ACT Constraints on Marginally Deformed Starobinsky Inflation

We investigate the inflationary phenomenology of a marginally deformed Starobinsky model, motivated by quantum corrections to the R² term, in light of the latest cosmological observations. In this framework, the inflationary potential acquires a small deformation parameter, $γ$, which shifts predictions away from the exact Starobinsky limit. Using the slow-roll formalism, we derive analytic expressions for the spectral index nₛ and tensor-to-scalar ratio r and confront them with constraints from Planck, ACT, and DESI data. Our analysis shows that nonzero values of $γ$ raise both nₛ and r, thereby alleviating the 2σ tension between the Starobinsky R² scenario and the ACT+DESI (P-ACT-LB) measurements, which favor nₛ 0.9743 ± 0.0034. For N ~ 60 e-foldings, the model consistently reproduces the observed amplitude of primordial perturbations while predicting tensor contributions within current observational bounds. We also demonstrate that the deformation softens the otherwise severe fine-tuning of the quartic self-coupling in minimally coupled inflation. The parameter range γ~ O(10⁻³)-O(10⁻²) emerges as phenomenologically viable, providing a natural extension of Starobinsky inflation compatible with present data. We conclude that marginally deformed R² inflation remains a compelling and testable candidate for the primordial dynamics of the Universe, with future CMB and gravitational-wave observations expected to further probe its parameter space.