Tabletop potentials for inflation from $f(R)$ gravity

TL;DR

This paper explores how certain modified gravity theories with $f(R)$ functions can produce scalaron potentials suitable for inflation, especially flattened hilltop or tabletop types, which align well with CMB data, offering new inflationary models.

Contribution

It demonstrates that MOG $f(R)$ models with two mass scales naturally lead to tabletop potentials compatible with observations, expanding the landscape of inflationary theories.

Findings

Tabletop potentials match CMB observations well.

Single-scale MOG models produce steep hilltop potentials.

Gravity can vanish asymptotically at large $R$, affecting universe evolution.

Abstract

We show that a large class of modified gravity theories (MOG) with the Jordan-frame Lagrangian $f(R)$ translate into scalar-field (scalaron) models with hilltop potentials in the Einstein frame. (A rare exception to this rule is provided by the Starobinsky model for which the corresponding scalaron potential is plateau-like for $\phi > 0$.) We find that MOG models featuring two distinct mass scales lead to scalaron potentials that have a flattened hilltop, or tabletop. Inflationary evolution in tabletop models agrees very well with CMB observations. Tabletop potentials therefore provide a new and compelling class of MOG-based inflationary models. By contrast, MOG models with a single mass scale generally correspond to steep hilltop potentials and fail to reproduce the CMB power spectrum. Inflationary evolution in hilltop/tabletop models can proceed in two alternative directions: towards the stable point at small $R$ describing the observable universe, or towards the asymptotic region at large $R$. The MOG models which we examine have several new properties including the fact that gravity can become asymptotically vanishing, with $G_{\rm eff} \to 0$, at infinite or large finite values of the scalar curvature $R$. A universe evolving towards the asymptotically vanishing gravity region at large $R$ will either run into a 'Big-Rip' singularity, or inflate eternally.