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.Comment: 41 pages in Latex style, 5 figure
