Eff-ACT-ive Starobinsky pre-inflation

We consider quantum corrections to a recently obtained perturbative form of Starobinsky model to extract information about the initial conditions of the universe leading to cosmological inflation. Integrating out graviton modes, we find higher-derivative instabilities that are shown to decay into scalarons, causing an effective kinetic-domination stage which is shown to lead naturally to inflation without the need for fine-tuning of initial inflaton amplitude. We find a perturbative upper bound on scalaron magnitude that matches Planck constraints on inflaton energy density near the pivot scale. This modified history also affects observables and resolves other anomalies such as the low-$\ell$ power deficit in the TT spectrum as well as the disfavorment of Starobinsky model based on updated predictions of scalar spectral index accounting for data from Atacama Cosmology Telescope (ACT).