Late Time Magnetogenesis from Ultralight Scalar Dark Matter

TL;DR

This work demonstrates that an ultralight scalar dark matter field coupled to electromagnetism via a gauge-kinetic function $I(\phi)$ can induce a tachyonic resonance in gauge modes just after recombination, generating cosmologically relevant magnetic fields without invoking early-universe mechanisms. For a dilaton-like coupling $I(\phi)=e^{2\beta\phi/M}$, the resonance band extends to $k<k_c$ with $k_c^2=(\sqrt{I})''/\sqrt{I}$, producing predominantly non-helical fields on Mpc scales and controlled by backreaction. The authors connect the mechanism to observable constraints from time variation of the fine-structure constant, parameterizing the effect with $\gamma=\beta\Phi/M$, which sets both the resonance edge and the amplitude of $\Delta\alpha/\alpha$, and derive bounds that identify a viable parameter window where $B_0$ at 1 Mpc can exceed the observational lower limit while respecting $\Delta\alpha/\alpha$ constraints. A key prediction is the absence of helicity and oscillating $\alpha$ for scalar DM, offering a distinguishable signature from pseudoscalar models; unresolved issues include backreaction details and plasma effects, which the authors plan to address in ongoing work.

Abstract

Assuming that Dark Matter is an ultralight scalar field which is coupled to electromagnetism via a gauge-kinetic function and which at the time of recombination is oscillating coherently over a Hubble patch, we show that there is a tachyonic instability for the gauge field modes which leads to the generation of magnetic fields on cosmological scales of sufficient amplitude to explain observations.