Hybrid Expansion Law Research Articles

FRW dark energy cosmological model with hybrid expansion law

In this work, we study a cosmological model of spatially homogeneous and isotropic accelerating universe which exhibits a transition from deceleration to acceleration. For this, Friedmann Robertson Walker(FRW) metric is taken and Hybrid expansion law a(t)=tαexp(βt) is proposed and derived. We consider the universe to be filled with two types of fluids barotropic and dark energy which have variable equations of state. The evolution of dark energy, Hubble, and deceleration parameters etc., have been described in the form of tables and figures. We consider 581 data’s of observed values of distance modulus of various SNe Ia type supernovae from union 2.1 compilation to compare our theoretical results with observations and found that model satisfies current observational constraints. We have also calculated the time and redshift at which acceleration in the Universe had commenced.

A Cosmological Scenario from the Starobinsky Model within the f(R,T) Formalism

In this paper we derive a novel cosmological model from the f(R,T) theory of gravitation, for which R is the Ricci scalar and T is the trace of the energy-momentum tensor. We consider the functional form f(R,T)=f(R)+f(T), with f(R) being the Starobinsky Model, named R+αR2, and f(T)=2γT, with α and γ being constants. We show that a hybrid expansion law form for the scale factor is a solution for the derived Friedmann-like equations. In this way, the model is able to predict both the decelerated and the accelerated regimes of expansion of the universe, with the transition redshift between these stages being in accordance with recent observations. We also apply the energy conditions to our material content solutions. Such an application makes us able to obtain the range of acceptability for the free parameters of the model, named α and γ.

Hypersurface-homogeneous modified holographic Ricci dark energy cosmological model by hybrid expansion law in Saez–Ballester theory of gravitation

The main motive of this investigation is to study the behavior of cosmological model in the presence of matter and a modified holographic Ricci dark energy for homogeneous hypersurface in the scalar tensor theory of gravitation, proposed by Saez–Ballester (Phys. Lett. A, 113, 467 (1986)). The hybrid expansion law (Akarsu et al., JCAP, 01, 022 (2014)) has been used to get a determinate solution. The physical condition that is shear scalar proportional to the expansion scalar is used to obtain the solution of the field equations. The various physical and geometrical aspects of the model are also discussed.

Accelerating universe with variable EoS parameter of dark energy in Brans–Dicke theory of gravitation

We have investigated the spatially homogeneous and isotropic Friedmann–Robertson–Walker (FRW) universe filled with barotropic fluid and dark energy in the framework of the Brans–Dicke theory of gravitation. Here we have discussed three models: (i) law of variation for Hubble’s parameter, which leads to a constant value of deceleration parameter, (ii) hybrid expansion law model, and (iii) special form of deceleration parameter model. We have found that among all these derived models, the most suitable standard cosmological model according to the recent cosmological observations is the model with special form of deceleration parameter.

Stability of Kaluza–Klein holographic dark energy cosmological models in f(R) theory of gravitation

We study Kaluza–Klein space–time in the context of f(R) theory of gravitation in the presence of minimally interacting holographic dark energy. Exact solutions of field equations are obtained for volumetric exponential expansion, power law expansion and hybrid expansion law. Stability of these models is discussed with the help of the square of sound speed relation. The physical parameters of the models are also discussed.

A comparative study of Kaluza–Klein model with magnetic field in Lyra manifold and general relativity

Kaluza–Klein (K–K) dark energy (DE) cosmological models in presence of magnetized anisotropic fluid have been examined in normal gauge for the field equations for Lyra’s manifold where gauge function β is taken as the function of time. We have chosen the scale factor a(t)=[tkexp(t)]1n (k ≥ 0, n ≥ 0) as the generalized Hybrid expansion law which yields the time-dependent deceleration parameter (DP), to obtain the deterministic solutions of field equations, speaking to a class of models which produce the transition of the universe from the early decelerating stage to the ongoing accelerating stage. The dark energy EoS (ω) is observed to be the time-dependent and its current range for inferred models is in great concurrence with the ongoing observations. For various estimations of (n, k), we can produce a class of physically suitable DE models in K–K theory. The anisotropy of the model vanishes and become isotropic under the suitable condition. The solutions are likewise compared with the solutions in general relativity and without magnetic field. The geometric and physical properties of both the models are likewise talked about in detail.

