Inflation in pure gravity with only massless spin-2 fields

TL;DR

This paper demonstrates that a higher derivative extension of pure Einstein gravity, involving only massless spin-2 fields, can naturally produce an inflationary phase with sufficient e-foldings due to curvature-dependent Newton's constant.

Contribution

It introduces a specific higher derivative gravity model with only massless spin-2 fields that achieves inflation without extra fields or degrees of freedom.

Findings

The model exhibits a quasi-de Sitter inflationary phase.

Inflation occurs without additional scalar or tensor fields.

The effective Newton's constant's curvature dependence is key.

Abstract

We show that without introducing additional fields or extra degrees of freedom, a specific higher derivative extension of Einstein's gravity that has only a massless spin-2 excitation in its perturbative spectrum, has an inflationary period, a quasi-de Sitter phase with enough number of e-foldings required to solve the horizon and related problems. The crucial ingredient in the construction is the curvature dependence of the effective Newton's constant.