Running of the spectral index and inflationary dynamics of F(R) gravity

Abstract

In this work we shall provide a model-independent general calculation of the running of the spectral index for vacuum F(R) gravities. We shall exploit the functional form of the spectral index and of the tensor-to-scalar ratio in order to present a general ns−r relation for vacuum F(R) gravity theories. As we show, viable F(R) gravity theories can be classified to two classes of models regarding their prediction for the running spectral index. The R2-attractor models predict a running of the spectral index in the range −10−3<as<−10−4, which classifies them in the same universality class that most inflationary scalar field models belong to. We provide three models of this sort, for which we verify our claims in detail. However there exist viable F(R) gravity models with running of spectral index outside the range −10−3<as<−10−4 and in some cases it can be positive. We also present an R2-corrected scalar field model, which also predicts a running of the spectral index in the range −10−3<as<−10−4. For all the cases we studied, we found no evidence for the most phenomenologically interesting scenario of having r<10−4 and a running as<−10−3, which in principle could be realized.