Isotropic Universe Model Research Articles

Investigation of dark energy scenarios in f(Q) gravitation theory

In order to solve the mystery of the accelerating and expanding universe model first, various dark energy candidates, such as Quintessence, Tachyon, k-essence, Phantom and DBI-essence, were investigated in detail in the [Formula: see text] modified symmetric teleparallel gravitation theory for homogeneous and isotropic FRLW universe model. The [Formula: see text] model is used to obtain solutions in [Formula: see text] theory. Also, the physical behaviors of some cosmological parameters such as pressure and density were studied. Dark energy candidates were analyzed for different values of n with various graphs.

Anisotropic Generalization of the ΛCDM Universe Model with Application to the Hubble Tension

Anisotropic Generalization of the ΛCDM Universe Model with Application to the Hubble Tension

Cosmic Evolution of Five‐dimensional Einstein Chern‐Simons Gravity Through Dark Energy Model

AbstractIn this paper, the cosmic evolution in the context of five‐dimensional Einstein Chern‐Simons gravity theory by using flat homogeneous and isotropic universe model is studied. The interaction is taken into account between dark sector of the evolving universe along with holographic dark energy model with Hubble horizon. Our study with well‐known models EChS ‐β5 and EChS ‐ for Einstein Chern‐Simons gravity theory in five dimensions are carried. Some cosmological parameters through graphical representations such as coincidence parameter u, equation of state parameter , deceleration parameter q and squared speed of sound parameter is analyzed. It is found that these cosmological parameters show consistent behavior with current cosmic scenario. Also, the behavior of generalized second law of thermodynamics in the underlying gravity theory with Bekenstein‐Hawking entropy and its validity for some fixed values of constants appearing in each model are explored.

Magnetized Strange Quark Models in Lyra Theory

In this study, the behavior of magnetized strange quark matter (MSQM) distribution in Lyra theory was investigated for homogeneous anisotropic Bianchi III, locally rotationally symmetric (LRS) Bianchi I, and Kantowski-Sachs universe models. We have used the equations of state, anisotropy, and linearly varying deceleration parameters to obtain the exact solutions of field equations in Lyra theory. When switching from the anisotropic universe model to the isotropic universe model, the magnetic field was not observed in the LRS Bianchi I universe. Besides, the graphs of the dynamic quantities obtained for each universe model were analyzed in detail. Finally, we inquire whether further research should be conducted.

Stability analysis of coupled phantom and tachyon field models in nonlinear electrodynamics

This paper is devoted to studying the phase space portrait of FRW universe by taking different linear forms of coupling between scalar field models and dark matter with nonlinear electromagnetic effects. We introduce normalized dimensionless quantities to construct an autonomous system of equations. We evaluate critical points and respective eigenvalues for different parameters to analyze the stability of the cosmos. These points show saddle/unstable behavior for tachyon coupled field with the nonaccelerating universe. In the case of phantom energy, the stability decreases for some critical points which also represent the nonaccelerating universe. We conclude that the dynamical stability of the isotropic and homogeneous universe model reduces in the presence of nonlinear electrodynamics for tachyon as well as the phantom field.

Thermodynamics of Modified Cosmic Chaplygin Gas

We examine the thermodynamic features of an exotic fluid known as modified cosmic Chaplygin gas in the context of homogeneous isotropic universe model. For this purpose, the behavior of physical parameters is discussed that help to analyze nature of the universe. Using specific heat formalism, the validity of third law of thermodynamics is checked. Furthermore, with the help of thermodynamic entities, the thermal equation of state is also discussed. The thermodynamic stability is explored by means of adiabatic, specific heat and isothermal conditions from classical thermodynamics. It is concluded that the considered fluid configuration is thermodynamically stable and expands adiabatically for an appropriate choice of parameters.

Study of Homogeneous and Isotropic Universe in f(R,Tφ) Gravity

This paper is devoted to study the cosmological behavior of homogeneous and isotropic universe model in the context of f(R,Tφ) gravity, where φ is the scalar field. For this purpose, we follow the first-order formalism defined by H=W(φ). We evaluate Hubble parameter, effective equation of state parameter (ωeff), deceleration parameter, and potential of scalar field for three different values of W(φ). We obtain phantom era in some cases for the early times. It is found that exponential expression of W(φ) yields ωeff independent of time for flat universe and independent of model parameter otherwise. It is concluded that our model corresponds to ΛCDM for both initial and late times.

Thermodynamical stability of FRW models with quintessence

In this paper, we study the thermodynamic stability of quintessence in the background of homogeneous and isotropic universe model. For the evolutionary picture, we consider two different forms of potentials and investigate the behavior of different physical parameters. We conclude that the quintessence model expands adiabatically and this expansion is thermodynamically stable for both potentials with suitable model parameters.

