ESA Science & Technology - Planck finds no new evidence for cosmic anomalies

The Cosmic Microwave Background: temperature and polarisation. Credit: ESA/Planck Collaboration

Planck's latest results come from an analysis of the polarisation of the Cosmic Microwave Background (CMB) radiation – the most ancient light in cosmic history, released when the Universe was just 380 000 years old.

The satellite's initial analysis, which was made public in 2013, concentrated on the temperature of this radiation across the sky. This allows astronomers to investigate the origin and evolution of the cosmos. While it mostly confirmed the standard picture of how our Universe evolves, Planck's first map also revealed a number of anomalies that are difficult to explain within the standard model of cosmology.

The anomalies are faint features on the sky that appear at large angular scales. They are definitely not artefacts produced by the behaviour of the satellite or the data processing, but they are faint enough that they could be statistical flukes – fluctuations which are extremely rare but not entirely ruled out by the standard model.

Alternatively, the anomalies might be a sign of 'new physics', the term used for as-yet unrecognised natural processes that would extend the known laws of physics.

The CMB polarisation on large angular scales. Credit: ESA/Planck Collaboration

To further probe the nature of the anomalies, the Planck team looked at the polarisation of the CMB, which was revealed after a painstaking analysis of the multi-frequency data designed to eliminate foreground sources of microwave emission, including gas and dust in our own Milky Way galaxy. This signal is the best measurement to date of the so-called CMB polarisation E-modes, and dates back to the time when the first atoms formed in the Universe and the CMB was released. It is produced by the way light scattered off electron particles just before the electrons were gathered into hydrogen atoms.

Polarisation provides an almost independent view of the CMB, so if the anomalies were also to show up there, this would increase astronomers' confidence that they could be caused by new physics rather than being statistical flukes.

While Planck was not originally designed to focus on polarisation, its observations have been used to create the most accurate all-sky maps of the CMB polarisation to date. These were published in 2018, greatly improving the quality of Planck's first polarisation maps, released in 2015.

When the Planck team looked at this data, they saw no obvious sign of the anomalies. At best, the analysis, published today in Astronomy and Astrophysics, revealed some weak hints that some of the anomalies may be present.

"Planck's polarisation measurements are fantastic," says Jan Tauber, ESA Planck project scientist. "Yet in spite of the great data we have, we don't see any significant traces of anomalies."

On the face of it, this would seem to make the anomalies more likely to be statistical flukes, but actually it does not rule out new physics because nature might be trickier than we imagine. As yet, there is no convincing hypothesis for what kind of new physics could be causing the anomalies. So, it could be that the phenomenon responsible only affects the temperature of the CMB, but not the polarisation. From this point of view, while the new analysis does not confirm that new physics is taking place, it does place important constraints on it.

The CMB temperature on large angular scales. Credit: ESA/Planck Collaboration

The most serious anomaly that showed up in the CMB temperature map is a deficit in the signal observed at large angular scales on the sky, around five degrees – as a comparison, the full Moon spans about half a degree. At these large scales, Planck's measurements are about ten per cent weaker than the standard model of cosmology would predict.

Planck also confirmed, with high statistical confidence, other anomalous traits that had been hinted at in previous observations of the CMB temperature, such as 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.

"We said at the time of the first release that Planck would be testing the anomalies using its polarisation data. The first set of polarisation maps which are clean enough for this purpose were released in 2018, now we have the results," says Krzysztof M. Górski, one of the authors of the new paper, from the Jet Propulsion Laboratory (JPL), Caltech, USA.

Unfortunately, the new data did not take the debate any further, as the latest results neither confirm nor deny the nature of the anomalies.

"We have some hints that, in the polarisation maps, there could be a power asymmetry similar to the one that is observed in the temperature maps, although it remains statistically unconvincing," adds Enrique Martínez González, also a co-author of the paper, from Instituto de Física de Cantabria in Santander, Spain.

While there will be further analysis of the Planck results taking place, it is unlikely that they will yield significantly new results on this subject. The obvious route to make progress is for a dedicated mission specially designed and optimised to study the CMB polarisation, but this is at least 10 to 15 years into the future.

"Planck has given us the best data we will have for at least a decade," says co-author Anthony Banday from Institut de Recherche en Astrophysique et Planétologie in Toulouse, France.

In the meantime, the mystery of the anomalies continues.

Notes for editors

"Planck 2018 results. VII. Isotropy and Statistics of the CMB" by the Planck Collaboration is published in Astronomy & Astrophysics.

The results are based on the Planck legacy data release, which was made public on 17 July 2018.

For further information, please contact:

Jan Tauber
ESA Planck Project Scientist
European Space Agency
Email: jan.tauberesa.int

Enrique Martínez González
Instituto de Física de Cantabria
Santander, Spain
Email: martinezifca.unican.es

Anthony Banday
Institut de Recherche en Astrophysique et Planétologie
Toulouse, France
Email: anthony.bandayirap.omp.eu

Calla Cofield (Media Relations)
Jet Propulsion Laboratory
Pasadena, California, USA
Email: calla.e.cofieldjpl.nasa.gov

Markus Bauer
ESA Science Programme Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.baueresa.int