Planck unveils the dynamic side of the Universe

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The Planck collaboration, which involves in particular the CNRS, CEA, CNES, several universities and French institutions, today reveals data issued from the four years of observation of the Planck satellite of the European Space Agency (ESA). The Planck mission is dedicated to the study of the fossil radiation, the light echo of the Big Bang. The measurements, made in nine frequency bands, allow to build maps relating to the sky temperature as well as its polarisation (1), which gives us additional information on both the very young Universe (380,000 years old) and the magnetic field of our Galaxy. These data and the associated articles are submitted to the journal Astronomy & Astrophysics and are available on the ESA website (2). These informations will lead to better know the material and energetic content of the universe, the time of birth of the first stars and the current rate of expansion of the universe.

From 2009 to 2013, ESA’s Planck satellite observed fossil radiation, the oldest image of the Universe. The legacy of this project includes a wealth of unique and essential data for several fields of astrophysics. These include a map of the polarized emission of interstellar dust, a catalogue of 13188 dense and cold clouds of our Galaxy and 1653 clusters of galaxies detected by their interaction with fossil radiation, but also information on how matter has gradually concentrated over the last ten billion years and, last but not least, a map of this fossil radiation all over the sky. This tool enables researchers to visualise the distribution of matter 380,000 years after the Big Bang. Thanks to this data, our knowledge of the young Universe becomes dynamic and allows us to explore all the workings of the cosmological model.

Crédits : ESA – collaboration Planck/E. Hivon/CNRS

Fossil radiation

On the map above, the colours indicate the deviations of the temperature of the fossil radiation from its mean value. The cooler blue areas and the warmer red areas indicate variations in the density of matter early in the history of the Universe. The direction and intensity of the polarization appears as a watermark on the temperature map. They form an imprint that shows the movement of matter that falls towards denser regions and away from less dense regions. These structures can be observed at different scales on the sky.

This new data has made it possible to accurately determine the material content of the Universe :

  • 4.9% of its energy today is ordinary matter,
  • 25.9% dark matter, the nature of which remains unknown,
  • 69.2% of another form of energy, distinct from dark matter and whose precise nature is more mysterious.

The time of the birth of the first stars, which is now estimated to be around 550 million years after the Big Bang, has also been better determined. Finally, thanks to these highly accurate data, researchers have been able to assess the current rate of expansion of space, leading to an estimate of the age of the Universe at 13.77 billion years.

But what has increased considerably with the data on the polarization of fossil radiation is the ability of cosmologists to test a number of hypotheses they make about the Universe, whether in relation to the physical laws governing it or the properties of its constituents (neutrinos and dark matter, for example (3) ). The new catalogue of galaxy clusters has also helped to refine the cosmological parameters governing the formation of structures in the Universe, such as the mass of neutrinos and the time of reionization (4). Today, these data provide researchers around the world with a particularly solid basis for exploring the earliest eras close to the Big Bang. In particular, the phenomenon known as cosmic inflation, which presumably transformed the initially very chaotic Universe into a relatively homogeneous medium but with tiny fluctuations in density that would later allow galaxies to form.

Planck’s view of the magnetism of our galaxy

Images of the polarization of interstellar dust. The colours indicate the intensity of the emission. The texture of the image reflects the polarization of the emission. Where it is regular, it shows the orientation of the magnetic field. Elsewhere the information represented in the image is more complex to analyse. Irregular patterns are associated with changes in the direction of the magnetic field. Credits: ESA/Planck/M.-A. Miville-Deschênes/CNRS Collaboration
Image of the polarization of the synchrotron emission. The colours indicate the intensity of the emission. The texture of the image reflects the polarization of the emission. Where it is regular, it shows the orientation of the magnetic field. Elsewhere the information represented in the image is more complex to analyse. Irregular patterns are associated with changes in the direction of the magnetic field. Credits: ESA/Planck/M.-A. Miville-Deschênes/CNRS Collaboration

The interstellar space of our Galaxy is not empty. It contains gas and tiny grains of dust: the material our Galaxy has at its disposal to form new stars and their planets. Interstellar dust radiates at the observation wavelengths of the Planck satellite. Like the Earth or the Sun, interstellar space is traversed by a magnetic field. The magnetic force tends to align the grains, polarizing their radiation. Planck measured this polarization for the first time over the entire sky.

The discovery of the magnetism of our Galaxy is linked to the discovery of cosmic rays. Without the magnetic field, these particles, accelerated by supernovae to speeds close to the speed of light, would quickly leave the Galaxy. The magnetic force holds them back. The magnetic field is held by interstellar matter. Matter, the magnetic field and cosmic rays form a dynamic whole: they interact with each other. The importance of the magnetic field in this trio has long been recognized, but the observations available to us to study it are still too fragmentary. Astrophysicists have long sought to understand how gravity plays with the magnetic field to initiate star formation.

