“To stabilize it in our simulations, we had to add dark matter.”
“In the early 1980s, I was studying the dynamics of galaxies. We were then moving from analytical calculations to the first numerical simulations to describe their shapes. These two-dimensional space simulations resulted in barred galaxies [les bras spiraux de la galaxie émergent aux extrémités d’une barre d’étoiles]. This surprised us because the calculations described galaxies that were all spirals, without bars. When this was possible, We moved to three-dimensional models to see if the bars remained. Not only were they still there, but we showed the existence of a halo, a bulge of stars, extending above and below the galactic disk. To stabilize it in our simulations, we had to add dark matter, otherwise the galaxy’s disk would heat up too much, destroying any structure. “
Thus, dark matter appears essential to account for observations, such as the shape of galaxies, but also the rotation speed of stars located on the periphery, much greater than that predicted if there were only classical matter. Initially, this matter is not considered exotic, it is just invisible. “It was only in 1985 that it was realised that no particle in the standard model was suitable. So we started looking for new ones. “
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The wimp hunt has so far yielded nothing
In the early 2000s, the hunt for wimps was organized at the LHC, the large particle collider installed at CERN in Geneva. “Wimps have been around since the beginning of the universe, so they’re very stable. We thought we might see them. “Energies of 100 times the mass of the proton have been reached. And nothing. In 2012, the Higgs boson was detected with a mass of 125 times that of the proton. But no wimp. “And so on: we went up to 1000 times the mass of the proton in vain. Today, particle physicists have given up on finding it. “
In their quest for wimps, other scientists had other strategies. For example, the Xenon experiments that began in 2005 aimed to detect the recoil of xenon nuclei when struck by wimps. They were conducted at the Gran Sasso National Laboratory in Italy. The first experiment used about 15 kg of liquid xenon. It didn’t work. The most recent version of the experiment, XENONnT, now includes almost 6 tons of xenon, which greatly increases the number of targets, and thus the probability of observing a wimp. Measurements began in 2021, but so far have been unsuccessful.
The last chance to prove their existence could have come from the stars. “In the dense regions of the Universe, at the center galaxies, we had the hope that wimps would meet and annihilate each other, which would have produced gamma-ray flashes. The mass of the wimps allowed us to calculate the energy of these rays. We looked for them in the hearts of galaxies, dwarf galaxies, and even in the Sun, which after all could have contained them. We detected nothing at all “
Developing a new theory of gravity
No direct evidence, no indirect evidence. The wimp hypothesis seems to have fizzled out, which is causing some dismay among astronomers. “That was our main candidate. All the other hypotheses seem a bit far-fetched. There is the hypothesis of sterile neutrinos, of intermediate mass, whose existence is very constrained. And also axions, another hypothetical particle but infinitely lighter than neutrinos, whose mass we already cannot measure. So, measure that of axions! And apart from that, there is nothing else. “
Nothing but “turning the tables” and considering that dark matter is perhaps only an illusion due to an incomplete knowledge of gravitation.This hypothesis is as old as exotic dark matter. It involves developing a new theory of gravity by modifying the equations of general relativity. Albert Einstein had considered the case of a strong gravitational field, like the one that reigns on Earth or in the Solar System. This made it possible to simplify the equations and make calculations. But if we consider weak fields, 11 orders of magnitude weaker than Earth’s gravity, it works less well. However, we find these weak fields at the edge of galaxies, precisely where we need dark matter to describe the behavior of stars.“
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Today, many astronomers are working to develop this modified gravity, in the hope of finally being able to do without dark matter. “A satellite like Euclidby mapping tens of millions of galaxies, should help us to elaborate this modified gravity that could explain not only dark matter, but also the dark energy responsible for the acceleration of the expansion of the Universe. In five years, we should finally have some answers to our questions… “
