Cosmic microwave background anisotropies and extra dimensions in string cosmology

A recently proposed mechanism for large-scale structure in string cosmology -based on massless axionic seeds-is further analyzed and extended to the ac oustic-peak region. Existence, structure, and height of the peaks turn out to depend crucially on the overall evolution of extra dimensions during the pre-big bang phase: Conversely, precise, cosmic microwave background aniso tropy data in the acoustic-peak region will provide a window on the extra d imensions front string theory before their eventual compactification.