The CMB temperature on large angular scales
Map of the cosmic microwave background (CMB) temperature 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 and larger – 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 (the two hemispheres are outlined by the large, roughly u-shaped curve in the image, the northern one being at the centre) and a so-called 'cold spot' – a large, low-temperature spot with an unusually steep temperature profile (also outlined in the lower right).
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 the anomalous features are clearly not artefacts as they are indeed present in the signal and not in the noise.
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.