Successful takeoff. This Saturday, July 1 at 5:12 p.m. (French time), Euclid, a revolutionary space telescope, took off from Cape Canaveral (United States) in the direction of space in order to observe nearly two billion galaxies. The goal: to determine the cause of the acceleration of the expansion of the cosmos, so far attributed by scientists to a mysterious entity called “dark energy” which, with dark matter, constitutes 95% of the Universe – but whose we know almost nothing.
Euclid, named after the father of geometry, “will be the first space mission to study the properties of dark energy,” said Michael Seiffert, scientific manager of the project for NASA, who is taking part in this mission. of the European Space Agency (ESA).
Six years of mission
The two-ton telescope will be placed 1.5 million kilometers from Earth. For six years, the probe will draw up a three-dimensional map of the Universe, encompassing approximately two billion galaxies, over a portion of a third of the celestial vault. The distant galaxies observed will make it possible to go back in time to 10 billion years ago – the time it took for their light to reach us. The immense amount of data collected will be made public.
This European mission costs 1.5 billion euros and must last until at least 2029. Nasa also plans to launch in a few years a mission dedicated to the exploration of dark matter, the Nancy Grace Roman space telescope.
Attraction and repulsion
Dark matter (which represents 26% of the Universe) and dark energy (69%) have opposite effects: when the first exerts an attraction that holds cosmic objects together, dark energy causes them to disperse . The remaining 5% is simply ordinary matter: stars, galaxies, gas clouds and ourselves.
In the case of dark matter, we know that it exists because of a mysterious observation: impossible to explain how a galaxy or a group of galaxies does not disperse by taking into account only the gravity of their visible elements. (planets, stars, etc.). “You have to assume an additional amount of matter, invisible to our telescopes, like a gravitational component that holds everything together,” says Michael Seiffert. This cosmic “cement” has been dubbed dark matter. Never observed directly, it could be subatomic particles, according to certain hypotheses.
Dark energy is perhaps even more enigmatic. Since the discoveries of the famous astronomer Edwin Hubble in the 1920s, we know that the Universe is expanding. And since the 1990s, this expansion has accelerated. But this “implies that on very large scales, gravity actually contains a repulsive component that pulls things apart”, explains Michael Seiffert. This force, this “great mystery of physics”, is dark energy.
Mapping the expansion of the Universe
Lack of knowledge of these two dark components has been described as an “embarrassing situation” by the head of the Euclid mission at ESA, Giuseppe Racca. However, the satellite does not aim to determine their nature – too ambitious – but first to better understand their properties, the way in which they act and evolve over time.
Thanks to its 3D map, the telescope will allow precise measurements on the distribution of galaxies and the expansion of the Universe. “Euclid will observe visible objects in unprecedented proportions and scales”, explains Stéphanie Escoffier, research director at the CNRS and French specialist in the dark universe for Futura Science.
From these observations, dark matter and dark energy will be deduced “indirectly”, explained Giuseppe Racca. Calculating dark matter can be done by “subtracting” visible matter. For dark energy, David Elbaz, astrophysicist at the Atomic Energy Commission (CEA) and member of the Euclid collaboration, draws the parallel with a balloon: dark energy is the breath that inflates the balloon. Observing how it grows will therefore be instructive.
The acceleration of the expansion of the Universe would have started six billion years ago. Going back 10 billion years, Euclid could observe the first effects of dark energy.
Big Crunch?
All of this data could also shed light on the fate of the Universe. The “Big Crunch” theory (contraction of the universe on itself) has been weakened since the discovery of the accelerated expansion of the Universe. But how it continues to expand – perhaps, in tens of billions of years, until it pushes planets away from any Sun or even tears atoms apart – will depend on the properties of dark energy, which Euclid must help to measure.
The telescope,1.2 meters long, carries two instruments: an imager observing in visible light (VIS) and a near-infrared spectro-imager (NISP). It only has its antenna to deploy in flight, and should be operational after about three months. “We don’t know what we’re going to find, but Euclid has the potential to completely upset our understanding of the laws of the Universe,” says, in the CNRS JournalYannick Mellier, the head of the Euclid Consortium, which brings together all the scientists involved in the mission.