Inflation in pure gravity with only massless spin-2 fields

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.