Vanilla Inflation Predicts Negative Running

Abstract

We show that the simplest, and currently favoured, theoretical realizations of cosmic inflation yield a sharp prediction for the running of the spectral index $\hat{I}{\pm }_{\mathrm{S}}$. Using latest cosmological data, we compute its marginalized posterior probability distribution over the space of nearly 300 models of single-field slow-roll inflation. The most probable value is $\hat{I}{\pm }_{\mathrm{S}}=-6.3\times 10^{-4}$, lying within the $98\%$ credible interval $-1.8\times 10^{-3}<\hat{I}{\pm }_{\mathrm{S}}<-9.1\times 10^{-5}$. Within the landscape of all the proposed slow-roll inflationary models, positive values for the running are therefore disfavoured at more than three-sigma.

6 pages, 4 figures, uses epl2. Misprints corrected, matches published version