119115655
Cosmological evolution of the interacting phantom (quintessence) model in loop quantum gravity
The dynamics of the interacting dark energy model in loop quantum cosmology (LQC) is studied in this paper. The dark energy has a constant equation of state wx and interacts with dark matter through a form 3cH(rho(x) + rho(m)). We find that for the quintessence model (w(x) > -1) the cosmological evolution in LQC is the same as that in classical Einstein cosmology, whereas for phantom dark energy (w(x) < -1), although there are the same critical points in LQC and classical Einstein cosmology, the loop quantum effect significantly reduces the parameter spacetime (c, w(x)) required by stability. If parameters c and wx satisfy the conditions that the critical points are existing and stable, the universe will enter an era dominated by dark energy and dark matter with a constant energy ratio between them, and accelerate forever; otherwise it will enter an oscillatory regime. Comparing our results with the observations we find at 1 sigma confidence level that the universe will accelerate forever.
124372997
Interacting generalized Chaplygin gas
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.
119285179
Oscillating universe in the DGP braneworld
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.
118596987
Latest cosmological constraints on Cardassian expansion models including the updated gamma-ray bursts
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. ﻿
119336402
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.
122767439
Vacuum polarization of scalar electrodynamics in flat space-time with nontrivial topology
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.
118699286
The ASY-EOS experiment at GSI: investigating the symmetry energy at supra-saturation densities
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.