Eff-ACT-ive Starobinsky pre-inflation

Abstract

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).

Figures (3)

• Figure 1: Evolution of field magnitude across the kination and inflation eras. Note that the 0 reference point is only for field magnitude and not $N$.
• Figure 2: We display the $D_\ell^{TT}$s obtained from Planck PR3 Planck:2019nip and ACT DR6 AtacamaCosmologyTelescope:2025blo. The black and red curves respectively show the best-fit theoretical $D_\ell^{TT}$s for the standard power law model and the pre-inflationary model using best-fit parameter values from \ref{['tab:cons_table']}. In the standard power law case, the cosmological parameters are taken from AtacamaCosmologyTelescope:2025blo for the P-ACT-LB combination. For the pre-inflationary model, we can see a decrease in power at low $\ell$.
• Figure 3: Since $n_s^{\rm eff}$ exhibits decaying oscillations about $n_s^{\rm BD}$, we can obtain the observed value by averaging over the oscillations in an observation window. This is accomplished by a Hilbert transform of the detected signal which extracts the envelope function. The same has been displayed here for the best-fit value $\delta N=5.7207$. The confidence intervals correspond to that of the standard spectral index obtained from the joint analysis of P-ACT-LB AtacamaCosmologyTelescope:2025blo.