Effective Starobinsky pre-inflation

We consider one-loop corrections to a recently obtained form of Starobinsky inflation in Jordan frame 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 and causing an effective kinetic-domination stage where the universe behaves as a stiff fluid. Through slow cosmological expansion, the kination stage leads to inflation. We find a natural cut-off for the scalaron magnitude ensuring positivity of the modified kinetic term which matches Planck constraints on inflaton energy density at the pivot scale. We, then, backtrack to find that even for arbitrary initial conditions, kination naturally leads to the inflationary epoch without the need for fine-tuning. Modifications to the scalar power spectrum are also shown to potentially explain low-$l$ anomaly in CMB observations as well as the updated prediction of spectral index $\approx0.9743$ accounting for data from Atacama Cosmology Telescope (ACT).