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"Brane supersymmetry breaking" occurs in String Theory when the onlyavailable combinations of D-branes and orientifolds are not mutually BPS andyet do not introduce tree-level tachyon instabilities. It is characterized bythe emergence of a steep exponential potential, and thus by the absence ofmaximally symmetric vacua. The corresponding low-energy supergravity admitsintriguing spatially-flat cosmological solutions where a scalar field is forcedto climb up toward the steep potential after an initial singularity, andadditional milder terms can inject an inflationary phase during the ensuingdescent. We show that, in the resulting power spectra of scalar perturbations,an infrared suppression is typically followed by a pre-inflationary peak thatreflects the end of the climbing phase and can lie well apart from theapproximately scale invariant profile. A first look at WMAP9 raw data showsthat, while the chi^2 fits for the low-l CMB angular power spectrum are clearlycompatible with an almost scale invariant behavior, they display nonetheless aneye-catching preference for this type of setting within a perturbative stringregime.

Pre - Inflationary Clues from String Theory ?