Repulsive gravity in the very early Universe

I present two examples in which the curvature singularity of a radiation-dominated Universe is regularized by (a) the repulsive effects of spin interactions, and (b) the repulsive effects arising from a breaking of the local gravitational gauge symmetry. In both cases the collapse of an initial, asymptotically flat state is stopped, and the Universe bounces towards a state of decelerated expansion. The emerging picture is typical of the pre-big bang scenario, with the main difference that the string cosmology dilaton is replaced by a classical radiation fluid, and the solutions are not duality-invariant.Comment: 9 pages, LATEX, one figure included using epsf. Awarded the Fourth Prize in the 1998 Awards for Essays on Gravitation (Gravity Research Foundation, Wellesley Hills, Ma). To appear in Gen. Rel. Grav. An updated collection of papers on the pre-big bang scenario is available at http://www.to.infn.it/~gasperi

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

Birth of the Universe in string cosmology

The decay of the string perturbative vacuum into our present cosmological state is associated to the transition from a phase of growing curvature and growing dilaton, to a phase of decreasing curvature and frozen dilaton. The possible approaches to a classical and quantum description of such a transition are introduced and briefly discussed.Comment: 17 pages, LATEX, seven figures included using EPSF, based on a talk given at the "Fourth Paris Cosmology Colloquium" (Observatoire de Paris, June 1997). To appear in the Proceedings (ed. by H. J. De Vega and N. Sanchez). An updated collection of papers on the pre-big bang scenario is available at http://www.to.infn.it/teorici/gasperin

Late-time effects of Planck-scale cosmology: dilatonic interpretation of the dark energy field

We present a model of dark energy based on the string effective action, and on the assumption that the dilaton is strongly coupled to dark matter. We discuss the main differences between this class of models and more conventional models of quintessence,uncoupled to dark matter. This paper is based on talks presented at the "VII Congresso Nazionale di Cosmologia" (Osservatorio Astronomico di Roma, Monte Porzio Catone, November 2002), and at the Meeting "Dark Energy Day" (University of Milano-Bicocca, November 2002). To appear in the Proc. of the International Conference on ``Thinking, Observing and Mining the Universe" (Sorrento, September 2003), eds. G. Longo and G. Miele (World Scientific, Singapore).Comment: 8 pages, latex, 4 figures included using eps

On the initial regime of pre-big bang cosmology

The production of a background of super-horizon curvature perturbations with the appropriate (red) spectrum needed to trigger the cosmic anisotropies observed on large scales is associated, in the context of pre-big bang inflation, with a phase of growing string coupling. The extension towards the past of such a phase is not limited in time by the dynamical backreaction of the quantum perturbations of the cosmological geometry and of its sources. A viable, slightly red spectrum of scalar perturbations can thus be the output of an asymptotic, perturbative regime which is well compatible with an initial string-vacuum state satisfying the postulate of "Asymptotic Past Triviality".Comment: 9 pages, no figures, a few typos corrected. Published in JCAP 09 (2017) 00

Cosmology and short-distance gravity

If the observed dark-energy density $\rho_\Lambda$ is interpreted as the net contribution of the energy density of the vacuum, $\rho_\Lambda \equiv \rho_V \sim M_V^4$, and the corresponding vacuum length scale $\lambda_V = M_V^{-1}$ as the cutoff scale controlling the low-energy, effective field-theory limit of gravity, it follows that the conventional cosmological scenario based on the effective gravitational equations may be valid only up to the Tev energy scale. Such a possibility would be strongly disfavored by the existence of a relic background of primordial gravitational radiation of intensity compatible with present (or near future) experimental sensitivities.Comment: 8 pages, no figures. Essay written for the 2015 Awards for Essays on Gravitationv (Gravity Research Foundation, Wellesley Hills, MA) and awarded with "Honorable Mention". To appear in the "October 2015 Special Issue" of Int. J. Mod. Phys.

Relic dilatons in string cosmology

The allowed mass windows for a cosmic background of relic dilatons are estimated in the context of the pre-big bang scenario. The dilatons are produced from the quantum fluctuations of the vacuum, and the extension of the windows is controlled by the string mass scale. The possible relaxation of phenomenological bounds due to an intermediate stage of reheating is discussed. Even without such a relaxation, the allowed range of masses includes a light sector in which the dilatons are not yet decayed, and could provide the dominant contribution to the present large scale density.Comment: 13 pages, LATEX, two figures included using epsf. To appear in Proc. of the 12th It. Conf. on Gen. Rel. and Grav. Phys. (Rome, Sept. 1996), ed. by M. Bassan et al. (World Scientific, Singapore). An updated collection of papers on the pre-big-bang scenario in string cosmology is available at http://www.to.infn.it/teorici/gasperini

Relic Gravitational Waves from String Cosmology

A large class of string-cosmology backgrounds leads to a spectrum of relic stochastic gravitational waves, strongly tilted towards high frequencies, and characterized by two basic parameters of the cosmological model. We estimate the required sensitivity for detection of the predicted gravitational radiation and show that a region of our parameter space is within reach for some of the planned gravitational-wave detectors.Comment: 14 pages, latex, 2 figures included as epsf file