Cosmic Strings Collision in Cosmological Backgrounds

The collisions of cosmic strings loops and the dynamics of junctions formations in expanding backgrounds are studied. The key parameter controlling the dynamics of junctions formation, the cosmic strings zipping and unzipping is the relative size of the loops compared to the Hubble expansion rate at the time of collision. We study analytically and numerically these processes for large super-horizon size loops, for small sub-horizon size loops as well as for loops with the radii comparable to the Hubble expansion rate at the time of collision.Comment: 24 pages, 13 figure

Fermionic Tachyons as a Source of Dark Energy

A model for the universe on the basis of a self interacting fermionic tachyon field is investigated here. It is shown that, devising a self interaction potential of a proper form, the fermionic tachyon field is capable of producing an accelerating expansion that at late time tends to a constant value which is in consistence with the cosmological constant. This way the introduced fermionic tachyon field can be interpreted as the source of dark energy.Comment: 5 figure

Fields Annihilation and Particles Creation in DBI inflation

We consider a model of DBI inflation where two stacks of mobile branes are moving ultra relativistically in a warped throat. The stack closer to the tip of the throat is annihilated with the background anti-branes while inflation proceeds by the second stack. The effects of branes annihilation and particles creation during DBI inflation on the curvature perturbations power spectrum and the scalar spectral index are studied. We show that for super-horizon scales, modes which are outside the sound horizon at the time of branes collision, the spectral index has a shift to blue spectrum compared to the standard DBI inflation. For small scales the power spectrum approaches to its background DBI inflation value with the decaying superimposed oscillatory modulations.Comment: First revision: minor changes, the background spectral index is corrected, new references are added. Second revision: minor changes, new references are added, accepted for publication in JCA