(ed.). Gravitational waves (IOP, 2001)(422s).

316 Elementary introduction to pre-big bang cosmologySuch sensitivities are not so far from the border, after all, but to get really insidewe have to wait for the cross-correlation of two spherical resonant-mass detectors[81], expected to reach G ∼ 10 −7 in the kilohertz range, or for the advancedinterferometers, expected to reach G ∼ 10 −10 in the range of 10 2 Hz. At lowerfrequencies, around 10 −2 –10 −3 Hz, the space interferometer LISA [36] seems tobe able to reach very high sensitivities, up to G ∼ 10 −11 . Work is in progress,however, for a more precise computation of their sensitivity to a cosmic stochasticbackground [82].Detectors able to reach, and to cross the limiting sensitivity (16.111), couldexplore for the first time the parameter space of string cosmology and of Planckscale physics. The detection of a signal from a pre-big bang background,extrapolated to the gigahertz range, could give a first experimental indication onthe value of the fundamental ratio M s /M p . Even the absence of a signal, insidethe allowed region, would be significant, as we could exclude some portion ofparameter space of the string cosmology models, obtaining in such a way directexperimental information about processes occuring at (or very near to) the stringscale.16.7 ConclusionThe conclusion of these lectures is very short and simple.There is a rich structure of stochastic, gravitational-wave backgrounds, ofcosmological origin, in the frequency range of present (or planned for the future)gravity-wave detectors.Among such backgrounds, the stronger one seems to be the backgroundpossibly predicted in the context of the pre-big bang scenario, in a stringcosmology context, originating at (or very near to) the fundamental string scale.Also, the maximal predicted intensity of the background seems to be accessibleto the sensitivity of the future advanced detectors.If this is the case, the future gravity wave detectors will be able to test stringtheory models, or perhaps models referring to some more fundamental unifiedtheory, such as D-brane theory, M-theory, and so on. In any case, such detectorswill give direct experimental information on Planck scale physics.AcknowledgmentsIt is a pleasure to thank the organizers of the Second SIGRAV School, and allthe staff of the Center ‘A. Volta’, in Villa Olmo, for their hospitality and perfectorganization of this interesting School.