ACT constraints on marginally deformed starobinsky inflation

We investigate the inflationary phenomenology of a marginally deformed Starobinsky model, motivated by quantum corrections to the R 2 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 n s 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 n s and r , thereby alleviating the ≳ 2 σ tension between the Starobinsky R 2 scenario and the ACT+DESI (P-ACT-LB) measurements, which favor n s ≃ 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. Moreover, the model yields a small running of the spectral index that remains fully consistent with the ACT 95 % confidence constraints. We conclude that marginally deformed R 2 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.