Spotting deviations from R^2 inflation

TL;DR

This paper evaluates the stability and observational constraints of generalized inflationary models beyond the Starobinsky paradigm, focusing on higher-order equations and their impact on viable inflation scenarios.

Contribution

It introduces a framework for analyzing stability and slow-roll parameters in broad classes of Ricci scalar and K-essence theories, linking them to observational data.

Findings

Higher-order equations constrain model stability

Planck/Bicep2 data limit deviations from Starobinsky inflation

Bounds on gravitational Lagrangians derived from observations

Abstract

We discuss the soundness of inflationary scenarios in theories beyond the Starobinsky model, namely a class of theories described by arbitrary functions of the Ricci scalar and the K-essence field. We discuss the pathologies associated with higher-order equations of motion which will be shown to constrain the stability of this class of theories. We provide a general framework to calculate the slow-roll parameters and the corresponding mappings to the theory parameters. For paradigmatic gravitational models within the class of theories under consideration we illustrate the power of the Planck/Bicep2 latest results to constrain such gravitational Lagrangians. Finally, bounds for potential deviations from Starobinsky-like inflation are derived.