Magnetogenesis from Cosmic String Loops

TL;DR

This paper reevaluates the potential of cosmic string networks to generate primordial magnetic fields, finding that loop contributions dominate and could explain galactic magnetic fields if amplified by dynamos.

Contribution

It provides improved estimates of magnetic flux from cosmic string loops and models their evolution in a Lambda-CDM universe, highlighting their role in magnetogenesis.

Findings

Loop population dominates magnetic field contribution.

String networks could produce galactic magnetic fields.

Magnetic fields require dynamo amplification.

Abstract

Large-scale coherent magnetic fields are observed in galaxies and clusters, but their ultimate origin remains a mystery. We reconsider the prospects for primordial magnetogenesis by a cosmic string network. We show that the magnetic flux produced by long strings has been overestimated in the past, and give improved estimates. We also compute the fields created by the loop population, and find that it gives the dominant contribution to the total magnetic field strength on present-day galactic scales. We present numerical results obtained by evolving semi-analytic models of string networks (including both one-scale and velocity-dependent one-scale models) in a Lambda-CDM cosmology, including the forces and torques on loops from Hubble redshifting, dynamical friction, and gravitational wave emission. Our predictions include the magnetic field strength as a function of correlation length, as well as the volume covered by magnetic fields. We conclude that string networks could account for magnetic fields on galactic scales, but only if coupled with an efficient dynamo amplification mechanism.