If there’s one thing physicists agree on, it’s that this issue is missing. American astronomer Vera Rubin demonstrated this in the 1970s by observing the rotation of galaxies: she measured that stars on the fringes were orbiting the galactic center faster than expected, without finding themselves being ejected into space by centrifugal force. This is a sign that these stars are kept stuck in the galaxy by gravity. But that’s not what stars do: they aren’t many enough to generate the same amount of gravity. Therefore, there is inevitably a “matter” other than the one we see. Matter 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, and therefore is impossible to see, but exerts a gravitational force and allows galaxies to hold together. It has been called dark matter, for lack of a better word.
What they are made of, and above all, how do you know? The search for dark matter is one of the broader topics in astrophysics, as it involves bringing to light an entity whose nature we do not know. You should be able to discover something you know nothing about. One of physicists’ first ideas was to invent “wimps,” a new type of particle that barely interacts with ordinary matter (protons, neutrons, and electrons) except by means of gravity. We have imagined, designed, financed and built machines to detect the few rare collisions that can occur between a weak atom and an ordinary atom. Nothing conclusive came of it. But it’s okay. If we still don’t know what dark matter is at the end of an unsuccessful experiment, we at least know what it isn’t. Present important failures.
Always more hypotheses
In parallel, the search for dark matter continued on the gray matter side. Theorists have produced more and more hypotheses. There can be particles called axions, which have different properties than isotropic, or still other particles that are very light (one inch of electron mass)…unless the opposite: what if dark matter is made up of massive stars that are invisible to our eyes because they don’t Emitting and not reflecting light? They can be simple black holes, brown dwarfs (corpses of stars), and wandering planets whose number in the universe has been greatly underestimated.
For each of these avenues, it is necessary to conduct experiments to create new tools capable of confirming this. Sometimes we look for direct evidence of a new particle, as in collision detectors. Other times, we work on indirect evidence, such as the effects of dark matter on astronomical observations. One of these effects is the “gravitational lensing” effect: certain lights from galaxies arrive at us distorted, stretched by the gravitational pull of a cloud of invisible matter that was in its path. The search for new theories coexists for decades with the design of tools to put them to the test, and the field of possibilities is only expanding without us being able to find the beginning of the solution.
‘We know very little about our universe’
In this ghost hunt, physicists also don’t rule out the absence of dark matter. Instead of relentlessly pursuing it in an infinity of great or infinite magnitude, perhaps we should search for it in… our miscalculations. Thus Mond’s theory proposes a slight modification of Newton’s second law, which would solve the problem of galaxies spinning very quickly. Recent studies have also found anomalies in the stars’ velocity, and suggest that the entire complexity of the laws of gravity may have been misunderstood.
“I don’t know if there’s dark matter, or if we need to change the definition of gravity, or something else. We know very little about our universe. But it’s fun” Vera Rubin said. I have already realized that, on the scale of a seeker’s life, the most important thing is not the removal of doubt, but the enjoyment of moving forward in the treasure hunt.
The fifth night of ENS takes place on Friday 9 September 2022 with conferences and roundtables to discover, understand and tame uncertainty.