic Cosmological Models Research Articles

Anisotropy Bianchi Type-III Cosmological Model in Brans-Dicke Theory of Gravitation

Objectives: To derive the multi-fluid anisotropic Bianchi type-III cosmological model filled with radiation and matter in the presence of Brans-Dicke scalar-tensor theory. Method: In this paper, we have calculated Brans-Dicke field equations in anisotropic Bianchi type -III space times in the presence of matter and radiation which are minimally interacting fields. To obtain the determinate solution of the model, we considered i) the state of the equation , ii) the shear scalar is directly proportional to the expansion scalar i.e., . Also, consider the deceleration parameter, , where with which yields models with constant deceleration parameter. Findings: This solution shows an anisotropic radiation model in Brans-Dicke's theory. Also, it shows accelerated expansion of the universe and transition of the universe from the deceleration phase to the accelerated phase. Novelty: The derived model's many parameters support recent cosmological findings, indicating the model's good discovery. Thus, within the framework of our established model, we are able to comprehend the cosmic development. Keywords: Bianchi type-III, Brans-Dicke theory, Multifluid, Deceleration parameter, Λ CDM

Kinematic parameters and Raychaudhuri equation in the Kantowski-Sachs model

In this work we present a review of the calculations of homogeneous and anisotropic cosmological models, particularly the Kantowski-Sachs cosmological model. As an innovative contribution, we calculate an exact solution to the field equations with a general expression for the material content of the Universe. We obtain the expressions for energy density in this general case. We also study the case involving a scalar field. We investigate the evolution dynamics of the model, encompassing the Hubble, density, and deceleration parameters. We calculate the kinematic parameters associated with the model, namely: the scalar expansion Θ, the shear scalar σ, the rotation parameter ω and the parameter R α β U α U β , also known as Raychaudhuri scalar or convergence condition, all these parameters expressed in the Raychaudhuri equation.

Anisotropic Bianchi Type-1 Cosmological Model Containing Dark Energy

Considering the LRS Bianchi type I metric here we studied the spatial homogeneous and anisotropic dark energy cosmological models in the frame work of B-D scalar tensor theory of gravity along with variable equation of parameter and deceleration parameter. Exact solutions of field equations are obtained with certain physical assumptions in different cases. The physical behavior of the model is also discussed in details.

A Study on Cosmological Model of Anisotropic Universe with Electromagnetic Field and Cloud Strings in the Frame Work of General Relativity

In this study, we have investigated the Bianchi type-III anisotropic cosmological model in the presence of cloud strings with the electromagnetic field in the general theory of relativity. Exact solutions of field equations are obtained using the fact that shear scalar is proportional to scalar expansion and constant decelerating parameter, which is derived from the variation law of Hubble parameter proposed by Berman(Nuovo Cimento B 74, 182 (1983))and linearly varying decelerating parameter proposed by Akarsu and Dereli(International Journal of Theoretical Physics, 51,612(2012)). The dynamics and significance of the physical parameters of the model are discussed using a graphical representation of these parameters. The dynamics and significance of the physical parameters of the models are discussed.

An anisotropic cosmological model with Bianchi type III universe in f(G) gravity

The paper is devoted to exploring the exact solution of the modified Einstein's field in the setting of spatially homogeneous and anisotropic Bianchi type III spacetime in modified f(G) gravity, where G is Gauss-Bonnet invariant. Here, by employing the hyperbolic hybrid scale factor a = emt (sinh(t))n in which m,n are positive constants, with the power-law f (G)=βGm+1 model where β,m are arbitrary constants, we computed the physical and geometrical properties of the cosmological parameters of the model. The results of the model parameters are well satisfied with recent cosmological observational data. To get the exact solution of the field equation of the Bianchi type III model in the presence of an anisotropic dark fluid, we consider the relation in which the shear scalar (σ) is proportional to the expansion scalar (θ), resulting in C = An. Furthermore, the energy conditions for the power-law f(G) model are graphically examined, and it turns out that the null energy condition (NEC), weak energy condition (WEC), and dominant energy condition (DEC) are well satisfied except for the strong energy condition (SEC). The violation of the strong energy conditions (SEC) indicates the f(G) model supports the universe's current expansion with negative pressure, having a quintessence model in the present and Λ cold dark matter (CDM) model in the future.

