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

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.

Detection of gravitational waves from the QCD phase transition with pulsar timing arrays

“…Furthermore, the GW background might have non-vanishing helicity, which would be an interesting phenomenon to investigate by itself [57].…”

Numerical simulations of the decay of primordial magnetic turbulence

“…In general, however, the evolution of a primordial magnetic field is a complex process influenced by MHD as well as by cosmological dynamics [25]. In particular, the presence of a magnetic field can dramatically affect the behavior of primordial turbulence (for example, turbulence associated with phase transition bubble motions) [26][27][28]. Also, the presence of a magnetic field might itself lead to the development of turbulent motions, see Refs.…”

Modified magnetohydrodynamics around the electroweak transition

“…The evolution of the chiral anomaly around the electroweak transition, naturally leading to the creation of helicity, could therefore also influence the growth of the magnetic field correlation length if turbulence develops. Also, the presence of helicity could change constraints on the amplitude of primordial magnetic fields from gravitational wave production [42].…”