Low-energy graceful exit in anisotropic string cosmology backgrounds

We discuss the possibility of a smooth transition from the pre- to the post-big bang regime, in the context of the lowest-order string effective action (without higher-derivative corrections), taking into account with a phenomenological model of source the repulsive gravitational effects due to the back-reaction of the quantum fluctuations outside the horizon. We determine a set of necessary conditions for a successful and realistic transition, and we find that such conditions can be satisfied (by an appropriate model of source), provided the background is higher-dimensional and anisotropic.Comment: 13 pages, latex, 2 figures included using epsfi

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

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

Gravitational waves in non-singular string cosmologies

We study the evolution of tensor metric fluctuations in a class of non-singular models based on the string effective action, by including in the perturbation equation the higher-derivative and loop corrections needed to regularise the background solutions. We discuss the effects of such higher-order corrections on the final graviton spectrum, and we compare the results of analytical and numerical computations.Comment: 24 pages, 7 figure

Recent developments in quantum string cosmology

In this talk we discuss the quantisation of a class of string cosmology models characterised by scale factor duality invariance. The amplitudes for the full set of classically allowed and forbidden transitions are computed by applying the reduced phase space and path integral methods. In particular, the path integral calculation clarifies the meaning of the instanton-like behaviour of the transition amplitudes that has been first pointed out in previous investigations.Comment: 4 pages, one eps figures, uses espcrc2.sty. Talk given at the Third Conference on Constrained Dynamics and Quantum Gravit

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

Relaxed Bounds on the Dilaton Mass in a String Cosmology Scenario

We discuss bounds on the dilaton mass, following from the cosmological amplification of the quantum fluctuations of the dilaton background, under the assumption that such fluctuations are dominant with respect to the classical background oscillations. We show that if the fluctuation spectrum grows with the frequency the bounds are relaxed with respect to the more conventional case of a flat or decreasing spectrum. As a consequence, the allowed range of masses may become compatible with models of supersymmetry breaking, and with a universe presently dominated by a relic background of dilaton dark matter.Comment: 12 pages, plain tex, 2 figures (available on request) DFTT-03/9

On the response of gravitational antennas to dilatonic waves

It is pointed out that the coupling of macroscopic test masses to the gravi-dilaton background of string theory is non geodesic, in general, and cannot be parametrized by a Brans-Dicke model of scalar-tensor gravity. The response of gravitational antennas to dilatonic waves should be analyzed through a generalized equation of geodesic deviation, taking into account the possible direct coupling of the background to the (composition-dependent) dilatonic charge of the antenna.Comment: 10 pages, Latex, no figure

S-duality Invariant Perturbations in String Cosmology

We investigate the generation of curvature and isocurvature (dilaton, moduli and axion) perturbations in a general class of axion-dilaton-moduli models,including the pre-big bang scenario. Allowing for an arbitrary coupling constant between the dilaton field and the axion field, we exploit the SL(2,R) symmetry of the theory to obtain the spectral indices of the field perturbations in a pre-big bang type scenario. Axion field fluctuations about a homogeneous background field can yield a scale-invariant (Harrison-Zel'dovich) spectrum. As an example we present a string-motivated case with SL(2,R)xSL(2,R) symmetry, where a second axion field arises from the compactification of the ten-dimensional theory to four dimensions.Comment: 15 pages, no figures, plain LaTe

Inhomogeneous Pre-Big Bang String Cosmology

An inhomogeneous version of pre--Big Bang cosmology emerges, within string theory, from quite generic initial conditions, provided they lie deeply inside the weak-coupling, low-curvature regime. Large-scale homogeneity, flatness, and isotropy appear naturally as late-time outcomes of such an evolution.Comment: 13 pages, Latex, references adde