Cosmology with a variable generalized Chaplygin gas

We investigate observational constraint on the variable generalized Chaplygin gas (VGCG) model as the unification of dark matter and dark energy by using the Union supernovae sample and the baryon acoustic oscillations data. Based on the best fit parameters for VGCG model it is shown that the current value of equation of state for dark energy is w_0de=−1.08<−1, and the universe will not end up with big rip in the future. In addition, we also discuss the evolution of several quantities in VGCG cosmology such as deceleration parameter, fractional density parameters, growth index and sound speed. Finally, the statefinder diagnostic is performed to discriminate the VGCG with other models.     

Statefinder Diagnostic and w?w′ Analysis for Interacting Polytropic Gas Dark Energy Model

The interacting polytropic gas dark energy model is investigated from the viewpoint of statefinder diagnostic tool and w−w′ analysis. The dependency of the statefinder parameters on the parameter of the model as well as the interaction parameter between dark matter and dark energy is calculated. We show that different values of the parameters of model and different values of interaction parameter result different evolutionary trajectories in s−r and w−w′ planes. The polytropic gas model of dark energy mimics the standard ΛCDM model at the early time.     

Statefinder Diagnostic for Modified Chaplygin Gas with the Sign-Changeable Interaction

Statefinder diagnostic is a useful method which can distinguish one dark energy model from the others. In this paper, we apply this method to the modified Chaplygin gas (MCG) model with the sign-changeable interaction in which the interaction term Q can change its sign from Q<0 to Q>0 as the universe expands. We plot the evolutionary trajectories of this model in the statefinder parameter-planes, and it is found that the coupling constant β plays a significant role from the statefinder viewpoint. Furthermore, we can conclude that the statefinder diagnostic can not only discriminate the model with different coupling constant but also distinguish the model from other dark energy models.     

Accelerating Universe in Brans-Dicke Theory in Presence of Chaplygin Gas

In this paper, we have considered a model for Brans-Dicke scalar field in presence of Chaplygin gas and interaction between them. We have shown that the BD parameter ω is constant or not, the Chaplygin gas provides early deceleration and late time acceleration of the universe. The graphical representation of statefinder parameters shows the total evolution of the universe starts from radiation era to phantom model.     

Statefinder Parameters for Different Dark Energy Models with Variable G Correction in Kaluza-Klein Cosmology

In this work, we have calculated the deceleration parameter, statefinder parameters and EoS parameters for different dark energy models with variable G correction in homogeneous, isotropic and non-flat universe for Kaluza-Klein Cosmology. The statefinder parameters have been obtained in terms of some observable parameters like dimensionless density parameter, EoS parameter and Hubble parameter for holographic dark energy, new agegraphic dark energy and generalized Chaplygin gas models.     

Evolution of Holographic Dark Energy in Interacting Modified Chaplygin Gas Model

We investigate the modified Chaplygin gas （MCG） with interaction between holographic dark energy proposed by Li and dark matter. In this model, evolution of the universe is described in detail, which is from deceleration to acceleration. Specifically, the evolutions of related cosmological quantities such as density parameter, the equation of state of holographic dark energy, deceleration parameter and transition redshift are discussed. Moreover, we also give their present values which are consistent with the lately observations. Furthermore, the results given by us show such a model can accommodate a transition of the dark energy from a normal state wx 〉 -1 to ωx 〈 -1 phantom regimes.     

Variable <Emphasis Type="Italic">G</Emphasis> Corrections to Statefinder Parameters of Dark Energy

Motivated by several observational and theoretical developments concerning the variability of Newton’s gravitational constant with time G(t), we calculate the varying G correction to the statefinder parameters for four models of dark energy namely interacting dark energy holographic dark energy, new-agegraphic dark energy and generalized Chaplygin gas.     

Modified holographic dark energy in non-flat Kaluza–Klein universe with varying <Emphasis Type="Italic">G</Emphasis>

The purpose of this paper is to discuss the evolution of modified holographic dark energy with variable G in non-flat Kaluza–Klein universe. We consider the non-interacting and interacting scenarios of the modified holographic dark energy with dark matter and obtain the equation of state parameter through logarithmic approach. It turns out that the universe remains in different dark energy eras for both cases. Further, we study the validity of the generalized second law of thermodynamics in this scenario. We also justify that the statefinder parameters satisfy the limit of ΛCDM model.     

