Non-local dilaton coupling to dark matter: cosmic acceleration and pressure backreaction

A model of non-local dilaton interactions, motivated by string duality symmetries, is applied to a scenario of "coupled quintessence" in which the dilaton dark energy is non-locally coupled to the dark-matter sources. It is shown that the non-local effects tend to generate a backreaction which -- for strong enough coupling -- can automatically compensate the acceleration due to the negative pressure of the dilaton potential, thus asymptotically restoring the standard (dust-dominated) decelerated regime. This result is illustrated by analytical computations and numerical examples.Comment: 11 pages, 1 figure ep

Why supersymmetry should be restored at the TeV scale

It is explained why the curvature associated to the vacuum energy density arising from SUSY breaking cannot be completely transferred to the extra spatial dimensions of a bulk space-time manifold, and it is shown -- without using hierarchy arguments but only the results of current large-scale observations -- why the Tev scale should correspond to the maximal allowed SUSY-breaking scale.Comment: 6 pages. Essay written for the 2008 Awards for Essays on Gravitation (Gravity Research Foundation, USA), and awarded with "Honorable Mention

Quantum Squeezing and Cosmological Entropy Production

The entropy growth in a cosmological process of pair production is completely determined by the associated squeezing parameter, and is insensitive to the number of particles in the initial state. The total produced entropy may represent a significant fraction of the entropy stored today in the cosmic black-body radiation, provided pair production originates from a change in the background metric at a curvature scale of the Planck order.Comment: 7 pages, plain TEX, to appear in Class.Quantum Grav., CERN-TH.6954/9

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

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

Fully Anisotropic String Cosmologies, Maxwell Fields and Primordial Shear

We present a class of exact cosmological solutions of the low energy string effective action in the presence of a homogeneous magnetic fields. We discuss the physical properties of the obtained (fully anisotropic) cosmologies paying particular attention to their vacuum limit and to the possible isotropization mechanisms. We argue that quadratic curvature corrections are able to isotropize fully anisotropic solutions whose scale factors describe accelerated expansion. Moreover, the degree of isotropization grows with the duration of the string phase. We follow the fate of the shear parameter in a decelerated phase where, dilaton, magnetic fields and radiation fluid are simultaneously present. In the absence of any magnetic field a long string phase immediately followed by radiation is able to erase large anisotropies. Conversely, if a short string phase is followed by a long dilaton dominated phase the anisotropies can be present, in principle, also at later times. The presence of magnetic seeds after the end of the string phase can induce further anisotropies which can be studied within the formalism reported in this paper.Comment: 19 pages in Revtex style, 14 Encapsulated figure

Homogeneous magnetic fields in fully anisotropic string cosmological backgrounds

We present new solutions of the string cosmological effective action in the presence of a homogeneous Maxwell field with pure magnetic component. Exact solutions are derived in the case of space-independent dilaton and vanishing torsion background. In our examples the four dimensional metric is either of Bianchi-type III and VI$_{-1}$ or Kantowski-Sachs.Comment: 4 page

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