Towards a Cosmological Dual to Inflation

TL;DR

This paper classifies all single-field cosmological models with unit sound speed that produce scale-invariant curvature perturbations, revealing three phases with identical spectra but differing in non-Gaussian features, especially in the adiabatic ekpyrotic phase.

Contribution

It identifies three distinct cosmological phases that generate identical power spectra but differ in non-Gaussian signatures, expanding understanding of early universe models.

Findings

Three phases produce identical scale-invariant spectra.

Adiabatic ekpyrosis predicts large non-Gaussianities.

Degeneracy broken at the 3-point correlation level.

Abstract

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.