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General conditions for scale-invariant perturbations in an expanding universe

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arxiv 1107.1241 v1 pith:P6KPKOWI submitted 2011-07-06 astro-ph.CO

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keywords expandingperturbationsscale-invariantspeedconditionsgeneralsounduniverse
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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.

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