We show that the recently released B-mode polarisation data from the South Pole Telescope (SPT) favour a non-vanishing contribution of primordial gravitational waves of inflationary origin which is in tension with the previous BICEP-Keck (BK) measurements. Our analysis uses the third-order slow-roll primordial power spectra, with theoretically motivated priors, on the multifrequency SPT likelihoods complemented by the latest Planck satellite data products. The SPT measurements provide 1. 0 bit of information gain on the first slow-roll parameter, which is higher than the 0. 9 bit provided by BK even though the SPT sensitivity is five times lower. Moreover, the Bayesian dimensionality on the same parameter exceeds 1. 5 for SPT vs. 0. 3 for BK showing that it is overconstrained by the SPT data. Even if this B B-tension could be the result of a yet to be understood foreground, our findings should motivate for a closer analysis of this unexpected B-modes excess.
Christophe Ringeval (2026) studied this question.