FRW bulk viscous cosmology with modified cosmic Chaplygin gas

In this paper we study FRW bulk viscous cosmology in presence of modified cosmic Chaplygin gas. We obtain generalized Friedmann equations due to bulk viscosity and modified cosmic Chaplygin gas. Then, we calculate time-dependent energy density and discuss Hubble expansion parameter.     

How effective is new variable modified Chaplygin gas to play the role of dark energy—a dynamical system analysis in RS II brane model

Motivated by some previous works of Rudra et al. we set to explore the background dynamics when dark energy in the form of New Variable Modified Chaplygin gas is coupled to dark matter with a suitable interaction in the universe described by brane cosmology. The main idea is to find out the efficiency of New variable modified Chaplygin gas to play the role of DE. As a result we resort to the technique of comparison with standard dark energy models. Here the RSII brane model have been considered as the gravity theory. An interacting model is considered in order to search for a possible solution of the cosmic coincidence problem. A dynamical system analysis is performed because of the high complexity of the system. 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 and get an insight into the effectiveness of the dark energy model. It is also seen that the background dynamics of New Variable Modified Chaplygin gas is consistent with the late cosmic acceleration. After performing an extensive mathematical analysis, we are able to constrain the parameters of new variable modified Chaplygin gas as m<n to produce the best possible results. Future singularities are studied and it is found that the model has a tendency to result in such singularities unlike the case of generalized cosmic Chaplygin gas. Our investigation leads us to the fact that New Variable Modified Chaplygin gas is not as effective as other Chaplygin gas models to play the role of dark energy.     

Dynamics of interacting generalized cosmic Chaplygin gas in brane-world scenario

In this work we explore the background dynamics when dark energy is coupled to dark matter with a suitable interaction in the universe described by brane cosmology. Here DGP and the RSII brane models have been considered separately. Dark energy in the form of Generalized Cosmic Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is considered 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 models. Graphs and phase diagrams are drawn to study the variations of these parameters. It is also seen that the background dynamics of Generalized Cosmic Chaplygin gas is consistent with the late cosmic acceleration, but not without satisfying certain conditions. It has been shown that the universe in both the models follows the power law form of expansion around the critical point, which is consistent with the known results. Future singularities were studied and our models were declared totally free from any types of such singularities. Finally, some cosmographic parameters were also briefly studied. Our investigation led to the fact that although Generalized cosmic Chaplygin gas with a far lesser negative pressure compared to other dark energy models, can overcome the relatively weaker gravity of RS II brane, with the help of the negative brane tension, yet for the DGP brane model with much higher gravitation, the incompetency of Generalized cosmic Chaplygin gas is exposed, and it cannot produce the accelerating scenario until it reaches the phantom era.     

Holographic dark energy scenario and variable modified Chaplygin gas

In this letter, we have considered that the universe is filled with normal matter and variable modified Chaplygin gas. Also we have considered the interaction between normal matter and variable modified Chaplygin gas in FRW universe. Then we have considered a correspondence between the holographic dark energy density and interacting variable modified Chaplygin gas energy density. Then we have reconstructed the potential of the scalar field which describes the variable modified Chaplygin cosmology.     

The sign-changeable interaction between variable generalized Chaplygin gas and dark matter

In this paper, we study a cosmological model with the sign-changeable interaction between variable generalized Chaplygin gas (VGCG) and dark matter. The dynamical analysis indicates that there exists a stable scaling attractor, which can help to alleviate the coincidence problem. Furthermore, when the parameters of the model take some fixed values, the attractor corresponds to the phase w=?0.939 and the equation of state of VGCG approaches it from either w>?1 or w<?1 depending on the choice of its initial cosmic density parameter and the ratio of pressure to critical energy density. So, the phantom divide can be crossed. We find the interaction term Q can change its sign from Q<0 to Q>0 as the universe expands, which is different from the usual interaction. Also, we place constraints on the parameters from the point of view of dynamics.     

Dynamics of modified Chaplygin gas in brane world scenario: phase plane analysis

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 brane cosmology. Here DGP and the RSII brane models have been considered separately. Dark energy in the form of modified Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is considered 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 models. Graphs and phase diagrams are drawn to study the variations of these parameters. It is also seen that the background dynamics of modified Chaplygin gas is completely consistent with the notion of an accelerated expansion in the late universe. Finally, it has been shown that the universe in both the models follows the power law form of expansion around the critical point, which is consistent with the known results.     

FRW bulk viscous cosmology with modified Chaplygin gas in flat space

In this paper we study FRW bulk viscous cosmology in presence of modified Chaplygin gas. We write modified Friedmann equations due to bulk viscosity and Chaplygin gas and obtain time-dependent energy density for the special case of flat space.     

