Vanilla Inflation Predicts Negative Running

We show that the simplest, and currently favoured, theoretical realizations of cosmic inflation yield a sharp prediction for the running of the spectral index ${\alpha }_{\mathrm{S}}$. 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 ${\alpha }_{\mathrm{S}}=-6.3\times {10}^{-4}$, lying within the $98\mathrm{\%}$ credible interval $-1.8\times {10}^{-3}<{\alpha }_{\mathrm{S}}<-9.1\times {10}^{-5}$. Within the landscape of all the proposed slow-roll inflationary models, positive values for the running are therefore disfavoured at more than three-sigma.