Dissipative Future Universe Without Big Rip

The present study deals with dissipative future universe without big rip in context of Eckart formalism. The generalised Chaplygin gas, characterised by equation of state p=-\fracAr^\frac1ap=-\frac{A}{\rho^{\frac{1}{\alpha}}}, has been considered as a model for dark energy due to its dark-energy-like evolution at late time. It is demonstrated that, if the cosmic dark energy behaves like a fluid with equation of state p=ωρ; ω<−1 as well as Chaplygin gas simultaneously then the big rip problem does not arise and the scale factor is found to be regular for all time.     

Fitting Cosmological Data to the Function <Emphasis Type="Italic">q</Emphasis>(<Emphasis Type="Italic">z</Emphasis>) from GR Theory: Modified Chaplygin Gas

In the Friedmann cosmology, the deceleration of the expansion q plays a fundamental role. We derive the deceleration as a function of redshift q(z) in two scenarios: ΛCDM model and modified Chaplygin gas (MCG) model. The function for the MCG model is then fitted to the cosmological data in order to obtain the cosmological parameters that minimize χ ². We use the Fisher matrix to construct the covariance matrix of our parameters and reconstruct the q(z) function. We use Supernovae Ia, WMAP5, and BAO measurements to obtain the observational constraints. We determined the present acceleration as q ₀ = − 0.65 ±0.19 for the MCG model using the Union2 dataset of SNeIa, BAO, and CMB and q ₀ = − 0.67 ±0.17 for the Constitution dataset, BAO and CMB. The transition redshift from deceleration to acceleration was found to be around 0.80 for both datasets. We have also determined the dark energy parameter for the MCG model: Ω _X0 = 0.81 ±0.03 for the Union2 dataset and Ω _X0 = 0.83 ±0.03 using the Constitution dataset.     

Statefinder Diagnostic for Variable Generalized Chaplygin Gas with the Sign-Changeable Interaction

In this paper, we apply the statefinder diagnostic to variable generalized Chaplygin gas (VGCG) with the sign-changeable interaction in which the interaction term Q can change its sign from Q<0 to Q>0 as the universe expands. The evolution trajectories of the statefinder pairs {r,s} and {r,q} are obtained under the circumstance where different values of model parameters are chosen. It is found that the coupling term does not affect the location of the late time attractor, but has an influence on the evolution of the statefinder parameters. Furthermore, it is shown that the evolution trajectories of our model in the r(s) diagram are different from those of other dark energy models.     

Cosmic-coincidence problem and variable constants of physics

The standard model of cosmology is investigated using a time-dependent cosmological constant Λ and Newton gravitational constant G. The total energy content is described by the modified Chaplygin gas equation of state. It is found that the time-dependent constants coupled with the modified Chaplygin gas interpolate between the earlier matter to the later dark-energy dominated phase of the universe. We also achieve a convergence of the parameter ω→−1, almost at the present time. Thus our model fairly alleviates the cosmic-coincidence problem, which demands ω=−1 at the present time.     

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.     

Interacting Generalized Cosmic Chaplygin Gas in Loop Quantum Cosmology: A Singularity Free Universe

In this work we investigate the background dynamics when dark energy is coupled to dark matter with a suitable interaction in the universe described by Loop quantum cosmology. Dark energy in the form of Generalized Cosmic Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is taken into account in order to at least alleviate (if not solve) the cosmic coincidence problem. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. A significant attempt towards the solution of the cosmic coincidence problem is taken. The statefinder parameters are also calculated to classify the dark energy model. Graphs and phase diagrams are drawn to study the variations of these parameters. It is seen that the background dynamics of Generalized Cosmic Chaplygin gas is completely consistent with the notion of an accelerated expansion in the late universe. From the graphs, generalized cosmic Chaplygin gas is identified as a dark fluid with a lesser negative pressure compared to Modified Chaplygin gas, thus supporting a ‘No Big Rip’ cosmology. It has also been shown that in this model the universe follows the power law form of expansion around the critical point, which is consistent with the known results. Future singularities that may be formed in this model as an ultimate fate of the universe has been studied in detail. It was found that the model is completely free from any types of future singularities.     

