For decades, astrophysicists have thought that some form of invisible dark matter must penetrate the galaxies and hold them together, even if its nature remains a mystery. Now, three physicists claim that their observations of empty patches of heaven preclude a possible explanation of the strange substance – that it is made of unusual particles called sterile neutrino. But others claim that the data shows no such thing.
“I think for most people in society, this is the end of history,” says study author Benjamin Safdi, an astroparticle physicist at the University of Michigan, Ann Arbor. But Kevork Abazajian, a theoretical physicist at the University of California, Irvine, says the new analysis is incorrect. “To be honest, this is one of the worst cases of cherry picking data I’ve seen,” he says. In unpublished work, another group looked at similar patches of sky and saw the same sign of sterile neutrinos that escaped Safdi.
Astrophysicists believe that each galaxy forms and is in a huge clump, or “halo” of dark matter, like the pit in a peach. The seriousness of the invisible substance helps to prevent the stars from flying into empty space from the inside. Theoretical physicists have dreamed up many hypothetical particles that may constitute dark matter, among them cousins of almost massless, barely detectable subatomic particles called neutrinos, which shine out of the sun and nuclear reactors. The particles that make up dark matter would be hypothetical “sterile” neutrinos, heavier and even more difficult to capture. An ordinary neutrino can interact with an atomic nucleus; Sterile neutrinos would only interact with other neutrinos, when a normal neutrino is transformed into a sterile by a process called neutrino mixing.
The idea that sterile neutrinos could be dark matter was spurred in 2014. Observations of nearby galaxies and the center of our own dairy path revealed a faint glow of X-rays with a specific energy, 3.5 kilo-electron volts (keV). That glow can be expected if sterile neutrinos with a mass of 7 keV penetrated the galaxies. Very rarely would a sterile neutrino decay into a regular neutrino and an x-ray, which would have an energy equal to half the sterile neutrino mass.
But a new analysis of astronomical observations shows that the incandescent glow cannot come from dark matter, Safdi and colleagues report today in Science. They looked at data not from distant galaxies, but from blank sky-stars between the stars in more than 4,000 stock images snapped by XMM-Newton, an X-ray space telescope launched in 1999 by the European Space Agency. If our own galaxy lies within a large cloud of sterile neutrinos, the telescope must peer through that cloud – and the sky between the stars should also glow slightly with 3.5-keV x-rays.
Safdi’s team found no signs of such a glow. The no-show suggests that the glow in distant galaxies does not come from dark matter, but from some more common source such as hot gas, Safdi says.
Alexey Boyarsky, an astroparticle theorist at Leiden University, is not convinced. “I think this is wrong,” he says. Boyarsky says he and his colleagues performed a similar, unpublished analysis in 2018, even with images from XMM-Newton, and saw a 3.5-keV glow from the empty sky, which was just expected from peeking through a halo of sterile neutrinos.
How do two groups look at the same data and come to opposite conclusions? The difference lies in their methods, says Boyarsky. As our galaxy is filled with a thin ionized gas, the sky emits X-rays, which can peak as specific energies even without the contribution of dark matter. The XMM-Newton telescope itself can also glow and emit X-rays at certain energies. And some X-rays also come from outside our galaxy. To see a 3.5-keV glow from dark matter, researchers must filter it from these background contributions.
To do this, Boyarsky and colleagues analyzed the full range of X-ray energies that XMM-Newton can detect, model the entire background, and subtract it from data. Crucially, Boyarsky says, his team removed known peaks at 3.3 keV and 3.7 keV to reveal the unexplained peak with 3.5 keV. Safdi says his team took a different approach. By borrowing statistical techniques developed by atomic scatterers, they analyzed the spectrum from each image separately and analyzed data only over a much narrower range of energies.
But that area of energy is not much wider than the top the team is looking for, says Abazajian. Boyarsky adds that since Sadfi and his team did not take out the other two background peaks, they may have mistaken a plateau created by the three overlapping peaks for a flat spectrum.
Not so, says Safdi. His team found that subtracting the other peaks and expanding the energy window did not change the result. If there is a peak of 3.5 keV, he says, his team’s more sophisticated technology would have revealed it.
Boyarsky says he will try to publish his blanket sky analysis. A physics journal rejected it, saying it was not “interesting enough”, he says. Now he says he will leave it at that Science. “I don’t care for it to be published, but I would like it to be reviewed,” he says. “They can’t say it’s not interesting.”