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A vehement controversy keeps physicists and astronomers bottled up: the X-ray signal that seemed to indicate that dark matter was made up of sterile neutrinos does not appear in new observations.
Dark and mysterious
Five times more abundant than ordinary matter, dark matter is an essential ingredient of the universe. It is essential to explain the rapid movements of stars at the periphery of galaxies and to explain the relative movements of galaxies in clusters. Dark matter played a crucial role in the formation and evolution of galaxies and galaxy clusters and has effects that can be measured against the cosmic microwave background (just as the Planck satellite did).
More than 80% of the mass of a galaxy is dark matter, but what is this mysterious matter made of? Astronomers' insistent attempts to detect it directly in space have been as unsuccessful as those of particle physicists trying to reveal it in large accelerators such as CERN's LHC .
3.5 kiloelectronvolts
In 2014, studies by Esra Bulbul (Center for Astrophysics, Harvard) and Alexey Boyarsky (University of Leiden), independently, found a clue that could give the long-awaited answer to questions about dark matter. These authors, when examining the X-rays emitted by dozens of galaxy clusters, found that the central regions of the clusters emitted a very weak signal focused on energies of 3.5 kiloelectronvolts (keV), one of them being the famous Perseus cluster.
The signal, collected by ESA's XMM-Newton Space Telescope, was so weak that, to detect it clearly (with more than 3 or 4 sigma, in scientific slang), the emissions of several of these clusters had to be added, using a technique known in astronomy as 'stacking'.
The signal at 3.5 keV found no explanation within known plasma physics. One could think of a spectral line, for example: that of an element like argon. But such an explanation was impossible, the argon had to emit a signal 30 times less than what was observed . More heterodox explanations had to be thought of, and theoretical physicists turned to it. In a few months, a hundred articles were published proposing alternative explanations.
Sterile neutrinos
Among the proposed hypotheses, the one that most prospered was the one that explained the signal at 3.5 keV as the result of the disintegration of a type of neutrinos called 'sterile'. Neutrinos, which invade the universe in huge quantities, were initially considered as possible constituents of dark matter, but this explanation had to be abandoned because the mass of all those neutrinos was insufficient. Sterile neutrinos would be more massive particles than the neutrinos known to date. A sterile neutrino of 7 keV of energy, when disintegrating, could produce an ordinary neutrino and a photon X of 3.5 keV, which would explain the origin of the mysterious signal from the galaxy clusters .
But this explanation suffered from several problems from the beginning. The first of all is that such sterile neutrinos are nothing more than hypothetical particles that have so far not been directly detected.
Furthermore, the mysterious signal at 3.5 keV was searched, but not detected in places where it must have been particularly intense, such as the Milky Way itself or the galaxy cluster in Virgo, a huge grouping of 2,000 galaxies ten times closer than the cluster. from Perseus. Various groups of astronomers questioned the works of Bulbul and Boyarsky. Neither NASA's Chandra telescope nor Japan's Hitomi (which, unfortunately, only ran for seven hours) also did not detect the signal in the Perseus cluster.
Missing
Seeking to settle the problem, a team led by Christopher Dessert (University of Michigan) has now analyzed hundreds of observations made, also by the XMM-Newton telescope, on empty halo fields in the Milky Way. In these fields there is less possibility of contamination by other stars, and the halo of our galaxy must unequivocally show the elusive signal at 3.5 keV. The cumulative exposure time for these authors was one year, but as reported in Dessert's recent publication in the prestigious journal Science , there is no sign of the signal.
So, business done? Of course not . Boyarsky has returned to the fray with his detections. In fact, this researcher had tried to publish an article based on the same data used by Dessert in which he presents an alternative analysis and detection in the Milky Way. Although this manuscript, which dates from February 2019, has not passed the usual revision filters, it can be viewed on the arXiv document server.
The substance of the matter
The controversy stems from the difficulties inherent in analyzing such weak signals. One of the greatest difficulties in reducing these data is 'background subtraction', that is, the elimination of the residual signal that exists in the sky to allow the emission of the galaxy's halo itself to appear, or that of a cluster of galaxies. In X-rays, this background has several components: the emission of other stars in the observation field, galactic cosmic rays, the solar wind, etc.
There is no single technique for bottom subtraction. Dessert claims to have used a new statistical technique similar to that used in accelerators like the LHC. Boyarsky argues that this technique uses a spectral window that is only 0.5 keV wide (centered on 3.5 keV) and that it is insufficient, while his group uses a window that ranges from 2 to 6 keV.
To move on
The real problem is that we are at the limit of what is measurable with current X-ray telescopes and, at this point, it is not possible to conclude on the existence (or not) of this supposedly weak signal from hypothetical sterile neutrinos. .
To reach the conclusion, more sensitive telescopes are needed, which are capable of measuring in this energy range with greater precision. Fortunately, a telescope of these characteristics, the so-called eROSITA, has been built by the Max Planck Institute for Extraterrestrial Physics (Munich) and was launched into space on July 13 of last year from the Baikonur (Kazakhstan) cosmodrome. And this will be followed by the Japanese XRISM that should be released in 2022.
Thanks to these new telescopes, at least we will be able, in just a few months, to better isolate the 3.5 keV signal, if it really exists, and measure its intensity. In parallel, particle physicists will continue to strive to bring out sterile neutrinos in the most powerful particle accelerators, such as the aforementioned CERN LHC .
Rafael Bachiller is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain .
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