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BaBy Cosmic Tension

Christophe Ringeval

TL;DR

The paper investigates whether the large-scale B-mode polarization observed by SPT signals a non-zero primordial gravitational-wave contribution from inflation, and whether this is in tension with BK measurements. It uses third-order slow-roll primordial power spectra with theoretically motivated priors, combined with Planck data and multifrequency SPT likelihoods, to perform Bayesian inference on the slow-roll parameters $\epsilon_1,\epsilon_2,\epsilon_3,\epsilon_4$. The analysis finds that SPT favors a non-vanishing $\epsilon_1$ (and thus $r \simeq 16\epsilon_1$) at roughly the 2$\sigma$ level, with information-theoretic and dimensionality metrics showing a more peaked SPT posterior compared to BK. While not statistically decisive, the results motivate closer scrutiny of the B-mode excess and call for independent measurements and improved foreground modelling to determine whether inflationary gravitational waves are detectable in current large-angle CMB data.

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 versus 0.3 for BK showing that it is overconstrained by the SPT data. Even if this BB-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.

BaBy Cosmic Tension

TL;DR

The paper investigates whether the large-scale B-mode polarization observed by SPT signals a non-zero primordial gravitational-wave contribution from inflation, and whether this is in tension with BK measurements. It uses third-order slow-roll primordial power spectra with theoretically motivated priors, combined with Planck data and multifrequency SPT likelihoods, to perform Bayesian inference on the slow-roll parameters . The analysis finds that SPT favors a non-vanishing (and thus ) at roughly the 2 level, with information-theoretic and dimensionality metrics showing a more peaked SPT posterior compared to BK. While not statistically decisive, the results motivate closer scrutiny of the B-mode excess and call for independent measurements and improved foreground modelling to determine whether inflationary gravitational waves are detectable in current large-angle CMB data.

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 versus 0.3 for BK showing that it is overconstrained by the SPT data. Even if this BB-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.

Paper Structure

This paper contains 4 sections, 4 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Data analysis
  3. Results
  4. Conclusion

Figures (4)

  • Figure 1: One-dimensional marginalised posterior distribution for the cosmological and slow-roll parameters obtained from either the SPT (red) or BK (black) $B$-mode data.
  • Figure 2: Two-dimensional marginalised posterior distribution for selected pairs of correlated parameters from the SPT (red) or BK (blue) $B$-mode data.
  • Figure 3: Two-dimensional marginalised posterior distribution of the derived power law parameters $n_{{{\mathrm{S}}}}^\epsilon$ and $r_\epsilon$ obtained by importance sampling from the slow-roll posteriors associated with the SPT (red) or BK (blue) data (at third-order).
  • Figure 4: Effects of including the $TT$ lowest multipoles from Planck on some selected slow-roll and derived power law parameters (to be compared with \ref{['fig:Psr1D']} and \ref{['fig:Ppl2D']}).