In the Abruzzo Mountains, an hour from Rome, physicists Davide Franco and Davide D’Angelo (their name similarity is a coincidence) regularly descend under the mountain. Under the protection of the de Gran Sasso tunnel (1400 meters of rock) and dressed in white suits and yellow helmets, the two have been researching invisible and ungraspable particles for the last five to six years, that could, if found, revolutionise science.
The mysterious substance that the two Davide are researching so passionately has a name straight out of a Sci-Fi book: dark matter. But the research surrounding it is very serious. It is difficult to explain what it is, because scientists do not know that. Davide Franco explains that « by observing the cosmos and taking other measures we have found out that normal matter, the matter we know, only makes up 5% of the universe. The other 95% are made up of dark energy [70%] and of dark matter [25%]. It is an invisible matter that produces no light, and who is completely dark. A particle that we have yet to discover, but also one that we are not sure exists. »
The Italian physicist, who is the Neutrinos specialist in the Cosmology and AstroParticle lab in CNRS Paris, notes that « a number of things in the universe » cannot be explained without reference to dark matter. A neutrino is a subatomic, elementary particle, which has no electric charge and has a unique relationship to gravity and weak interactions. Its light mass allows it to pass through matter without interacting with it, which makes it almost undetectable to scientists.
But first, we must talk about the universe’s creation 13 billion years ago: « in the beginning, the universe was a very homogeneous primordial plasma, with many small irregularities, which we think provided the germs for our galaxies. But the only interaction possible with these germs was gravity, which is insufficient for the creation of stars and planets. The only explanation is the existence of gravity wells, made up of a matter that produces no light, and capable of creating enough gravity excess to allow the particles that create atoms to regroup. » Then, we have the fact that galaxies are near each other (near enough to create galaxy clusters): « their speed is too fast for them to stay bound together without supplementary gravitational attraction, without dark matter. » This « dark matter » is equally invoked to explain why stars stay in their galaxies, instead of leaving them at great speed.
« We don’t know what dark matter is, but we can be sure that it exists around us. Simply put, it cannot interact with light nor weak or strong interactions » notes Davide D’Angelo with his musical accent. The particle physicist explains that « there are three ways to research dark matter. By looking at astrophysical objects, by indirect observation; the search for irregularities in the cosmos or the collision of normal particles in an accelerator, like the CERN’s Large Hadron Collider (LHC), by direct observation; looking at normal and dark matter particles using land-based detectors. » The LHC is the world’s most powerful particle accelerator and is situated near Geneva.
Journey to the Center of the Earth
But to directly observe dark matter particles in the best conditions, you must first avoid the cosmic rays that are constantly coming from space to Earth and that saturate dark matter detectors. « We have to be absolutely certain that there is no noise (interactions) which could be accidentally considered as dark matter. This is why we go underground, because the rock acts as shield against rays that could disturb our findings » Davide Franco explains.
Most scientists (including those that work under the Gran Sasso mountain) think that dark matter particles are WIMPs (weakly interactive massive particles), which is to say that they are elementary particles whose existence was theorised in the 80s. They would almost never interact with ordinary particles (nucleons and electrons), but would have an important mass (similar to an atomic core) and would always be in pairs, according to the universe’s Superstring theory.
The astrophysicist David Elbaz does not hesitate in proclaiming that, « if we discover dark matter and it correspond to WIMPs, we will have solved one the biggest mysteries in today’s physics. » Head of the Cosmology and Galaxy Evolution group, committee member for the selection of the European Space Agency’s future missions, this researcher observes dark matter using the indirect method by scrutinizing galaxies and galaxy clusters for irregularities. « I very closely follow the experiments that have happened underground, because dark matter could resolve paradoxes in the domain of the infinitely large (why stars and galaxies are moving too quickly), but also in the domain of the infinitely small, by answering to a problem that is linked with the big forces invoked by quantum physics », he explains from his CEA centre laboratory in Saclay.
