Beyond the Starobinsky model after ACT

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. $R^3, R^4,\cdots$) can bring the model into better agreement with current data and impose nontrivial constraints on the post-inflationary dynamics.

💡 Research Summary

The paper revisits the Starobinsky inflationary model in light of the latest ACT‑LB‑BK18 cosmic‑microwave‑background data, which indicate a modest but statistically significant tension (≈ 2 σ) with the original $R^2$ prediction for the scalar spectral index $n_s$. The authors extend the $f(R)$ action to include a cubic curvature term, \