Anisotropic Bianchi Type VI<sub>0</sub> Cosmological Models in a Modified <i>f</i>(<i>R,T</i>) Gravity

We investigate the spatially homogeneous and anisotropic Bianchi type VI0 cosmological models with modified Holographic Ricci Dark Energy subjected to f(R,T) gravity. We have obtained the solution of the field equations with the help of the power law relation among the scale factors. Dynamical parameters are calculated by using the connection between pressure and energy density. The kinematical implications of the models are also examined. Graphical presentation of different cosmological parameters is shown for distinct values of the parameter of the model

Anisotropic Fractional Cosmology: K-Essence Theory

In the particular configuration of the scalar field k-essence in the Wheeler–DeWitt quantum equation, for some age in the Bianchi type I anisotropic cosmological model, a fractional differential equation for the scalar field arises naturally. The order of the fractional differential equation is β=2α2α−1. This fractional equation belongs to different intervals depending on the value of the barotropic parameter; when ωX∈[0,1], the order belongs to the interval 1≤β≤2, and when ωX∈[−1,0), the order belongs to the interval 0<β≤1. In the quantum scheme, we introduce the factor ordering problem in the variables (Ω,ϕ) and its corresponding momenta (ΠΩ,Πϕ), obtaining a linear fractional differential equation with variable coefficients in the scalar field equation, then the solution is found using a fractional power series expansion. The corresponding quantum solutions are also given. We found the classical solution in the usual gauge N obtained in the Hamiltonian formalism and without a gauge. In the last case, the general solution is presented in a transformed time T(τ); however, in the dust era we found a closed solution in the gauge time τ.

Observational constraints on anisotropic cosmological model in Lyra’s manifold

In the present study, we have explored an axially symmetric Bianchi I universe with Lyra’s geometry. We assumed variable displacement vector as component of Lyra’s geometry in the Einstein’s field equations. We developed and presented exact solutions of the field equations of proposed model. The parameters of model have been extracted using latest observational finding of OHD, Pantheon and BAO data sets. The best fit values are estimated using MCMC method. Features like deceleration parameter, particle horizon, age of universe and jerk parameter are explored. Further Om and statefinder diagnosis of the assumed model are also discussed for geometrical comparison with the standard ΛCDM model.

Cloud String Cosmological Models Using Different Decelerating Parameters with Electromagnetic Field in General Theory of Relativity

In this study, we have investigated the Bianchi type-III anisotropic cosmological model in the presence of cloud strings with electro- magnetic field in general theory of relativity. Exact solutions of field equations are obtained using the fact that shear scalar is proportional to scalar expansion and constant decelerating parameter which is derived from variation law of Hubble parameter proposed by Berman[1] and linearly varying decelerating parameter proposed by Akarsu and Dereli [2]. The dynamics and significance of physical parameters of the model are discussed using a graphical representation of these parameters.The dynamics and significance of physical parameters of the models are discussed.

Is the Universe Anisotropic Right Now? Comparing the Real Universe with the Kasner’s Space-Time

We investigate possible astronomical manifestations of space-time anisotropy. The homogeneous vacuum Kasner solution was chosen as a reference anisotropic cosmological model because there are no effects caused by inhomogeneity in this simple model with a constant degree of anisotropy. This anisotropy cannot become weak. The study of its geodesic structure made it possible to clarify the properties of this space-time. It showed that the degree of manifestation of anisotropy varies significantly depending on the travel time of the light from the observed object. For nearby objects, for which it does not exceed half the age of the universe, the manifestations of anisotropy are very small. Distant objects show more pronounced manifestations; for example, in the distribution of objects over the sky and over photometric distances. These effects for each of the individual objects decrease with time but, in general, the manifestations of anisotropy in the Kasner space-time remain constant due to the fact that new sources come from beyond the cosmological horizon. We analyze observable signatures of the Kasner-type anisotropy and compare it to observations. These effects were not found in astronomical observations, including the study of the CMB. We can assume that the Universe has always been isotropic or almost isotropic since the recombination era. This does not exclude the possibility of its significant anisotropy at the moment of the Big Bang followed by rapid isotropization during the inflationary epoch.

