Cosmic Neutrino Background as a Ferromagnet

Abstract If cosmic background neutrinos interact very weakly with each other, through spin–spin interactions, then they may have experienced a phase transition, leading to a ferromagnetic ordering. The small magnetic field resulting from ferromagnetic ordering – if present before galaxy formation – could act as a primordial seed of the magnetic fields observed in several galaxies. Our findings suggest that the magnetization could occur in the right epoch, if the exchange boson of neutrino–neutrino interaction is a massless boson beyond the Standard Model, with a coupling constant of 2.2 × 10 − 13 ( m ν 10 − 4 eV ) 2 g 2.3 × 10 − 7 . The estimation of the magnetic seed is 2.3 × 10 − 27 G ≲ B CNB ≲ 6.8 × 10 − 10 G .