Speedy sound and cosmic structure.

TLDR: If the speed of sound were vastly larger in the early Universe, a near scale-invariant spectrum of density fluctuations could have been produced even if the Universe did not submit to conventional solutions to the horizon problem.

Abstract: If the speed of sound were vastly larger in the early Universe, a near scale-invariant spectrum of density fluctuations could have been produced even if the Universe did not submit to conventional solutions to the horizon problem. We examine how the mechanism works, presenting full mathematical solutions and their heuristics. We then discuss several concrete models based on scalar fields and hydrodynamical matter that realize this mechanism, but stress that the proposed mechanism is more fundamental and general.