http://arxiv.org/abs/2210.01828
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 free, with $G_{\rm eff} \to 0$, at infinite or large finite values of the scalar curvature $R$. Interestingly a universe evolving towards the asymptotically free gravity region at large $R$ will either run into a ‘Big-Rip’ singularity, or inflate eternally.
Y. Shtanov, V. Sahni and S. Mishra
Thu, 6 Oct 22
60/77
Comments: 32 pages, 11 figures