Scalar field cosmology in f ( R , T ) $f(R,T)$ gravity via Noether symmetry

This paper investigates the existence of Noether symmetries of isotropic universe model in \(f(R,T)\) gravity admitting minimal coupling of matter and scalar fields. The scalar field incorporates two dark energy models such as quintessence and phantom models. We determine symmetry generators and corresponding conserved quantities for two particular \(f(R,T)\) models. We also evaluate exact solutions and investigate their physical behavior via different cosmological parameters. For the first model, the graphical behavior of these parameters indicate consistency with recent observations representing accelerated expansion of the universe. For the second model, these parameters identify a transition form accelerated to decelerated expansion of the universe. The potential function is found to be constant for the first model while it becomes \(V(\phi )\approx \phi ^{2}\) for the second model. We conclude that the Noether symmetry generators and corresponding conserved quantities appear in all cases.

Study of GSLT in Curvature-Matter Coupling Gravity

In this work, we study the first and generalized second laws of thermodynamics at the apparent horizon of homogeneous and isotropic universe model in the context of f (G, T) gravity (G and T represent the Gauss-Bonnet invariant and trace of the energymomentum tensor, respectively). We formulate the corresponding field equations as well as determine the radius, temperature and entropy to analyze these laws. An extra term associated with entropy production is appeared in the first law due to the non-equilibrium treatment of thermodynamics. It is found that the universal condition is obtained to preserve the generalized second law of thermodynamics.

Thermodynamics in f(G,T) Gravity

This paper explores the nonequilibrium behavior of thermodynamics at the apparent horizon of isotropic and homogeneous universe model in f(G,T) gravity (G and T represent the Gauss-Bonnet invariant and trace of the energy-momentum tensor, resp.). We construct the corresponding field equations and analyze the first as well as generalized second law of thermodynamics in this scenario. It is found that an auxiliary term corresponding to entropy production appears due to the nonequilibrium picture of thermodynamics in first law. The universal condition for the validity of generalized second law of thermodynamics is also obtained. Finally, we check the validity of generalized second law of thermodynamics for the reconstructed f(G,T) models (de Sitter and power-law solutions). We conclude that this law holds for suitable choices of free parameters.

Cosmological reconstruction and stability in F(T,TG) gravity

This study investigates the reconstruction scheme and stability of some well-known cosmological models in [Formula: see text] gravity, where [Formula: see text] and [Formula: see text] represent the torsion scalar and Gauss–Bonnet invariant torsion term, respectively. For this purpose, we consider isotropic homogeneous universe model and develop the corresponding field equations. It is found that we can reproduce cosmological evolution for power-law, de Sitter solutions, phantom/nonphantom era and [Formula: see text] cold dark matter by applying reconstruction scheme in this gravity. Finally, we discuss stability of the reconstructed power-law and de Sitter solutions as well as two well-known [Formula: see text] models. It is concluded that all these models provide stable solutions for suitable choices of the constants except power-law solutions.

Stability analysis of some reconstructed cosmological models inf(G,T)gravity

Abstract The aim of this paper is to reconstruct and analyze the stability of some cosmological models against linear perturbations in f ( G , T ) gravity ( G and T represent the Gauss–Bonnet invariant and trace of the energy–momentum tensor, respectively). We formulate the field equations for both general as well as particular cases in the context of isotropic and homogeneous universe model. We reproduce the cosmic evolution corresponding to de Sitter universe, power-law solutions and phantom/non-phantom eras in this theory using reconstruction technique. Finally, we study stability analysis of de Sitter as well as power-law solutions through linear perturbations.

Inflationary dynamics in f(𝒢) gravity

This paper investigates inflationary dynamics for isotropic and homogeneous universe model in the background of [Formula: see text] gravity. We construct Hubble-flow functions, slow-roll parameters, amplitude of scalar power spectrum, spectral index and tensor–scalar ratio for a particular form of equation of state which describes quasi-de Sitter expansion. The dynamics of inflationary epoch is analyzed for scalar field as well as fluid cosmology with viable power-law [Formula: see text] model. We obtain different potential functions that correspond to chaotic inflation and Starobinsky type models. The graphical behavior of these parameters shows compatible results with observational Planck 2015 data in this gravity.

Can we remove the systematic error due to isotropic inhomogeneities?

Usually, we assume that there is no inhomogeneity isotropic in terms of our location in our uni- verse. This assumption has not been observationally confirmed yet in sufficient accuracy, and we need to consider the possibility that there are non-negligible large-scale isotropic inhomogeneities in our universe. The existence of large-scale isotropic inhomogeneities affects the determination of the cosmological parameters. In particular, from only the distance-redshift relation, we can not dis- tinguish the inhomogeneous isotropic universe model from the homogeneous isotropic one, because of the ambiguity in the cosmological parameters. In this paper, in order to avoid such ambiguity, we consider three observables, the distance-redshift relation, the fluctuation spectrum of the cosmic microwave background radiation(CMBR) and the scale of the baryon acoustic oscillation(BAO), and compare these observables in two universe models; One is the inhomogeneous isotropic uni- verse model with the cosmological constant and the other is the homogeneous isotropic universe model with the dark energy other than the cosmological constant. We show that these two universe models can not predict the same observational data of all three observables but the same ones of only two of three, as long as the perturbations are adiabatic. In principle, we can distinguish the inhomogeneous isotropic universe from the homogeneous isotropic one through appropriate three observables, if the perturbations are adiabatic.

