Letter: Statefinder Parameters for Interacting Dark Energy

We argue that the recently introduced statefinder parameters (Sahni et al., JETP Lett. 77, 201 (2003)), that include the third derivative of the cosmic scale factor, are useful tools to characterize interacting quintessence models. We specify the statefinder parameters for two classes of models that solve, or at least alleviate, the coincidence problem.     

Loop Quantum Corrections to Statefinder Parameters of Dark Energy

In this paper, we have calculated the statefinder parameters for the Friedmann-Robertson-Walker (FRW) Universe in the gravitational framework of loop quantum cosmology (LQC). As examples, we study two types of dark energy models namely Holographic dark energy and New-Agegraphic dark energy.     

Statefinder Description in Generalized Holographic and Ricci Dark Energy Models

We have considered the generalized holographic and generalized Ricci dark energy models for acceleration of the universe. If the universe filled with only GHDE/GRDE the corresponding deceleration parameter, EOS parameter and statefinder parameters have been calculated. Next we have considered that the mixture of GHDE/GRDE and dark matter in interacting and non-interacting situations. Also the mixture of GHDE/GRDE and generalized Chaplygin gas have been analyzed during evolution of the universe. The natures of above mentioned parameters have been investigated for interacting and non-interacting situations. Finally, it follows that the prescribed models derive the acceleration of the universe.     

Reconstruction of Scalar Field Dark Energy Models in Kaluza-Klein Universe

This paper is devoted to study the modified holographic dark energy model by taking its different aspects in the flat Kaluza-Klein universe.We construct the equation of state parameter which evolutes the universe from quintessence region towards the vacuum.It is found that the modified holographic model exhibits instability against small perturbations in the early epoch of the universe but becomes stable in the later times.We also develop its correspondence with some scalar field dark energy models.It is interesting to mention here that all the results are consistent with the present observations.     

Constraining f(G) Gravity Models Using Energy Conditions

Recently, energy condition inequalities in the context of modified Gauss-Bonnet gravity have been derived in Garcia et al. (Phys. Rev. D, 83:104032, 2011). Using these general inequalities, we examine the viability of specific forms of f(G) models proposed in De Felice and Tsujikawa (Phys. Lett. B, 675:1, 2009) that can be responsible for the late-time cosmic acceleration following the matter era. In doing so we also use the recent estimated values of the deceleration, jerk and snap parameters to obtain the bounds from the weak and strong energy conditions on the parameters of the above mentioned forms of f(G) gravity theories.     

Kaluza-Klein Cosmology: Friedmann Models with Phenomenological Matter

We reduce the FRIEDMANN models in generalized KALUZA -KLEIN cosmologies, in which the ordinary space-time is supplemented by internal factor spaces, to the motion of a tensorial mass particle in a scalar field. Some general properties of these models as well as exact solutions for the case of one internal space are discussed for different mixtures of phenomenological matter components.     

Dark Energy Models with Variable Equation of State Parameter

The dark energy models with variable equation of state parameter ω are investigated by using law of variation of Hubble’s parameter that yields the constant value of deceleration parameter. Here the equation of state parameter ω is found to be time dependent and its existing range for this model is consistent with the recent observations of SN Ia data, SN Ia data (with CMBR anisotropy) and galaxy clustering statistics. The physical significance of the dark energy models have also been discussed.     

Dark Energy Cosmology with a Rarita-Schwinger Field

Recent observations of the Cosmic Microwave Background, Supernovae and Sloan Digital Sky Survey (SDSS) show that our universe has a critical energy density, and roughly 2/3 of it is dark energy, which drives the accelerating expansion of the cosmos. In view of the astrophysical data, we find that the equation of state parameter of the dark energy lies in a narrow range around w = −1. In this paper, we construct a cosmology model with a Rarita-Schwinger field to realize the equation of state parameter w < −1 or w > −1 and discuss its stability.     

关于宇宙中的暗能量

近年中，WMAP卫星通过对微波背景辐射的各向异性的详尽测量，推断出了宇宙物质各种主要组分的密度．它引起了人们广泛的兴趣和关注．其中两点倍受注意：一是真空对宇宙总密度的贡献最大，约占70％；二是实物只约占30％，而其5/6是非常规的暗物质．这些事实向物理学提出了两个很深刻的问题：什么是真空能？以及什么是非重子暗物质？我在上一篇文章中已讨论了后一个问题，即暗物质问题．本文要讨论的是前一个问题，它原叫真空能问题，现在人们常叫它暗能量问题．这问题的物理渊源较复杂．本文的意图是对它的来龙和去脉做些初步的说明．     

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.     

Holographic Dark Energy in Brans-Dicke Cosmology with Granda-Oliveros Cut-off

Motivated by the recent works of one of us (Karami and Fehri, Int. J. Theor. Phys. 49:1118, 2010; Phys. Lett. B 684:61, 2010), we study the holographic dark energy in Brans-Dicke gravity with the Granda-Oliveros cut-off proposed recently in literature. We find out that when the present model is combined with Brans-Dicke field the transition from normal state where w _D >−1 to the phantom regime where w _D <−1 for the equation of state of dark energy can be more easily achieved for than when resort to the Einstein field equations is made. Furthermore, the phantom crossing is more easily achieved when the matter and the holographic dark energy undergo an exotic interaction. We also calculate some relevant cosmological parameters and their evolution.     

