Seeds of large-scale anisotropy in string cosmology - art. no. 043511.

Pre-big-bang cosmology predicts tiny first-order dilaton and metric perturb ations at very large scales. Here we discuss the possibility that other, mo re copiously generated, perturbations may act, at second order, as scalar s eeds of large-scale structure and CMB anisotropies. We study, in particular , the cases of electromagnetic and axionic seeds. We compute the stochastic fluctuations of their energy-momentum tensor and determine the resulting c ontributions to the multipole expansion of the temperature anisotropy. In t he axion case it is possible to obtain a flat or slightly tilted blue spect rum that fits present data consistently, both for massless and for massive (but very light) axions. [S0556-2821(99)03102-1].