Bianchi type-I Universe Research Articles

Cosmographic analysis of anisotropic Kaniadakis holographic dark energy model

In this paper, we investigate the anisotropic and spatially homogeneous Bianchi type-I universe with Kaniadakis holographic dark energy in Saez–Ballester [Phys. Lett. A 113, 467 (1986)] theory of gravitation. We determine Kaniadakis holographic dark energy model by assuming a correlation between the metric potentials to solve the field equations of the model. This results in a dynamical deceleration parameter which demonstrates an accelerating expansion of the universe. Our model’s equation of state parameter [Formula: see text] close to [Formula: see text] ([Formula: see text] model) at late-times and is in agreement with the most recent observations. Next, we obtained the squared sound speed ([Formula: see text]) and found that it is positive, implying stability against perturbations. The [Formula: see text] plane is constructed to investigate the evolution of the models’ EoS parameter turned out to be in a freezing zone. As should be the case in an expanding universe, the strong energy conditions of the model are violated. Statefinders [Formula: see text], and [Formula: see text] planes were also examined. Our model includes the Chaplygin gas, [Formula: see text] limit, and is inclined towards the steady-state model.

Cosmological first-order vacuum phase transitions in an expanding anisotropic universe

We examine the anisotropy originated from a first-order vacuum phase transitions through three-dimensional numerical simulations. We apply Bianchi Type-I metric to our model that has one scalar field minimally coupled to the gravity. We calculate the time evolution of the energy density for the shear scalar and the directional Hubble parameters as well as the power spectra for the scalar field and the gravitational radiation although there are a number of caveats for the tensor perturbations in Bianchi Type-I universe. We run simulations with different mass scales of the scalar field, therefore, in addition to investigation of anisotropy via the shear scalar, we also determine at which mass scale the phase transition completes successfully, hence, neglecting the expansion of the Universe does not significantly affect the results. Finally, we showed that such an event may contribute to the total anisotropy depending on the mass scale of the scalar field and the initial population of nucleated bubbles.

Anisotropic f(Q) gravity model with bulk viscosity

This study investigates the dynamics of a spatially homogeneous and anisotropic LRS Bianchi type-I Universe with viscous fluid in the framework of [Formula: see text] symmetric teleparallel gravity. We assume a linear form for [Formula: see text] and introduce hypotheses regarding the relationship between the expansion and shear scalars, as well as the Hubble parameter and bulk viscous coefficient. The model is constrained using three observational datasets: the Hubble dataset (31 data points), the Pantheon SN dataset (1048 data points), and the BAO dataset (6 data points). The calculated cosmological parameters indicate expected behavior for matter-energy density and bulk viscous pressure, supporting the universe’s accelerating expansion. Diagnostic tests suggest that the model aligns with a [Formula: see text]CDM model in the far future and resides in the quintessence region. These findings are consistent with recent observational data and contribute to our understanding of cosmic evolution within the context of modified gravity and bulk viscosity.

Observation constraints on scalar field cosmological model in anisotropic universe

In this study, we have explored a scalar field cosmological model in the axially symmetric Bianchi type-I universe. In this study, our aim is to constrain the scalar field dark energy model in an anisotropic background. For this purpose, the explicit solution of the developed field equations for the model is determined and analyzed. Constraints on the cosmological model parameters are established utilizing Markov Chain Monte Carlo (MCMC) analysis and using the latest observational datasets of OHD, BAO, and Pantheon. For the combined dataset (OHD, BAO, and Pantheon), the best-fit values of Hubble and density parameters are estimated as [Formula: see text], [Formula: see text] [Formula: see text] and [Formula: see text]. The model shows a flipping nature and redshift transition occurs at [Formula: see text], and the present value of decelerated parameter is computed to be [Formula: see text] for the combined dataset. We have explored characteristics like the universe’s age, particle horizon, deceleration parameter, and jerk parameter. The dynamical properties, such as energy density [Formula: see text], scalar field pressure [Formula: see text], and equation of state parameter [Formula: see text], are analyzed and presented. We have also described the behavior of the scalar potential [Formula: see text] and scalar field. Furthermore, the authors also described the behavior of energy conditions in scalar-tensor cosmology. The scenario of the present accelerated expansion of the universe is described by the contribution of the scalar field.

