Beyond the Starobinsky model for inflation

Understand

We single out the Starobinsky model and its extensions among generic $f(R)$ gravity as attractors at large field values for chaotic inflation.

• Treating a $R^3$ curvature term as a perturbation of the Starobinsky model, we impose the phenomenological bounds on the additional term satisfying the successful inflationary predictions.
• We find that the scalar spectral index can vary in both the red or blue tilted direction, depending on the sign of the coefficient of the $R^3$ term, whereas the tensor-to-scalar ratio is less affected in the Planck-compatible region.
• We also discuss the role of higher order curvature term for stability and the reheating dynamics for the unambiguous prediction for the number of efoldings up to the $R^3$ term.