Adiabatic fluctuations from cosmic strings in a contracting universe

TL;DR

This paper proposes that cosmic strings in a contracting universe can generate adiabatic, scale-invariant density fluctuations, which could explain the primordial fluctuations observed in the universe today.

Contribution

It introduces a novel mechanism where cosmic strings produce adiabatic fluctuations during contraction, linking topological defects to primordial density perturbations.

Findings

Cosmic strings generate super-Hubble adiabatic fluctuations in a contracting universe.

Isocurvature perturbations from cosmic strings seed curvature fluctuations on large scales.

Fluctuations evolve as adiabatic in the expanding phase after symmetry restoration.

Abstract

We show that adiabatic, super-Hubble, and almost scale invariant density fluctuations are produced by cosmic strings in a contracting universe. An essential point is that isocurvature perturbations produced by topological defects such as cosmic strings on super-Hubble scales lead to a source term which seeds the growth of curvature fluctuations on these scales. Once the symmetry has been restored at high temperatures, the isocurvature seeds disappear, and the fluctuations evolve as adiabatic ones in the expanding phase. Thus, cosmic strings may be resurrected as a mechanism for generating the primordial density fluctuations observed today.