Cold dark matter and holographic dark energy in f(R,T) theory of gravity

In this study, we have examined the cosmological model of the locally rotationally symmetric (LRS) Bianchi Type-I universe within the framework of the [Formula: see text] theory of gravity. The [Formula: see text] theory incorporates the Ricci scalar (R) and the trace of the energy–momentum tensor (T) in its formulation. Our investigation focuses on the cold dark matter (CDM) and holographic dark energy (DE) cosmological models, particularly in relation to the occurrence of a big rip singularity. To solve the field equation, we have assumed a specific relationship between the parameters, [Formula: see text] and [Formula: see text], where [Formula: see text] is proportional to [Formula: see text] and m is held constant. We have examined the existence of both the Big Bang and Big Rip singularities within this cosmological model. Additionally, we have analysed the state-finder parameter, which provides insights into the dynamics and evolution of the universe. Furthermore, we have discussed the physical and geometrical parameters of the universe in this context. These parameters help characterize and describe the behavior of the universe, shedding light on its underlying properties and dynamics. Through our investigation, we aim to deepen our understanding of the LRS Bianchi Type-I cosmological model within the [Formula: see text] theory of gravity, exploring the interplay between CDM, holographic DE, singularities, and the various parameters that contribute to the understanding of the universe.

An Investigation into LRS Bianchi I Universe in Brans-Dicke Theory

In this study, the homogenous and anisotropic locally rotationally symmetric (LRS) Bianchi type-I universe filled with the bulk viscous and the string cloud matter has been investigated in the Brans-Dicke (BD) scalar-tensor theory. The modified Einstein field equations of the constructed model have been solved by using the relation of the scale factors $A=B^m$ and considering the deceleration parameter to be $q=m-1$. It is found that the string does not survive for the model due to the obtained zero string energy density ($\rho_s=0$) and agrees with some studies in recent years. Moreover, it is possible to say that string matter may be turned into phantom energy over time, depending on the obtained negative rest energy density of the matter. When BD scalar field $\Phi$ is assumed constant, the attained solutions are reduced to General Relativity (GR) solutions for the static vacuum. Thus, the constructed model has not allowed the reduction of the BD solutions to GR solutions, including the matter distributions. In addition, some expansion models for choosing a value of constant $m$ have been obtained and provided. All the results have been analyzed in detail.

Renyi Holographic Dark Energy Model in f(R) Gravity with Hubble's IR Cut-Off

In the present study, a homogeneous and anisotropic LRS Bianchi type-I universe model is considered with an interacting dark matter and Renyi holographic dark energy model (RHDE) in f(R) gravity. The deceleration parameter (DP) shows a signature flipping for a universe which was decelerating in past and accelerating at present epoch. Therefore, the DP is a most physically justified parameter to analyze the solution of cosmological model. In order to find an exact solution of the field equations of the model, the shear scalar is considered to be proportional to the expansion scalar. We have considered f(R) = b Rn, the depiction model of f(R) which is the function of Ricci scalar R. The physical and geometrical characteristics of the universe model have been studied.

Squeezed Thermal State Representation of the Inflaton and Particle Production in Bianchi Type-I Universe

Squeezed Thermal State Representation of the Inflaton and Particle Production in Bianchi Type-I Universe

Spin-2 dark matter from an anisotropic universe in bigravity

Bigravity is one of the natural extensions of general relativity and contains an additional massive spin-2 field which can be a good candidate for dark matter. To discuss the production of spin-2 dark matter, we study fixed point solutions of the background equations for axisymmetric Bianchi type-I Universes in two bigravity theories without Boulware-Deser ghost, i.e., Hassan-Rosen bigravity and Minimal Theory of Bigravity. We investigate the local and global stability of the fixed points and classify them. Based on the general analysis, we propose a new scenario where spin-2 dark matter is produced by the transition from an anisotropic fixed point solution to isotropic one. The produced spin-2 dark matter can account for all or a part of dark matter and can be directly detected by laser interferometers in the same way as gravitational waves.

