Gauge field production in SUGRA inflation: local non-Gaussianity and primordial black holes

TL;DR

The paper analyzes gauge-field production during inflation in supergravity models via a pseudoscalar inflaton coupled to gauge fields, showing this mechanism generically generates non-Gaussian and non-scale perturbations and influences reheating. It identifies two main realizations: massless gauge fields can produce strong CMB signatures only for large couplings and risk primordial black hole overproduction, while massive gauge fields—generated by a light curvaton-like field—can yield observable local-type non-Gaussianity without PBH overproduction. It also develops a stochastic δN framework for the light curvaton and discusses reheating temperatures, underscoring a rich set of observational predictions (non-Gaussianity, gravitational waves) constrained by PBH bounds. The results guide viable parameter choices in SUGRA inflation with χ F̃F-type couplings and emphasize potential tests with current and future cosmological data.

Abstract

When inflation is driven by a pseudo-scalar field χcoupled to vectors as α/4 χF \tilde F, this coupling may lead to a copious production of gauge quanta, which in turns induces non-Gaussian and non-scale invariant corrections to curvature perturbations. We point out that this mechanism is generically at work in a broad class of inflationary models in supergravity hence providing them with a rich set of observational predictions. When the gauge fields are massless, significant effects on CMB scales emerge only for relatively large α. We show that in this regime, the curvature perturbations produced at the last stages of inflation have a relatively large amplitude that is of the order of the upper bound set by the possible production of primordial black holes by non-Gaussian perturbations. On the other hand, within the supergravity framework described in our paper, the gauge fields can often acquire a mass through a coupling to additional light scalar fields. Perturbations of these fields modulate the duration of inflation, which serves as a source for non-Gaussian perturbations of the metric. In this regime, the bounds from primordial black holes are parametrically satisfied and non-Gaussianity of the local type can be generated at the observationally interesting level f_NL =O(10).

Figures (8)

• Figure 1: The evolution of the inflaton field $\chi$, as a function of the number of e-folds $N$ left to the end of inflation (time is moving to the left) for $\xi[N=60]=2.2$. The result in dashed blue does take backreaction from the sources in equations \ref{['kg']} and \ref{['fr']} into account, the result in red does not. It is clear that backreaction prolongs inflation.
• Figure 2: The evolution of the Hubble scale $H$ as a function of $N$ for $\xi[N=60]=2.2$. Again the dashed blue line is the result corrected for backreaction from the sources in equations \ref{['kg']} and \ref{['fr']}.
• Figure 3: Evolution of $(\beta-1)$ as function of $N$, for $\xi(N=60)=2.2$.
• Figure 4: Evolution of the power spectrum as function of $N$, for $\xi(N=60)=2.2$. The expression \ref{['spectrum']} that does not take backreaction into account is in tinily dashed blue. In solid red is the estimate \ref{['psest']}. When backreaction becomes significant this estimate coincides with the late-time estimate $(2 \pi \xi[N])^{-2}$, in largely dashed green.
• Figure 5: Evolution of our estimate for the power spectrum as a function of $N$. In dashed red is the result for $\xi[N=64]=2.2$. Other lines are for $\xi[N=64]=2.5$ (solid brown), $\xi[N=64]=2$ (solid blue), $\xi[N=64]=1.5$ (solid green), $\xi[N=64]=1$ (solid yellow) and $\xi[N=64]=0.5$ (solid orange). The black hole bound is in dashed black.