Can the observed large scale magnetic fields be seeded by helical primordial fields?

TL;DR

This paper investigates whether helical primordial magnetic fields can serve as seeds for observed cosmic magnetic fields, showing that helicity and inverse cascade effects can relax previous constraints, especially for fields generated at QCD or inflation.

Contribution

It demonstrates that helicity-induced inverse cascade can allow certain primordial magnetic fields to seed cosmic magnetism, challenging prior limitations from gravitational wave constraints.

Findings

Helical fields at QCD or inflation can seed galactic magnetic fields.

Electroweak phase transition fields are excluded as large-scale seeds.

Helical fields produce a characteristic gravitational wave spectrum.

Abstract

Gravitational wave production induces a strong constraint on the amplitude of a primordial magnetic field. It has been shown that the nucleosynthesis bound for a stochastic gravitational wave background implies that causally generated fields cannot have enough power on large scales to provide the seeds necessary for the observed magnetic fields in galaxies and clusters, even by the most optimistic dynamo amplification. Magnetic fields generated at inflation can have high enough amplitude only if their spectrum is very red. Here we show that helicity, which leads to an inverse cascade, can mitigate these limits. In particular, we find that helical fields generated at the QCD phase transition or at inflation with red spectrum are possible seeds for the dynamo. Helical fields generated at the electroweak phase transition are instead excluded as seeds at large scales. We also calculate the spectrum of gravitational waves generated by helical magnetic fields.