Extended interactions in the Palatini-$R^2$ inflation

TL;DR

This paper explores an extended Palatini-$R^2$ inflation model with a scalar field-dependent $R^2$ term, analyzing its inflationary predictions and implications for observables like the spectral index and tensor-to-scalar ratio.

Contribution

It introduces a logarithmic field dependence in the $R^2$ coefficient within Palatini inflation, showing how this extension affects inflationary predictions and observational compatibility.

Findings

Model predictions align with observational data.

Field dependence decreases $n_s$ and may increase $r$.

Potential to circumvent suppression of tensor modes.

Abstract

The Palatini formulation of the Starobinsky model does not yield a propagating scalaron that can assume the role of the inflaton field as in the conventional metric formulation. In the so-called Palatini-${R}^2$ models this role is assumed by a fundamental scalar field nonminimally coupled to gravity. In this article we consider an extension of the interactions of this field by introducing a field dependence to the $R^2$ coefficient in the form of logarithmic corrections. We examine the resulting predictions of the inflationary observables in the framework of slow-roll approximation and investigate the possible implications of the reheating process on these predictions. We find that the model predictions lie in the favoured region of the observations and that the assumed scalar field dependence of the $R^2$ coupling tends to decrease the value of the spectral index $n_s$ and potentially enhance the tensor-to-scalar ratio $r$, circumventing the highly suppressed values commonly cited for models considered in the same framework.