The Planck mission now reveals two previously unpublished maps of the polarization of the sky: one of the synchrotron (5) emission of electrons from cosmic radiation and one of the emission of interstellar dust. The data reveal the structure of the Galactic magnetic field in unprecedented detail. The polarization of the synchrotron emission, like that of the dust, indicates the direction of the magnetic field. Interpretation of the observations is complex because we only have access to a projection of a structure that is essentially three-dimensional. The data must be compared with models and numerical simulations to understand the interaction between matter and the magnetic field. This work has already started within the Planck consortium but much remains to be done as the data are so dense in information.


(1) Polarization is a property of light in the same way as colour or direction of propagation. This property is invisible to the human eye but is familiar to us (sunglasses with polarized lenses, 3D glasses in the cinema, for example). A propagating light beam is actually the result of tiny vibrations of an electric and magnetic field. If the electric field oscillates preferentially in a given direction, then the light is polarized. Certain physical phenomena naturally produce polarized light, such as fossil radiation. The polarization is measured by the two instruments on board the satellite in its seven channels from 30 to 353 GHz. Information is currently available in four of these seven channels: in the three bands of the low-frequency instrument and in the 353 GHz channel of the high-frequency instrument.

(2) The results are available at this address:

(3) See the new INSU of 01/12/2014 “Planck: new revelations on dark matter and fossil neutrinos”.

(4) The primordial Universe was ionized – electrons and protons were not bound. The emission of fossil radiation corresponds to the formation of atoms: the Universe became neutral. But we know from quasar studies that the Universe is now ionised, and has been for more than 12 or 13 billion years. Between 380,000 years and 1 billion years ago, the Universe was thus reionized.

(5) Synchrotron emission is the radiation emitted by any charged particle in the presence of a magnetic field. Its name refers to particle accelerators where it is particularly intense. The intensity of the radiation depends on the energy of the electrons and the strength of the magnetic field.

The main French laboratories involved in the Planck mission

The following French laboratories were involved in the construction and then in the analysis of the HFI instrument data (from raw measurements to frequency maps), as well as in the astrophysical and cosmological interpretation of the Planck mission data set. These results are derived in particular from measurements made with this instrument, assembled under the direction of the Institut d’Astrophysique Spatiale (CNRS/Université Paris-Sud) and operated under the direction of the Institut d’Astrophysique de Paris (CNRS/UPMC) by various laboratories involving CEA, CNRS and universities and institutions:

  • APC, AstroParticle and Cosmology (Université Paris Diderot/CNRS/CEA/Observatoire de Paris), in Paris.
  • IAP, Institut d’Astrophysique de Paris (CNRS/UPMC), in Paris.
  • IAS, Institute of Space Astrophysics (University Paris-Sud/CNRS), Orsay.
  • Institut Néel (CNRS), Grenoble.
  • IPAG, Institut de Planétologie et d’Astrophysique de l’Observatoire des Sciences de l’Univers de Grenoble (CNRS/Université Joseph Fourier), Grenoble.
  • IRAP, Institut de recherche en astrophysique et planétologie de l’Observatoire Midi-Pyrénées (Université Paul Sabatier/CNRS), in Toulouse.
  • CEA-IRFU, Research Institute on the Fundamental Laws of the Universe of the CEA, in Saclay.
  • LAL, Linear Accelerator Laboratory (CNRS/Université Paris-Sud), in Orsay.
  • LERMA, Laboratoire d’étude du rayonnement et de la matière en astrophysique (Observatoire de Paris/CNRS/ENS/Université Cergy-Pontoise/UPMC), in Paris.
  • LPSC, Laboratory of Subatomic Physics and Cosmology (Université Joseph-Fourier/CNRS/Grenoble-INP), in Grenoble.
  • CC-IN2P3 of the CNRS, Computing Centre of the National Institute of Nuclear and Particle Physics (IN2P3) of the CNRS.

Additional Resources

  • An article from CNRSleJournal that retraces in video the latest results of the Planck mission.
  • The Planck mission’s general public website:
  • The CNES website on Planck:
  • Mission Planck” films: 2013, images of the Universe in formation, Planck 2014, new results and Planck 2014, seeing the invisible, directed by Véronique Kleiner, produced by CNRS Images
  • These videos are available from the CNRS video library,
  • Frequently asked questions about the results and the Planck mission, to download (PDF)

IRAP Contact

  • Ludovic Montier:



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