Inflationary magnetogenesis from non-minimal coupling in large- and small-field potentials

Abstract

We investigate inflationary magnetogenesis in a scenario where conformal invariance of electromagnetism is broken through a \emph{non-minimal Yukawa-like coupling between the inflaton and the Ricci scalar}. We account for electromagnetic backreaction and the Schwinger effect, analyzing both standard single-field inflation and a generalized K-essence framework, \emph{dubbed quasi-quintessence}. We consider inflationary potentials compatible with Planck satellite constraints, including Starobinsky and $\alpha$-attractor models for large fields, as well as hilltop scenarios for small fields. Moreover, we explore very different functional electromagnetic couplings, introducing a novel ansatz modeled for small-fields. We show that the non-minimal coupling plays a central role in controlling the dynamics, \emph{acting as a timing parameter that regulates the onset of electric backreaction and the Schwinger regime}. This leads to a deep modification of the magnetogenesis process. Indeed, the amplitude of the generated magnetic fields can be enhanced by several orders of magnitude with respect to the minimally coupled case, reaching present-day values up to $B_0 \sim 10^{-13}\,\mathrm{G}$ in large-field scenarios, \emph{which appear as the only ones compatible with observational bounds}. Conversely, small-field models yield negligible magnetic amplitudes and appear non-predictive within our non-minimal framework.