Late-Time Magnetogenesis Driven by Axionlike Particle Dark Matter and a Dark Photon

Abstract

We propose a mechanism generating primordial magnetic fields after the e+e- annihilations. Our mechanism involves an ultralight axionlike particle (ALP) which constitutes the dark matter and a dark U(1)X gauge boson introduced to bypass the obstacle placed by the conductivity of cosmic plasma. In our scheme, a coherently oscillating ALP amplifies the dark photon field, and part of the amplified dark photon field is concurrently converted to the ordinary magnetic field through the ALP-induced magnetic mixing. For the relevant ALP mass range 10-21 eV≲mφ≲10-17 eV, our mechanism can generate B∼10-24 G(mφ/10-17 eV)5/4 with a coherent length λ∼(mφ/10-17 eV)-1/2 kpc, which is large enough to provide a seed of the galactic magnetic fields. The mechanism also predicts a dark U(1)X electromagnetic field EX∼BX∼80 nG(mφ/10-17 eV)-1/4, which can result in interesting astrophysical or cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.