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

TL;DR

This paper proposes a novel mechanism where an ultra-light ALP dark matter amplifies a dark photon field, which then converts into observable magnetic fields after the electron-positron annihilation era, potentially seeding galactic magnetic fields.

Contribution

It introduces a new late-time magnetogenesis mechanism involving ALP dark matter and dark photons, enabling magnetic field generation despite cosmic plasma conductivity.

Findings

Generates primordial magnetic fields of ~10^{-24} G for relevant ALP masses.

Predicts a dark electromagnetic field of ~80 nG that could influence astrophysical phenomena.

Provides a coherent length scale for the generated magnetic fields based on ALP mass.

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 $10^{-21} {\rm eV}\lesssim m_\phi\lesssim 10^{-17}{\rm eV}$, our mechanism can generate $B\sim 10^{-24} \,{\rm G} \,(m_\phi/10^{-17} {\rm eV})^{5/4}$ with a coherent length $\lambda \sim (m_\phi/10^{-17} {\rm eV})^{-1/2}$ 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 $E_X \sim B_X\sim 80\,{\rm nG}\, (m_\phi/10^{-17}{\rm eV})^{-1/4}$, which can result in interesting astrophysical/cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.