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.

Thermodynamical aspects of Bianchi type-I Universe in quadratic form of [formula omitted] gravity and observational constraints

In this paper, we discuss the Bianchi type-I cosmological model in the framework of symmetric teleparallel gravity say f(Q) gravity in which the non-metricity term Q is responsible for the gravitational interaction. We consider a special form of the f(Q) function which can be cast as f(Q)=λQn, where λ and n both are the dynamical model parameters. Such a choice can be viewed as a hybrid scale factor that leads to a relation between cosmic time and redshift as t=(αt0β)W[βαeβ−ln⁡(1+z)α] which describes a ΛCDM model of the Universe with the expansion evolving from decelerating to an acceleration phase. The best values for the model parameters i.e. α and β that would accord with the most current observational datasets are then estimated. We make use of 57 points from the Hubble dataset, 1048 points from the supernovae of type Ia dataset and 6 points from the BAO dataset. We use the Markov Chain Monte Carlo (MCMC) technique in conjunction with Bayesian analysis and the likelihood function. Further, we study the validity of our model with the investigation of the thermodynamical quantities, energy conditions along with some physical variables such as the EoS, and jerk parameters. Next, our results are discussed in light of current observational data and trends.

Squeezed Number State Representation of the Inflaton and Particle Production in the FRW Universe

In this study, we use a single-mode squeezed thermal vacuum state formalism and examine the nature of a massive homogeneous scalar field, minimally coupled to the gravity in the framework of semiclassical gravity in the Bianchi type-I universe. We have obtained an estimate leading solution to the semiclassical Einstein equation for the Bianchi type-I universe shows, each scale factor in its respective direction obeys t^(2/3) power-law expansion. The mechanism of the nonclassical thermal cosmological particle production is also analyzed in the Bianchi type-I universe.

Role of constant jerk parameter in f(R,T) gravity

In this study, a homogeneous and anisotropic Bianchi type-I universe model is considered taking the cosmological constant as a variable in the context of f (R, T) gravity. The presented model relies on a unique condition of constant jerk parameter as j = 1, which corresponds to a flat [Formula: see text]CDM model that leads to a variable deceleration parameter. The exact solution of the field equations is obtained under that condition, and the physical and geometrical features of the model are discussed through the paper. The model has been shown to be successful in explaining recent cosmological observations such as the universe’s expansion decelerates in the early universe and accelerates in the late universe, in agreement with their current data.

Barrow holographic dark energy models in f(Q) symmetric teleparallel gravity with Lambert function distribution

The paper presents Barrow holographic dark energy (infrared cut-off is the Hubble horizon) suggested by Barrow [The area of a rough black hole, Phys. Lett. B 808 (2020) 135643] recently in an anisotropic Bianchi type-I Universe within the framework of [Formula: see text] symmetric teleparallel gravity, where the non-metricity scalar [Formula: see text] is responsible for the gravitational interaction. We consider two cases: Interacting and non-interacting models of pressureless dark matter and Barrow holographic dark energy by solving [Formula: see text] symmetric teleparallel field equations. To find the exact solutions of the field equations, we assume that the time-redshift relation follows a Lambert function distribution as [Formula: see text], where [Formula: see text], [Formula: see text] and [Formula: see text] are non-negative constants and [Formula: see text] represents the age of the Universe. Moreover, we discuss several cosmological parameters such as energy density, equation of state (EoS) and skewness parameters, squared sound speed, and [Formula: see text] plane. Finally, we found the values of the deceleration parameter (DP) for the Lambert function distribution as [Formula: see text] and [Formula: see text] which are consistent with recent observational data, i.e. DP evolves with cosmic time from initial deceleration to late-time acceleration.

Time-dependent spinor field in a static cylindrically symmetric space-time

Within the scope of a static cylindrically symmetric space-time we study the behavior of a nonlinear spinor field that depends on time and radial coordinates. It is found that the presence of nontrivial non-diagonal components of the energy-momentum tensor (EMT) imposes some restrictions on both the metric functions and the spinor field. While for the time independent spinor field there occur three way restrictions resembling the Bianchi type-I Universe, in this case things become more complicated.

