BaBy cosmic tension

Cosmology, Universe and Relativity at Louvain (CURL), Institute of Mathematics and Physics, University of Louvain 2 Chemin du Cyclotron, 1348 Louvain-la-Neuve, Belgium

Received: 10 October 2025
Accepted: 13 January 2026

Abstract

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 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.