ESA Science & Technology: Simple but challenging: the Universe according to Planck

ESA Science & Technology: Simple but challenging:

 Universe according to Planck.

However, when we try and make sense of the data on the grandest scale, more than about six degrees across the sky, we realise that we might be in trouble,” he adds.

The view of the Universe presented in the standard model may not be able to fully explain the richness of detail present in the CMB at the largest scales on the sky, as cosmologists revealed a number of ‘anomalies’ in the all-sky CMB map that do not fit very well with this model’s predictions. While the observations on small and intermediate angular scales agree extremely well with the model predictions, the fluctuations detected on large angular scales on the sky – between 90 and six degrees – are about 10 per cent weaker than the best fit of the standard model to Planck data would like them to be. Another, perhaps related, anomalous signal appears as a substantial asymmetry in the CMB signal observed in the two opposite hemispheres of the sky: one of the two hemispheres appears to have a significantly stronger signal on average. An additional peculiar element in the data is the presence of a so-called ‘cold spot’: one of the low-temperature spots in the CMB extends over a patch of the sky that is much larger than expected.

One of the possible ways to explain the anomalies present in the large-scale pattern of the CMB invites cosmologists to reexamine one of the pillar assumptions of the standard model – isotropy. An existing theoretical framework that describes such a Universe is known as Bianchi models. If the fabric of the cosmos is not isotropic on scales so large that extend beyond the horizon of the ‘patch’ of the Universe that we can access with observations, its global geometry would be rather complex: this could force bundles of light rays into highly intricate paths where they would be significantly focussed. As CMB photons have travelled across the Universe for most of its history, they might have experienced this effect, resulting in the anomalous pattern of the CMB observed across the sky.

When we take into account the large-scale anisotropy described by the Bianchi models in the analysis of the Planck data, several anomalies are simultaneously reduced by a significant amount,” explains Krzysztof M. Górski from the Jet Propulsion Laboratory (JPL), Caltech, U.S.A.

However, it is not possible to merge this very specific anisotropic scenario with the standard model that holds very well on ‘local’ scales. Therefore, this exercise remains a tantalising demonstration of how attempts to reach a satisfactory description of the Universe at large scales might require us to get more creative in developing plausible extensions of the standard model,” he adds.

The new data from Planck are based on the first 15.5 months of its all-sky surveys. Launched in 2009, Planck was designed to map the sky in nine frequencies using two state-of-the-art instruments: the Low Frequency Instrument (LFI), which includes the frequency bands 30–70 GHz, and the High Frequency Instrument (HFI), which includes the frequency bands 100–857 GHz. HFI completed its survey in January 2012, while LFI continues to operate.

Planck’s first all-sky image was released in 2010 and the first scientific data were released in 2011. Since then, scientists have been extracting the foreground emissions that lie between us and the Universe’s first light to reveal the CMB presented in this release. The next set of cosmology data will be released in early 2014.

ESA Member States also provided key technologies such as the innovative cooler that enabled the mission’s instrumentation to be maintained at just one-tenth of a degree above absolute zero (–273.15°C). Important technologies and payload elements were also contributed by NASA.

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