Transitioning Scenario of Bianchi-I Universe Within f (R,T) Formalism

In this paper, we report the existence of transitioning scenario of Bianchi type I universe in the context of $f\left(R,T\right)$ gravity with special case $f\left(R,T\right) = f_{1}(R) + f_{2}(R)f_{3}(T)$ and it's functional forms $f_{1}(R) = f_{2}(R) = R$ and $f_{3}(T) = \alpha T$ with $\alpha$ being constant. The exact solution of the Einstein's field equations are derived by using the generalized hybrid expansion law that yields the model of transitioning universe from early deceleration phase to current acceleration phase. Under this specification, we obtain the singular as well as non singular solution of Bianchi type I model depending upon the particular choice of the value of problem parameters. We also notice that the validation of weak energy condition and dominant energy condition and violation of strong energy condition occurs for these values of problem parameters. The deceleration parameter is found to be negative at present (z = 0) in the derived model which is supported by recent observations.

Transit cosmological models with perfect fluid and heat flow in Sáez-Ballester theory of gravitation

In this paper, the Bianchi-V universe has been applied to the transitional universe. Exact solutions of Einstein’s modified field equations in the framework of Saez-Ballester theory are obtained with heat conduction and perfect fluid. We have applied the hybrid expansion law for the average scale factor $$a = k t^{\alpha }e^{\beta t}$$ , (where $$\alpha \ge 0$$ , $$k > 0$$ , and $$\beta \ge 0$$ are constants). This results into a new class of transit models from decelerating universe to the current accelerating universe. The present work also elucidates some of the physical, geometric and kinematic properties of the universe and found them in good agreement with recent observations.

Dynamical Anisotropic Dark Energy Cosmological Model in General Relativity

ABSTRACTWe have investigated an anisotropic dark energy cosmological model in the presence of anisotropic dark energy fluid coupled with mass less scalar field in KantowskiSachs space-time in general theory of relativity. A physically viable dark energy cosmological model is presented using (i) a hybrid expansion law proposed by Akarsu et al. (JCAP. 01, 022: 2014) and (ii) a relation between the metric potential. The dark energy density, the equation of state (EoS) parameter and the skewness parameters of the model are determined and their physical siguificance is pointed out. The deceleration parameter and the jerk parameter of the model show that there is a smooth transition of the uuiverse from early deceleration to late time acceleration which confirms the observations modem cosmology.

Anisotropic Dark Energy Models with Hybrid Expansion Law in Lyra’s Manifold

Field equations of the locally rotationally symmetric (LRS) Bianchi type-I metric with anisotropic fluid are constructed in the framework of Lyra’s manifold. By assuming a hybrid expansion law (HEL) for the average scale factor that yields power-law and exponential-law cosmologies, we model Bianchi type-I space time for the time-dependent displacement field which is proportional to a power-law form of the Hubble parameter. The model provides an elegant description of the transition from cosmic deceleration to acceleration. We discuss the physical behaviors of the derived models with observational constraints applied to late-time acceleration as well as early stages of the Universe. It is observed that HEL Bianchi type I universe is anisotropic at early stage of evolution and becomes isotropic at late times.

Transit dark energy string cosmological models with perfect fluid in F(R,T)-gravity

In this paper, spatially homogeneous and anisotropic Bianchi type-[Formula: see text] dark energy (DE) cosmological transit models with string fluid source in [Formula: see text] gravity [T. Harko et al., Phys. Rev. D 84 (2011) 024020], where [Formula: see text] is the Ricci scalar and [Formula: see text] the trace of the stress energy–momentum tensor, have been studied in the context of early time decelerating and late-time accelerating expansion of the Universe as suggested by the recent observations. The exact solutions of the field equations are obtained first by using generalized hybrid expansion law (HEL) [Formula: see text] which yields a time-dependent deceleration parameter [Formula: see text] and second by considering the metric coefficient [Formula: see text]. By using recent constraints from supernovae type-Ia union data [Cunha, arXiv:0811.2379[astro-ph]], we obtain [Formula: see text] and [Formula: see text] for transit model [Formula: see text]. The Universe has an initial singularity and is anisotropic closed and it tends to be flat at the late time, i.e. our Universe is in accelerating expansion. Our model shows a phase transition property from decelerating to accelerating. It is remarkable to mention here that our Universe is homogeneous and anisotropic in the early phase whereas it becomes homogeneous and isotropic for [Formula: see text]. We have also discussed the stability of the background solution with respect to perturbations of the metric along with the properties of future singularities in the Universe dominated by DE including the phantom-type fluid. Various physical and dynamical parameters are also calculated and investigated in terms of time and redshift both.

