Nearly Starobinsky inflation from modified gravity

Abstract

We study inflation induced by (power-low) scalar curvature corrections to General Relativity. The class of inflationary scalar potentials $V(\ensuremath{\sigma})\ensuremath{\sim}\text{exp}[n\ensuremath{\sigma}]$, $n$ general parameter, is investigated in the Einstein frame, and the corresponding actions in the Jordan frame are derived. We found the conditions for which these potentials are able to reproduce viable inflation according to the last cosmological data and lead to large scalar curvature corrections that emerge only at a mass scale larger than the Planck mass. The cosmological constant may appear or be set equal to zero in the Jordan frame action without changing the behavior of the model during inflation. Moreover, polynomial corrections to General Relativity are analyzed in detail. When de Sitter space-time emerges as an exact solution of the models, it is necessary to use perturbative equations in the Jordan framework to study their dynamics during the inflation. In this case, we demonstrate that the Ricci scalar decreases after a correct amount of inflation, making the models consistent with the observable evolution of the Universe.