Late-time magnetogenesis driven by ALP dark matter and dark photon

Abstract

We propose a mechanism generating primordial magnetic fields after the $e^{+}{e}^{-}$ annihilations. Our mechanism involves an ultra-light axion-like 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 $10^{-21}\mathrm{eV}\lesssim m_{\varphi }\lesssim 10^{-17}\mathrm{eV}$, our mechanism can generate $B\sim 10^{-24}\mathrm{G}(m_{\varphi }/10^{-17}\mathrm{eV}{)}^{5/4}$ with a coherent length $\lambda \sim (m_{\varphi }/10^{-17}\mathrm{eV}{)}^{-1/2}$ 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 $E_X\sim B_X\sim 80\mathrm{nG}(m_{\varphi }/10^{-17}\mathrm{eV}{)}^{-1/4}$, which can result in interesting astrophysical/cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.

Comments: 6 pages, 2 figures; discussions rearranged, minor numerical errors fixed, conclusion unchanged; discussion improved, accepted for publication in PRL