F(R,T)=f(R)+lambda T gravity models as alternatives to cosmic acceleration

This article presents cosmological models that arise in a subclass of f(R,T)=f(R)+f(T) gravity models, with different f(R) functions and fixed T-dependence. That is, the gravitational lagrangian is considered as f(R,T)=f(R)+lambda T, with constant lambda . Here R and T represent the Ricci scalar and trace of the stress-energy tensor, respectively. The modified gravitational field equations are obtained through the metric formalism for the Friedmann–Lemaître–Robertson–Walker metric with signature (+,-,-,-). We work with f(R)=R+alpha R^2-frac{mu ^4}{R}, f(R)=R+kln (gamma R) and f(R)=R+me^{[-nR]}, with alpha , mu , k, gamma , m and n all free parameters, which lead to three different cosmological models for our Universe. For the choice of lambda =0, this reduces to widely discussed f(R) gravity models. This manuscript clearly describes the effects of adding the trace of the energy-momentum tensor in the f(R) lagrangian. The exact solution of the modified field equations are obtained under the hybrid expansion law. Also we present the Om diagnostic analysis for the discussed models.

Dark energy cosmological models with cosmic string

In this paper we have studied the anisotropic Kantowski-Sachs, locally rotationally symmetric (LRS) Bianchi type-I and LRS Bianchi type-III geometries filled with dark energy and one dimensional cosmic string in the Saez-Ballester theory of gravitation. To get physically valid solution we take hybrid expansion law of the average scale factor which is a product of power and exponential type of functions that results in time dependent deceleration parameter ( $q$ ). The equation of state parameter of dark energy ( $\omega _{\mathit{de}}$ ) has been discussed and we have observed that for the three models it crosses the phantom divide line ( $\omega _{\mathit{de}} = -1$ ) and shows quintom like behavior. The density of dark energy ( $\rho _{\mathit{de}}$ ) is an increasing function of redshift and remains positive throughout the evolution of the universe for the three models. Moreover in Kantowski-Sachs and LRS Bianchi type-I geometries the dark energy density dominates the string tension density ( $\lambda $ ) and proper density ( $\rho $ ) throughout the evolution of the universe. The physical and geometrical aspects of the statefinder parameters ( $r,s$ ), squared speed of sound ( $v_{s}^{2} $ ) and $\omega _{\mathit{de}}$ – $\omega ^{\prime }_{\mathit{de}}$ plane are also discussed.

Bianchi type-III holographic dark energy model with quintessence

In this study, we investigate an anisotropic Bianchi type-[Formula: see text] space-time in the presence of matter and holographic dark energy components within the framework of general relativity. We obtained the solution of the field equations by assuming (i) the expansion scalar [Formula: see text] in the model is proportional to shear scalar [Formula: see text], (ii) hybrid expansion law for average scale factor (keeping an eye on the recent scenario of accelerating nature of the universe). We develop cosmological parameters like deceleration and equation of state parameters. These parameters are plotted versus redshift [Formula: see text] for different values of power component of average scale factor [Formula: see text]. We observe that the equation of state varies in quintessence region ([Formula: see text]) and ultimately tends to [Formula: see text]CDM model ([Formula: see text]). The deceleration parameter exhibits a smooth transition from early decelerated epoch to present accelerated era. In addition, we establish the correspondence between our holographic dark energy model and quintessence scalar field. We also express the self-interacting potential [Formula: see text] and scalar field [Formula: see text] of quintessence model as functions of cosmic time [Formula: see text], which describes the accelerated expansion of the universe.

