Vanilla Inflation Predicts Negative Running

TL;DR

This paper demonstrates that simple single-field slow-roll inflation models predict a negative spectral index running, which is strongly supported by current cosmological data, disfavoring positive running at over three sigma.

Contribution

It provides a comprehensive analysis of the predicted spectral index running in simple inflation models and compares it with the latest observational data.

Findings

Most probable running: -6.3 x 10^{-4}

Negative running is within 98% credible interval

Positive running is disfavored at over three sigma

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 $\alpha_\mathrm{S}$. 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 $98\%$ 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.