For decades, astrophysicists around the world have been relentlessly trying to find dark matter, that strange substance that permeates galaxies and without which they could not keep their structures intact. But the nature of dark matter, five times more abundant than ordinary matter (from which all the planets, stars, and galaxies we can see are made), remains a mystery. No one has yet been able to detect it directly, and no one knows what kind of subatomic particles it might be made of.
Now, to top it all, a recent study appeared in Science and carried out by physicists from the Berkeley Lab and the University of Michigan has thrown a new jug of cold water on what was considered the best clue so far. No, the article says, dark matter is also not made of sterile neutrinos, an unusual type of subatomic particle that many had high hopes for.
The publication of this work has sparked a bitter scientific controversy among its authors and those who, on the contrary, affirm that the study must be wrong.
“I think for most people in the scientific community this is the end of the story,” says Benjamin Safdi, lead author of the Science study. But others, like Kevork Abazajian, a theoretical physicist at the University of California, think the new analysis is very flawed. “To be honest,” says this researcher, “this is one of the worst cases of data selection I’ve ever seen.” In support of his opinion, this physicist points to an unpublished work in which another group of researchers, observing the same parts of the sky, did manage to discover signs of the sterile neutrinos that Safdi denies.
The “glue” of galaxies
According to the most widely accepted theory, all existing galaxies form and reside within vast halos of dark matter. And it is precisely the gravity of that invisible substance that helps prevent the stars that make up the galaxies from sticking together instead of moving freely and disorderly through space.
For many years now, physicists have proposed a large number of hypothetical particles (that is, never observed in the laboratory) as candidates for dark matter. Among them are sterile neutrinos, heavier and even more elusive than their “cousins”, neutrinos, tiny, almost massless particles that roam throughout the Universe.
Unlike “normal” neutrinos, which can interact (although they do very rarely) with atomic nuclei, sterile neutrinos only interact with other neutrinos, and arise when an ordinary neutrino is transformed into a sterile one through a process called ” neutrino mix. ”
The idea that sterile neutrinos could be the components of dark matter got a huge boost in 2014, when observations of nearby galaxies and the center of our own Milky Way Galaxy revealed a faint X-ray glow with very specific energy, 3, 5 kiloelectronvolts (keV).
That ghostly glow would be expected if a huge amount of sterile neutrinos with a mass of 7 keV were everywhere in the galaxies. Very occasionally, some of them would decompose into an ordinary neutrino and an X-ray emission that would have an energy equal to half the mass of the sterile neutrino, that is, 3.5 keV, which is just what they observed. the scientists.
Deny the greatest
The work of Safdi and his colleagues, however, debunks this idea. And in his study of Science he makes sure that that faint glow cannot come from dark matter. To reach that conclusion, the researchers decided not to focus on distant galaxies, but on “dark spaces” in our own galaxy, black spots between the stars. They used more than 4,000 images from the archive of the XMM-Newton, an X-ray space telescope launched in 1999 by the European Space Agency. If our own galaxy is within a huge cloud of sterile neutrinos, the researchers reason, then the telescope, for its distant observations, must be looking through that cloud, and the dark sky spaces between the stars should also shine. faintly with 3.5 keV X-rays.
But Safdi and his team found no trace of that faint glow, which led them to claim that the brightness detected in distant galaxies does not come from dark matter, but from some much more common source, such as cloud clouds. very hot gas.
Another study says otherwise
“I think that work is wrong,” says Alexey Boyarski, from Leiden University, who in 2018 carried out a similar study (still unpublished), also using images from XMM-Newton, and they did see a brightness of 3.5 keV in the empty spaces, which revealed the presence of sterile neutrinos.
How is it possible that two different groups reach opposite conclusions based on the same data? For Boyarski, the difference is in the analysis methods used. Since our galaxy is filled with a faint ionized gas, the sky emits X-rays, which can reach certain energies even without a contribution from dark matter. Some of those rays, indeed, may also come from beyond our own galaxy. To be able to see the long-awaited 3.5 keV brightness, researchers must therefore “clean it up” before all those possible outside contributions.
So Boyarski and his colleagues analyzed the entire spectrum of X-ray energies that the XMM-Newton is capable of detecting, discarded the emissions that came from further away, and subtracted them from the data. In this way they managed to reveal the 3.5 keV emission.
In his defense, Safdi, the author of the Science article explains that his team took a different approach. Using statistical techniques developed in particle colliders, they analyzed the spectrum of each image separately and studied the data only over a much narrower range of energies, paying no attention to other parts of the spectrum.
For Boyarski, that is precisely the problem. Safdi and his team, he says, did not eliminate radiation spikes from far space, and that could have falsified their results.
But Safdi considers that this is not the case. His team found that subtracting the other emissions and expanding the window of analyzed energy does not change the result. If there is a 3.5 keV peak, he says, the sophisticated technique used in his study would have revealed it.
Now Boyarski will try to publish his own analysis, which was rejected by a physics magazine as “uninteresting”. Given the controversy aroused, the researcher now intends to present it in Science.
We will see who wins this particular battle and whether or not we can continue to rely on sterile neutrinos as the main component of dark matter. .