Nonminimally Assisted Inflation: A General Analysis

TL;DR

This paper investigates how a nonminimally coupled scalar assistant field influences single-field inflation models, providing analytical formulas and showing it can reconcile models with observational data by adjusting key inflationary parameters.

Contribution

It introduces a general framework for analyzing the effects of a nonminimally coupled assistant field on inflationary observables, expanding the understanding of model viability.

Findings

The assistant field generally lowers $n_s$ and $r$, aligning models with observations.

In some cases, the assistant field increases $n_s$, improving model compatibility.

Demonstrated effects on loop, power-law, and hybrid inflation models.

Abstract

The effects of a scalar field, known as the "assistant field," which nonminimally couples to gravity, on single-field inflationary models are studied. The analysis provides analytical expressions for inflationary observables such as the spectral index ($n_s$), the tensor-to-scalar ratio ($r$), and the local-type nonlinearity parameter ($f_{\rm NL}^{(\rm local)}$). The presence of the assistant field leads to a lowering of $n_s$ and $r$ in most of the parameter space, compared to the original predictions. In some cases, $n_s$ may increase due to the assistant field. This revives compatibility between ruled-out single-field models and the latest observations by Planck-BICEP/Keck. The results are demonstrated using three example models: loop inflation, power-law inflation, and hybrid inflation.