To measure cosmological parameters, the Euclid space telescope will use two main probes: gravitational lensing (Weak Gravitational Lensing) and galaxy distribution (Galaxy Clustering). These measurements will allow us to study dark energy and dark matter, which affect the growth of cosmic structures and the accelerated expansion of the Universe.

In addition to its implications on instrumental developments and data processing, Irfu is actively involved in the development of algorithms needed to prepare the extraction of cosmological parameters that will be derived from Euclid measurements.

Coordinated by Valeria Pettorino, physicist at Irfu's CosmoStat laboratory, in collaboration with Tom Kitching (UCL[1]) and Ariel Sanchez (MPE[2]), an international team from the Euclid collaboration with complementary expertise in theory and observation has just completed a 3-year study characterizing the performances expected from Euclid for these observational probes.

Publication an Arxiv:

[1] University College London ; [2] Max Planck Institute for extraterrestrial physics

The missing mass of the universe or non-baryonic dark matter is probably made up of particles that remain to be discovered. Massive and neutral, with very weak interactions, they still escape a detection that would identify them. While conventional photons are massless, dark matter could be made up of particles of a new type, similar to massive photons. New experimental results on the search for non-baryonic matter in this form, obtained by a team of three Irfu members, have just been published in Physical Review Letters[1].

‘First light’ for the Dark Energy Spectroscopic Instrument (DESI): as the installation phase nears completion, this new instrument is due to undergo final tests before starting to create a giant map of the sky in early 2020, a mission that is scheduled to run for five years. CEA, the French National Centre for Scientific Research (CNRS), Aix-Marseille University and French company Winlight System have played a significant role in the project to develop this international instrument which aims to scan the sky in an attempt to understand the effects of dark energy.

The installation of DESI, the Dark Energy Spectroscopic Instrument at the Kitt Peak Observatory in Arizona, has just passed an important milestone: with 6 operational spectrographs on site, the minimum configuration required to meet the scientific objectives of the project has been reached. At the end, DESI will have 10 spectrographs and will commit itself from 2020 to the spectroscopic survey of 35 million galaxies and quasars, to study the dark component of the Universe. Irfu, responsible for the cryogenic part of the spectrographs, has made a major contribution to the success of this installation and is currently finalizing the qualification of the cameras of the last spectrograph in Saclay. In parallel, other essential milestones for the construction of the instrument are achieved.

Several decades after its discovery, dark matter remains enigmatic. Researchers from IRFU have tested three models of dark matter in which the formation of large structures was modeled using supercomputing. The reconstruction of large structures from observations of quasar spectra favors the hypothesis of a standard "cold" dark matter and sets some of the strongest constraints on these invisible masses.

An international team from the Sloan Digital Sky Survey (SDSS) has carried out the first large-scale spectroscopic analysis of quasars, and was able to create a full 3D map of the universe and its large structures as it was 6 billion years ago. For now, the standard model of Cosmology, based upon Einstein's general theory of relativity, is confirmed.


Clusters and superclusters billions of light-years away

An international team, including scientists from the Astrophysics Department-AIM and the Particle Physics Department of CEA-Irfu, has just used the Planck satellite to discover galaxy clusters with characteristics that were previously unknown. These clusters, which contain up to a thousand galaxies, are the largest structures in the Universe. Many of them are located very far away from us, and we still know relatively little about them. Astrophysicists were able to detect the new clusters thanks to the imprint left in the background radiation of the universe by the hot gas from the clusters. Of the 189 clusters detected by Planck at distances from 1 to 5 billion light-years, 20 were previously unknown. Thanks to a joint program with the XMMNewton x-ray satellite, some of these new clusters could be observed, revealing weaker luminosity and a highly perturbed gas distribution. These must therefore be clusters with different characteristics.
These results were presented at a scientific colloquium on results from the Planck satellite, held from 10th to 14th January 2011 in Paris. They were published in a special issue of Astronomy & Astrophysics.

For a more detailed account, see also the French version.

The scientific community had to wait 18 months for the data collected by Planck, the European Space Agency satellite. Now, the first scientific results are in. The first edition of the compact sources catalog (ERCSC, Early Release Compact Sources Catalogue), with several thousand sources detected by Planck, has been published and presented in the context of an international colloquium, held from 11th to 14th January 2011 at the Cité des Sciences et de l'Industrie in La Villette (Paris).

Read the joint press release from CNES, CNRS, CEA, and ESA

Also refer to the program of the colloquium






J. Bonnet-bidaud

Edelweiss-ID: innovative detectors for tracking dark matter in the Milky Way


The new generation of detectors from the Edelweiss experiment, which is searching for dark matter, have just delivered their first results.  Remarkably reliable and robust, they have proved excellent at removing interference signals. Although only just installed and not yet perfected, these new detectors have improved the experiment's sensitivity by a factor of 10 in terms of its capacity to measure an interaction with a "wimp"1 , a weakly interacting massive particle, which is one of the candidates for dark matter. 





 Article submitted to Phys Lett. B (online)




In 2010 the usable mass of detectors will be tripled in order to improve their discovery potential still further.





This jump in sensitivity puts this experiment in amongst the leading group of experiments worldwide that are seeking to detect these new particles. These first results have just been submitted for publication in the journal Physics Letters B

The Nobel Prize for Physics 2008 rewarded Makoto Kobayashi and Toshihide Maskawa for having realised that the weak interaction does not affect particles and antiparticles in the same way1. In this theory, it was expected that the strong interaction would exhibit the same type of asymmetry between quarks and antiquarks.

However the asymmetry is not there! A problem! To explain this anomaly of the strong interaction, theoreticians have postulated the existence of a new particle known as the "axion", named after a detergent because it will help to clean up the problem. Expected to be both neutral and light, this particle will be analogous to a photon, with which it could be coupled. On the other hand it will only interact slightly with matter, so slightly that, to date, it has never been observed.

CAST2 is an experiment designed to detect this hypothetical particle which may be produced abundantly by the sun. The collaboration recently published their results ("Journal of Cosmology and Astroparticle Physics") which have enabled the limits of the predicted mass of the axion to be reduced. 

The search for axions continues, thanks to improvement in the performance of the CAST Micromegas detectors, enabling the level of noise to be reduced to an extremely low level, in the  low energy regime between 1 and 10 keV, and so to further reduce the lower limit for detection of these hypothetical particles.



Why are we interested in axions?




Axions are particles which were introduced by theoreticians to explain the apparent symmetry between matter and antimatter in the strong interaction. These axions, neutral and of very low mass, are also candidates to make up the dark matter in our universe. Theoretical models and astrophysical observations limit the mass of the axion to being in the range between several µeV/c2 and several eV/c2.

The absence of observation of axions in the CAST experiment has enabled an experimental upper limit to be established for the axion-photon coupling constant, for masses less than 0.4 eV/c2 (figure 1). For the first time this limit has reached the yellow band favoured by the theoretical models.  





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