In science, the shining example of black holes – Liberation

And if the most important thing for a researcher was not to remove uncertainty, but to enjoy playing with…

If there’s one thing physicists agree on, it’s that something is missing. The American astronomer Vera Rubin showed this in the 1970s by observing the rotation of galaxies: she measured that the stars in the periphery rotate faster around the galactic center than expected, without, however, finding themselves hurled into space by centrifugal force. This is a sign that these stars are held in the galaxy by gravity. But not that which the stars exert: they are not numerous enough to create so much attraction. There is therefore inevitably another “case” than the one we see. A substance six times more present (in terms of mass) than the ordinary matter of stars, planets and gas nebulae. A substance that does not emit or absorb light, therefore impossible to see, but which exerts a gravitational force and allows the coherence of galaxies. It has been called dark matter, for lack of a better word.

What is it made of, and above all, how do you know? The hunt for dark matter is one of the broadest topics in astrophysics because it involves highlighting an entity whose nature we do not know. You must be able to discover something you know nothing about. One of the physicists’ first ideas was to invent “wimps”, a new type of particle that would hardly interact with ordinary matter (protons, neutrons, electrons) except via gravity. We envisioned, designed, funded, and built machines to detect the few rare collisions that would occur between a bitch and an ordinary atom. Nothing decisive came of it. But it does not matter. If we still don’t know what dark matter is at the end of a failed experiment, at least we know what it isn’t. Mistakes advance the search.

Always more assumptions

At the same time, the hunt for dark matter continued on the gray matter side. Theorists have produced more and more hypotheses. There could be particles called axions, with properties different from wimps, or still other particles that would be ultralight (an inch of the mass of an electron)… Unless it’s the opposite: what if dark matter was composed of massive stars that were invisible to our eyes because they neither emit nor reflect light? These could be simple black holes, brown dwarfs (dead bodies of stars) and wandering planets, whose abundance in the universe would have been largely underestimated.

For each of these pathways, it is necessary to construct experiments to create new instruments capable of confirming it. Sometimes we look for direct evidence of the existence of a new particle, as in collision detectors. And we work at other times on indirect evidence, such as the effects of dark matter on astronomical observations. One of these effects is “gravitational lensing”: certain light from galaxies arrives at us distorted, stretched by the gravitational attraction of a cloud of invisible matter that was in their path. The pursuit of new theories has coexisted for decades with the design of instruments to test them, and the field of possibilities only expands without us finding the beginnings of a solution.

“We know so little about our universe”

In this ghost hunt, physicists also do not rule out that dark matter does not exist. Instead of relentlessly pursuing it in the infinitely large or the infinitely small, perhaps we should look for it in … our miscalculations. The Mond theory thus proposes a slight modification of Newton’s second law that would solve the problem of galaxies rotating too fast. Recent studies have also found anomalies in the stars’ velocities, suggesting that the full complexity of the laws of gravity may have been misunderstood.

“I don’t know if there is dark matter or if we need to change the definition of gravity or something else. We know so little about our universe. But it’s fun,” said Vera Rubin. She had already understood that the most important thing, on the scale of a researcher’s life, is not to remove uncertainty, but to take pleasure in furthering the treasure hunt.

ENS’s 5th night will take place on Friday 9 September 2022 with conferences and roundtables to discover, understand and tame uncertainty.

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