Constraints on the Generalized Chaplygin Gas Model from Recent Supernova Data and Baryonic Acoustic Oscillations
Constraints from the Gold sample Type Ia supernova (SN Ia) data, the Supernova Legacy Survey (SNLS) SN Ia data, and the size of the baryonic acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS) on the generalized Chaplygin gas (GCG) model, proposed as a candidate for the unified dark matter-dark energy scenario (UDME), are examined in the cases of both a spatially flat and a spatially curved universe. Our results reveal that the GCG model is consistent with a flat universe up to the 68% confidence level, and the model parameters are within the allowed parameter ranges of the GCG as a candidate for UDME. Meanwhile, we find that in the flat case, both the Gold sample + SDSS BAO data and the SNLS sample + SDSS BAO data break the degeneracy of As and α and allow for the scenario of a cosmological constant plus dark matter (α = 0) at the 68% confidence level, although they rule out the standard Chaplygin gas model (α = 1) at the 99% confidence level. However, for the case without a flat prior, the SNLS SN Ia + SDSS BAO data do not break the degeneracy between As and α, and they allow for ΛCDM (α = 0) and the standard Chaplygin gas model (α = 1) at a 68% confidence level, while the Gold SN Ia + SDSS BAO break the degeneracy of As and α and rule out ΛCDM at a 68% confidence level and the standard Chaplygin gas model at a 99% confidence level.

title: Avoidance of big rip in phantom cosmology by gravitational back reaction
abstract: The effects of the gravitational back reaction of cosmological perturbations are investigated in a cosmological model where the universe is dominated by phantom energy. We assume a COBE normalized spectrum of cosmological fluctuations at the present time and calculate the effective energy–momentum tensor of the gravitational back-reactions of cosmological perturbations whose wavelengths at the time when the back-reactions are evaluated are larger than the Hubble radius. Our results reveal that the effects of gravitational back-reactions will counteract that of phantom energy sooner or later and can become large enough to terminate the phantom dominated phase before the big rip as the universe evolves. This arises because the phase space of infrared modes grows very rapidly as we come close to the big rip.

title: Vacuum polarization of scalar electrodynamics in flat space-time with nontrivial topology
abstract: The one-loop vacuum polarization is calculated for scalar electrodynamics in a flat space-time with the topology S1×R3. The effect of vacuum polarization upon photon propagation is considered. It is found that photon propagation becomes anisotropic with some photon modes acquiring a positive topological mass and thus travelling at a subluminal speed.

title: Latest cosmological constraints on Cardassian expansion models including the updated gamma-ray bursts
abstract: We constrain the Cardassian expansion models from the latest observations, including the updated Gamma-ray bursts (GRBs), which are calibrated using a cosmology independent method from the Union2 compilation of type Ia supernovae (SNe Ia). By combining the GRB data with the joint observations from the Union2 SNe Ia set, along with the results from the Cosmic Microwave Background radiation observation from the seven-year Wilkinson Microwave Anisotropy Probe and the baryonic acoustic oscillation observation galaxy sample from the spectroscopic Sloan Digital Sky Survey Data Release, we find significant constraints on the model parameters of the original Cardassian model Ω M0 = 0.282 +0.015 -0.014 , n = 0.03 +0.05 -0.05 ; and n = −0.16 +0.25 -3.26 , β = 0.76 +0.34 -0.58 of the modified polytropic Cardassian model, which are consistent with the ΛCDM model in a 1-σ confidence region. From the reconstruction of the deceleration parameter q(z) in Cardassian models, we obtain the transition redshift z T = 0.73 ± 0.04 for the original Cardassian model and z T = 0.68 ± 0.04 for the modified polytropic Cardassian model.

title: Oscillating universe in the DGP braneworld
abstract: With a method in which the Friedmann equation is written in a form such that evolution of the scale factor can be treated as that of a particle in a potential, we classify all possible cosmic evolutions in the DGP braneworld scenario with the dark radiation term retained. By assuming that the energy component is pressureless matter, radiation or vacuum energy, respectively, we find that in the matter or vacuum energy dominated case, the scale factor has a minimum value ${a}_{0}$. In the matter dominated case, the big bang singularity can be avoided in some special circumstances, and there may exist an oscillating universe or a bouncing one. If the cosmic scale factor is in the oscillating region initially, the universe may undergo an oscillation. After a number of oscillations, it may evolve to the bounce point through quantum tunneling and then expand. However, if the universe contracts initially from an infinite scale, it can turn around and then expand forever. In the vacuum energy dominated case, there exists a stable Einstein static state to avoid the big bang singularity. However, in certain circumstances in the matter or vacuum energy dominated case, a new kind of singularity may occur at ${a}_{0}$ as a result of the discontinuity of the scale factor. In the radiation dominated case, the universe may originate from the big bang singularity, but a bouncing universe which avoids this singularity is also possible.

title: Interacting generalized Chaplygin gas
abstract: A model with the interaction between generalized Chaplygin gas (GCG) and dark matter is studied. We show that the stationary attractor exists and the universe will enter a de Sitter phase in which the energy densities of both the GCG and dark matter are constant. Because of the interaction between the GCG and the dark matter, the equation of state parameter, w, of the GCG can cross the phantom divide and remains less than −1 once crossed. The larger the GCG model parameter α, the earlier the w crosses the divide and the universe enters the accelerating era, and the larger the present value of w and the present acceleration. By investigating the deceleration parameter, we find that a small α is favored by observations.

title: The ASY-EOS experiment at GSI: investigating the symmetry energy at supra-saturation densities
abstract: The elliptic-flow ratio of neutrons with respect to protons in reactions of neutron rich heavy-ions systems at intermediate energies has been proposed as an observable sensitive to the strength of the symmetry term in the nuclear Equation Of State (EOS) at supra-saturation densities. The recent results obtained from the existing FOPI/LAND data for $^{197}$Au+$^{197}$Au collisions at 400 MeV/nucleon in comparison with the UrQMD model allowed a first estimate of the symmetry term of the EOS but suffer from a considerable statistical uncertainty. In order to obtain an improved data set for Au+Au collisions and to extend the study to other systems, a new experiment was carried out at the GSI laboratory by the ASY-EOS collaboration in May 2011.