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Planck reveals the screen of polarized dust in front of the CMB

Nearly two years after the end of its mission, the ESA Planck satellite continues to deliver valuable information regarding the deep universe that it has observed during nearly three years, in the submillimeter and microwave fields. On September 22, the Planck consortium to which several IRAP researchers (Paul Sabatier University of Toulouse and CNRS) contribute published a new map of the level of polarized emission from the galactic dust towards the Galactic poles. It thus appears that the polarized emission coming from the dust in our Galaxy is large enough to contaminate significantly the polarized signal coming from the CMB, on the whole sky. This result thus cast doubt on the discovery of gravitational waves proclaimed by the BICEP2 team in March 2014.

The instruments on board the Planck satellite, among which the HFI instrument to which the IRAP contributed, have collected, for nearly three years (2009-2012), the emission coming from our Milky Way and from the cosmic microwave background, in the sub-millimeter and microwave fields, allowing in particular to measure the polarized signal coming from the sky in this area of wavelength, in the entire sky and with a revolutionary sensibility.

The presence of a magnetic field within our Galaxy results in the alignment of the interstellar dust grains, and thus in the polarization of the light which the dust emits. The processing of the data collected by the Planck satellite resulted in the mapping of the Galactic polarization of the entire sky. It thus appeared that the magnitude of this polarization could in no way be ignored, whatever is the portion of the sky considered, not even at the poles, and is a major foreground of the cosmological signal.


Carte du signal galactique, en unités de signal cosmologique, estimé à partir des données de Planck. Plus les zones sont bleues, mois le fond diffus cosmologique est masqué. Le vert correspond à une amplitude du signal galactique de l'’ordre du signal détecté par BICEP2 qui a étudié la partie du ciel indiquée par le contour noir.  Crédits : ESA –& collaboration Planck.

The access to the information contained in the CMB signal, such as the presence of gravitational waves generated during the Big Bang and whose signature is sought in the polarized signal of the CMB, therefore is much more complicated than the BICEP2 collaboration hoped. The detection of these gravitational waves will therefore first require the separation of the cosmological and galactic contributions to the polarized signal of the sky.

Parallel to the analysis that the researchers of the Planck collaboration lead to characterize the polarization of the CMB over the whole sky, a collaborative work is underway between the Planck and BICEP2 teams to combine their two sets of data in the field of view of BICEP2. They hope, together, to be able to discover the presence of gravitational waves generated during the Big Bang, which would validate the existence of the inflation phase and account for the uniformity of the cosmic microwave background.

Further Resources  :

IRAP Contacts :

IRAP researchers involved in the Planck mission :

  • D. Alina, T. Banday,J.-P. Bernard,O. Berné, K. Ferrière,I. Florès-Cacho,O. Forni, M. Giard, T. Jaffe, L. Montier, E. Pointecouteau, I. Ristorcelli, A. Sauvé
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