Dynamics of axially symmetric anisotropic modified holographic Ricci dark energy model in Brans-Dicke theory of gravitation

In this paper, we have derived field equations of Brans-Dicke (Phys. Rev. 124, 925 (1961)) theory of gravitation with the help of an axially symmetric anisotropic Bianchi-type space-time in the presence of dark matter and anisotropic modified holographic Ricci dark energy. We have presented a cosmological model solving the field equations. We have used i) the hybrid expansion law, ii) a relation between metric potentials and iii) the modified holographic Ricci dark energy defined by Chen and Jing (Phys. Lett. B 679, 144 (2009)) to solve the field equations. We have determined the cosmological parameters, namely, EoS parameter, matter energy density, anisotropic dark energy density, Skewness parameter, deceleration and jerk parameters. A detailed physical discussion of these dynamical parameters is presented through a graphical representation. We observe that we have a quintessence model which exhibits a smooth transition from decelerated phase to an accelerated phase of the universe. This situation is quite in agreement with the scenario of modern cosmology.

FRW type Kaluza\u2013Klein modified holographic Ricci dark energy models in Brans\u2013Dicke theory of gravitation

In this paper, we investigate non-Ricci, non-compact Friedmann–Robertson–Walker type Kaluza–Klein cosmology in the presence of pressureless matter and modified holographic Ricci dark energy in the frame work of Brans and Dicke (Phys Rev 124:965, 1961) scalar–tensor theory of gravitation. We solve the field equations of this theory using a hybrid expansion law for the five dimensional scale factor. We have also used a power law and a form of logarithmic function of the scale factor for the Brans–Dicke scalar field. Consequently, we obtain two interesting cosmological models of the Kaluza–Klein universe. We have evaluated the cosmological parameters, namely, the equation of state parameter, the deceleration parameter, and the density parameters. To check the stability of our models we use the squared speed of sound. Some well-known cosmological (omega _{de}–omega ^{prime }_{de} and statefinder) planes are constructed for our models. We have also analyzed the physical behavior of these parameters through graphical representation. It is observed that the FRW type Kaluza–Klein dark energy models presented are compatible with the present day cosmological observations.

Kantowski–Sachs modified holographic Ricci dark energy model in Saez–Ballester theory of gravitation

We have considered Kantowski–Sachs space–time in the presence of matter and anisotropic modified holographic Ricci dark energy components in the scalar–tensor theory of gravitation formulated by Saez and Ballester (Phys. Lett. A, 113, 467, 1986) and derived the field equations of the theory. We have used (i) hybrid expansion law proposed by Akarsu et al. (JCAP, 022, 2014), (ii) a relation between metric potentials, and (iii) modified holographic Ricci dark energy density given by Chen and Jing (Phys. Lett. B, 679, 144, 2009) to obtain an exact solution of the field equations that describes a Kantowski–Sachs holographic modified Ricci dark energy universe in this theory. Physical and kinematical parameters are also computed and their physical behavior is discussed.

Plane Symmetric Inflationary Universe with Hybrid Expansion Law and Time Varying Λ

Plane symmetric inflationary cosmological model using Hybrid Expansion Law (HEL) with flat potential and time varying in General Theory of Relativity has been studied. The solution of the Einstein’s field equations is obtained under the assumption of HEL which yields a time-dependent deceleration parameter presenting transition of the universe from the early decelerating phase to the recent accelerating phase. The physical and kinematical parameters of the models have been studied and discussed.

Kaluza-Klein Cosmological Models with Barotropic Fluid Distribution

Kaluza-Klein cosmological models have been investigated for barotropic fluid distribution with time dependent gravitational constant and cosmological constant in the context of general relativity. We have used Hybrid Expansion Law (HEL), a product of power-law and exponential type of functions, to obtain determinate solutions of the Einstein’s field equations. In this paper, we have mainly discussed the graphical behavior of the results for three different cases of interest in modern cosmology.

Anisotropic ghost dark energy cosmological model with hybrid expansion law

In this paper, we study the anisotropic Bianchi type-VI0 metric filled with dark matter and anisotropic ghost dark energy. We have solved the Einstein's field equations by considering hybrid expansion law (HEL) for the average scale factor. It is found that at later times the universe becomes spatially homogeneous, isotropic and flat. From a state finder diagnosis, it is found that our model is having similar behavior like ɅCDM model at late phase of cosmic time.