Anisotropic Cosmological Model in f (R,T) Theory of Gravity with a Quadratic Function of T

In this paper, we study spatially homogeneous and anisotropic Bianchi type-I space-time filled with perfect fluid within the framework of f(R,T)  theory of gravity for the functional form f(R,T)=R+2f(T)  with f(T)=αT+βT2, where α and β are constants. Exact solutions of the gravitational field equations are obtained by assuming the average scale factor to obey a hybrid expansion law and some cosmological parameters of the model are derived. Two special cases, leading to the power-law expansion and the exponential expansion, are also considered. We investigate the physical and geometrical properties of the models by studying the evolution graphs of some relevant cosmological parameters such as the Hubble parameter (H), the deceleration parameter ( q) etc.

Anisotropic Cosmology in the Local Limit of Nonlocal Gravity

Within the framework of the local limit of nonlocal gravity (NLG), we investigate a class of Bianchi type I spatially homogeneous but anisotropic cosmological models. The modified field equations are presented in this case, and some special solutions are discussed in detail. This modified gravity theory contains a susceptibility function S(x) such that general relativity (GR) is recovered for S = 0. In the modified anisotropic cosmological models, we explore the contribution of S(t) and its temporal derivative to the local anisotropic cosmic acceleration. The implications of our results for observational cosmology are briefly discussed.

Bianchi type-III holographic dark energy cosmological model in [formula omitted] theory of gravitation

In this paper, we have studied the spatially homogeneous and anisotropic Bianchi type-III cosmological model in the presence of pressureless matter and Holographic dark energy within the framework of f(R,T) gravity. We have constructed a cosmological model with an appropriate choice of function f(T). Field equations are solved using the relation between the metric potentials A=Cn and using a simple power-law form of a metric potential C=tn. The main objective behind this paper is to explore some physically significant discussions on the evolution of the universe. It is observed that the Hubble parameter H, scalar expansion θ and shear scalar σ diverge at the initial epoch while they approach zero for a large value of time and the anisotropic parameter Am is constant. The negative value of the deceleration parameter indicates the present acceleration of the universe.

Dynamics of cosmological model with domain walls and massive scalar fields in f(R,T) gravity

A spatially homogenous and anisotropic locally rotationally symmetric (LRS) Bianchi type-I space-time is considered in the presence of a massive scalar field containing domain walls in the framework of the [Formula: see text] gravity proposed by Harko et al. [Phys. Rev. D 84, 024020, (2011)]. Solving the field equations of the model using a relation between metric potentials and power law between the scalar field and average scale factor of the model, an anisotropic cosmological model with massive scalar fields and domain walls in [Formula: see text] theory is presented. These conditions result in a model solution that provides a dynamic deceleration parameter. The model’s geometrical and physical properties are also examined. The universe exhibits a smooth transition from its early decelerated phase to its current accelerated phase, as shown by the study of the deceleration parameter. The statefinder plane corresponds to the Chaplygin gas era and the model finally approaches [Formula: see text]CDM model.

The Big Bang could be anisotropic. The case of Bianchi I model

We consider an evolution of anisotropic cosmological model on the example of the Bianchi type I homogeneous Universe. It is filled by the mixture of matter and dark energy with an arbitrary barotropic equation of state (EoS). The general solution for this case is found and analyzed. A complete list of possible future singularities for this model is given. Some new solution were obtained for a particular EoSs, e.g. for the Bianchi type I ΛCDM homogeneous model. It is shown that all special cases corresponding to different EoSs have common properties, provided that now or at another moment of time the Universe is expanding, and the density of the mixture is positive. Then the evolution always begins with an anisotropic ‘Big Bang’ which happened a finite time ago. After that the Universe is constantly expanding and in all cases, with rare exceptions, becomes more isotropic. A particularly strong isotropization is associated with the epoch of inflation. After its completion, the expansion of the Universe becomes almost isotropic, and this case cannot be distinguished from isotropic by astronomical observations. This fact allows us to consider an anisotropic cosmological model as a possible candidate for the description of the observed Universe despite the isotropic pattern of expansion.

