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

TL;DR

This paper explores how a coupling between a gauge field and the inflaton during slow-roll inflation can produce observable non-Gaussianities in the cosmic microwave background, with implications for magnetogenesis and model constraints.

Contribution

It demonstrates that gauge field production during inflation can generate detectable non-Gaussianity with distinctive angular dependence, within a controlled perturbative regime.

Findings

Observable non-Gaussianity with quadrupolar angular dependence.

Amplification of gauge field fluctuations influences scalar and tensor perturbations.

Connection to magnetogenesis models, highlighting potential issues with strong coupling.

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.