Constraints on the strength of primordial magnetic fields from big bang nucleosynthesis reexamined

Progress on cosmological magnetic fields

A variety of observations impose upper limits at the nano Gauss level on magnetic fields that are coherent on inter-galactic scales while blazar observations indicate a lower bound ∼10^-16G. Such magnetic fields can play an important astrophysical role, for example at cosmic recombination and during structure formation, and also provide crucial information for particle physics in the early Universe. Magnetic fields with significant energy density could have been produced at the electroweak phase transition. The evolution and survival of magnetic fields produced on sub-horizon scales in the early Universe, however, depends on the magnetic helicity which is related to violation of symmetries in fundamental particle interactions. The generation of magnetic helicity requires new CP violating interactions that can be tested by accelerator experiments via decay channels of the Higgs particle.

Limit on primordial small-scale magnetic fields from cosmic microwave background distortions

Spatially varying primordial magnetic fields may be efficiently dissipated prior to the epoch of recombination due to the large viscosity of the baryon-photon fluid. We show that this dissipation may result in observable chemical potential &mgr; and Compton y distortions in the cosmic microwave background spectrum. Current upper limits on &mgr; and y from FIRAS constrain magnetic fields to have strength B0<3x10(-8) G (scaled to the present) between comoving coherence length approximately 400 pc and approximately 0.6 Mpc. These represent the strongest upper limits on small-scale primordial magnetic fields to date.