Gauge field production in SUGRA inflation: local non-Gaussianity and primordial black holes (1212.1693v4)

Abstract: When inflation is driven by a pseudo-scalar field \chi coupled to vectors as \alpha/4 \chi F \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 \alpha. 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).

Gauge Field Production in SUGRA Inflation: Local Non-Gaussianity and Primordial Black Holes

This paper presents an analysis of gauge field production in supergravity (SUGRA) inflationary models where the inflaton is modeled as a pseudo-scalar field, denoted by $\chi$. The mechanism involves the coupling of $\chi$ to gauge fields, producing significant theoretical and observational implications. Researchers Linde, Mooij, and Pajer investigate both the impacts of this mechanism on cosmic microwave background (CMB) scales and the consequences for primordial black hole production, producing a framework with implications for inflationary cosmology.

Theoretical Foundations

The inflation model under consideration leverages a pseudo-scalar inflaton field $\chi$ with a potential applicable to chaotic inflation in SUGRA, characterized by the coupling $(\alpha/4) \chi F \tilde F$. This coupling results in the production of gauge quanta, which in turn produce observable non-Gaussian and non-scale invariant corrections in curvature perturbations. The authors highlight that such a coupling is not arbitrary but expected in a range of supergravity models due to permitted symmetries, such as a shift symmetry in $\chi$.

Phenomenological Implications

1. CMB Observations:
   • The production of gauge fields is especially impactful when $\alpha$ is large enough to significantly generate gauge quanta around the horizon scales.
   • The analysis shows that the parameters affecting this mechanism are such that detectable non-Gaussian signals in the CMB spectrum, specifically equilateral non-Gaussianity, can be produced within constraints set by WMAP and the broader Planck satellite observations.
   • The non-Gaussianity parameter $f_{\text{NL}}$ is calculated to be of an observationally interesting level $f_{\text{NL}} \sim \mathcal{O}(10)$.
2. Primordial Black Hole Bounds:
   • The paper explores the conditions under which the produced curvature perturbations can lead to primordial black hole formation.
   • The amplitude of curvature perturbations produced in this inflationary framework is near the upper bounds imposed by current non-observations of primordial black holes.
   • For massless gauge fields, the authors predict the perturbations exceed the critical density threshold for black hole formation, introducing potential constraints on the range of acceptable parameters for $\alpha$.
3. Alternative Mechanisms:
   • The paper discusses an alternative utilizing a light, scalar field $h$ coupled to vector fields, which are proposed to acquire mass through a Higgs-like mechanism.
   • This approach naturally limits excessive gauge quanta production, thus parametrically evading the threats from black hole overproduction.
   • They propose that perturbations in these fields impact the duration of inflation, leading to local-type non-Gaussianity through a $\delta N$ formalism.

Implications and Speculation on Future Developments

The results presented in this paper are indicative of the rich phenomenology that SUGRA-based inflationary models can exhibit. They prompt further investigation into whether primordial gravitational waves could corroborate such a coupling and whether alternate fields, as $\chi$, play a broader role in particle physics. Advances in high-precision CMB studies, gravitational wave detection (Advanced LIGO, Virgo), and constraints on light scalar particles could further refine or critique the scenarios outlined in this paper, potentially impacting our understanding of the inflationary paradigm.

The work underscores the importance of using exact solutions and numerical simulations to address backreaction effects and post-inflationary dynamics for better alignment with observational cosmology. Moreover, the project highlights the potential intersection of primordial non-Gaussianities with particle physics constraints such as those on the gravitino mass, reflecting the intertwining paths of cosmology and high-energy physics in contemporary theoretical pursuits. Future research would benefit from exploring more resilient frameworks that maintain observational viability while accounting for such complex interactions.