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.

41 pages, 12 figures

References in corpus (7)

• Gravitational wave generation from bubble collisions in first-order phase transitions: an analytic approach
• Strong magnetic fields in normal galaxies at high redshifts
• Evolution of Magnetic Fields in Freely Decaying Magnetohydrodynamic Turbulence
• Helical Magnetic Fields from Sphaleron Decay and Baryogenesis
• Large-scale magnetic fields from inflation due to Chern-Simons-like effective interaction
• Property of the spectrum of large-scale magnetic fields from inflation
• Magnetizing the universe