Robustness of Starobinsky inflation in a minimal two-field scalar-tensor completion

TL;DR

This paper investigates the stability of Starobinsky inflation within a minimal two-field scalar-tensor model, showing that the inflationary predictions remain robust against certain radiative deformations.

Contribution

It demonstrates that a class of initial conditions leads to attractor solutions with negligible entropy effects, preserving Starobinsky-like inflationary observables.

Findings

Entropy mode remains suppressed, negligible sourcing of curvature perturbation.

Inflationary observables are effectively unchanged from the original Starobinsky model.

The model exhibits an attractor behavior connecting to the Starobinsky solution.

Abstract

We study a minimal two-field scalar-tensor completion of Starobinsky inflation motivated by the one-loop effective action of scalar-tensor gravity. The model admits an exact Starobinsky branch, but the relevant question is whether nearby trajectories generate observable multifield effects. We show that a non-trivial class of initial conditions relaxes to an attractor-connected slow-roll branch continuously connected to the Starobinsky solution. We then solve the coupled adiabatic and entropy perturbation equations numerically. On the branch studied here, the entropy mode remains sufficiently suppressed that its sourcing of the curvature perturbation is negligible, while the tensor sector is unchanged. The inflationary observables therefore remain effectively Starobinsky-like, providing a robustness test of Starobinsky inflation against a minimal radiative scalar-tensor deformation.