Reinterpretation of the Starobinsky model

TL;DR

This paper explains the peculiar form of the Starobinsky inflation model through extra-dimensional compactification, predicting observable inflationary parameters and suggesting quantum gravity effects at high energy scales.

Contribution

It offers a novel explanation for the Starobinsky model's action form based on extra dimensions, reducing the need for fine-tuning of higher-order terms.

Findings

Predicted spectral index and its runnings consistent with observations

Estimated tensor-to-scalar ratio within observable range

Proposed quantum gravity effects at high energy scale

Abstract

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. Finally, we discuss a possibility that quantum gravity may appear at the scale $\Lambda \gtrsim 5 \times 10^{15}$ GeV.