“Imagine filling the Old Port of Marseille with a hundred lines, each almost the height of the Eiffel Tower. » The abyss physicist Paschal Coyle has the sense of the formula to describe the new Provence Méditerranée underwater laboratory of which he is the director. And which now hosts a new generation giant telescope at the bottom of the sea. The latter is capable of detecting the most mysterious elementary particles: neutrinos.
“Every second, billions of these particles emitted by the sun pass through our bodies,” explains Mathieu Perrin-Terrin, CNRS researcher and scientific manager of this new instrument called “KM3NeT”. These particles do not interact or very little with matter, fortunately for the earthlings that we are in a certain way. Their role is however essential to understand the Big Bang. “Neutrinos could be the key to many mysteries of the cosmos, continues the researcher. First, because they play a fundamental role in the explosion of stars at the end of their life. Then, because they could explain why the universe today consists essentially of matter, whereas with the Big Bang it should have remained filled with energy. »
Eventually, 2,070 “eyes” in the abyss
To track these neutrinos coming from the sky, the scientists decided to immerse their instruments in the deep seabed. The abyss being as black as space, it is the ideal place to observe the trail of bluish light that the neutrinos leave in the water. They have also developed glass spheres that resist the pressure of the abyss and are a true concentration of technologies. “We don’t immerse cameras, but photomultiplier tubes,” explains Mathieu Perrin-Terrin.
Marseille hosts one of the control rooms of the submarine base, but it is installed 40 km off Toulon, at a depth of nearly 2,500 meters. It currently hosts about fifteen lines, the objective being to deploy 115 within a few years, or ultimately 2,070 “eyes”. These lines are anchored to the ground and kept vertical by a head float. “The spheres are superimposed every nine meters on these ladder lines, and this over a depth of 200 meters to instrument as much volume of water as possible”, explains Vincent Bertin, CNRS researcher at the Marseille particle physics center.
Earthquakes, marine animals, radioactivity
It is there, in these premises at the foot of the Calanques National Park, that the team prepares the optical modules (the spheres) and the lines which, by a clever ball system, will then be deployed under water. The descent to the abyss takes several hours. An underwater robot is then responsible for connecting them to the 40 km electro-optical cable, which is the other feat of the base. It allows to retrieve data in real time. “It’s as if you had an electrical outlet and very high-speed Internet at a depth of 2,500 meters, which is not normal, smiles Paschal Coyle. The idea of this platform is that it is also open to other users. »
This CNRS base is in fact co-piloted with Aix-Marseille University and Ifremer, and the giant neutrino telescope is itself developed by an international collaboration of 250 researchers from 17 countries. The underwater laboratory will therefore not only probe neutrinos. It will also collect valuable data on ocean acidification, underwater radioactivity, monitoring cetacean populations, bioluminescent animals and even seismic activity: an earthquake in Mexico can be measured up to here.