Generalized Chaplygin gas model: Constraints from Hubble parameter versus redshift data

We examine observational constraints on the generalized Chaplygin gas (GCG) model for dark energy from the 9 Hubble parameter data points, the 115 SNLS Sne Ia data and the size of baryonic acoustic oscillation peak at redshift, z=0.35$z=0.35$. At a 95.4% confidence level, a combination of three data sets gives 0.67?As?0.83$0.67?A_s?0.83$ and −0.21?α?0.42$-0.21?\alpha ?0.42$, which is within the allowed parameters ranges of the GCG as a candidate of the unified dark matter and dark energy. It is found that the standard Chaplygin gas model (α=1$\alpha =1$) is ruled out by these data at the 99.7% confidence level.     

Chaplygin inflation on the brane

Brane inflationary universe model in the context of a Chaplygin gas equation of state is studied. General conditions for this model to be realizable are discussed. In the high-energy limit and by using a chaotic potential we describe in great details the characteristic of this model. The parameters of the model are restricted by using recent astronomical observations.     

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.     

Dynamical Evolution of Modified Chaplygin Gas

Based our previous work [Mod. Phys. Lett. A 22 （2007） 783, Gen. Relat. Gray. 39 （2007） 653], some properties of modified Chaplygin gas （MCG） as a dark energy model continue to be studied mainly in two aspects： one is the change rates of the energy density and energy transfer, and the other is the evolution of the growth index. It is pointed that the density of dark energy undergoes the change from decrease to increase no matter whether the interaction between dark energy and dark matter exists or not, but the corresponding transformation points are different from each other.Eurthermore, it is stressed that the MCG model even supports the existence of interaction between dark energy and dark matter, and the energy of transfer flows from dark energy to dark matter. The evolution of the interaction term with an ansatz 3Hc^2ρ is discussed with the MCG model. Moreover, the evolution of the growth index f in the MCG model without interaction is illustrated, from which we find that the evolutionary trajectory of f overlaps with that of the ACDM model when a 〉 0.7 and its theoretical value f ≈ 0.566 given by us at z = 0.15 is consistent with the observations.     

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.     

Dynamical Stability and Attractor of the Variable Generalized Chaplygin Gas Model

For the variable generalized Chaplygin gas （VGCG） as a dynamical system, its stability is analyzed and the related dynamical attractors are investigated. By analysis it is shown that there are two critical points corresponding to the matter-dominated phase and the VGCG dark energy-dominated phase, respectively. Moreover, when the parameters n, a and γ take some fixed values, the phase with ωVGCG = --0.92 is a dynamical attractor and the equation of state of VGCG reaches it from either ωVGCG 〉 --1 or ωVGCG 〈 --1, independent of the initial values of the dynamical system. This shows a satisfactory cosmological model： the early matter-dominated era, followed by the dark energy-dominated era. Meanwhile, the evolutions of density parameters Ωγ and ΩVGCG are quite different from each other. For different initial values of x and y, Ωγ decreases and ωVGCG increases as the time grows, they will eventually approach Ωγ = 0 and ωVGCG ---- 1. Furthermore, since different values of n or a may lead to different equation-of-state parameters ωVGCG, we also discuss the constraints on the parameters n and by the observation data.     

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.     

Phantom Energy with Variable G and Λ

We investigate a cosmological model of a phantom energy with a variable cosmological constant （∧） depending on the energy density （ρ） as ∧∝ρ^α,α=const and a variable gravitational constant G. The model requires α 〈 0 and a negative gravitational constant. The cosmological constant evolves with time as ∧ ∝ t^-2. For ω 〉 - 1 and α 〈 -1 the cosmological constant ∧ 〈 0, G 〉 0 and ρ decrease with cosmic expansion. For ordinary energy （or dark energy）, i.e.ω 〉 -1, we have -1 〈 α〈 0 and β 〉 0 so that G〉0 increases with time and p decreases with time. Cosmic acceleration with dust particles is granted, provided -2/3 〈α〈 0 and ∧〉0.     

Charge non-conservation,dequantisation, and induced electric dipole moments in varying-α theories

We note that in extensions of the Standard Model that allow for a varying fine structure constant, α, all matter species, apart from right-handed neutrinos, will gain an intrinsic electric dipole moment (EDM). In a large subset of varying-α theories, all such particle species will also gain an effective electric charge. This charge will, in general, not be quantised and can result in macroscopic non-conservation of electric charge.     

