Bianchi Type-V Dark Energy Model with Varying EoS Parameter

Within the scope of an anisotropic Bianchi type-V cosmological model we have studied the evolution of the universe. The assumption of a diagonal energy-momentum tensor leads to some severe restriction on the metric functions, which on its part imposes restriction on the components of the energy momentum tensor. This model allows anisotropic matter distribution. Further using the proportionality condition that relates the shear scalar (σ) in the model with the expansion scalar (?) and the variation law of Hubble parameter, connecting Hubble parameter with volume scale. Exact solution to the corresponding equations are obtained. The EoS parameter for dark energy as well as deceleration parameter is found to be the time varying functions. A qualitative picture of the evolution of the universe corresponding to different of its stages is given using the latest observational data.     

LRS Bianchi type I models with anisotropic dark energy and constant deceleration parameter

Locally rotationally symmetric Bianchi type I cosmological models are examined in the presence of dynamically anisotropic dark energy and perfect fluid. We assume that the dark energy (DE) is minimally interacting, has dynamical energy density, anisotropic equation of state parameter (EoS). The conservation of the energy-momentum tensor of the DE is assumed to consist of two separately additive conserved parts. A special law is assumed for the deviation from isotropic EoS, which is consistent with the assumption on the conservation of the energy-momentum tensor of the DE. Exact solutions of Einstein’s field equations are obtained by assuming a special law of variation for the mean Hubble parameter, which yields a constant value of the deceleration parameter. Geometrical and kinematic properties of the models and the behaviour of the anisotropy of the dark energy have been carried out. The models give dynamically anisotropic expansion history for the universe that allows to fine tune the isotropization of the Bianchi metric, hence the CMB anisotropy.     

Holographic Dark Energy (DE) Cosmological Models with Quintessence in Bianchi Type-V Space Time

In this paper, author studied homogeneous and anisotropic Bianchi type-V universe filled with matter and holographic dark energy (DE) components. The exact solutions to the corresponding Einstein’s field equations are obtained for exponential and power-law volumetric expansion. The holographic dark energy (DE) EoS parameter behaves like constant, i.e. ω _Λ =?1, which is mathematically equivalent to cosmological constant (Λ) for exponential expansion of the model, whereas the holographic dark energy (DE) EoS parameter behaves like quintessence for power-law expansion of the model. A correspondence between the holographic dark energy (DE) models with the quintessence dark energy (DE) is also established. Quintessence potential and dynamics of the quintessence scalar field are reconstructed, which describe accelerated expansion of the universe. The statefinder diagnostic pair {r,s} is adopted to characterize different phases of the universe.     

A Tachyonic Scalar Field with Mutually Interacting Components

We investigate the tachyonic cosmological potential V(?) in two different cases of the quasi-exponential expansion of universe and discuss various forms of interaction between the two components—matter and the cosmological constant—of the tachyonic scalar field, which lead to the viable solutions of their respective energy densities. The distinction among the interaction forms is shown to appear in the O _m (x) diagnostic. Further, the role of the high- and low-redshift observations of the Hubble parameter is discussed to determine the proportionality constants and hence the correct form of matter–cosmological constant interaction.     

Holographic DBI-Essence Dark Energy via Power-Law Solution of the Scale Factor

The present work reports a Holographic reconstruction of Dirac–Born–Infeld (DBI)-essence Dark Energy (DE) in a flat FRW universe. The scale factor a(t) is chosen in power law form. We have reconstructed the scalar field and potential and subsequently the equation of state (EoS) parameter ω of the DBI-essence DE. The corresponding plots show increasing scalar field, decaying tension and decaying potential. The reconstructed EoS parameter stays below ?1, showing a phantom-like behavior. The stability of the reconstructed DBI-essence DE is investigated through squared speed of sound $v_{s}^{2}$ : its negative sign reveals that the holographically reconstructed DBI-essence is classically unstable.     

