Big Bounce Singularity of a Simple Five—Dimensional Cosmological Model

The big bounce singularity of a simple five-dimensional cosmological model is studied.Contrary to the standard big bang space-time singularity,this big bounce singularity is found to be an event horizon at which the scale factor and the mass density of the universe are finite,while the pressure undergoes a sudden transition from negative infinity to positive infinity.By using coordinate transformation it is also shown that before the bounce the universe contracts deflationary.According to the proper-time,the universe may have existed for an infinitely long time.     

Comparison of Supernovae Datasets Constraints on Dark Energy

Cosmological measurements suggest that our universe contains a dark energy component. In order to study the dark energy evolution, we constrain a parameterized dark energy equation of state ω（z） = ω0 ＋ ω1 1＋z/z using the recent observational datasets： 157 Gold type Ia supernovae and the newly released 182 Gold type Ia supernovae by the maximum likelihood method. It is found that the best fit ω（z） crosses -1 in the past and the present best fit value of ω（0） 〈 -1 obtained from 157 Gold-type Ia supernovae. The crossing of-1 is not realized and ω0 = -1 is not ruled out in 1σ confidence level for the 182 Gold-type Ia supernovae. It is also found that the range of parameter ω0 is wide even in 1σ confidence level and the best fit ω（z） is sensitive to the prior of Ωm.     

Observational constraints on the accelerating universe in the framework of a 5D bounce cosmological model

In the framework of a five-dimensional (5D) bounce cosmological model, a useful function f(z) is obtained by giving a concrete expression of deceleration parameter q(z)=q₁+{q₂}/{1+ln (1+ z)}. Then using the obtained Hubble parameter H(z) according to the function f(z), we constrain the accelerating universe from recent cosmic observations: the 192 ESSENCE SNe Ia and the 9 observational H(z) data. The best fitting values of transition redshift z_T and current deceleration parameter q₀ are given as z_T= 0.65_-0.12^0.25 and q₀ = - 0.76_-0.15^+0.15 (1σ). Furthermore, in the 5D bounce model it can be seen that the evolution of equation of state (EOS) for dark energy w_de can cross over -1 at about z=0.23 and the current value w_0de= - 1.15<- 1. On the other hand, by giving a concrete expression of model-independent EOS of dark energy w_de, in the 5D bounce model we obtain the best fitting values z_T= 0.66_0.08^+0.11 and q₀ = - 0.69_0.10^+0.10 (1σ) from the recently observed data: the 192 ESSENCE SNe Ia, the observational H(z) data, the 3-year Wilkinson Microwave Anisotropy Probe (WMAP), the Sloan Digital Sky Survey (SDSS) baryon acoustic peak and the x-ray gas mass fraction in clusters.     

Statefinder Diagnostic for Phantom Model with V（φ） = Voexp（-λφ^2）

We investigate the phantom field with potential V（φ） = Voexp（-λφ^2） and dark matter in the spatially flat Friedman-Robertson-Walker model It has been shown by numerical calculation that there is a attractor solution in this model We also apply tile statefinder diagnostic to this phantom model. It is shown that tile evolving trajectories of this scenario in tile s - r diagram is quite different from other dark energy models.     

Cosmological Constant or Variable Dark Energy？

Selection statics of the Akaike information criterion （AIC） model and the Bayesian information criterion （BIC） model are applied to the A-cold dark matter （ACDM） cosmological model, the constant equation of state of dark energy, w =constant, and the parametrized equation of state of dark energy, w（z） = wo ＋ wlz/（1 ＋ z）, to determine which one is the better cosmological model to describe the evolution of the universe by combining the recent cosmic observational data including She Ia, the size of baryonic acoustic oscillation （BAO） peak from SDSS, the three-year WMAP CMB shift parameter. The results show that AIC, BIC and current datasets are not powerful enough to discriminate one model from the others, though odds suggest differences between them.     

Reconstructing dark energy potentials from parameterized deceleration parameters

In this paper, the properties of dark energy are investigated according to the parameterized deceleration parameter q(z), which is used to describe the extent of the accelerating expansion of the universe. The potential of dark energy V(φ) and the cosmological parameters, such as the dimensionless energy density \varOmega_φ, \varOmega_m, and the state parameter w_φ, are connected to it. Concretely, by giving two kinds of parameterized deceleration parameters q(z)=a+bz/(1+z) and q(z)=1/2+(az+b)/(1+z)^2, the evolution of these parameters and the reconstructed potentials V(φ) are plotted and analysed. It is found that the potentials run away with the evolution of universe.     

Constraints on the unified model of dark matter and dark energy

Using recently observed data:the Constitution dataset of type supernovae Ia (SNIa),the observational Hubble data (OHD),the measurement results of baryon acoustic oscillation (BAO) from the Sloan Digital Sky Survey (SDSS) and the Two Degree Field Galaxy Redshift Survey (2dFGRS),and the current cosmic microwave background (CMB) data from the five-year Wilkinson Microwave Anisotropy Probe (WMAP),we apply the Markov Chain Monte Carlo method to investigate the observational constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy.For this unified model,the constraints on GCG mixture are discussed by considering the different expressions of current matter density or considering constraints as being independent of the matter quantity Ωm.