Reinterpretation of the Starobinsky model

Understand

The Starobinsky model of inflation, consistent with Planck 2015, has a peculiar form of the action, which contains the leading Einstein term $R$, the $R^2$ term with a huge coefficient, and negligible higher-order terms.

• We propose an explanation of this form based on compactification of extra dimensions.
• Once tuning of order $10^{-4}$ is accepted to suppress the linear term $R$, we no longer have to suppress higher-oder terms, which give nontrivial corrections to the Starobinsky model.
• We show our predictions of the spectral index, its runnings, and the tensor-to-scalar ratio.