Effects of modified Chaplygin gas on structure formation in the universe

In this paper, the structure formation theories for the modified Chaplygin gas (MCG) model are established in the linear and non-linear regimes. Concretely, for the linear regime, the evolutions of the growth index $f$ and the growth variable $T$ are illustrated for the interacting MCG (IMCG) model and MCG model without interaction between dark energy and dark matter, which can give reasonable predictions for structure formation. While for the non-linear regime, by supposing the homogeneity and conservation of dark energy when the system reaches virialization, we can point out that MCG reaches the state of turn around later than GCG, the value of the collapse factor in MCG model is bigger than the fiducial value 0.5 in Einstein-de Sitter universe, namely $\eta >0.5$ , because of the effective repulsive force of dark energy, and the density contrast of the virialization tends to the Einstein-de Sitter value $18\pi ^{2}$ . Furthermore, the evolutions of the cluster number counts in LCDM, GCG and MCG models without and with interaction between dark energy and dark matter are illustrated by extending the Press-Schechter framework, which can exhibit the differences among the three models.     

Anisotropic Universe Models with Perfect Fluid and Dark Energy with Time Varying G and Λ

We have investigated general Bianchi type I cosmological models which containing a perfect fluid and dark energy with time varying G and Λ that have been presented. The perfect fluid is taken to be one obeying the equation of state parameter, i.e., p=ωρ; whereas the dark energy density is considered to be either modified polytropic or the Chaplygin gas. Cosmological models admitting both power-law which is explored in the presence of perfect fluid and dark energy too. We reconstruct gravitational parameter G, cosmological term Λ, critical density ρ _c , density parameter Ω, cosmological constant density parameter Ω _Λ and deceleration parameter q for different equation of state. The present study will examine non-linear EOS with a general nonlinear term in the energy density.     

Observational constraint on generalized Chaplygin gas model

We investigate observational constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy from the latest observational data: the Union SNe Ia data, the observational Hubble data, the SDSS baryon acoustic peak and the five-year WMAP shift parameter. The result is obtained that the best-fit values of the GCG model parameters with their confidence level are A _s=0.73_−0.06^+0.06 (1σ) _−0.09^+0.09 (2σ), α=−0.09_−0.12^+0.15 (1σ) _−0.19^+0.26 (2σ). Furthermore, in this model, we can see that the evolution of equation of state (EOS) for dark energy is similar to quiessence, and its current best-fit value is w _0de=−0.96 with the 1σ confidence level −0.91≥w _0de≥−1.00.     

Constraining the coupling constant between dark energy and dark matter

We have investigated constraints on the coupling between dark matter and the interacting Chaplygin gas. Our results indicate that the coupling constant c between these two entities can take arbitrary values, which can be either positive or negative, thus giving arbitrary freedom to the inter-conversion between Chaplygin gas and dark matter. Thus, our results indicate that the restriction 0<c<1 on the coupling constant occurs as a very special case. Our analysis also supports the existence of phantom energy under certain conditions on the coupling constant.     

Co-Existence of Modified Chaplygin Gas and Other Dark Energies in the Framework of Fractal Universe

In this work, we consider a non-flat universe in the framework of fractal cosmology. We have investigated the co-existence of different kinds of dark energy models such as tachyonic field, DBI-essence, hessence, k-essence, dilaton, quintessence with the modified Chaplygin gas (MCG) in fractal universe and obtained the statefinder parameters. The natures of the scalar fields and the concerned potentials have been analyzed by the correspondence scenario in the fractal universe.     

Cosmological Implications of Interacting Polytropic Gas Dark Energy Model in Non-flat Universe

The polytropic gas model is investigated as an interacting dark energy scenario. The cosmological implications of the model including the evolution of EoS parameter w _Λ, energy density Ω_Λ and deceleration parameter q are investigated. We show that, depending on the parameter of model, the interacting polytropic gas can behave as a quintessence or phantom dark energy. In this model, the phantom divide is crossed from below to up. The evolution of q in the context of polytropic gas dark energy model represents the decelerated phase at the early time and accelerated phase later. The singularity of this model is also discussed. Eventually, we establish the correspondence between interacting polytropic gas model with tachyon, K-essence and dilaton scalar fields. The potential and the dynamics of these scalar field models are reconstructed according to the evolution of interacting polytropic gas.