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We derive all single-field cosmologies with unit sound speed that generate scale invariant curvature perturbations on a dynamical attractor background. We identify three distinct phases: slow-roll inflation; a slowly contracting adiabatic ekpyrotic phase, described by a rapidly varying equation of state; and a novel adiabatic ekpyrotic phase on a slowly expanding background. All of these yield identical power spectra. The degeneracy is broken at the 3-point level: unlike the nearly Gaussian spectrum of slow-roll inflation, adiabatic ekpyrosis predicts large non-Gaussianities on small scales.
Date Posted: 14 July 2011
This document has been peer reviewed.