C-field Cosmology with Variable G in the Flat Friedmann--Robertson--Walker Model

The C-field cosmological model based on the Hoyle-Narlikar theory with variable gravitational constant G is investigated. To obtain the deterministic value of C=dC/dt we present certain constant values for integration constants. The creation field is proportional to time. G and p （density） decrease with time and the universe represents an expanding universe. The creation field increases as time increases. We find C=√1/2πf t where f〉0,Thus C=1 when f=1/2π〉0.     

Viscous Dark Energy Models with Variable G and Λ

We consider a cosmological model with bulk viscosity η and variable cosmological A ∝p^-α, alpha = const and gravitational G constants. The model exhibits many interesting cosmological features. Inflation proceeds du to the presence of bulk viscosity and dark energy without requiring the equation of state p =-p. During the inflationary era the energy density p does not remain constant, as in the de-Sitter type. Moreover, the cosmological and gravitational constants increase exponentially with time, whereas the energy density and viscosity decrease exponentially with time. The rate of mass creation during inflation is found to be very huge suggesting that all matter in the universe is created during inflation.     

Hypermagnetic baryogenesis

We study a new scenario for baryogenesis due to the spontaneous breaking of the CPT   invariance through the interaction between a baryon current and a hypermagnetic helicity. The hypermagnetic helicity (Chern–Simons number) of UY₍₁₎$U{(1)}_Y$ provides a CPT violation background for the generation of baryons via sphaleron processes, which protects these baryons from the sphaleron wash-out effect in thermal equilibrium. It is shown that if the present amplitude of the resultant magnetic fields are sufficiently large, for a wide range mass scale (from TeV to the Planck scale), the observational magnitude of the baryon asymmetry of the Universe can be realized.     

Varying alpha driven by the Dirac–Born–Infeld scalar field

Since about ten years ago, varying α theories attracted many attentions, mainly due to the first observational evidence from the quasar absorption spectra that the fine structure “constant” might change with cosmological time. In this Letter, we investigate the cosmic evolution of α   driven by the Dirac–Born–Infeld (DBI) scalar field. To be general, we consider various couplings between the DBI scalar field and the electromagnetic field. We also confront the resulting Δα/α$\mathrm{\Delta }\alpha /\alpha$ with the observational constraints, and find that various cosmological evolution histories of Δα/α$\mathrm{\Delta }\alpha /\alpha$ are allowed. Comparing with the case of varying α driven by quintessence, the corresponding constraints on the parameters of coupling have been relaxed, thanks to the relativistic correction of the DBI scalar field.     

Prospects of inflation with perturbed throat geometry

We study brane inflation in a warped deformed conifold background that includes general possible corrections to the throat geometry sourced by coupling to the bulk of a compact Calabi–Yau space. We focus specifically, on the perturbation by chiral operator of dimension 3/2 in the CFT. We find that the effective potential in this case can give rise to required number of e-foldings and the spectral index nS$n_S$ consistent with observation. The tensor to scalar ratio of perturbations is generally very low in this scenario. The COBE normalization, however, poses certain difficulties which can be circumvented provided model parameters are properly fine tuned. We find the numerical values of parameters which can give rise to enough inflation, observationally consistent values of density perturbations, scalar to tensor ratio of perturbations and the spectral index nS$n_S$.     

Quintessential inflation from a variable cosmological constant in a 5D vacuum

We explore an effective 4D cosmological model for the universe where the variable cosmological constant governs its evolution and the pressure remains negative along all the expansion. This model is introduced from a 5D vacuum state where the (space-like) extra coordinate is considered as noncompact. The expansion is produced by the inflaton field, which is considered as nonminimally coupled to gravity. We conclude from experimental data that the coupling of the inflaton with gravity should be weak, but variable in different epochs of the evolution of the universe.     

Circular Loop Equation of a Cosmic String in Gauss--Bonnet--de

We perform the analysis of evolution of cosmic string loops in the background of Gauss-Bonnet-de Sitter. The equation of motion of cosmic string loops in this spacetime is derived. Having solved the equation numerically, we investigate the dependence of the loop evolution on the values of a, related to the Gauss-Bonnet coupling. In the Gauss-Bonnet-de Sitter spacetimes with different dimensionality there exists a special parameter αm. In the environment with α 〉 αm, all the cosmic string loops will collapse to form black holes. Within the region 0 〈 α 〈 αm, the stronger Gauss-Bonnet effect will lead more cosmic string loops, including smaller ones, to form black holes. The larger the value of a is, the smaller the special values that exist, and only the cosmic string loops with initial radius larger than the special values can expand and evolve instead of becoming black holes.