Resurrecting Quadratic Inflation with a non-minimal coupling to gravity

TL;DR

This paper revisits quadratic inflation models by incorporating a non-minimal coupling to gravity, demonstrating that with a specific coupling strength, the model aligns with current observational data and predicts detectable tensor-to-scalar ratios.

Contribution

It introduces a viable non-minimal coupling parameter that restores quadratic inflation's compatibility with observations, and analyzes its implications for future experiments and gravity formulations.

Findings

Model with $\xi=O(10^{-3})$ fits Planck and BICEP2/Keck data.

Predicts tensor-to-scalar ratio $0.01 \,\leq r < 0.12$.

Future experiments may confirm or rule out the scenario.

Abstract

We study Quadratic Inflation with the inflaton field $\phi$ coupled non-minimally to the curvature scalar $R$, so that the potential during inflation is of the form $V\propto m^2\phi^2+\xi R\phi^2$. We show that with a suitable choice of the non-minimal coupling strength, $\xi=\mathcal{O}(10^{-3})$, one can resurrect the success of the scenario when compared against the Planck and BICEP2/Keck Array data, and that in the region of the parameter space which is still allowed the model predicts values of the tensor-to-scalar ratio in the range $0.01\leq r < 0.12$, making it possible to either confirm the scenario or rule it out already by the current or near-future experiments, such as BICEP3 or LiteBIRD. However, we show that in this case the near-future observations are unlikely to be able to distinguish between the metric and Palatini formulations of gravity.