The Euclid space telescope, whose mission is to probe the expansion of the universe and dark matter, was launched from Cape Canaveral Air Force Station (USA) late on Saturday, July 1st. This new ESA mission involves many significant space experts in Europe, including the Centre Spatial de Liège (CSL), which has tested some of the instruments, and astrophysicists from ULiège, who will be part of the team analysing the data sent by the telescope.
T
he European Space Agency's (ESA) telescope, Euclid's mission is to reveal the properties and nature of dark matter and dark energy - 95% of the content of the Universe. For the first time, Euclid will survey several billion galaxies. This unprecedentedly precise mapping will help us to understand how the Universe has been structured from the Big Bang to the present day. Researchers and engineers from the University of Liège have contributed to this new space project. Firstly, at the Centre Spatial de Liège (CSL), where the telescope was placed in a tank to simulate Euclid's ability to withstand the drastic conditions of the stellar vacuum. Then, at the Institut d'Astrophysique de l'Université de Liège, where astrophysicists will analyse the data sent by Euclid from November 2023.
The 'magic' ingredient of the Universe
Discovered in 1998, the acceleration of cosmic expansion is still largely misunderstood. Its physical origin escapes us. Is it "simply" the cosmological constant introduced and then denied by Einstein? Is it a new physical interaction yet to be discovered? Or do we need a modified theory of gravitation to describe the infinitely large? By studying its influence on the distribution of galaxies, the Euclid mission will help us to understand the nature of the dark energy that has dominated the cosmos for five billion years and will preside over the ultimate destiny of the Universe.
Mysterious dark matter
It is invisible, and its nature is still unknown, but its gravitational effects are observable. It plays a significant role in forming the Universe's major structures. Thanks to Euclid, Belgian researchers will study the properties of dark matter in dwarf galaxies and on cosmological scales. Particular attention will be paid to one candidate, primordial black holes, which could have formed from lumps in the primordial soup less than a millisecond after the Big Bang.
Back to the Big Bang
Although Euclid's direct observations concern galaxies present in the Universe at least two billion years after the Big Bang, their statistical distribution will make it possible to constrain the primordial inhomogeneities that appeared a tiny fraction of a second after the Big Bang. This is an invaluable mine of information for selecting viable models of cosmic inflation, probing the primordial history of the Universe, the existence of unknown particles or measuring the mass of neutrinos for the first time.
Gravitational lenses, or the Universe under the magnifying glass of dark matter
Dark matter in the universe can deflect light rays from distant galaxies, causing them to appear distorted. This physical effect can become a tool for probing the properties and granularity of dark matter in galaxies. Dark matter thus becomes a tool, a microscope for observing very distant lensed galaxies, to measure the Universe's rate of expansion or probe the existence of primordial black holes... Euclid's sharp vision will reveal details of the structure of an unprecedented sample of galaxies. The characteristics of the most massive galaxies will enable us to probe more deeply into the mysteries of their formation and evolution, while dwarf galaxies will allow us to probe the properties of dark matter on a smaller scale. "Thanks to its characteristics, Euclid is a formidable discoverer of gravitational lenses. It should enable us to discover ten times more systems than we currently know about! This will mean an incredible leap forward in our knowledge of the dark matter content of the central regions of galaxies", rejoices Dominique Sluse, an astrophysicist at the ORCa Laboratory (STAR Research Unit / Faculty of Science) at ULiège.
Medical examination for Euclid at the Centre Spatial de Liège (CSL)
Once launched, there is no way back. It was, therefore, essential to test and verify Euclid's ability to cope with the vacuum of space while maintaining its visual acuity. The CSL was chosen to carry out this "medical examination", involving almost 60 days of intensive vacuum tests, to be carried out in 2021. As with other space missions, CSL's unique expertise in this field has helped to verify and guarantee the telescope's performance in space. "Three major challenges had to be met during this campaign," explains Christophe Grodent, an electromechanical civil engineer, commercial director and member of the CSL guidance council. First of all, we had to ensure that the Focal 5 tank (5 m in diameter) was compatible with the instrument's dimensions and its ground-based measurement resources - it was a matter of centimetres! Secondly, cooling the environment to -260°C on such a large surface required perfect optimisation of the test setup to minimise heat loss. Finally, specific precautions had to be taken during the heating phases to avoid any risk of molecular contamination."
Scientists are already looking forward to the data the telescope is expected to send from November 2023, when it arrives at the L2 Lagrange point.
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