Limits for primordial magnetic fields

TL;DR

This paper reviews constraints on primordial magnetic fields, discussing their origins, evolution, and observational bounds, and suggests inflation-generated fields with a red spectrum as plausible seeds for current cosmic magnetism.

Contribution

It provides a comprehensive overview of current upper bounds on primordial magnetic fields and evaluates their origins, highlighting inflation as a favored generation mechanism.

Findings

Current bounds from CMB and gravitational waves limit magnetic field amplitude.

Inflation-generated fields with a red spectrum are favored as seeds.

Primordial magnetic fields could explain present-day cosmic magnetism.

Abstract

A possible explanation for the origin of the magnetic fields observed today in matter structures is that they were generated in the primordial universe. After briefly revising the model of a primordial stochastic magnetic field and sketching the main features of its time evolution in the primordial plasma, we illustrate the current upper bounds on the magnetic field amplitude and spectral index from Cosmic Microwave Background observations and gravitational wave production. We conclude that a primordial magnetic field generated by a non-causal process such as inflation with a red spectrum seems to be favoured as a seed for the magnetic fields observed today in structures.