Cosmic Inflation at the Crossroads

Abstract: The capability of Cosmic Inflation to explain the latest Cosmic Microwave Background and Baryonic Acoustic Oscillation data is assessed by performing Bayesian model comparison within the landscape of nearly three-hundred models of single-field slow-roll inflation. We present the first Bayesian data analysis based on the third-order slow-roll primordial power spectra. In particular, the fourth Hubble-flow function $\epsilon_4$ remains unbounded while the third function verifies, at two-sigma, $\epsilon_{3}\in[-0.4,0.5]$, which is perfectly compatible with the slow-roll predictions for the running of the spectral index. We also observe some residual excess of $B$-modes within the BICEP/Keck data favoring, at a non-statistically significant level, non-vanishing primordial tensor modes: $\log(\epsilon_{1}) > -3.9$, at $68\%$ confidence level. Then, for 283 models of single-field inflation, we compute the Bayesian evidence, the Bayesian dimensionality and the marginalized posteriors of all the models' parameters, including the ones associated with the reheating era. The average information gain on the reheating parameter $R_\mathrm{reh}$ reaches $1.3 \pm 0.18$ bits, which is more than a factor two improvement compared to the first Planck data release. As such, inflationary model predictions cannot meet data accuracy without specifying, or marginalizing over, the reheating kinematics. We also find that more than $40\%$ of the scenarios are now strongly disfavored, which shows that the constraining power of cosmological data is winning against the increase of the number of proposed models. In addition, about $20\%$ of all models have evidences within the most probable region and are all favored according to the Jeffreys' scale of Bayesian evidences.