title: Adiabatic ekpyrosis: scale-invariant curvature perturbations from a single scalar field in a contracting universe.
abstract: The Universe can be made flat and smooth by undergoing a phase of ultraslow (ekpyrotic) contraction, a condition achievable with a single, canonical scalar field and conventional general relativity. It has been argued, though, that generating scale-invariant density perturbations requires at least two scalar fields and a two-step process that first produces entropy fluctuations and then converts them to curvature perturbations. In this Letter we identify a loophole in the argument and introduce an ekpyrotic model based on a single, canonical scalar field that generates nearly scale-invariant curvature fluctuations through a purely "adiabatic mechanism" in which the background evolution is a dynamical attractor. The resulting spectrum can be slightly red with distinctive non-Gaussian fluctuations.
authors.affiliation: Center for Particle Cosmology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
authors.forename: Justin
authors.initials: J
authors.lastname: Khoury
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Steinhardt