Primordial magnetic fields

Large scale magnetic fields represent a triple point where cosmology, high-energy physics and astrophysics meet for different but related purposes. After reviewing the implications of large scale magnetic fields in these different areas, the role of primordial magnetic fields is discussed in various physical processes occurring prior to the decoupling epoch with particular attention to the big bang nucleosynthesis (BBN) epoch and to the electroweak (EW) epoch. The generation of matter--antimatter isocurvature fluctuations, induced by hypermagnetic fields, is analyzed in light of a possible increase of extra-relativistic species at BBN. It is argued that stochastic GW backgrounds can be generated by hypermagnetic fields at the LISA frequency. The problem of the origin of large scale magnetic fields is also scrutinized.