Nonminimally assisted inflation: a general analysis

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 (ns ), the tensor-to-scalar ratio (r), and the local-type nonlinearity parameter (f NL (local)). The presence of the assistant field leads to a lowering of ns and r in most of the parameter space, compared to the original predictions. In some cases, ns 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.