B-mode constraints from Planck low multipole polarisation data

TL;DR

This paper uses Planck satellite data and advanced foreground removal techniques to constrain primordial B modes, finding no significant detection and setting upper limits on the tensor-to-scalar ratio.

Contribution

It introduces a Bayesian foreground cleaning method and compares two likelihood approaches for analyzing low multipole polarisation data.

Findings

Tensor-to-scalar ratio r < 0.274 at 95% confidence with pixlike

Likelihood-approximation yields a weaker limit r < 0.408

No significant primordial B mode detection from Planck low multipole data

Abstract

We present constraints on primordial B modes from large angular scale cosmic microwave background polarisation anisotropies measured with the Planck satellite. To remove Galactic polarised foregrounds, we use a Bayesian parametric component separation method, modelling synchrotron radiation as a power law and thermal dust emission as a modified blackbody. This method propagates uncertainties from the foreground cleaning into the noise covariance matrices of the maps. We construct two likelihoods: (i) a semi-analytical cross-spectrum-based likelihood-approximation scheme (momento) and (ii) an exact polarisation-only pixel-based likelihood (pixlike). Since momento is based on cross-spectra it is statistically less powerful than pixlike, but is less sensitive to systematic errors correlated across frequencies. Both likelihoods give a tensor-to-scalar ratio, r, that is consistent with zero from low multipole (2 <= ell < 30) Planck polarisation data. From full-mission maps we obtain r_0.05<0.274, at 95 per cent confidence, at a pivot scale of k = 0.05 Mpc^-1, using pixlike. momento gives a qualitatively similar but weaker 95 per cent confidence limit of r_0.05<0.408.