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Dark Matter 'Strikes Back' in Galactic Mystery

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Fermi’s gamma-ray map of the sky.
Fermi’s gamma-ray map of the sky.
Image: NASA/DOE/Fermi LAT Collaboration.

Dark matter could be responsible for the mysterious observation of gamma rays in the center of our galaxy, according to a new paper.

Astrophysicists have known for a decade that the center of the Milky Way is sending too many gamma rays our way. Mysterious stuff called dark matter was once considered the prime candidate for this excess, but subsequent data analysis implicated something more mundane. But now, a new analysis could once again swing the pendulum in favor of dark matter as the explanation.

“If it turns out that the signal is dark matter, that’s huge for our understanding of the universe,” Rebecca Leane, postdoctoral researcher in the Center for Theoretical Physics at MIT, told Gizmodo. “We know that dark matter makes up most of the universe, but we don’t have much of an idea of what it is... if there’s a dark matter signal, that would have a massive impact, and getting to the bottom of it would be very interesting.”


The Fermi Gamma-ray Space Telescope first measured a mysterious excess of high-energy light called gamma rays beaming from the center of the galaxy back in 2009. Since then, mathematical models have suggested two prime candidates for the excess: spinning neutron stars called pulsars and colliding dark matter particles annihilating one another, where dark matter is the unidentified mass that appears to form the scaffolding of the universe.

But back in 2016, two papers published in the journal Physical Review Letters presented two different mathematical models that seemed to favor an undiscovered population of pulsars as a better fit for the data, without the need to invoke dark matter. The dark matter hypothesis started to fall out of favor.


Then, this past April, Leane and Tracy Slatyer, another theoretical physicist at MIT and an author on one of the 2016 papers, dropped a paper on the arXiv physics paper server titled “Dark Matter Strikes Back at the Galactic Center.” The paper explains that the 2016 models might have been wrong.

The researchers first reviewed the non-Poissonian template fitting (NPTF) method used to calculate the various origins of light in the night sky, which treats the total light as a combination of disparate sources each modeled by a “template.” All of the templates together contribute to a probability that a certain number of photons will be observed in each pixel of an image. But in their analysis on simulated data, it appeared that the sum of the templates may not be correctly describing what’s really going on. They demonstrated that a contribution of gamma rays from dark matter might easily be misinterpreted as coming from “point sources,” that is, star-like objects in the sky, most likely pulsars. When they injected a fake signal that looked like the effect of dark matter into the real data, the model misattributed the dark matter signal to point sources.


“This suggests that the data are mis-modeled in some way, and that dark matter may be the dominant contribution to the galactic center excess after all—though this does not constitute positive evidence for dark matter, and the [excess] could still be composed mainly of point sources,” the authors write in the paper, which was published in Physical Review Letters this week.

On top of that, just this past month, a separate team of scientists published a paper on the arXiv that refutes the conclusions of the second 2016 paper.


In essence, this work casts doubt on the analysis previously used to attribute the excess to pulsars, by demonstrating a flaw in the initial data analysis methods.

These results are energizing dark matter researchers. “I’m approximately as bullish on the dark matter interpretation on this signal now as I’ve ever been,” Dan Hooper, senior scientist and the head of the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory, told Gizmodo. He said that he hoped dedicated experiments would now look for similar sources of emissions in the dwarf galaxies that orbit the Milky Way.


Mariangela Lisanti, a Princeton professor who worked on the 2016 paper with Slatyer, agreed that “it is very important to carefully assess the uncertainties involved in any search for dark matter, especially one focusing on gamma rays from the Milky Way’s galactic center,” she told Gizmodo in an email. However, she did not think that the tests invalidated the conclusions of the 2016 work, which she and her collaborators showed in a paper they published on the arXiv over the summer.

“My collaborators and I are continuing to study the problem carefully, improving the statistical methodology as well as the treatment of cosmic-ray contamination that can complicate any analysis of gamma rays at the galactic center,” she told Gizmodo. “It’s a challenging problem, but so far, our results remain generally consistent with the conclusions of the original 2016 study.”


But this back-and-forth demonstrates that modeling space is hard and can lead to mistakes in interpretation—you may have heard the saying, “All models are wrong, but some are useful.” It’s going to take more concrete evidence before the true identity of the source of the mysterious excess reveals itself.