Interacting Modified Cosmic Chaplygin Gas with Variable Cosmological Constant and Viscosity

In this paper we study interaction between modified cosmic Chaplygin gas and pressureless matter in presence of both bulk and shear viscosities as a model of our Universe. Also we consider variable cosmological constant and investigate some cosmological parameters such as sound speed and time-dependent density. We investigate stability of model by using first order linear perturbation.     

Viscous New Varying Modified Cosmic Chaplygin Gas with Cosmological Constant in Non-flat Universe

In this paper we study new varying modified cosmic Chaplygin gas which has viscosity in presence of cosmological constant and space curvature. By using well-known forms of scale factor in Friedmann equation we obtain behavior of dark energy density numerically. We use observational data to fix solution and discuss about stability of our system.     

Variable Modified Chaplygin Gas in Anisotropic Universe with Kaluza-Klein Metric

In this work, we have considered Kaluza-Klein Cosmology for anisotropic universe where the universe is filled with Variable Modified Chaplygin Gas (VMCG). Here we find normal scalar field ? and the self interacting potential V(?) to describe the VMCG Cosmology. We have also graphically analyzed the geometrical parameters named Statefinder Parameters in anisotropic Kaluza-Klein model. Next, we have considered a Kaluza-Klein model of interacting VMCG with dark matter in the Einstein gravity framework. Here we construct the three dimensional autonomous dynamical system of equations for this interacting model with the assumption that the dark energy and the dark matter interacts between themselves and for that we also choose the interaction term. We convert that interaction term to its dimensionless form and perform stability analysis and solve them numerically. We obtain a stable scaling solution of the equations in Kaluza-Klein model and graphically represent solutions.     

Interacting Viscous Modified Chaplygin Gas Cosmology in Presence of Cosmological Constant

Here, we consider interacting viscous modified Chaplygin gas in presence of cosmological constant. We assumed bulk viscosity as a function of density. We consider interaction between modified Chaplygin gas and baryonic matter. Then, the effects of viscosities on the cosmological parameters such as energy, density, Hubble expansion parameter, scale factor and deceleration parameter investigated. This model may be considered as a toy model of our universe.     

Universe Described by Variable Modified Chaplygin Gas with Statefinder Diagnostic in General Relativity

The universe filled with variable modified Chaplygin gas having the equation of state p=Aρ?B/ρ ^α , where 0≤α≤1, A is a positive constant and B is a positive function of the average scale factor a(t) of the universe (i.e. B=B(a)) is studied within the framework of general relativity. The new class of exact solutions of Einstein’s field equations is derived by using a time dependent deceleration parameter. The cosmic jerk parameter in our derived model is 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 behavior of different stages of the universe is studied. The physical and kinematical properties of cosmological models are also discussed.     

Viscous Varying Generalized Chaplygin Gas with Cosmological Constant and Space Curvature

In this paper we study varying generalized Chaplygin gas which has viscosity in presence of cosmological constant and space curvature. By using well-known forms of scale factor in non-linear differential equation we obtain behavior of dark energy density numerically. We use observational data to fix solution and discuss about stability of our system.     

Cosmological Evolution of Modified Chaplygin Gas with the Sign-Changeable Interactions

The cosmological evolution of modified Chaplygin Gas (MCG) model with the sign-changeable interactions is studied. The key point of the new interaction is the deceleration parameter $q\equiv-\ddot{a}a/\dot{a}^{2}$ in the interaction term Q. This new feature gives the possibility that interaction Q can change its sign when the universe changes from deceleration (q>0) to acceleration (q<0) and brings different evolution to cosmology. We find that there exist some stable scaling attractors, which can alleviate the coincidence problem. The equation of state (EoS) of MCG approaches the attractor phase from either w _g >?1 or w _g $\frac{2\kappa^{2}}{9H^{3}}Q$ with the proper parameters and find the new interaction Q has a transition from Q<0 to Q>0 as the universe expands, which is different from the usual interaction. The numerical calculation shows that a heteroclinic orbit (solution of dynamical system) can interpolate between MCG matter-dominated phase (an unstable critical point) and MCG vacuum-energy-dominated attractor.     

FRW Cosmological Model with Modified Chaplygin Gas and Dynamical System

The Friedmann-Robertson-Walker (FRW) model with dynamical Dark Energy (DE) in the form of modified Chaplygin gas (MCG) has been investigated. The evolution equations are reduced to an autonomous system on the two dimensional phase plane and it can be interpreted as the motion of the particle in an one dimensional potential. Also the dynamical system analysis has been extended to examine the critical points at infinity with will exist provided the equation of state parameter $\omega<-\frac{1}{3}$ . Finally, theoretical points are asymptotically stable or unstable.     

Role of Modified Chaplygin Gas in Bianchi Type-I Universe

We have studied the evolution of a homogeneous, anisotropic universe given by a Bianchi type-I cosmological model with modified Chaplygin gas. We have assumed that the equation of state of this modified model is valid from the radiation era to the ΛCMD model. We have used state-finder parameters in characterizing different phase of the model.     