The study of anisotropic perfect fluid cosmological models in f(R, T) gravity

The study of anisotropic perfect fluid cosmological models in f(R, T) gravity

Transit string dark energy models in f(Q) gravity

In this paper, we have investigated an anisotropic cosmological model in [Formula: see text] gravity with string fluid in LRS Bianchi type-I universe. We have considered the arbitrary function [Formula: see text], where [Formula: see text] is model free parameter and [Formula: see text] is the cosmological constant. We have established a relationship between matter energy density parameter [Formula: see text] and dark energy density parameter [Formula: see text] through Hubble function using constant equation of state parameter [Formula: see text]. We have made observational constraint on the model using [Formula: see text]-test with observed Hubble datasets [Formula: see text] and SNe Ia datasets, and obtained the best fit values of cosmological parameters. We have used these best fit values in the result and discussion. We have discussed our result with cosmographic coefficients and found a transit phase dark energy model. Also, we analyzed the Om diagnostic function for anisotropic universe and found that our model is quintessence dark energy model.

Cosmic expansion of the universe characterized by anisotropic viscous fluid cosmological models

In this paper, the cosmic expansion of the universe filled with viscous fluid has been discussed in Bianchi type-I space-time. To solve Einstein’s field equations, a time-varying deceleration parameter, ( q( t)) yields a time variable scale factor [Formula: see text], with " α" and " n" are positive constants. We also consider a time-varying gravitational constant ( G) via the formulation 8π G = G0 a m , where " G0" and " m" are positive constants. Further, based on the third constraint, two different viscous cosmological models, one by considering the perfect gas equation of state p = ωρ and the other by considering time-varying bulk viscosity coefficient ( ξ( t)), have been constructed. It is mentioned here that this particular choice of scale factor generates a class of accelerating models for n < 1, while for n > 1, the transition of the universe from early deceleration to current acceleration takes place. An important feature of the models is decreasing dark energy candidate, i.e., cosmological constant (Λ) with time, fine-tuning with the results from recent supernovae Ia observations. The various physical parameters involved in the models have been discussed analytically and graphically and are found in good agreement with recent observations. For the physical acceptability of both the models, energy conditions are shown graphically. The models are found to be physically acceptable, particularly in identifying the universe’s transition. We have discussed the statefinder and observed that all the trajectories indicate dark energy candidates as Chaplygin gas and quintessence for different constraints ( α, n). We have also calculated the cosmographic parameters such as jerk, lerk, and maxout to analyze the physical behavior of the DE models.

SPATIALLY HOMOGENEOUS AND ANISOTROPIC TWO FORMS DARK ENERGY COSMOLOGICAL MODEL

We have examined spatially homogeneous and anisotropic cosmological models in the presence of two forms of dark energy (DE). We have calculated the exact solutions of the Einsteins field equations by assuming a special law of variation of Hubbles parameter [3] . Some physical parameters of the model are determined. From the point of kinematical behaviour we have also extended our work by deriving the analytic expressions for the look back time.

Anisotropic behavior of perfect fluid in fractal cosmology

We investigate a perfect fluid anisotropic plane symmetric cosmological model with the help of fractal cosmology. The field equations are obtained in the framework of fractal cosmology. To get a deterministic model we have considered a linear relationship between the directional Hubble’s parameter H3=kH1, where k≥0 is a constant and assumed a fractal parameter η≠0. Also, we have discussed some physical and geometrical properties of the model in details using graphs. Finally we present the state-finder diagnosis of the model