On the Quantum-To-Classical Transition for Ekpyrotic Perturbations

TL;DR

This paper investigates how quantum fluctuations during an ekpyrotic phase become classical density perturbations, showing that decoherence and squeezing lead to observable, nearly scale-invariant structures similar to inflationary predictions.

Contribution

It demonstrates that ekpyrotic models can produce classical, scale-invariant curvature perturbations through quantum decoherence, paralleling inflationary mechanisms.

Findings

Entropic perturbations evolve into a highly squeezed quantum state.

Quantum coherence is efficiently lost during the conversion phase.

Curvature perturbations inherit semi-classicality from entropic modes.

Abstract

We examine the processes of quantum squeezing and decoherence of density perturbations produced during a slowly contracting ekpyrotic phase in which entropic perturbations are converted to curvature perturbations before the bounce to an expanding phase. During the generation phase, the entropic fluctuations evolve into a highly squeezed quantum state, analogous to the evolution of inflationary perturbations. Subsequently, during the conversion phase, quantum coherence is lost very efficiently due to the interactions of entropy and adiabatic modes. Moreover, while decoherence occurs, the adiabatic curvature perturbations inherit their semi-classicality from the entropic perturbations. Our results confirm that, just as for inflation, an ekpyrotic phase can generate nearly scale-invariant curvature perturbations which may be treated as a statistical ensemble of classical density perturbations, in agreement with observations of the cosmic background radiation.