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 ( n s ), 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 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.