Vector Perturbations in a Contracting Universe

TL;DR

This paper demonstrates that vector perturbations grow in contracting universes, potentially causing perturbation theory breakdown and offering a mechanism for primordial magnetic field generation via vorticity.

Contribution

It reveals the growth of vector perturbations in contracting phases and discusses their implications for bouncing cosmological models and magnetic field origins.

Findings

Vector perturbations grow during contraction.

Growth can lead to perturbation theory breakdown.

Primordial vector perturbations may seed large-scale magnetic fields.

Abstract

In this note we show that vector perturbations exhibit growing mode solutions in a contracting Universe, such as the contracting phase of the Pre Big Bang or the Cyclic/Ekpyrotic models of the Universe. This is not a gauge artifact and will in general lead to the breakdown of perturbation theory -- a severe problem that has to be addressed in any bouncing model. We also comment on the possibility of explaining, by means of primordial vector perturbations, the existence of the observed large scale magnetic fields. This is possible since they can be seeded by vorticity.