Observable non-gaussianity from gauge field production in slow roll inflation, and a challenging connection with magnetogenesis

TL;DR

This work demonstrates that a simple slow-roll inflation model in which the inflaton φ couples to a U(1) gauge field via a dilaton-like I^2(φ) F^2 term can generate observable non-Gaussianity through the feed-in of gauge-field fluctuations into inflaton perturbations. The mechanism yields a nearly local bispectrum with a characteristic quadrupolar angular dependence and logarithmic time evolution, alongside a subdominant tensor signal; the power spectrum acquires a calculable sourced contribution, and the non-Gaussianity parameter scales as f_NL ∼ O(10–100) within a perturbative regime. A key feature is the logarithmic enhancement tied to the number of e-folds and the super-horizon growth of entropy modes, controlled by N_CMB and N_tot; this framework also connects to primordial magnetogenesis, though identifying the gauge field with the electromagnetic field runs into a strong coupling problem that remains unresolved in this setup. Overall, the paper provides explicit expressions for P_ζ, B_ζ, and P_GW including sourced components, clarifies perturbativity and backreaction bounds, and outlines observable signatures and limitations for future CMB/LSS analyses.

Abstract

In any realistic particle physics model of inflation, the inflaton can be expected to couple to other fields. We consider a model with a dilaton-like coupling between a U(1) gauge field and a scalar inflaton. We show that this coupling can result in observable non-gaussianity, even in the conventional regime where inflation is supported by a single scalar slowly rolling on a smooth potential: the time dependent inflaton condensate leads to amplification of the large-scale gauge field fluctuations, which can feed-back into the scalar/tensor cosmological perturbations. In the squeezed limit, the resulting bispectrum is close to the local one, but it shows a sizable and characteristic quadrupolar dependence on the angle between the shorter and the larger modes in the correlation. Observable non-gaussianity is obtained in a regime where perturbation theory is under control. If the gauge field is identified with the electromagnetic field, the model that we study is a realization of the magnetogenesis idea originally proposed by Ratra, and widely studied. This identification (which is not necessary for the non-gaussianity production) is however problematic in light of a strong coupling problem already noted in the literature.