String Gas Cosmology and Structure Formation
  It has recently been shown that a Hagedorn phase of string gas cosmology may provide a causal mechanism for generating a nearly scale-invariant spectrum of scalar metric fluctuations, without the need for an intervening period of de Sitter expansion. A distinctive signature of this structure formation scenario would be a slight blue tilt of the spectrum of gravitational waves. In this paper we give more details of the computations leading to these results. 
Robert H. Brandenberger (McGill), Ali Nayeri (Harvard), Subodh P.   Patil (McGill) and Cumrun Vafa (Harvard)