Fredagskollokvium: Searches for violations of statistical isotropy: power asymmetry in WMAP and Planck sky maps as measured by a local variance estimator
Yashar Akrami, postdoctoral researcher, ITA
The principles of isotropy and homogeneity of the Universe on large scales are two cornerstones of the cosmological concordance model. Despite the fact that these assumptions have so far been in good agreement with most cosmological observations of both the cosmic microwave background (CMB) anisotropies and the large scale structure (LSS) of the Universe, subtle hints of the contrary have been claimed by some studies of the CMB and LSS data. Any violation of these cosmological principles may have strong impacts on our current understanding of the Universe, and therefore, it is of crucial importance to verify whether such claims hold against the tide of various high-quality data or they are only the results of systematic errors or statistical flukes. In this talk, I will present a powerful but conceptually and practically simple numerical tool for the statistical analysis of various cosmological data sets in searches for signatures of violations of statistical isotropy. I will show how this principle can be tested in a model-independent way by measuring the local variances over the sky and on disks of various sizes. For the 2013 Planck temperature sky map we find that the data, in comparison with simulated isotropic maps, suggests the presence of an anisotropic signature formally significant at least at the 3.3σ level. Similar results are obtained from analyzing the WMAP 9-year sky map. The preferred directions for the asymmetry are in good agreement with previous reports of the same hemispherical power asymmetry. I will finally discuss the implications of this finding for theoretical cosmology and the prospects for applying the same technique to the forthcoming Planck CMB polarization data and measurements of the LSS by future generations of galaxy surveys such as the ESA Euclid mission.