ESA Science & Technology - The CMB polarisation on large angular scales

Map of the cosmic microwave background (CMB) polarization amplitude as observed by ESA's Planck satellite. While fluctuations in the CMB are present and were observed by Planck down to very small angular scales, these images have been filtered to show mostly the signal detected on fairly large scales in the sky, around 5 degrees – as a comparison, the full Moon spans about half a degree.

On these large scales, a number of anomalies are observed in the CMB temperature – these are features that are difficult to explain within the standard model of cosmology, which relies on the assumption that the Universe, on large scales, has the same properties when observed in all directions.

The most serious anomaly is a deficit in the signal observed on scales around 5 degrees, which is about ten per cent weaker than predicted. Other anomalous traits are a significant discrepancy of the signal as observed in the two opposite hemispheres of the sky and a so-called 'cold spot' – a large, low-temperature spot with an unusually steep temperature profile (see the annotated version of this image).

Such anomalies were not detected, at least not at any significant level, in Planck's observations of the CMB polarisation.

A comparison between the top map, showing the total Planck measurement – comprising both signal and noise – with the bottom map, showing only the noise, indicates that some anomalous features may be present, such as for example a power asymmetry between the two hemispheres, but they are statistically unconvincing.

The lack of statistically significant anomalies in the polarisation maps does not rule out the potential relevance of those seen in the temperature, but makes it even more challenging to understand the origin of these puzzling features.

Regions shown in grey in the maps were masked out in the analysis to avoid residual foreground emission from our Milky Way or extragalactic sources affecting the cosmological results.