Warm generalized cosmic Chaplygin gas inflation inspired by generalized dissipative coefficient

We examine the warm inflationary universe model in the presence of generalized cosmic Chaplygin gas and standard scalar field. We assume the generalized dissipative coefficient and corresponding weak and strong dissipative regimes. In this framework, we analyze the characteristics of inflationary dynamics under the slow-roll approximations. Under these approximations, we formulate the important inflationary parameters, such as scalar spectral index, scalar and tensor power spectrum, tensor-to-scalar ratio etc. It is interesting to note that our results for these inflationary parameters are well corroborated with the recent observational data like WMAP7, WMAP9 and Planck data.     

Dieterici gas as a unified model for dark matter and dark energy

The dominance of dark energy in the universe has necessitated the introduction of a repulsive gravity source to make q₀ negative. The models for dark energy range from a simple Λ term to quintessence, Chaplygin gas, etc. We look at the possibility of how change of behaviour of missing energy density, from DM to DE, may be determined by the change in the equation of state of a background fluid instead of a form of potential. The question of cosmic acceleration can be discussed within the framework of theories which do not necessarily include scalar fields.     

Interaction between phantom field and modified Chaplygin gas

In this letter, we have considered a flat FRW universe. Instead of considering only one candidate for the dark energy, we have considered the interaction between phantom field and modified Chaplygin gas. It has been shown that the potential of the phantom field increases from a lower value with evolution of the universe. It has been observed that the field has an increasing tendency and the potential has also an increasing tendency with passage of cosmic time. In the evolution of the universe the crossing of w=−1 has been realized by this interacting model.     

Correspondence between DBI-essence and modified Chaplygin gas and the generalized second law of thermodynamics

In this work, we have considered the DBI-essence dark energy model in FRW Universe. We have found the exact solutions of potential, warped brane tension and DBI scalar field. We also calculate the statefinder parameters for our model that make it distinguishable among numerous dark energy models. Moreover, we establish correspondence between DBI-essence and modified Chaplygin gas (MCG) and hence reconstruct the potential and warped brane tension. By this reconstruction, we observe that DBI scalar field and potential increase and warped brane tension decreases during evolution of the Universe. Finally, we investigate the validity of the generalized second law (GSL) of thermodynamics in the presence of DBI-essence and modified Chaplygin gas. It is observed that the GSL breaks down for DBI-essence model but GSL always satisfied for MCG model.     

Interacting modified Chaplygin gas in loop quantum cosmology

We investigate the background dynamics when dark energy is coupled to dark matter in the universe described by loop quantum cosmology. We consider dark energy of the form modified Chaplygin gas. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. It henceforth resolves the famous cosmic coincidence problem in modern cosmology. The statefinder parameters are also calculated to classify this dark energy model.     

The generalized second law for the interacting generalized Chaplygin gas model in non-flat universe enclosed by the apparent horizon

We investigate the validity of the generalized second law of gravitational thermodynamics in a non-flat FRW universe containing the interacting generalized Chaplygin gas with the baryonic matter. The boundary of the universe is assumed to be enclosed by the dynamical apparent horizon. We show that for the interacting generalized Chaplygin gas as a unified candidate for dark matter and dark energy, the equation of state parameter can cross the phantom divide. We also present that for the selected model under thermal equilibrium with the Hawking radiation, the generalized second law is always satisfied throughout the history of the universe for any spatial curvature, independently of the equation of state of the interacting generalized Chaplygin gas model.     

Holographic dark energy model with linearly varying deceleration parameter and generalised Chaplygin gas dark energy model in Bianchi type-I universe

The paper deals with a spatially homogeneous and anisotropic Bianchi type-I universe filled with two minimally interacting fluids; matter and holographic dark energy components. The nature of the holographic dark energy for Bianchi type-I space time is discussed. An exact solution to Einstein’s field equations in Bianchi type-I line element is obtained using the assumption of linearly varying deceleration parameter. Under the suitable condition, it is observed that the anisotropy parameter of the universe approaches to zero for large cosmic time and the coincidence parameter increases with increasing time. We established a correspondence between the holographic dark energy models with the generalised Chaplygin gas dark energy model. We also reconstructed the potential and dynamics of the scalar field which describes the Chaplygin cosmology. Solution of the field equations shows that a big rip type future singularity will occur for this model. It has been observed that the solutions are compatible with the results of recent observations.     

Modified Chaplygin gas and solvable F-essence cosmologies

The Modified Chaplygin Gas (MCG) model belongs to the class of a unified models of dark energy and dark matter. In this paper, we have modeled MCG in the framework of f-essence cosmology. By constructing an equation connecting the MCG and the f-essence, we solve it to obtain explicitly the pressure and energy density of MCG. As special cases, we obtain both positive and negative pressure solutions for suitable choices of free parameters. We also calculate the state parameter which describes the phantom crossing.     

