Symbiotic Magnetogenesis during Radiation Domination

Abstract

We present a late-time magnetogenesis mechanism in which a coupled axion-dilaton system sources a dark $U(1)$ gauge field. The dilaton's exponential coupling drives tachyonic amplification by reshaping the instability band, while the axion controls the helicity structure of the field. Together, they amplify both gauge helicities and produce a moderately chiral dark magnetic field without fine-tuning in either scalar sector. The field is generated in the dark sector, thus the mechanism avoids plasma-conductivity suppression in the visible sector, while the model remains robust across a broad range of scalar-masses and couplings. Numerical evolution from $z=10^5$ to matter-radiation equality, combined with a parameter search over the axion mass, dilaton initial conditions, and dilaton coupling, shows that astrophysically relevant amplification persists across fuzzy-dark-matter and ultralight-axion regimes. A benchmark case yields $B\approx0.9 ~\mathrm{nG}$ on $\lambda_0\sim1 \mathrm{Mpc}$, with kinetic mixing transferring the field to the visible sector over a broad range of mixing parameters.