Models of G time variations in diverse dimensions

A review of different cosmological models in diverse dimensions leading to a relatively small time variation in the effective gravitational constant G is presented. Among them: the 4-dimensional (4-D) general scalar-tensor model, the multidimensional vacuum model with two curved Einstein spaces, the multidimensional model with the multicomponent anisotropic “perfect fluid”, the S-brane model with scalar fields and two form fields, etc. It is shown that there exist different possible ways of explaining relatively small time variations of the effective gravitational constant G compatible with present cosmological data (e.g. acceleration): 4-dimensional scalar-tensor theories or multidimensional cosmological models with different matter sources. The experimental bounds on Ġ may be satisfied either in some restricted interval or for all allowed values of the synchronous time variable.      

Power Law and Logarithmic Entropy Corrected Ricci Dark Energy Models in Brans-Dicke Chameleon Cosmology

In this work, we study the power-law and the logarithmic entropy corrected versions of the Ricci Dark Energy (RDE) model in the framework of the Brans-Dicke cosmology non-minimally coupled with a chameleon scalar field ?. Considering the presence of interaction between Dark Energy (DE) and Dark Matter (DM), we derived the expressions of some relevant cosmological parameters, i.e. the equation of state parameter ω _D , the deceleration parameter q and the evolution of the energy density parameter $\varOmega'_{D}$ .     

Interacting Tachyon Dark Energy Model in Gauss-Bonnet Cosmology

We investigate the tachyon scalar filed model of dark energy in the framework of Gauss-Bonnet cosmology. We consider a spatially non-flat universe containing interacting tachyon dark energy with pressureless dark matter. We obtain the equation of state and deceleration parameters. We also reconstruct the potential and the dynamics for the tachyon scalar field model, which describe accelerated expansion of the universe.     

Higher Dimensional Cosmology with Some Dark Energy Models in Emergent, Logamediate and Intermediate Scenarios of the Universe

We have considered N-dimensional Einstein field equations in which four-dimensional space-time is described by a FRW metric and that of extra dimensions by an Euclidean metric. We have chosen the exponential forms of scale factors a and d numbers of b in such a way that there is no singularity for evolution of the higher dimensional Universe. We have supposed that the Universe is filled with K-essence, Tachyonic, Normal Scalar Field and DBI-essence. Here we have found the nature of potential of different scalar field and graphically analyzed the potentials and the fields for three scenario namely Emergent Scenario, Logamediate Scenario and Intermediate Scenario. Also graphically we have depicted the geometrical parameters named statefinder parameters and slow-roll parameters in the higher dimensional cosmology with the above mentioned scenarios.     

Interacting Ricci Logarithmic Entropy-Corrected Holographic Dark Energy in Brans-Dicke Cosmology

In the derivation of Holographic Dark Energy (HDE), the area law of the black hole entropy assumes a crucial role. However, the entropy-area relation can be modified including some quantum effects, motivated from the Loop Quantum Gravity (LQG), string theory and black hole physics. In this paper, we study the cosmological implications of the interacting logarithmic entropy-corrected HDE (LECHDE) model in the framework of Brans-Dicke (BD) cosmology. As system’s infrared (IR) cut-off, we choose the average radius of Ricci scalar curvature, i.e. R ^?1/2. We obtain the Equation of State (EoS) parameter ω _D , the deceleration parameter q and the evolution of energy density parameter $\varOmega'_{D}$ of our model in a non-flat universe. Moreover, we study the limiting cases corresponding to our model without corrections and to the Einstein’s gravity.     

Carmeli's Cosmology Fits Data for an Accelerating and Decelerating Universe Without Dark Matter or Dark Energy

A new relation for the density parameter Ω is derived as a function of expansion velocity υ based on Carmeli's cosmology. This density function is used in the luminosity distance relation D _L. A heretofore neglected source luminosity correction factor (1 − (υ/c)²)^−1/2 is now included in D _L. These relations are used to fit type Ia supernovae (SNe Ia) data, giving consistent, well-behaved fits over a broad range of redshift 0.1 < z < 2. The best fit to the data for the local density parameter is Ω_m = 0.0401 ± 0.0199. Because Ω_m is within the baryonic budget there is no need for any dark matter to account for the SNe Ia redshift luminosity data. From this local density it is determined that the redshift where the universe expansion transitions from deceleration to acceleration is z _t = 1.095^+0.264 _−0.155. Because the fitted data covers the range of the predicted transition redshift z _t, there is no need for any dark energy to account for the expansion rate transition. We conclude that the expansion is now accelerating and that the transition from a closed to an open universe occurred about 8.54 Gyr ago.