Ekpyrotic collapse with multiple fields

TL;DR

This paper explores how multiple-field ekpyrotic models generate a scale-invariant isocurvature spectrum during collapse, revealing the role of a tachyonic instability and the transition to a single-field dominated phase.

Contribution

It demonstrates that scale-invariant isocurvature perturbations arise from a saddle point in multi-field ekpyrotic models and identifies the late-time attractor as a single-field collapse.

Findings

Scale-invariant isocurvature spectrum generated during collapse

Tachyonic instability causes the scale invariance in the phase space

Late-time attractor is a single-field ekpyrotic collapse with a blue spectrum

Abstract

A scale invariant spectrum of isocurvature perturbations is generated during collapse in the scaling solution in models where two or more fields have steep negative exponential potentials. The scale invariance of the spectrum is realised by a tachyonic instability in the isocurvature field. We show that this instability is due to the fact that the scaling solution is a saddle point in the phase space. The late time attractor is identified with a single field dominated ekpyrotic collapse in which a steep blue spectrum for isocurvature perturbations is found. Although quantum fluctuations do not necessarily to disrupt the classical solution, an additional preceding stage is required to establish classical homogeneity.