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

TL;DR

This study investigates whether large-scale cosmic magnetic fields could originate from primordial helical fields. By modeling the evolution of helical fields with an inverse cascade, computing the associated gravitational-wave spectrum, and applying nucleosynthesis constraints, the authors derive robust upper bounds on the primordial magnetic-field amplitude. They find that helicity relaxes the large-scale limits, making seeds from inflation with a red spectrum or from the QCD phase transition viable, while electroweak-phase-transition helicity remains disfavored for large scales. The results have implications for dynamos in galaxies and clusters and for discerning the primordial origin of cosmic magnetism. Overall, gravitational-wave bounds provide a model-independent probe of primordial magnetism and help identify the most promising seed-generation scenarios.

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.