Non-Singular Bouncing Universes in Loop Quantum Cosmology

Non-perturbative quantum geometric effects in Loop Quantum Cosmology predict a $\rho^2$ modification to the Friedmann equation at high energies. The quadratic term is negative definite and can lead to generic bounces when the matter energy density becomes equal to a critical value of the order of the Planck density. The non-singular bounce is achieved for arbitrary matter without violation of positive energy conditions. By performing a qualitative analysis we explore the nature of the bounce for inflationary and Cyclic model potentials. For the former we show that inflationary trajectories are attractors of the dynamics after the bounce implying that inflation can be harmoniously embedded in LQC. For the latter difficulties associated with singularities in cyclic models can be overcome. We show that non-singular cyclic models can be constructed with a small variation in the original Cyclic model potential by making it slightly positive in the regime where scalar field is negative.Comment: Minor changes and one figure added to improve presentation. References added. To appear in Physical Review

Physical constants and the Gurzadyan-Xue formula for the dark energy

We consider cosmological implications of the formula for the dark energy density derived by Gurzadyan and Xue which predicts a value fitting the observational one. Cosmological models with varying by time physical constants, namely, speed of light and gravitational constant and/or their combinations, are considered. In one of the models, for example, vacuum energy density induces effective negative curvature, while another one has an unusual asymptotic. This analysis also explicitely rises the issue of the meaning and content of physical units and constants in cosmological context.Comment: version corrected to match the one to appear in Modern Physics Letters

On the kinetic treatment of pair productionin strong electric fields

We investigate the behavior of the electron-positron plasma created by a strong electric field using a kinetic approach. Assuming a uniform and unbound field, the system under consideration is uniform and homogeneous in the physical space and axially symmetric in the momentum space with the axis of symmetry given by the direction of the initial field. The relativistic Boltzmann-Vlasov equation for pairs is solved numerically for different starting values of the field with particular attention to the momentum distribution of pairs produced from the field. Then we solve the system of coupled Vlasov-Boltzmann equation for pairs and Boltzmann equation for photons including collision terms for all the two-particle interactions between pairs and photons: electron-positron annihilation into two photons and its inverse process, Compton and Coulomb scatterings. We compare the two cases and discuss the role of the interactions

GRBs and the thermalization process of electron-positron plasmas

We discuss the temporal evolution of the pair plasma created in Gamma-Ray Burst sources. A particular attention is paid to the relaxation of the plasma into thermal equilibrium. We also discuss the connection between the dynamics of expansion and the spatial geometry of the plasma. The role of the baryonic loading parameter is emphasized.Comment: 4 pages, 3 figures, in the Proceedings of the "Gamma Ray Bursts 2007" meeting, November 5-9, 2007, Santa Fe, New Mexico, US