Anisotropic Dark Energy Bianchi Type-II Cosmological Models in Lyra Geometry

The spatially homogeneous and totally anisotropic Bianchi type-II cosmological model has been discussed in general relativity in the presence of a hypothetical anisotropic dark energy fluid with constant deceleration parameter within the frame work of Lyra’s manifold with uniform and time varying displacement field vector. With the help of special law of variation for Hubble’s parameter proposed by Bermann (Nuovo Cimento 74B:182, 1983) a dark energy cosmological model is obtained in this theory. We use the power law relation between average Hubble parameter H and average scale factor R to find the solution. The assumption of constant deceleration parameter leads to two models of universe, i.e. power law model and exponential model. Some physical and kinematical properties of the model are also discussed.     

Bianchi Type-II Dark Energy Model in f(R,T) Gravity

The spatially homogeneous and totally anisotropic Bianchi Type-II space-time dark energy model with EoS parameter is considered in the presence of a perfect fluid source in the framework of f(R,T) gravity proposed by Harko et al. (Phys. Rev. D, 84:024020, 2011). With the help of special law of variation for Hubble’s parameter proposed by Berman (Nuovo Cimento B, 74:182, 1983) a dark energy cosmological model is obtained in this theory. We consider f(R,T) model and investigate the modification R+f(T) in Bianchi type-II cosmology with an appropriate choice of a function f(T)=λT. We use the power law relation between average Hubble parameter H and average scale factor R to find the solution. The assumption of constant deceleration parameter leads to two models of universe, i.e. power law model and exponential model. Some physical and kinematical properties of the model are also discussed.     

Anisotropic Bianchi-I universe with phantom field and cosmological constant

We study an anisotropic Bianchi-I universe in the presence of a phantom field and a cosmological constant. Cosmological solutions are obtained when the kinetic energy of the phantom field is of the order of anisotropy and dominates over the potential energy of the field. The anisotropy of the universe decreases and the universe transits to an isotropic flat FRW universe accommodating the present acceleration. A class of new cosmological solutions is obtained for an anisotropic universe in case an initial anisotropy exists which is bigger than the value determined by the parameter of the kinetic part of the field. Later, an autonomous system of equations for an axially symmetric Bianchi-I universe with phantom field in an exponential potential is studied. We discuss the stability of the cosmological solutions.      

Pilgrim dark energy in f(T) gravity

We discuss the interacting f(T) gravity with pressureless matter in an FRW spacetime. We construct an f(T) model by following the correspondence scheme incorporating a recently developed pilgrim dark energy model and taking the Hubble horizon as the IR cutoff. We use constructed model to discuss the evolution trajectories of the equation-of-state parameter, the ω _T -ω′ _T phase plane, and state-finder parameters in the evolving universe. It is found that the equation-of-state parameter gives a phantom era of the accelerated universe for some particular range of the pilgrim parameter. The ω _T -ω′ _T plane represents freezing regions only for an interacting framework, while the ΛCDM limit is attained in the state-finder plane. We also investigate the first and second laws of thermodynamics assuming equal temperatures at and inside the horizon in this scenario. Due to the violation of the first law of thermodynamics in f(T) gravity, we explore the behavior of the entropy production term. The validity of a generalized second law of thermodynamics depends on the present-day value of the Hubble parameter.     

Bianchi Type-VI Anisotropic Dark Energy Model with Varying EoS Parameter

Within the scope of an anisotropic Bianchi type-VI cosmological model we have studied the evolution of the universe filled with perfect fluid and dark energy. To get the deterministic model of Universe, we assume that the shear scalar (σ) in the model is proportional to expansion scalar (?). This assumption allows only isotropic distribution of fluid. Exact solution to the corresponding equations are obtained. The EoS parameter for dark energy as well as deceleration parameter is found to be the time varying functions. Using the observational data qualitative picture of the evolution of the universe corresponding to different of its stages is given. The stability of the solutions obtained is also studied.     

