On cosmological Inflation In Palatini $F(R,\phi)$ Gravity

Abstract

Single field inflationary models are investigated within Palatini quadratic gravity represented by $R+\alpha R^2$ along with a non-minimal coupling of the form $f(\phi) R$ between the inflaton field $\phi$ and the gravity. The treatment is performed in the Einstein frame, where the minimal coupling to gravity is recovered through conformal transformation. We consider various limits of the model with different inflationary scenarios characterized as canonical slow-roll inflation in the limit $\alpha \dot{\phi}^2\ll (1+f(\phi)) $, constant-roll k-inflation for $\alpha \ll 1$, and slow-roll K-inflation for$ \alpha \gg 1$ . A cosine and exponential potential are examined with the limits mentioned above and different well-motivated non-minimal couplings to gravity. We compare the theoretical results, exemplified by the tensor-to-scalar $r$ ratio and spectral index $n_s$, with the recent observational results of Planck 2018 $\&$ BICEP/Keck . Furthermore, we include the results of a new study forecast precision with which $n_s$ and $r$ can be constrained by currently envisaged observations, including CMB (Simons Observatory, CMB-S4, and LiteBIRD)