String Gas Baryogenesis

We describe a possible realization of the spontaneous baryogenesis mechanism in the context of extra-dimensional string cosmology and specifically in the string gas scenario.Comment: arXiv admin note: substantial text overlap with 0808.0746 by different autho

Perturbations in a Bouncing Brane Model

The question of how perturbations evolve through a bounce in the Cyclic and Ekpyrotic models of the Universe is still a matter of ongoing debate. In this report we show that the collision between boundary branes is in most cases singular even in the full 5-D formalism, and that first order perturbation theory breaks down for at least one perturbation variable. Only in the case that the boundary branes approach each other with constant velocity shortly before the bounce, can a consistent, non singular solution be found. It is then possible to follow the perturbations explicitly until the actual collision. In this case, we find that if a scale invariant spectrum developed on the hidden brane, it will get transferred to the visible brane during the bounce.Comment: 15 pages, minor modifications, a few typos correcte

Formation of High Redshift Objects in a Cosmic String Theory with Hot Dark Matter

Using a modification of the Zel'dovich approximation adapted to hot dark matter, the accretion of such matter onto moving cosmic string loops is studied. It is shown that a large number of $10^{12}M_\odot$ nonlinear objects will be produced by a redshift of $z=4$. These objects could be the hosts of high redshift quasars.Comment: 21 pages, 1 figure, uses phyzzx and epsf macro

Matter Bounce in Horava-Lifshitz Cosmology

Horava-Lifshitz gravity, a recent proposal for a UV-complete renormalizable gravity theory, may lead to a bouncing cosmology. In this note we argue that Horava-Lifshitz cosmology may yield a concrete realization of the matter bounce scenario, and thus give rise to an alternative to inflation for producing a scale-invariant spectrum of cosmological perturbations. In this scenario, quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and the spectrum is transformed into a scale-invariant one on super-Hubble scales before the bounce because the long wavelength modes undergo squeezing of their wave-functions for a longer period of time than shorter wavelength modes. The scale-invariance of the spectrum of curvature fluctuations is preserved during and after the bounce. A distinctive prediction of this scenario is the amplitude and shape of the bispectrum.Comment: 6 pages, 1 figure, a couple of minor wording change

Thermal Equilibrium of String Gas in Hagedorn Universe

The thermal equilibrium of string gas is necessary to activate the Brandenberger-Vafa mechanism, which makes our observed 4-dimensional universe enlarge. Nevertheless, the thermal equilibrium is not realized in the original setup, a problem that remains as a critical defect. We study thermal equilibrium in the Hagedorn universe, and explore possibilities for avoiding the issue aforementioned flaw. We employ a minimal modification of the original setup, introducing a dilaton potential. Two types of potential are investigated: exponential and double-well potentials. For the first type, the basic evolutions of universe and dilaton are such that both the radius of the universe and the dilaton asymptotically grow in over a short time, or that the radius converges to a constant value while the dilaton rolls down toward the weak coupling limit. For the second type, in addition to the above solutions, there is another solution in which the dilaton is stabilized at a minimum of potential and the radius grows in proportion to $t$. Thermal equilibrium is realized for both cases during the initial phase. These simple setups provide possible resolutions of the difficulty.Comment: 23 pages,19 figure

A Mechanism for Baryogenesis in Low Energy Supersymmetry Breaking Models

A generic prediction of models where supersymmetry is broken at scales within a few orders of magnitude of the weak scale and is fed down to the observable sector by gauge interactions is the existence of superconducting cosmic strings which carry baryon number. In this paper we propose a novel mechanism for the generation of the baryon asymmetry which takes place at temperatures much lower than the weak scale. Superconducting strings act like ``bags'' containing the baryon charge and protect it from sphaleron wash-out throughout the evolution of the Universe, until baryon number violating processes become harmless. This mechanism is efficient even if the electroweak phase transition in the MSSM is of the second order and therefore does not impose any upper bound on the mass of the Higgs boson. (13kb)Comment: LaTeX file, 17 pages. One ref. adde

On the Spectrum of Fluctuations in an Effective Field Theory of the Ekpyrotic Universe

We consider the four-dimensional effective field theory which has been used in previous studies of perturbations in the Ekpyrotic Universe, and discuss the spectrum of cosmological fluctuations induced on large scales by quantum fluctuations of the bulk brane. By matching cosmological fluctuations on a constant energy density hypersurface we show that the growing mode during the very slow collapsing pre-impact phase couples only to the decaying mode in the expanding post-impact phase, and that hence no scale-invariant spectrum of adiabatic fluctuations is generated. Note that our conclusions may not apply to improved toy models for the Ekpyrotic scenario.Comment: 8 pages, few sentences added. Conclusions unchanged. Added references. Missing name added to Ref. 5

Processing of Cosmological Perturbations in a Cyclic Cosmology

The evolution of the spectrum of cosmological fluctuations from one cycle to the next is studied. It is pointed out that each cycle leads to a reddening of the spectrum. This opens up new ways to generate a scale-invariant spectrum of curvature perturbations. The large increase in the amplitude of the fluctuations quickly leads to a breakdown of the linear theory. More generaly, we see that, after including linearized cosmological perturbations, a cyclic universe cannot be truly cyclic.Comment: 5 pages, 1 figur