Planck illuminates dark matter and detects fossil neutrinos

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This Monday, December 1, the Planck collaboration to which IRAP researchers (Paul Sabatier University in Toulouse and CNRS) contribute reveals the results of four years of observation of the Planck satellite of the European Space Agency (ESA), dedicated to the study of the “background radiation” of the universe. For the first time, the oldest image of our universe is accurately measured by two parameters – light polarisation1 and intensity – over the whole sky. This primordial light reveals the most elusive traces, namely those of dark matter and fossil neutrinos.

From 2009 to 2013, the Planck satellite observed the CMB, the oldest image of the universe, also called cosmic microwave background. Today, the full analysis of the data allows to draw a map whose quality is such that the footprints left by the dark matter and primordial neutrinos are clearly visible.

30×30 degrees map of the signal polarized at 353 GHz. The colors draw the thermal emission of dust while the reliefs refer to the galactic magnetic field. Credits: ESA-Planck Collaboration

As soon as 2013 the map of variations in light intensity was unveiled, informing us about the localization of matter 380,000 years after the Big Bang. By measuring the polarization of the light, Planck is able to observe the movement of this matter. Our vision of the early universe thus becomes dynamic. This new dimension and the quality of the data lead to test many parameters of the standard model of cosmology. In particular, they today throw light on the most elusive content of the universe: dark matter and neutrinos.

Fossil radiation temperature map based on data from the entire Planck mission. The unit is the deviation from the mean temperature of 2.7255 kelvin (COBE) in millionths of a degree. No smoothing was applied. Credits: ESA- Planck collaboration

New constraints on dark matter

Results of the Planck Collaboration now allow to rule out a whole class of dark matter models, in which the dark matter – dark antimatter annihilation would be important. The annihilation between a particle and its antiparticule3 means joint disappearance of the one and the other, and the release of energy.

The idea of dark matter begins to be widely accepted, but the nature of the particles that compose it remains unknown. Models are numerous in particle physics and one of the current goals is to reduce the range of possibilities by multiplying the exploration routes, for example by looking for the effects of this mysterious matter on ordinary matter and light. Planck observations show that it is not necessary to postulate the existence of a strong annihilation of dark matter – dark antimatter to explain the dynamics of the early universe. Indeed, such a mechanism would produce an amount of energy that would affect the evolution of the fluid light-matter, particularly near the time of emission of the CMB. However, the most recent observations do not bear traces of it.

These new results are even more interesting when confronted with measurements made by other instruments. The Pamela and Fermi satellites, like the AMS-02 experiment aboard the International Space Station, observed an excess of cosmic rays, which can be interpreted as a consequence of dark matter annihilation. Given the results of Planck, we have to prefer an alternative explanation (eg the emission of undetected pulsars) if we – reasonably – assume that the properties of the dark matter particle are stable over time.

Furthermore, the Planck collaboration confirms that dark matter occupies a little more than 26% of the current universe (value from its analysis in 2013), and specifies the map of the density of matter a few billion years after the Big Bang through measurements of temperature and polarization in B modes.

Répartition des densités de chaque type de composant (matière, rayonnement, constante cosmologique) en fonction du temps

Neutrinos of the first moments detected

The new results of the Planck Collaboration also cover other types of very elusive particles: neutrinos. These “ghostly” elementary particles, produced in abundance in the Sun for example, cross our planet with almost no interaction, which makes them extremely difficult to detect. It is therefore not possible to directly detect the first neutrinos produced less than one second after the Big Bang, which are extremely low in energy. Yet for the first time, Planck has detected unambiguously the effect of these primordial neutrinos on the map of the microwave background.

The primordial neutrinos detected by Planck were released about one second after the Big Bang, as the universe was still opaque to light but already transparent to these particles which can escape freely from an opaque medium to photons, as the heart of the sun. 380,000 years later, when the light of the background radiation was released, it bore the imprint of neutrinos since photons interacted gravitationnally4 with these particles. So looking at the oldest photons led to check the properties of neutrinos.

Planck’s observations are consistent with the standard model of particle physics. They practically exclude the existence of a fourth family of neutrinos5 which the final data from the WMAP satellite, the US Planck predecessor, led to envisage. Finally, Planck allowed to set an upper limit on the sum of the masses of neutrinos, which is now set to 0.23 eV (electron volts)6.

The data of the full mission and the associated articles will to be submitted to the journal Astronomy & Astrophysics (A & A) and available from the 22nd December 2014 on the ESA website. These results are particularly issued from measurements made with the high-frequency instrument HFI designed and built 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 different laboratories involving the CEA, the CNRS and universities, with funding from CNES and CNRS.


1 Polarization is a property of light as well as the color or the direction of propagation. This property is invisible to the human eye but it is familiar to us (sunglasses with polarized glasses, 3D glasses in cinema, for example). A photon which propagates is associated with an electrical field (E) and a magnetic field (B) both orthogonal to each other and to the direction of propagation. If the electric field remains in the same plane, we say that the photon is linearly polarized. This is the case for the CMB.

In the three frequency bands of the low-frequency instrument and in the 353 GHz channel of the high frequency instrument.

In some models, the dark matter particle is its own anti-particle.

4 Within the framework of general relativity, even if they have no mass, photons are sensitive to gravity that bends space-time.

5 There are three neutrino families in the standard model of particle physics.

The electronvolt, noted eV, is a unit of energy used in particle physics to express masses, the equality E = m c2 linking energy and mass (c is the speed of light). The lightest known particle after the photon and the neutrino weighs 511 keV, that is to say, more than 2 million times more than the sum of the masses of the three neutrinos.

Further Resources

The main French laboratories involved in the Planck mission

The following French laboratories were involved in the construction then the analysis of the HFI data (from raw measurements to frequency cards), and in the astrophysical and cosmological interpretation of the whole data of the Planck mission:

  • APC, AstroParticule et cosmologie (Université Paris Diderot/CNRS/CEA/Observatoire de Paris), à Paris.
  • IAP, Institut d’astrophysique de Paris (CNRS/UPMC), à Paris.
  • IAS, Institut d’astrophysique spatiale (Université Paris-Sud/CNRS), à Orsay
  • Institut Néel (CNRS), à Grenoble.
  • IPAG, Institut de planétologie et d’astrophysique de l’Observatoire des sciences de l’Univers de Grenoble (CNRS/Université Joseph Fourier), à Grenoble.
  • IRAP, Institut de recherche en astrophysique et planétologie de l’Observatoire Midi-Pyrénées (Université Paul Sabatier/CNRS), à Toulouse.
  • CEA-IRFU, Institut de recherche sur les lois fondamentales de l’Univers du CEA, à Saclay.
  • LAL, Laboratoire de l’accélérateur linéaire (CNRS/Université Paris-Sud), à Orsay.
  • LERMA, Laboratoire d’étude du rayonnement et de la matière en astrophysique (Observatoire de Paris/CNRS/ENS/Université Cergy-Pontoise/UPMC), à Paris
  • LPSC, Laboratoire de physique subatomique et de cosmologie (Université Joseph-Fourier/CNRS/Grenoble-INP), à Grenoble.
  • CC-IN2P3 du CNRS, Centre de calcul de l’Institut national de physique nucléaire et de physique des particules (IN2P3) du CNRS.

IRAP Contact :

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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