Newtonian self-gravitating system in a relativistic huge void universe model

We consider a test of the Copernican Principle through observations of the large-scale structures, and for this purpose we study the self-gravitating system in a relativistic huge void universe model which does not invoke the Copernican Principle. If we focus on the the weakly self-gravitating and slowly evolving system whose spatial extent is much smaller than the scale of the cosmological horizon in the homogeneous and isotropic background universe model, the cosmological Newtonian approximation is available. Also in the huge void universe model, the same kind of approximation as the cosmological Newtonian approximation is available for the analysis of the perturbations contained in a region whose spatial size is much smaller than the scale of the huge void: the effects of the huge void are taken into account in a perturbative manner by using the Fermi-normal coordinates. By using this approximation, we derive the equations of motion for the weakly self-gravitating perturbations whose elements have relative velocities much smaller than the speed of light, and show the derived equations can be significantly different from those in the homogeneous and isotropic universe model, due to the anisotropic volume expansion in the huge void. We linearize the derived equations of motion and solve them. The solutions show that the behaviors of linear density perturbations are very different from those in the homogeneous and isotropic universe model.

Modified Bekenstein-Hawking System in f(R) Gravity

The present work deals with four alternative formulation of Bekenstein system on event horizon in $f(R)$ gravity. While thermodynamical laws holds in universe bounded by apparent horizon, these laws break down on event horizon. With alternative formulation of thermodynamical parameters (temperature and entropy), thermodynamical laws hold on event horizon in Einstein Gravity. With this motivation, we extend the idea of generalised Hawking temperature and modified Bekenstein entropy in homogeneous and isotropic model of universe on event horizon and examine whether thermodynamical laws hold in f(R) gravity. Specifically, we examine and compare validity of generalised second law of thermodynamics (GSLT) and thermodynamical equilibrium (TE) in four alternative modified Bekenstein scenarios. As Dark energy is a possible dominant candidate for matter in the univerese and Holographic Dark Energy (HDE) can give effective description of f(R) gravity, so matter in the universe is taken as in the form interacting HDE. In order to understand the complicated expressions, finally the above laws are examined from graphical representation using three Planck data sets and it is found that generalised/modified Hawking temperature has a crucial role in making perfect thermodynamical system.

The view of chaotic inflationary universe from f ( R ) $f(R)$ gravity

This paper is devoted to study the chaotic inflationary scenario for isotropic and homogeneous flat universe model in the framework of \(f(R)\) gravity. We investigate different chaotic potential models such as quadratic, quartic and fractional potentials. We explore quasi-de Sitter solutions and formulate bounding values of inflaton field for each model. We evaluate slow-roll parameters, number of e-folds and other significant observational parameters like spectral index, scalar power spectrum as well as tensor-scalar ratio and discuss their consistency with Planck constraints. It is concluded that all considered models are compatible with Planck data whereas the value of tensor-scalar ratio is larger than Planck’s result but for quadratic potential, the measured value is very close to Planck constraint.

Evolution of density perturbations in a large void universe

We study the evolution of linear density perturbations in a large spherical void universe which accounts for the acceleration of the cosmic volume expansion without introducing dark energy. The density contrast of this void is not large within the light cone of an observer at the center of the void. Therefore, we describe the void structure as a perturbation with a dimensionless small parameter $\ensuremath{\kappa}$ in a homogeneous and isotropic universe within the region observable for the observer. We introduce additional anisotropic perturbations with a dimensionless small parameter $ϵ$, whose evolution is of interest. Then, we solve perturbation equations up to order $\ensuremath{\kappa}ϵ$ by applying second-order perturbation theory in the homogeneous and isotropic universe model. By this method, we can know the evolution of anisotropic perturbations affected by the void structure. We show that the growth rate of the anisotropic density perturbations in the large void universe is significantly different from that in the homogeneous and isotropic universe. This result suggests that the observation of the distribution of galaxies may give a strong constraint on the large void universe model.

The idea of the universe as a black hole revisited

The Friedmann equation for a homogeneous and isotropic universe model with a cosmological constant yields the eventual maximal expansion of a closed dust universe. On the other hand, from the line element for the false vacuum outside a spherically matter distribution we obtain the generalized Schwarzschild radius, i.e., the position of the event horizon. It has been argued that the mathematical similarity of these two quantities cannot be a coincidence, but means that the universe is inside a black hole. We show that this interpretation is wrong. Matching the two metrics, we find what we intuitively expect: our expanding universe cannot be inside the event horizon of the vacuum metric.