Beyond the Starobinsky model after ACT

Abstract

We revisit higher order corrections to the Starobinsky inflationary model using the most recent P-ACT-LB-BK18 data, which exhibits a mild but definite tension with the predictions of the original model. Our results demonstrate how even small higher order deformations of the Ricci scalar (e.g. $R^3,R^4,\dots$) can bring the model into better agreement with current data and impose nontrivial constraints on the post-inflationary dynamics.