Despite decades of searching, no one has yet revealed the secret of dark matter. One hypothesis that explains this is based on strange X-rays emanating from distant galaxies and galaxy clusters, but new paper seems to rule out that dark matter creates these mysterious X-rays.
The way distant objects interact in the universe implies that there is much more mass than scientists can actually see – maybe five to six times more. Researchers call this unexplained stuff dark matter. As early as 2014, scientists discovered an “unidentified X-ray line” in distant galaxies and galaxy clusters – a source of X-ray emission with constant energy. Theorists soon realized that this line could be explained by the energy released by the decay of a popular dark matter candidate, the sterile neutrino. A team of scientists believe that they have ruled out dark matter to explain this line, although others want more evidence before they can be convinced.
In February 2014, a team of scientists led by Esra Bulbul, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics, reported the detection of an unidentified X-ray emission in data from 73 galaxy clusters from the XMM- Newtonian telescope. This emission, which is now referred to as the unidentified 3.5 keV X-ray line after its 3.5 kilo electron-volt energy, persisted when observing galaxies and galaxy clusters. Theoreticians realized that a candidate for explaining the dark matter candidate, the hypothetical sterile neutrino could produce this X-ray emission if it decays.
But How Can You Confirm Or Exclude Dark Matter As The Cause Of The X-Ray Line? Well, an atrium of dark matter should surround the center of our galaxy, the Milky Way. If sterile neutrino decays generated these X-rays and were the main constituent of dark matter, any image of an empty space taken by a telescope aimed at the supposed halo should reveal the presence of this unidentified line.
A new paper by astronomers at the University of Michigan and the University of California at Berkeley does just that. Scientists have compiled data on empty skies from 752 observations, a total of over 30 million seconds of observation on the XMM Newtonian space telescope. They have seen no evidence of the line in our galaxy, and according to the article published today in Science they "exclude" decaying dark matter as an interpretation of the line in distant galaxies.  "If this 3.5 Kev line came from dark matter because there is dark matter in our own galaxy, we should have seen it and we didn't," said Benjamin Safdi, an assistant professor at the university from Michigan to Gizmodo. “It should have been very clear, because this is a powerful way to look for dark matter. it should be obvious and it wasn't there at all. Unfortunately, that put a pretty definitive nail in the coffin for this line, which comes from dark matter. “
However, when this paper first appeared on the arXiv preprint server for physics over a year ago, some physicists questioned its results. A team led by astrophysicist Alexey Boyarsky from the University of Leiden found clues to the line in XMM observations of the Milky Way halo. Boyarsky told Gizmodo that the new paper was "completely wrong". He disagreed with the way the new paper dealt with XMM-Newton's background, the data it records, which is not the signal, and said it obscured the signal that his team was covering had seen.
Physics and astronomy professor Kevork Abazajian from the University of California at Irvine thought it was a case to select the data – the frequency range the team was looking for was too thin, possibly removing the signal. "In short, they don't have enough information to draw a clear conclusion," he said.
Nicholas Rodd, another co-author of the new newspaper, emailed Gizmodo that he knew Boyarsky and others. Concerns about the paper. He agreed that the differences were in statistics, but said: “The Boyarsky team has suggested various changes that we could make to our analytical and alternative analytical frameworks. Examples of this include the modeling of speculative instrument lines. We have performed each of these tests … and every time our analysis remains robust: no line is created and the explanation of dark matter for the 3.5 keV line is excluded. “
Some scientists who were not involved in the study agreed with the results given the large amount of data. "One thing I thought they did well was to show whether they were injecting this dark matter decay signal. If they pretend this signal in the data set, the method can restore it," said Kerstin Perez, a physics professor at MIT , opposite Gizmodo. And yet her analysis didn't see the signal. "I think that's pretty convincing," she said.
Tesla Jeltema, associate professor of physics at the University of California at Santa Cruz, emailed Gizmodo that this new paper, as well as the papers that first discovered the line, were all very careful analyzes of the data. However, Jeltema said: "Regardless of who you think is" right ", I would argue that if you can model the data in different, sensible ways and sometimes get an excess and sometimes not, the evidence of the need for new physics is not there. In other words, if the presence or absence of a phenomenon is highly dependent on the statistical model you are using, there is no clear evidence that dark matter is the cause.
Bulbul told Gizmodo that she did not believe this. The science paper was the end of the 3.5 keV story. She said it was extremely difficult to model the backgrounds of XMM-Newton. Therefore, it is difficult to make an analysis of the empty space regardless of whether the X-rays are present or not. She is looking forward to observing the eROSITA telescope and the XRISM telescope, which is scheduled to start in 2022 to confirm whether the line is due to dark matter or just an unrecorded astrophysical phenomenon. "Until then, I won't be convinced that the decay of dark matter to explain the line is impossible," she said.
The team behind the new paper told Gizmodo that they plan to continue searching the frequency domain for evidence of X-ray signals in the empty sky data using both XMM Newtonian and upcoming telescopes.
Regardless, it is clear that many people in the field are not yet ready to give up the decaying dark matter as the cause of the unidentified X-rays.