On the Dynamics of Dark Energy with Higher Time Derivatives of Hubble Parameter in El-Nabulsi Fractional Action Cosmology

In this work, we discussed a new dark energy density model which contains one term proportional to the Hubble parameter H squared, one to the first and one to second time derivative of the Hubble parameter H based on El-Nabulsi fractional action cosmology (FAC). Some cosmological parameters, like the Hubble parameter, the Equation of State (EoS) parameter ω _DE and the deceleration parameter q have been reconstructed and studied. Finally, through a test made using the squared speed of sound $v_{s}^{2}$ , the proposed reconstruction model results to be classically unstable.     

Two-Fluid Dark Energy Models in Bianchi Type-III Universe with Variable Deceleration Parameter

Some new exact solutions of Einstein’s field equations have come forth within the scope of a spatially homogeneous and anisotropic Bianchi type-III space-time filled with barotropic fluid and dark energy by considering a variable deceleration parameter. We consider the case when the dark energy is minimally coupled to the perfect fluid as well as direct interaction with it. Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution of the universe, the equation of state (EoS) for dark energy ω ^(de), in both cases, tends to ?1 (cosmological constant, ω ^(de)=?1), by displaying various patterns as time increases, which is consistent with recent observations. The cosmic jerk parameter in our derived models are in good agreement with the recent data of astrophysical observations under appropriate condition. It is observed that the universe starts from an asymptotic Einstein static era and reaches to the ΛCDM model. So from recently developed Statefinder parameters, the behaviour of different stages of the universe has been studied. The physical and geometric properties of cosmological models are also discussed.     

Bianchi Type-II Dark Energy Model in Scale Covariant Theory of Gravitation

Spatially Homogeneous and anisotropic Bianchi type-II space time with variable equation of state (EoS) parameter and constant deceleration parameter has been investigated in scale covariant theory of gravitation formulated by Canuto et al. (Phys. Rev. Lett. 39:429, 1977). With the help of special law of variation for Hubble’s parameter proposed by Bermann (Nuovo Cimento 74B:182, 1983) a dark energy cosmological model is obtained in this theory. We use the power law relation between scalar field ? and scale factor R to find the solutions. Some physical and kinematical properties of the model are also discussed.     

The Cosmological Constant Constrained with Union2.1 Supernovae Type Ia Data

We derive several, detailed relationships in terms of the Friedmann-Robertson-Walker (FRW) generalization which describe the Universe during both the radiation and matter dominated epochs. We explicitly provide for the influence of radiation, rather than burying this term within the matter term. Several models allow the cosmological constant (CC) to vary with universe expansion in differing manners. We evaluate these and other popular models including the ΛCDM(standard model), quintessence as presented by Vishwakarma, Equation of State (EoS) and the Carmeli model with data from the 580 Union2.1 supernovae type Ia collection, using several minimization routines and find models built about the CC, the ΛCDM models, fare no better than those without.     

Cosmological implications of different baryon acoustic oscillation data

In this work, we explore the cosmological implications of different baryon acoustic oscillation(BAO) data, including the BAO data extracted by using the spherically averaged one-dimensional galaxy clustering(GC) statistics(hereafter BAO1) and the BAO data obtained by using the anisotropic two-dimensional GC statistics(hereafter BAO2). To make a comparison, we also take into account the case without BAO data(hereafter NO BAO). Firstly, making use of these BAO data, as well as the SNLS3 type Ia supernovae sample and the Planck distance priors data, we give the cosmological constraints of the ΛCDM, the w CDM, and the Chevallier-Polarski-Linder(CPL) model. Then, we discuss the impacts of different BAO data on cosmological consquences, including its effects on parameter space, equation of state(Eo S), figure of merit(Fo M), deceleration-acceleration transition redshift,Hubble parameter H(z), deceleration parameter q(z), statefinder hierarchy S_3(1)(z), S_4(1)(z) and cosmic age t(z). We find that:(1)NO BAO data always give a smallest fractional matter density ?_(m0), a largest fractional curvature density ?k0and a largest Hubble constant h; in contrast, BAO1 data always give a largest ?_(m0), a smallest ?_(k0) and a smallest h.(2) For the w CDM and the CPL model, NO BAO data always give a largest Eo S w; in contrast, BAO2 data always give a smallest w.(3) Compared with the case of BAO1, BAO2 data always give a slightly larger Fo M, and thus can give a cosmological constraint with a slightly better accuracy.(4) The impacts of different BAO data on the cosmic evolution and the comic age are very small, and cannot be distinguished by using various dark energy diagnoses and the cosmic age data.     

