Despite decades of searching, no one has yet cracked the mystery of dark matter. One hypothesis to explain it relies on strange X-rays that originate from distant galaxies and galaxies, but a new paper seems to rule out dark matter producing these mysterious X-rays.
The long distance in the universe interacts, which means that there is much more mass than the scientists can actually see – maybe five to six times more. Scientists call this unexplained stuff dark matter. Already in 2014, researchers discovered an “unidentified X-ray line” in distant galaxies and galaxies – that is, a source of X-rays with a uniform energy. Theorists soon realized that this line could have been explained by energy released in the decay by a popular candidate for dark matter, called sterile neutrino. A team of researchers believe they have excluded dark matter as a way of explaining this line, although others want more evidence before they can be convinced.
In February 2014, a team of researchers led by Esra Bulbul, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics, reported the discovery of unidentified X-ray emission in data taken from 73 galaxy clusters of the XMM-Newton telescope. This emission, now called the 3.5 keV unidentified X-ray line after its 3.5 kilo-electron volts energy, continued in galaxy and galaxy cluster monitoring observations. Theorists realized that a candidate to explain dark matter candidate, the hypothesis sterile neutrino, could produce this X-ray emission if it expires.
But how can you confirm or exclude dark matter as the cause of the X-ray line? Well, a halo of dark matter should surround the center of our galaxy, Milky Way. If sterile neutrino decay produced these X-rays and was the primary component of dark matter, then any image of empty space taken by a telescope directed at the putative halo should reveal the existence of this unidentified line.
A new article by astronomers at the University of Michigan and the University of California, Berkeley, does exactly that. Researchers compiled celestial sky data from 752 observations, a total of over 30 million observations of seconds on the XMM-Newton space telescope. They saw no evidence of the line in our galaxy, and according to the newspaper published today in Science, they “exclude” decaying dark matter as the interpretation of the line seen 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,” Benjamin Safdi, assistant professor at the University of Michigan, told Gizmodo . “It should have been very clear as this is a powerful way to look for dark matter; it should be obvious, and it wasn’t there at all. So that put a pretty definite nail in the coffin for this line that comes from dark matter, unfortunately. “
But when this paper first appeared on the arXiv physics suppression server over a year ago, some physicists took issue with their results. A team led by astrophysicist Alexey Boyarsky at Leiden University found evidence of the line in XMM observations of the Milky Way halo. Boyarsky told Gizmodo that the new paper was “totally wrong.” He did not agree with how the new paper handled XMM-Newton’s backgrounds, which meant that the data it recorded was not the signal, and said it was hiding the signal his team saw.
Physics and astronomy professor Kevork Abazajian at the University of California, Irvine thought it was a case of cherry picking data – that the frequency range the team was chasing was too thin and potentially removing the signal. “The short thing is that they don’t have enough information to make a strong conclusion,” he said.
Nicholas Rodd, another co-author of the new magazine, told Gizmodo via email that he was aware of Boyarsky and others’ concerns about the newspaper. He agreed that the differences were in the statistics but said that “The Boyarsky team has proposed to us several modifications we could make in our analysis and alternative analysis frameworks. Examples include modeling speculative instrument lines, among many more. We have performed each of these checks … and each time our analysis remains robust: No line is created and the explanation of dark matter for the 3.5 keV line remains excluded.
Some researchers not involved in the study agreed with the results, given the large amount of data included. “One thing I think they did nicely was to show if they inject this signal of dark matter decay, if they falsified this signal in the dataset, the method could recover it,” Kerstin Perez, a physics professor at MIT, told Gizmodo. And yet their analysis still did not see the signal. “I think it’s pretty convincing,” she said.
Tesla Jeltema, associate professor of physics at the University of California, Santa Cruz, told Gizmodo in an email that this new essay, as well as the essays that first discovered the line, were all very thorough analyzes of the data. But, Jeltema said, “No matter who you think is ‘right’, if there was such a thing, I would argue that if you can model the data in different, reasonable ways and sometimes you get a surplus and sometimes not, the evidence of the need for new physics does not exist. “In other words, if the occurrence or absence of a phenomenon is strongly dependent on the statistical model you use, there is no strong evidence that dark matter is the cause.
Bulbul told Gizmodo that she didn’t think the Science article was the end of the 3.5 keV story. She said it is extremely difficult to model XMM-Newton’s backgrounds, so it is difficult to produce an analysis of the empty space whether or not the x-rays are present. She looks forward to observations from eROSITA telescope and from the XRISM telescope scheduled to launch in 2022, which should be able to confirm whether the line is due to dark matter or just an unregistered astrophysical phenomenon. “Until then, I will not be convinced of the decaying dark matter to explain line is excluded, “she said.
The team behind the new magazine told Gizmodo that they plan to continue sweeping the frequency range for X-ray evidence in blank sky data, both with XMM-Newton and with upcoming telescopes.
However, it is clear that many people in the field are not ready to abandon decaying dark matter as the cause of the unidentified X-rays yet.