According to David Elbaz, « this new type of matter is every physicist’s Grail, the missing link that could reconcile the infinitely large and infinitely small, Einstein’s relativity theory with the quantum field theory and the string theory. » At the University of Milan in Italy, Davide D’Angelo goes even further: « The impact of this discovery would be colossal. Dark matter plays a role in the formation of the universe, of galaxies, because it is responsible for galaxies’ structural formation. But it is also essential in the model of particle physic, because finding dark matter would create a new type of physics. »
Buried kilometers below the sea level, Gran Sasso is known for being the biggest underground laboratory in the world, with five dark matter detectors (DAMA/LIBRA, CRESST2, XENON1T, WARP, and DarkSide). But it is not the only underground center that hides researchers on the hunt for WIMPs. Every in the world, under mountains or in abandoned mines, particle astrophysicists are looking for dark matter. In total, globally, there are about forty experienced teams with the same goal – Canada has the SNOLAB (with 4 detectors), Sudan has the American project titled CDMS (Cryogenic Dark Matter Search), Japan has a lab in Kamioka, France’s lab is under the Frejus tunnel, and Spain’s lab is under the abandoned train tunnel of Canfranc.
But research refuse to speak of a dark matter « race». « A few years ago, it was clearly a competition. And for good reason: the experiments that have been ongoing for the last 5 to 6 years all rely on different techniques – detectors look for WIMPs through nuclear interactions liquid xenon, with subterranean crystals, or even with liquid argon as is the case with DarkSide, on which I am working. But we ended up understanding that it is very difficult to know if what you are detecting is dark matter: we quickly have to confirm a discovery by using another technique, or by using the same instruments coming from a different physics lab. Thus, the competition was transformed into cooperation », says Davide Franco, who is stationed at CNRS in Paris.
An experiment to confirm an experiment
At the Gran Sasso laboratory, under the Abruzzo mountains, Davide D’Angelo confirms his colleague’s tale. The particle physicist, who has been conducting neutrinos experiments for the last seven years has also been part of the international SABRE (Sodium Iodide with Active Background Rejection Experiment) program for the last three years. DAMA/LIBRA researchers announced that they had discovered traces of black matter in 2003 by using a detector that uses sodium iodide crystals, which are activated using Thalium, and analyses potential particle variations each year, depending on the Earth’s position around the Sun. « But at this time, no other experiment of the same type has been able to confirm this discovery, even with sensors that are a hundred times more sensitive » adds the Italian scientist.
« Many researchers are sceptical and think that the original source the Italian team saw was not dark matter, but came instead from radioactivity, the lab’s walls, or other types of rays. But because doubt remains, our mission with SABRE is to conduct the same experiment, here in Gran Sasso, and under a old gold mine in Australia, which is to say in two hemispheres, in order to find direct proof of dark matter particles – based on light produced when energy is freed by its collision with Sodium Iodine crystals », Davide D’Angelo explains.
The Italian researcher, whose interest in dark matter stemmed from his involvement in the 2013 DarkSide experiment (who was worked on by his colleague Davide Franco), is optimistic. « There is a good chance that SABRE may end with failure, that the DAMA/LIBRA team was wrong, and that we have been looking for ghosts our whole lives. But we will discover dark matter sooner or later! It’s a difficult quest, and I don’t know when we will find it, but I am certain it is there somewhere. »
At the dawn of a “scientific revolution”?
Over at the APC lab, Davide Franco is just as optimistic, because he is betting on the creation of new tools, which could go as far as change the available data: « it is an exciting challenge, because it pushes us to create new technology – like the Time Projecting Chambers (TPC), which use gas to perform a three-dimensional reconstruction of a particle trajectory. Filled with argon gas, DarkSide is one such detector, and between now and 2020 it should be ten times as powerful, which will allow us to finally find WIMPs. »
What if research ended in failure? « We are a number of scientists to think that what we are calling dark matter will provoke a scientific revolution, one way or another. When Copernicus or Einstein changed the way we see the world, every time they had for constant a strange coincidence; something which didn’t make sense, a glitch in our representation of the universe. And today, that glitch represents 95% of the universe’s make-up, which pushes us to think that it will certainly invoke a new way to describe physics, the universe, nature… Even if people believe in the existence of dark matter or not! », David Elbaz continues.
The scientist adds that« in all scenarios, we are at the dawn of something revolutionary. Because, even if dark matter does not exist, we will still have to revise all we know about the laws of physics, starting with the law of gravity. » Whatever the astrophysicists discover underground or in the dark of space, it will be our understanding of the whole universe that will inevitably be shaken. All we have to do now is wait for the big reveal.