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

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

Prolongation of Friction Dominated Evolution for Superconducting Cosmic Strings

This investigation is concerned with cosmological scenarios based on particle physics theories that give rise to superconducting cosmic strings (whose subsequent evolution may produce stable loop configurations known as vortons). Cases in which electromagnetic coupling of the string current is absent or unimportant have been dealt with in previous work. The purpose of the present work is to provide quantitative estimates for cases in which electromagnetic interaction with the surrounding plasma significantly affects the string dynamics. In particular it will be shown that the current can become sufficiently strong for the initial period of friction dominated string motion to be substantially prolonged, which would entail a reinforcement of the short length scale end of the spectrum of the string distribution, with potentially observable cosmological implications if the friction dominated scenario lasts until the time of plasma recombination.Comment: 10 pages Late

Dynamical Relaxation of the Cosmological Constant and Matter Creation in the Universe

In this Letter we discuss the issues of the graceful exit from inflation and of matter creation in the context of a recent scenario \cite{RHBrev} in which the back-reaction of long wavelength cosmological perturbations induces a negative contribution to the cosmological constant and leads to a dynamical relaxation of the bare cosmological constant. The initially large cosmological constant gives rise to primordial inflation, during which cosmological perturbations are stretched beyond the Hubble radius. The cumulative effect of the long wavelength fluctuations back-reacts on the background geometry in a form which corresponds to the addition of a negative effective cosmological constant to the energy-momentum tensor. In the absence of an effective scalar field driving inflation, whose decay can reheat the Universe, the challenge is to find a mechanism which produces matter at the end of the relaxation process. In this Letter, we point out that the decay of a condensate representing the order parameter for a ``flat'' direction in the field theory moduli space can naturally provide a matter generation mechanism. The order parameter is displaced from its vacuum value by thermal or quantum fluctuations, it is frozen until the Hubble constant drops to a sufficiently low value, and then begins to oscillate about its ground state. During the period of oscillation it can decay into Standard Model particles similar to how the inflaton decays in scalar-field-driven models of inflation.Comment: 6 page

Dilaton stabilization by massive fermion matter

The study started in a former work about the Dilaton mean field stabilization thanks to the effective potential generated by the existence of massive fermions, is here extended. Three loop corrections are evaluated in addition to the previously calculated two loop terms. The results indicate that the Dilaton vacuum field tend to be fixed at a high value close to the Planck scale, in accordance with the need for predicting Einstein gravity from string theory. The mass of the Dilaton is evaluated to be also a high value close to the Planck mass, which implies the absence of Dilaton scalar signals in modern cosmological observations. These properties arise when the fermion mass is chosen to be either at a lower bound corresponding to the top quark mass, or alternatively, at a very much higher value assumed to be in the grand unification energy range. One of the three 3-loop terms is exactly evaluated in terms of Master integrals. The other two graphs are however evaluated in their leading logarithm correction in the perturbative expansion. The calculation of the non leading logarithmic contribution and the inclusion of higher loops terms could made more precise the numerical estimates of the vacuum field value and masses, but seemingly are expected not to change the qualitative behavior obtained. The validity of the here employed Yukawa model approximation is argued for small value of the fermion masses with respect to the Planck one. A correction to the two loop calculation done in the previous work is here underlined.Comment: 18 pages, 5 figures, the study was extended and corrections on the former calculations and redaction were done. The paper had been accepted for publication in "Astrophysics and Space Science

Unified Superfluid Dark Sector

We present a novel theory of a unified dark sector, where late-time cosmic acceleration emerges from the dark matter superfluid framework. The system is described by a superfluid mixture consisting of two distinguishable states with a small energy gap, such as the ground state and an excited state of dark matter. Given their contact in the superfluid, interaction between those states can happen, converting one state into the other. This long range interaction within the superfluid couples the two superfluid phonon species through a cosine potential motivated by Josephson/Rabi interactions. As a consequence of this potential, a new dynamics of late-time accelerated expansion emerges in this system, without the need of dark energy, coming from a universe containing only this two-state DM superfluid. Because the superfluid species are non-relativistic, their sound speeds remain suitably small throughout the evolution. We calculate the expansion history and growth of linear perturbations, and compare the results to $\Lambda$CDM cosmology. For the fiducial parameters studied here, the predicted expansion and growth function are close to those of $\Lambda$CDM, but the difference in the predicted growth rate is significant at late times. The present theory nicely complements the recent proposal of dark matter superfluidity to explain the empirical success of MOdified Newtonian Dynamics (MOND) on galactic scales, thus offering a unified framework for dark matter, dark energy, and MOND phenomenology.Comment: 27 pages, 4 figures. v2: Version accepted in JCA

Looking Beyond Inflationary Cosmology

In spite of the phenomenological successes of the inflationary universe scenario, the current realizations of inflation making use of scalar fields lead to serious conceptual problems which are reviewed in this lecture. String theory may provide an avenue towards addressing these problems. One particular approach to combining string theory and cosmology is String Gas Cosmology. The basic principles of this approach are summarized.Comment: invited talk at "Theory Canada 1" (Univ. of British Columbia, Vancouver, Canada, June 2 - 4, 2005) (references updated

Scattering off an SO(10) cosmic string

The scattering of fermions from the abelian string arising during the phase transition $SO(10) \rightarrow SU(5) \times Z_2$ induced by the Higgs in the 126 representation is studied. Elastic cross-sections and baryon number violating cross-sections due to the coupling to gauge fields in the core of the string are computed by both a first quantised method and a perturbative second quantised method. The elastic cross-sections are found to be Aharonov-Bohm type. However, there is a marked asymmetry between the scattering cross-sections for left and right handed fields. The catalysis cross-sections are small, depending on the grand unified scale. If cosmic strings were observed our results could help tie down the underlying gauge group.Comment: 20 page