String cosmology versus standard and inflationary cosmology

This paper presents a review of the basic, model-independent differences between the pre-big bang scenario, arising naturally in a string cosmology context, and the standard inflationary scenario. We use an unconventional approach in which the introduction of technical details is avoided as much as possible, trying to focus the reader's attention on the main conceptual aspects of both scenarios. The aim of the paper is not to conclude in favour either of one or of the other scenario, but to raise questions that are left to the reader's meditation. Warnings: the paper does not contain equations, and is not intended as a complete review of all aspects of string cosmology.Comment: 22 pages, Latex (IOP Style), three figures included using epsfig. To appear in Class. Quantum Grav. (Topical Review Section). Two misprints correcte

Inflation and initial conditions in the pre-big bang scenario

The pre-big bang scenario describes the evolution of the Universe from an initial state approaching the flat, cold, empty, string perturbative vacuum. The choice of such an initial state is suggested by the present state of our Universe if we accept that the cosmological evolution is (at least partially) duality-symmetric. Recently, the initial conditions of the pre-big bang scenario have been criticized as they introduce large dimensionless parameters allowing the Universe to be "exponentially large from the very beginning". We agree that a set of initial parameters (such as the initial homogeneity scale, the initial entropy) larger than those determined by the initial horizon scale, H^{-1}, would be somewhat unnatural to start with. However, in the pre-big bang scenario, the initial parameters are all bounded by the size of the initial horizon. The basic question thus becomes: is a maximal homogeneity scale of order H^{-1} necessarily unnatural if the initial curvature is small and, consequently, H^{-1} is very large in Planck (or string) units? In the impossibility of experimental information one could exclude "a priori", for large horizons, the maximal homogeneity scale H^{-1} as a natural initial condition. In the pre-big bang scenario, however, pre-Planckian initial conditions are not necessarily washed out by inflation and are accessible (in principle) to observational tests, so that their naturalness could be also analyzed with a Bayesan approach, in terms of "a posteriori" probabilities.Comment: 4 pages, Latex, one figure. Many references added. The text has been improved in many points. To appear in Phys. Rev.

Non-Abelian T-duality in Pre-Big-Bang Cosmology

We study the impact of non-Abelian T-duality transformations on a string based cosmological model. The implementation of the pre-big-bang scenario is investigated. We found a region of the dual phase where such a picture is possible.Comment: 15 pages, LaTeX, 3 figure

Sensitivity of spherical gravitational-wave detectors to a stochastic background of non-relativistic scalar radiation

We analyze the signal-to-noise ratio for a relic background of scalar gravitational radiation composed of massive, non-relativistic particles, interacting with the monopole mode of two resonant spherical detectors. We find that the possible signal is enhanced with respect to the differential mode of the interferometric detectors. This enhancement is due to: {\rm (a)} the absence of the signal suppression, for non-relativistic scalars, with respect to a background of massless particles, and {\rm (b)} for flat enough spectra, a growth of the signal with the observation time faster than for a massless stochastic background.Comment: four pages, late

Towards a future singularity?

We discuss whether the future extrapolation of the present cosmological state may lead to a singularity even in case of "conventional" (negative) pressure of the dark energy field, namely $w=p/\rho \geq -1$. The discussion is based on an often neglected aspect of scalar-tensor models of gravity: the fact that different test particles may follow the geodesics of different metric frames, and the need for a frame-independent regularization of curvature singularities.Comment: 8 pages. Essay written for the "2004 Awards for Essays on Gravitation" (Gravity Research Foundation, Wellesley Hills, MA, USA), and selected for "Honorable Mention

Friedmann Universes and Exact Solutions in String Cosmology

We show that the classical null strings generate the Hilbert-Einstein gravity corresponding to D-dimensional Friedmann universes.Comment: 8 pages, LATE

Peak and end point of the relic graviton background in string cosmology

Using general arguments we determine the allowed region for the end point frequency and the peak energy density of the stochastic background of gravity waves expected in string cosmology. We provide an accurate estimate of the minimal experimental sensitivity required to detect a signal in the Hz to GHz range.Comment: 11 pages, LATEX, one figure included using eps. A complete collection of papers and references on the pre-big-bang scenario in string cosmology is available at http://www.to.infn.it/teorici/gasperini

Particle production in string cosmology models

We compute spectra of particles produced during a dilaton-driven kinetic inflation phase within string cosmology models. The resulting spectra depend on the parameters of the model and on the type of particle and are quite varied, some increasing and some decreasing with frequency. We use an approximation scheme in which all spectra can be expressed in a nice symmetric form, perhaps hinting at a deeper symmetry of the underlying physics. Our results may serve as a starting point for detailed studies of relic abundances, dark matter candidates, and possible sources of large scale anisotropy.Comment: 20 pages, no figures, latex, RevTe

Macroscopic Quantum Tunneling and the "cosmic" Josephson effect

We discuss the possible influence of a cosmic magnetic field on the macroscopic quantum tunneling process associated, in a cosmological context, to the decay of the "false vacuum." We find a close analogy with the effects of an external magnetic field applied to a Josephson junction in the context of low-temperature/high-temperature superconducting devices.Comment: 4 pages, 1 pdf figure. Added references and an inset in Fig. 1, results unchanged. To appear in Phys. Rev. D (Brief Report

Tensor perturbations in high-curvature string backgrounds

We derive a generalized equation for the evolution of tensor perturbations in a cosmological background, taking into account higher-curvature contributions and a tree-level coupling to the dilaton in the string frame. The equation is obtained by perturbing the gravi-dilaton string effective action, expanded up to first order in $\alpha'$. The $\alpha'$ corrections can modify the low-energy perturbation spectrum, but the modifications are shown to be small when the background curvature keeps constant in the string frame.Comment: 9 pages, REVTEX, three figures included using EPSFIG. An updated collection of papers on the pre-big bang scenario in string cosmology is a available at http://www.to.infn.it/teorici/gasperin