The universe revealed by Planck – perfect, but not quite

March 22, 2013
ESA's Planck satellite has delivered its first all-sky image of the Cosmic Microwave Background (CMB), providing the most precise picture of the early Universe so far. For the most part, the data agree extremely well with the 'standard model of cosmology' and allow for a much improved determination of its parameters. At the same time, the extraordinary quality of the Planck data reveals the presence of subtle anomalies. Two fundamental assumptions of the standard model are rigorously tested by the Planck CMB maps: isotropy and Gaussianity. Scientists at the Max Planck Institute for Extraterrestrial Physics and other institutes have now applied their statistical analysis methods to the Planck data and find that the temperature fluctuations seen in the CMB are indeed not a purely random, Gaussian field but that there are phase correlations on large scales.

The Cosmic Microwave Background Radiation (CMB) is an important probe of the very early Universe as it was emitted just 380 000 years after the Big Bang, when the cosmos became transparent. In the analysis of the CMB, it is usually assumed that the small temperature fluctuations are distributed randomly in a Gaussian field. If this is the case, then the analysis can be based on the so-called “power spectrum” of the distribution, which describes the amount of fluctuations seen on different scales – all information about any higher-order correlations is not considered.

These non-Gaussianities, however, contain important information about a process in the very early Universe called “inflation”, when the Universe experienced extremely rapid expansion just tiny fractions of a second after the Big Bang. While some inflationary scenarios predict that the fluctuations are nearly Gaussian, more complex models predict deviations from a Gaussian field.

Christoph Räth and his team at the Max Planck Institute for Extraterrestrial Physics have developed a method to test for and quantify these non-Gaussianities and applied this to the new CMB data. As in the normal CMB analysis a power spectrum is determined from the real data and then used as input to create “surrogate maps” in which possible phase correlations are randomized in a scale-dependent manner while exactly preserving the power spectrum. Suitable statistics are then used to compare the original data with these surrogate maps and to infer the presence of any higher-order correlations.

“We find non-Gaussianities and deviations from isotropy regardless of what statistic we use,” states Christoph Räth. “We had already seen the same pattern in the WMAP data, and with Planck the results are even more solid.”

The most significant anomalies found with WMAP have been confirmed with Planck data and it is clear that these anomalies represent real features of the CMB sky. “The anomalies are there – but we don’t yet understand the physical effects responsible for them,” says Christoph Räth.

However, the scientists expect that the polarization data that will become available with the 2014 data release should provide further valuable information on the nature of the CMB anomalies.

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