Thermodynamics of New Modified Chaplygin Gas in Magnetic FRW Universe

We study the generalized second law of thermodynamics in the presence of non-interacting magnetic field and new modified Chaplygin gas with FRW universe. In this scenario, we investigate the validity of this law on Hubble, apparent, particle and event horizons. It is found that this law is respected on all horizons for specific values of the model parameters except on the event horizon where it does not hold for short time but remains valid otherwise. Finally, we explore the statefinders and Om diagnostic to check the viability of the present cosmological model.     

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.     

Letter: Density Perturbations in a Universe Dominated by the Chaplygin Gas

We study the fate of density perturbations in a Universe dominate by the Chaplygin gas, which exhibit negative pressure. In opposition to other models of perfect fluid with negative pressure, there is no instability in the small wavelength limit, due to the fact that the sound velocity for the Chaplygin gas is positive. We show that it is possible to obtain the value for the density contrast observed in large scale structure of the Universe by fixing a free parameter in the equation of state of this gas. The negative character of pressure must be significant only very recently.     

Viscous Chaplygin Gas in Non-flat Universe

In this work we study viscous Chaplygin gas and obtain modified Friedmann equations due to viscosity. We calculate time-dependent energy density for the case of non-flat universe. By using stability condition and speed of sound we find critical value of viscosity coefficient where speed of sound is finite.     

Early Universe with Variable Gravitational and Cosmological Constants

Einstein field equations are considered in zero-curvature Robertson–Walker (R–W) cosmology with perfect fluid source and time-dependent gravitational and cosmological “constants.” Exact solutions of the field equations are obtained by using the ’gamma-law' equation of state p = (γ − 1)ρ in which γ varies continuously with cosmological time. The functional form of γ (R) is used to analyze a wide range of cosmological solutions at early universe for two phases in cosmic history: inflationary phase and Radiation-dominated phase. The corresponding physical interpretations of the cosmological solutions are also discussed.     

Constraining the Parameters of New Variable Modified Chaplygin Gas Model

Assuming the flat FRW universe in Einstein’s gravity filled with New Variable Modified Chaplygin gas (NVMCG) dark energy and dark matter having negligible pressure. In this research work we analyze the viability on the basis of recent observation. Hubble parameter H is expressed in terms of the observable parameters H ₀, $\varOmega_{m}^{0}$ and the model parameters A ₀, B ₀, C ₀, m, n, α and the red shift parameter z. Here we find a best fitted parameter range of A ₀, B ₀ keeping 0≤α≤1 and using Stern data set (12 points) by minimizing the χ ² test at 66 %, 90 % and 99 % confidence levels. Next we do the joint analysis with BAO and CMB observations. Again evaluating the distance modulus μ(z) vs redshift (z) curve obtained in the model NVMCG with dark matter with the best fitted value of the parameters and comparing with that derived from the Union2 compilation data.     

Viscous Generalized Chaplygin Gas in Non-flat Universe

In this paper we study viscous generalized Chaplygin gas and obtain modified Friedmann equations due to viscosity. In the case of non-flat universe we calculate time-dependent energy density of generalized Chaplygin gas. By using stability condition and speed of sound we find critical value of viscosity coefficient where speed of sound is finite.     

Eleven Spherically Symmetric Constant Density Solutions with Cosmological Constant

Einstein's field equations with cosmological constant are analysed for a static, spherically symmetric perfect fluid having constant density. Five new global solutions are described. One of these solutions has the Nariai solution joined on as an exterior field. Another solution describes a decreasing pressure model with exterior Schwarzschild-de Sitter spacetime having decreasing group orbits at the boundary. Two further types generalise the Einstein static universe. The other new solution is unphysical, it is an increasing pressure model with a geometric singularity.     

Bianchi Type-I Space-Time with Variable Cosmological Constant

A spatially homogeneous and anisotropic Bianchi type-I perfect fluid model is considered with variable cosmological constant. Einstein’s field equations are solved by using a law of variation for mean Hubble’s parameter, which is related to average scale factor and that yields a constant value of deceleration parameter. An exact and singular Bianchi-I model is presented, where the cosmological constant remains positive and decreases with the cosmic time. It is found that the solutions are consistent with the recent observations of type Ia supernovae. A detailed study of physical and kinematical properties of the model is carried out.     

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.     

Evolution of Variable Generalized Chaplygin Gas

We consider the variable Generalized Chaplygin gas （VGCG） proposal for unification of dark matter and dark energy with p = pdc and ρ= pdm ＋ ρdc. The equation of state of the VGCG is given by p = -A0a^-n/ρ^α, where a is the scale factor. Some cosmological quantities such as the fractional contributions of different components of the universe Ωi （i respectively denotes baryons, dark matter and dark energy） to the critical density, the deceleration parameter q are all obtained. The transition from deceleration to acceleration is described in this model. In addition, we find the behaviour of variable Generalized Chaplgin gas is similar to dust-like matter at early times and will be quiessence or phantom at late stage.