Bulk Viscosity and Decaying Vacuum Density in Friedmann Universe

We present an isotropic and homogeneous flat cosmological model for bulk viscous fluid distribution. We consider the vacuum density proportional to Hubble expansion parameter and time dependent bulk viscosity related to the velocity and acceleration of universe. The behaviour of resulting solutions are in accordance with recent astronomical observations. The model obtained evolves with a decelerating expansion followed by late time acceleration. Cosmological term Λ being very large at initial epoch relaxes to a genuine cosmological constant asymptotically. Presence of bulk viscosity prevents the matter density to vanish asymptotically and the matter density continues to be of the order of vacuum density after a finite time. Thus, we obtain a universe having the possibility of cosmic coincidence.     

Friedman-like Robertson–Walker model in generalized metric space-time with weak anisotropy

A generalized FRW model of space-time is studied, taking into consideration the anisotropic structure of fields which are depended on the position and the direction (velocity). The Raychaudhouri and Friedman-like equations are investigated assuming the Finslerian character of space-time. A long range vector field of cosmological origin is considered in relation to a physical geometry where the Cartan connection has a fundamental role. The Friedman equations are produced including extra anisotropic terms. The variation of anisotropy z _t is expressed in terms of the Cartan torsion tensor of the Finslerian manifold. A physical generalization of the Hubble and other cosmological parameters arises as a direct consequence of the equations of motion.     

A Study on the New Agegraphic Dark Energy Model in the Framework of Generalized Uncertainty Principle for Different Scale Factors

In this paper, we consider the New Agegraphic Dark Energy (NADE) model interacting with pressureless Dark Matter (DM) in the framework of generalized uncertainty principle. We consider different expressions of the scale factor a(t) pertaining to the emergent, the intermediate and the logamediate scenarios of the universe. We have derived the expressions for various cosmological parameters in all the three cases and plotted the equation of state (EoS) parameter ω _D and squared speed of the sound $v_{s}^{2}$ to check the stability of the model in each case. We have observed that for emergent and intermediate cases, the EoS parameter has a quintom-like behavior and in the logamediate case it has quintessence-like behavior. The negative squared speed of sound in all of the three cases has indicated that the model is classically unstable for each choice of scale factor.     

A Cyclic Model of Universe with Energy Exchange between Radiation,Matter and Vacuum Energy

We further extend the cosmological scenario with energy exchange by Barrow and Clifton and our previous work to the more complex case with energy exchange between three fluids: radiation, matter and vacuum energy. By prescribing the form of energy exchange function, we construct an infinitely cyclic cosmological model, in which the universe undergoes an endless sequence of cosmic epoch and each consisting of expansion and contraction, and the cosmological parameters, such as the Hubble parameter H, deceleration parameter q, transition red-shift Z _T, and densities ρ _r ,ρ _m , and ρ _Λ are consistent with the present observed values.     

Polytropic Gas Scalar Field Models of Dark Energy

In this work we investigate the polytropic gas dark energy model in the non flat universe. We first calculate the evolution of EoS parameter of the model as well as the cosmological evolution of Hubble parameter in the context of polytropic gas dark energy model. Then we reconstruct the dynamics and the potential of the tachyon and K-essence scalar field models according to the evolutionary behavior of polytropic gas model.