General conditions for scale-invariant perturbations in an expanding universe
6. July 2011
G. Geshnizjani, W. H. Kinney, A. Moradinezhad Dizgah, JCAP 1111 (2011) 049
We investigate the general properties of expanding cosmological models which generate scale-invariant curvature perturbations in the presence of a variable speed of sound. We show that in an expanding universe, generation of a super-Hubble, nearly scale-invariant spectrum of perturbations over a range of wavelengths consistent with observation requires at least one of three conditions: (1) accelerating expansion, (2) a speed of sound faster than the speed of light, or (3) super-Planckian energy density.