The universe revealed by Planck – perfect, but not quite
The cosmic microwave background radiation as revealed by Planck.
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.
Measure of the non-Gaussianities detected in the Planck data. These anomalies seem to indicate that even though the cosmological standard model fits the overall CMB data extremely well, there are some aspects that are not yet understood and might point towards profound physical phenomena that are not yet understood.
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 same non-Gaussianities were already found in the CMB from WMAP, and have now been confirmed with an even greater level of confidence.
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.