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CERN Accelerating science

Plancks results: Theoretical cosmologists' perspective

Julien Lesgourgues and Jan Hamann, respectively staff and post-doctoral fellows  of the TH unit here at CERN, are members of the Planck collaboration, and have contributed to the analysis of the Planck data presented in the series of articles published by collaboration in March. Another staff member of the TH unit, Leonardo Senatore, is a very active researcher in theoretical cosmology, has has followed closely the Planck release. We asked them to share a few comments about the released data and  their implications.


For Julien Lesgourgues the most striking message from Planck is the surprising robustness of the standard cosmological model when it is confronted to highly precise data. The current cosmological model  (called “flat LambdaCDM”) was established in 1998 on the basis of very unprecise data compared to what is available now. In the mind of most cosmologists, this model was meant to provide a rough first-order description of our observable Universe, that could have been refined with further observations. In 1998, not many people would have bet that this simplistic model would keep providing a consistent description of our Universe after improving observational error bars by few orders of magnitude, as it is the case with Planck. Julien Lesgourgues stresses that the large-scale features and the asymmetry are not significant enough for being conclusively considered as anomalies.  Moreover, even if the cosmological model needed to be adapted in order to account a departure from isotropy and homogeneity on large scales, very little of what we have learnt from Planck data on small scales would be affected. Julien Lesgourgues looks forward to see what the polarisation data will allow cosmologists to say about possible anomalies, neutrino masses or inflation.


For Jan Hamann it has been enormously satisfying to see so much personal and collective effort by so many people finally coming to fruition. Planck has delivered what it was built for and set a new standard for many years to come. He adds "If you had asked me a year ago what I would have liked to see in the Planck data, I might have wished for the presence of an additional, sub-eV particle species or perhaps a feautre in the power spectrum, bearing a mesage from the time of inflation. Alas, it was not to be. Nature appears to prefer simplicity and perhaps that's a good thing. But who knows, maybe next year's full mission data is still hiding a surprise or two!"



For Leonardo Senatore, the fantastic Planck measurements of the CMB have allowed us to better test the inflationary model. It is quite remarkable that the simplest model of Inflation fit this plethora of data with percent precision using just six parameters. This is an unbelievable success if we just think of the status of cosmology 15 years ago. One of the interesting results of the Planck measurements are related to the test of the non-Gaussianities of the primordial perturbations. These go in one-to-one correspondence with testing the interactions of the inflationary field, that is the part of the Inflaton Lagrangian which is the most non-trivial. Inflationary fluctuations can be described as the fluctuations of the Goldstone boson of time-translation, with self interactions suppressed by some high-energy scale. The Planck team has already looked for only a fraction of the possible non-Gaussian signals. Unfortunately, Planck’s absence of detection of these signals does not allow us to push the high-energy scale suppressing these interactions much higher than what the WMAP satellite had already done, just by a mere factor of two, so the pursue of testing the interactions of the inflaton will have to wait for the next Planck data analysis or for the next cosmological probes. Another very interesting aspect of the Planck results is the lack of power at large angular scales. This is might be connected to some remnants of the epoch when inflation was just starting, possibly even to the quite mysterious eternal inflationary phase. A better understanding of this signal might therefore shed light on what happened `before the beginning’ of the universe.



Julien Lesgourgues is co-author of the book "Neutrino Cosmology"  that was recently published by Cambridge University Press. In this book Julien Lesgourgues along wiith Gianpiero Mangano, Gennaro Miele and Sergio Pastor bring together all aspects of the role of neutrinos in cosmology, spanning from leptogenesis to primordial nucleosynthesis and from their role in CMB and structure formation, to the problem of their direct detection.


Further readings

Planck 2013 Results: I Overview of products and scientific results.

ESA: The Planck Mission.


Note from the editor: Licia Verde has recently delivered a CERN Colloquium  on the "Neutrino properties from cosmology". An overview of her talk was published in the last issue of the PH-newsletter (click here). You can also watch the full video of her talk by clicking here.