Researchers have explored axion inflation, a cosmological model where a scalar field (the inflaton) interacts with an Abelian gauge field. This axial coupling can induce tachyonic amplification of one gauge-field helicity. Traditionally, this has been studied with a massless vector, where modes with physical momentum $k/a \sim |\xi|H$ are exponentially enhanced, providing significant friction for the homogeneous inflaton.

This new work extends the mechanism to a massive vector field $m$. The amplification instability only occurs if the coupling parameter $|\xi|$ is greater than the normalized vector mass $\bar{m} \equiv m/H$. In the heavy vector regime, the mode amplitude scales as $\exp[\pi(|\xi|-\bar{m})]$. Since the amplified modes remain well within the Hubble radius when $\bar{m} \gg 1$, their contribution to long-wavelength curvature perturbations is strongly suppressed for fixed background backreaction.

Analytical calculations indicate that, in the weak-backreaction regime, the power of gauge-field-induced curvature perturbations is $\mathcal{P}^{\rm id}_\zeta \propto \bar{m}^{-2}$. Including gauge-induced friction on scalar perturbations modifies this scaling to $\mathcal{P}^{\rm id}_\zeta \propto \bar{m}^{-3}$. These estimates suggest that a value of $\mathcal{P}^{\rm id}_\zeta \lesssim 10^{-9}$ on Cosmic Microwave Background (CMB) scales is compatible with gauge field backreaction for $\bar{m}$ values greater than a few hundred. Furthermore, the first lattice simulations using a massive-vector extension of the Pencil Code, including strongly backreacting regimes, were performed, validating the analytical mode functions and backreaction estimates.