The fine tuning problem in pre-big-bang inflation

We examine the effect of spatial curvature in the pre-big-bang inflationary model suggested by string theory. We study $O(\alpha ')$ corrections and we show that, independently of the initial curvature, they lead to a phase of exponential inflation. The amount of inflation in this phase is long enough to solve the horizon and flatness problems if the evolution starts deeply into the weak coupling regime. There is a region of the parameter space of the model where such a long inflationary phase at the string scale is consistent with COBE anisotropies, millisecond pulsar timing and nucleosynthesis constraints. We discuss implications for the spectrum of relic gravitational waves at the frequencies of LIGO and Virgo.Comment: revised Sept. 97; the new version includes a discussion of the compatibility between a long inflationary phase at the string scale and the experimental bounds on the gravitational wave spectru

Supersymmetric Vacuum Configurations in String Cosmology

We examine in a cosmological context the conditions for unbroken supersymmetry in N=1 supergravity in D=10 dimensions. We show that the cosmological solutions of the equations of motion obtained considering only the bosonic sector correspond to vacuum states with spontaneous supersymmetry breaking. With a non vanishing gravitino-dilatino condensate we find a solution of the equations of motion that satisfies necessary conditions for unbroken supersymmetry and that smoothly interpolates between Minkowski space and DeSitter space with a linearly growing dilaton, thus providing a possible example of a supersymmetric and non-singular pre-big-bang cosmology.Comment: 4 pages, Latex, 2 figure

Extracting the three- and four-graviton vertices from binary pulsars and coalescing binaries

Using a formulation of the post-Newtonian expansion in terms of Feynman graphs, we discuss how various tests of General Relativity (GR) can be translated into measurement of the three- and four-graviton vertices. In problems involving only the conservative dynamics of a system, a deviation of the three-graviton vertex from the GR prediction is equivalent, to lowest order, to the introduction of the parameter beta_{PPN} in the parametrized post-Newtonian formalism, and its strongest bound comes from lunar laser ranging, which measures it at the 0.02% level. Deviation of the three-graviton vertex from the GR prediction, however, also affects the radiative sector of the theory. We show that the timing of the Hulse-Taylor binary pulsar provides a bound on the deviation of the three-graviton vertex from the GR prediction at the 0.1% level. For coalescing binaries at interferometers we find that, because of degeneracies with other parameters in the template such as mass and spin, the effects of modified three- and four-graviton vertices is just to induce an error in the determination of these parameters and, at least in the restricted PN approximation, it is not possible to use coalescing binaries for constraining deviations of the vertices from the GR prediction.Comment: 10 pages, 5 figures; v2: an error corrected; references adde

Gravitational Waves from Electroweak Phase Transitions

Gravitational waves are generated during first-order phase transitions, either by turbolence or by bubble collisions. If the transition takes place at temperatures of the order of the electroweak scale, the frequency of these gravitational waves is today just within the band of the planned space interferometer LISA. We present a detailed analysis of the production of gravitational waves during an electroweak phase transition in different supersymmetric models where, contrary to the case of the Standard Model, the transition can be first order. We find that the stochastic background of gravitational waves generated by bubble nucleation can reach a maximum value h0^2 Omega_{gw} of order 10^{-10} - 10^{-11}, which is within the reach of the planned sensitivity of LISA, while turbolence can even produce signals at the level h0^2 Omega_{gw} \sim 10^{-9}. These values of h0^2 Omega_{gw} are obtained in the regions of the parameter space which can account for the generation of the baryon asymmetry at the electroweak scale.Comment: 30 pages, 20 figure

Loop corrections and graceful exit in string cosmology

We examine the effect of perturbative string loops on the cosmological pre-big-bang evolution. We study loop corrections derived from heterotic string theory compactified on a $Z_N$ orbifold and we consider the effect of the all-order loop corrections to the Kahler potential and of the corrections to gravitational couplings, including both threshold corrections and corrections due to the mixed Kahler-gravitational anomaly. We find that string loops can drive the evolution into the region of the parameter space where a graceful exit is in principle possible, and we find solutions that, in the string frame, connect smoothly the superinflationary pre-big-bang evolution to a phase where the curvature and the derivative of the dilaton are decreasing. We also find that at a critical coupling the loop corrections to the Kahler potential induce a ghost-like instability, i.e. the kinetic term of the dilaton vanishes. This is similar to what happens in Seiberg-Witten theory and signals the transition to a new regime where the light modes in the effective action are different and are related to the original ones by S-duality. In a string context, this means that we enter a D-brane dominated phase.Comment: 39 pages, Latex, 17 eps figure

Graviton production from D-string recombination and annihilation

Fundamental superstrings (F-strings) and D-strings may be produced at high temperature in the early Universe. Assuming that, we investigate if any of the instabilities present in systems of strings and branes can give rise to a phenomenologically interesting production of gravitons. We focus on D-strings and find that D-string recombination is a far too weak process for both astrophysical and cosmological sources. On the other hand if D-strings annihilate they mostly produce massive closed string remnants and a characteristic spectrum of gravitational modes is produced by the remnant decay, which may be phenomenologically interesting in the case these gravitational modes are massive and stable.Comment: 28 pages, 3 figure