The influence of gravitational binding energy on cosmic expansion dynamics: new perspectives for cosmology

Confronted with microwave background observations by WMAP and with consternating supernova locations in the magnitude–redshift diagram modern cosmology feels enforced to call for cosmic vacuum energy as a necessary cosmological ingredient. Most often this vacuum energy is associated with Einstein’s cosmological constant Λ or with so-called “dark energy”. A positive value of Λ describes an inflationary action on cosmic dynamics which in view of recent cosmological data appears as an absolute need. In this article, however, we question the hypothesis of a constant vacuum energy density since not justifiable on physical grounds. Instead we show that gravitational binding energy of cosmic matter, connected with ongoing structure formation during cosmic expansion, acts similar to vacuum energy, since it reduces the effective gravitating proper mass density. Thus one may be encouraged to believe that actions of cosmic vacuum energy and gravitational binding energy concerning their cosmological effects are closely related to each other, perhaps in some respects even have identical phenomenologies.     

Gamma-ray bursts as dark energy–matter probes in the context of the generalized Chaplygin gas model

In this paper we consider the use of gamma-ray bursts (GRBs) as distance markers to study the unification of dark energy and dark matter in the context of the so-called generalized Chaplygin gas (GCG) model. We consider that the GRB luminosity may be estimated from its variability and time-lag, and we also use the so-called Ghirlanda relation. We evaluate the improvements expected once more GRBs and their redshift become available. We show that although GRBs allow for extending the Hubble diagram to higher redshifts, its use as a dark energy probe is limited when compared to Type Ia supernovae. We find that the information from GRBs can provide some bounds on the amount of dark matter and dark energy independently of the equation of state. This is particularly evident for XCDM-type models, which are, for low redshifts  ( z ≤ 2)  , degenerate with the GCG.     

The differential cosmic ray energy flux in the light of an ultrarelativistic generalized Lorentzian thermodynamics

The ultrarelativistic generalized Lorentzian quasi-equilibrium thermodynamic energy distribution is tentatively applied to the energy spectrum of galactic cosmic ray fluxes. It is found that the inferred power law slopes contain a component which evolves with cosmic ray energy in steps of thirds, resembling the sequence of structure functions in fully developed Kolmogorov turbulence. Within the generalized thermodynamics the chemical potential is estimated from the deviation of the fluxes at decreasing energy, presumably throwing light on the cosmic ray acceleration mechanism. A relation between involved turbulent volumina and structure functions is obtained. The conclusions drawn hold only within the realm of applicability of thermodynamics to cosmic ray spectra.     

Interaction between tachyon dark energy and modified Chaplygin gas

In the present work we assume that the universe is dominated with a two component mixture which do not evolve separately but interact non-gravitationally with one another. we consider the issue of the tachyon as a source of the dark energy and modified Chaplygin gas as background fluid. So we study the interacting between tachyon field and modified Chaplygin gas in different forms of interactions term Q in both flat and non-flat FRW universe. Then we reconstruct the potential and the dynamics of the tachyon field which describe tachyon cosmology. Also we find a equivalence potential for MCG in this model. Next we study two dark components respect to redshift and we find the conditions that are required for the stability of this model.     

About universes with scale‐related total masses and their abolition of presently outstanding cosmological problems

The most recently celebrated cosmological implications of the cosmic microwave background studies with WMAP (2006), though fascinating by themselves, do, however, create some extremely hard conceptual challenges for the present‐day cosmology. These recent extremely refined WMAP observations seem to reflect a universe which was extremely homogeneous at the recombination age and thus is obviously causally closed at the time of the cosmic recombination era. From the very tiny fluctuations apparent at this early epoch the presently observable nonlinear cosmic density structures can, however, only have grown up, if in addition to a mysteriously high percentage of dark matter an even higher percentage of dark energy is admitted as drivers of the cosmic evolution. The required dark energy density, on the other hand, is nevertheless 120 orders of magnitude smaller then the theoretically calculated value. These are outstanding problems of present day cosmology onto which we are looking here under new auspices. We shall investigate in the following, up to what degree a universe simply abolishes all these outstanding problems in case it reveals itself as an universe of constant total energy. As we shall show basic questions like: How could the gigantic mass of the universe of about 10⁸⁰ proton masses at all become created? – Why is the presently recognized and obviously indispensable cosmic vacuum energy density so terribly much smaller than is expected from quantum theoretical considerations, but nevertheless terribly important for the cosmic evolution? – Why is the universe within its world horizon a causally closed system? –, can perhaps simply be answered, when the assumption is made that the universe has a constant total energy with the consequence that the total mass density of the universe (matter and vacuum) scales with . Such a scaling of matter and vacuum energy abolishes the horizon problem, and the cosmic vacuum energy density can easily be reconciled with its theoretical expectation values. In this model the mass of the universe increases linearly with the world extension R_u and can grow up from a Planck mass as a vacuum fluctuation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)