Why is the expansion of the Universe speeding up? This question is one of the greatest physics mysteries that Euclid, the ESA satellite set to launch in 2022, is aiming to solve. Its mission is to understand the nature of dark energy, a form of energy we know nothing about but which physicists think is driving the Universe’s accelerating expansion. To unlock the secrets of dark energy, Euclid is going to compile a 3D map of several hundred million galaxies. The light from these galaxies is bent by the gravitational interaction of the matter lying between them and Earth, so measuring deformations in the images obtained by Euclid will allow scientists to determine how this matter is distributed throughout the Universe and how it is evolving over time. This information will give them new insights into this mysterious dark energy.

To accomplish this gigantic mapping effort, Euclid will be carrying a 4.5-m telescope with a 1.2-m-diameter mirror and two instruments: VIS (VISible Imager), a 600-million-pixel camera that will acquire very-high-resolution images of more than a billion galaxies; and NISP (Near-Infrared Spectrometre and Photometre), an imaging spectrometre that will map the large-scale structures of the Universe. These instruments are being developed by an international consortium led by Yannick Mellier from the IAP astrophysics institute in Paris, in which no fewer than 10 French research laboratories are involved (attached to the national scientific research centre CNRS and CEA, the French atomic energy and alternative energies commission), supported by CNES.

The 10 French research laboratories participating in Euclid are:
CPPM particle physics laboratory (Marseille)
LAIM astrophysics, instrumentation and modelling laboratory (Saclay)
APC astroparticles and cosmology laboratory (Paris)
IAP astrophysics institute (Paris)
IAS space astrophysics institute (Orsay)
IPNL nuclear physics institute (Lyon)
IRAP astrophysics and planetology research institute (Toulouse)
LAM astrophysics laboratory (Marseille)
Lagrange Laboratory (Nice)
Observatoire de Paris