Despite decades of searching, no one has yet cracked the mystery of dark matter. One hypothesis to explain it relies on strange x-rays emanating from distant galaxies and galaxy clusters, but a new paper appears to rule out dark matter as producing these mysterious x-rays.
The way distant objects in the universe interact implies that there’s a lot more mass than scientists can actually see—perhaps five to six more times. Researchers call this unexplained stuff dark matter. Back in 2014, scientists discovered an “unidentified x-ray line” in distant galaxies and galaxy clusters—that is, a source of x-ray emission with a consistent energy. Theorists soon realised that this line could have been explained by energy released in the decay of a popular dark matter candidate, called the sterile neutrino. One team of scientists thinks that they’ve ruled out dark matter as a way to explain this line, though 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 Centre for Astrophysics, reported the detection of unidentified x-ray emission in data taken from 73 galaxy clusters by the XMM-Newton telescope. This emission, now called the 3.5 keV unidentified x-ray line after its 3.5 kilo-electronvolt energy, persisted in follow-up observations of galaxies and galaxy clusters. Theorists realised that one candidate to explain dark matter candidate, the hypothesised sterile neutrino, could produce this x-ray emission if it decayed.
But how can you confirm or rule out dark matter as the cause of the x-ray line? Well, a halo of dark matter should surround the centre of our galaxy, the Milky Way. If sterile neutrino decays produced these x-rays and were the primary component of dark matter, then any image of empty space taken by a telescope pointed towards 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, Berkeley, does exactly that. Scientists compiled blank-sky data from 752 observations, totaling over 30 million observing seconds on the XMM-Newton space telescope. They did not see any evidence for the line in our galaxy, and, according to the paper published today in Science, they “rule out” decaying dark matter as the interpretation of the line seen in distant galaxies.
“If this 3.5 Kev line was coming from dark matter, since there is dark matter in our own galaxy, we should have seen it, and we did not,” Benjamin Safdi, assistant professor at the University of Michigan, told Gizmodo. “It should have been abundantly clear because this is a powerful way of looking for dark matter; it should be obvious, and it wasn’t there at all. So that put a pretty definitive nail in the coffin for this line arising from dark matter, unfortunately.”
But when this paper first appeared on the arXiv physics preprint server over a year ago, some physicists took issue with its results. One team led by astrophysicist Alexey Boyarsky at Leiden University did find evidence of 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 handled XMM-Newton’s backgrounds, meaning the data it records that isn’t the signal, and said that it obscured the signal his team did see.
Physics and astronomy professor Kevork Abazajian at the University of California, Irvine thought it was a case of cherrypicking the data—that the range of frequencies the team hunted for was too thin, potentially removing the signal. “The short of it is, they don’t have enough information to make a strong conclusion,” he said.
Nicholas Rodd, another coauthor of the new paper, told Gizmodo via email that he was aware of Boyarsky and others’ concerns about the paper. He agreed that the differences were in the statistics but said that “the Boyarsky team has suggested to us several modifications we could make to our analysis and alternate analysis frameworks. Examples include modelling speculative instrumental lines, amongst many more. We have performed every one of these checks… and each time our analysis remains robust: No line emerges, and the dark matter explanation for the 3.5 keV line remains excluded.”
Some scientists not involved in the study did agree with the results, given the large amount of data included. “One thing I think they did nicely was show if they inject this signal of dark matter decay, if they faked this signal in the dataset, the method would be able to recover it,” Kerstin Perez, physics professor at MIT, told Gizmodo. And yet, their analysis still didn’t see the signal. “I think that that’s pretty compelling,” she said.
Tesla Jeltema, associate professor of physics at University of California, Santa Cruz, told Gizmodo in an email that this new paper, as well as the papers that first discovered the line, were all very careful analyses of the data. But, said Jeltema, “Regardless of who you think is ‘right,’ if there were such a thing, I would argue that if you can model the data in different, reasonable ways and sometimes you get an excess and sometimes you don’t, the evidence for the need for new physics is not there.” In other words, if the presence or absence of a phenomenon relies strongly on which statistical model you’re using, then there isn’t strong evidence that dark matter is the cause.
Bulbul told Gizmodo that she didn’t think the Science paper was the end of the 3.5 keV line story. She said it’s extremely difficult to model XMM-Newton’s backgrounds, so producing an analysis of the empty space is difficult whether the x-rays are present or not. She is looking forward to observations from the 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 unrecorded astrophysical phenomenon. “Until then, I will not be convinced that decaying dark matter line is excluded,” she said.
The team behind the new paper told Gizmodo that they plan to continue sweeping the frequency range for evidence of x-ray signals in the blank sky data, both with XMM-Newton and with upcoming telescopes.
Regardless, it’s clear that many people in the field aren’t ready to abandon decaying dark matter as the cause of the unidentified x-rays just yet.