Skyrme fluid in anisotropic Universe

Cosmological solutions are obtained in an anisotropic Kantowski-Sachs (KS) and Bianchi Type-I universes considering a cosmological constant with Skyrme fluid. Interestingly, the solutions obtained here in both the KS and Bianchi-I anisotropic universes are found isotropize at late time due to the presence of the Skyrme fluid even in the absence of $\Lambda$ term or any inflationary mechanism involving the inflaton field. A comparative study of both the anisotropic cosmological models are carried out here and found that Bianchi-I universe admits oscillatory solutions for a given matter configuration. We also note that the emergent universe model can be obtained with Skyrme fluid. The anisotropy, deceleration and jerk parameters have been studied along with the linear perturbative stability to explore efficacy of the model. Both the cosmological models are stable in the absence of cosmological constant besides their compatibility with observational data. Thus, we claim Skyrme fluid a possible source for isotropization of an anisotropic universe via accelerated expansion, which is capable of reproducing some observed features of the universe.

Stability analysis of anisotropic Bianchi type-I cosmological model in teleparallel gravity

In this work, we study a cosmological model of Bianchi type-I Universe in teleparallel gravity for a perfect fluid. To obtain the cosmological solution of the model, we assume that the deceleration parameter (DP) is a linear function of the Hubble parameter H i.e. q = −1 + βH (where β as a positive constant). Consequently, we get a model of our Universe, where it goes from the initial phase of deceleration to the current phase of acceleration. We have discussed some physical and geometric properties such as Hubble parameter, DP, energy density, pressure, and equation of state parameter and study their behavior graphically in terms of redshift and compare it with observational data such as Type Ia supernovae. We also discussed the behavior of other parameters such as the jerk parameter, statefinder parameters and we tested the validity of the model by studying the stability analysis and energy conditions.

Holographic dark energy in Gauss-Bonnet gravity with Granda-Oliveros cut-off

In this paper, we have studied a model of holographic dark energy (HDE) with a homogeneous and anisotropic Bianchi type-I Universe in the framework of Gauss-Bonnet (GB) gravity or f(G) gravity. To find an exact solution of the field equations, we assume that the deceleration parameter varies with cosmic time (t). We plot some physical quantities and give interpretations of the results obtained. Finally, we compared our model with the ΛCDM model by analyzing the Jerk parameter, and we examine the equivalence between the HDE of the present work and the generalized HDE.

Microphysical manifestations of viscosity and consequences for anisotropies in the very early universe

It has been known that a non-perfect fluid that accounts for dissipative viscous effects can evade a highly anisotropic chaotic mixmaster approach to a singularity. Viscosity is often simply parameterised in this context, so it remains unclear whether isotropisation can really occur in physically motivated contexts. We present a few examples of microphysical manifestations of viscosity in fluids that interact either gravitationally or, for a scalar field for instance, through a self-coupling term in the potential. In each case, we derive the viscosity coefficient and comment on the applicability of the approximations involved when dealing with dissipative non-perfect fluids. Upon embedding the fluids in a cosmological context, we then show the extent to which these models allow for isotropisation of the universe in the approach to a singularity. We first do this in the context of expansion anisotropy only, i.e., in the case of a Bianchi type-I universe. We then include anisotropic 3-curvature modelled by the Bianchi type-IX metric. It is found that a self-interacting scalar field at finite temperature allows for efficient isotropisation, whether in a Bianchi type-I or type-IX spacetime, although the model is not tractable all the way to a singularity. Mixmaster chaotic behaviour, which is well known to arise in anisotropic models including anisotropic 3-curvature, is found to be suppressed in the latter case as well. We find that the only model permitting an isotropic singularity is that of a dense gas of black holes.

Two Minimally Interacting Fluids: Matter and Holographic Dark Energy in Bianchi Type-I Universe

Two Minimally Interacting Fluids: Matter and Holographic Dark Energy in Bianchi Type-I Universe