Assisted Contraction

TL;DR

This paper analyzes the dynamics of a contracting universe with two scalar fields and exponential potentials, identifying conditions for generating scale-invariant curvature and isocurvature perturbations, relevant for string-inspired cosmology.

Contribution

It provides analytical solutions for background and perturbations in a two-field contracting universe model with exponential potentials, highlighting conditions for scale-invariant spectra.

Findings

Scale-invariant curvature perturbations occur only in dust-like collapse.

Conditions for scale-invariant isocurvature perturbations are identified.

Analytical solutions are derived for the background and perturbations.

Abstract

We consider the dynamics of a contracting universe ruled by two minimally coupled scalar fields with general exponential potentials. This model describes string-inspired scenarios in the Einstein frame. Both background and perturbations can be solved analytically in this model. Curvature perturbations are generated with a scale invariant spectrum only for a dust-like collapse, as happens for a single field model with an exponential potential. We find the conditions for which a scale invariant spectrum for isocurvature perturbation is generated.