Reinterpretation of the Starobinsky model

The Starobinsky model of inflation, consistent with Planck 2015, has a peculiar form of the action, which contains the leading Einstein term R⁠, the R2 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-order 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. Finally, we discuss a possibility that quantum gravity may appear at the scale Λ≳5×1015 GeV⁠.