Producing a Scale-Invariant Spectrum of Perturbations in a Hagedorn Phase of String Cosmology

We study the generation of cosmological perturbations during the Hagedorn phase of string gas cosmology. Using tools of string thermodynamics we provide indications that it may be possible to obtain a nearly scale-invariant spectrum of cosmological fluctuations on scales which are of cosmological interest today. In our cosmological scenario, the early Hagedorn phase of string gas cosmology goes over smoothly into the radiation-dominated phase of standard cosmology, without having a period of cosmological inflation.Comment: 4 pages, 1 figur

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

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

On the new string theory inspired mechanism of generation of cosmological perturbations

Recently a non-inflationary mechanism of generation of scale-free cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and Vafa in the context of the string gas cosmology. We discuss various problems of their model and argue that the cosmological perturbations of metric produced in this model have blue spectrum with a spectral index n = 5, which strongly disagrees with observations. We conclude that this model in its present form is not a viable alternative to inflationary cosmology.Comment: 11 pages, 1 figur

Observational Constraints on Theories with a Blue Spectrum of Tensor Modes

Motivated by the string gas cosmological model, which predicts a blue tilt of the primordial gravitational wave spectrum, we examine the constraints imposed by current and planned observations on a blue tilted tensor spectrum. Starting from an expression for the primordial gravitational wave spectrum normalized using cosmic microwave background observations, pulsar timing, direct detection and nucleosynthesis bounds are examined. If we assume a tensor to scalar ratio on scales of the CMB which equals the current observational upper bound, we obtain from these current observations constraints on the tensor spectral index of $n_{T} \lesssim 0.79$, $n_{T} \lesssim 0.53$, and $n_{T} \lesssim 0.15$ respectively.Comment: 12 pages, 1 figure, 2 references added, relationship of this work with Ref. 20 adde

N=1 Supersymmetric Cosmic Strings

We investigate the microphysics of supersymmetric cosmic strings. In particular we focus on the vortices admitted by N=1 supersymmetric abelian Higgs models. We find the vortex solutions and demonstrate that the two simplest supersymmetric cosmic string models admit fermionic superconductivity. Further, by using supersymmetry transformations, we show how to solve for the fermion zero modes giving rise to string superconductivity in terms of the background string fields.Comment: 16 pages, RevTe

Observing the temperature of the Big Bang through large scale structure

It is widely accepted that the Universe underwent a period of thermal equilibrium at very early times. One expects a residue of this primordial state to be imprinted on the large scale structure of space time. In this paper we study the morphology of this thermal residue in a universe whose early dynamics is governed by a scalar field. We calculate the amplitude of fluctuations on large scales and compare it to the imprint of vacuum fluctuations. We then use the observed power spectrum of fluctuations on the cosmic microwave background to place a constraint on the temperature of the Universe before and during inflation. We also present an alternative scenario where the fluctuations are predominantly thermal and near scale-invariant

Evolution of Fields in a Second Order Phase Transition

We analyse the evolution of scalar and gauge fields during a second order phase transition using a Langevin equation approach. We show that topological defects formed during the phase transition are stable to thermal fluctuations. Our method allows the field evolution to be followed throughout the phase transition, for both expanding and non-expanding Universes. The results verify the Kibble mechanism for defect formation during phase transitions.Comment: 12 pages of text plus 17 diagrams available on request, DAMTP 94-8