Power-law $F(R)$ Gravity as Deformations to Starobinsky Inflation in View of ACT

In this work we aim to provide a new parametrization of power-law $F(R)$ gravity inflation framework in the Jordan frame. It is known in the literature that the power-law $F(R)$ gravity inflation of the form $F(R)=R+\beta R^n$ is non-viable and produces a power-law evolution. We demonstrate that the standard approach in power-law $F(R)$ gravity inflation has some parametrization issues that may lead to inconsistencies and we introduce a new parametrization which elevates the role of power-law $F(R)$ gravity deformations of the Starobinsky inflation, making it viable and compatible with both the Planck and ACT data. In our approach the power-law $F(R)$ gravity inflation is disentangled from a power-law evolution.