Primordial Magnetic Fields, Anomalous Matter-Antimatter Fluctuations, and Big Bang Nucleosynthesis

We show that the presence of primordial stochastic (hypercharge) magnetic fields before the elec-troweak (EW) phase transition induces isocurvature fluctuations (baryon number inhomogeneities).Depending on the details of the magnetic field spectrum and on the particle physics parameters(such as the strength of the EW phase transition and electron Yukawa couplings) these fluctuationsmay survive until the Big Bang nucleosynthesis (BBN). Their lenghtscale may exceed the neutrondiffusion length at that time, while their magnitude can be so large that sizable antimatter domainsare present. This provides the possibility of a new type of initial conditions for non-homogeneousBBN or, from a more conservative point of view, stringent bounds on primordial magnetic fields.