Daniela Saadeh (University College London, United Kingdom)
Large scales in the Cosmic Microwave Background (CMB) may break statistical isotropy. Bianchi models are often invoked as a possible explanation for these low-\ell features: they provide an anisotropic underlying pattern over which the usual stochastic fluctuations are superimposed. However, the Bianchi models generally employed in the analysis of CMB data — despite mimicking the anomalies in the CMB temperature map — overproduce B-mode polarisation due to the very specific way in which they break isotropy. In this work, we consider a more physical class of Bianchi models that satisfy polarisation constraints and test for their signatures in Planck temperature and polarisation maps; WMAP data are also analysed for comparison. We take into account all the possible ways to break Friedmann-Robertson-Walker isotropy, whilst preserving homogeneity. We also test for the well-known Bianchi models that are more commonly employed in the literature, and show that we are able to improve constraints on these models by extending the likelihood to high \ell. Nested sampling techniques are employed to determine whether the Bayesian evidence favours anisotropic universes over the standard Lambda-CDM scenario.