Abstract
We calculate the density and gravitational wave spectrums generated in a version of string cosmology termed pre-big bang scenario. The large scale structures are originated from quantum fluctuations of the metric and dilaton field during a pole-like inflation stage driven by a potential-less dilaton field realized in the low-energy effective action of string theory. The generated classical density field and the gravitational wave in the second horizon crossing epoch show tilted spectrums with n ⋍ 4 and n T ⋍ 3 , respectively. These differ from the observed spectrum of the large angular scale anisotropy of the cosmic microwave background radiation which supports scale invariant ones with n ⋍ 1 and n T ⋍ 0 . This suggests that the pre-big bang stage is not suitable for generating the present day observable large scale structures, and suggests the importance of investigating the quantum generation processes during stringy era with higher order quantum correction terms.
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