Dark Matter May Not Solve This Galactic Mystery After All

Physicists would love to find hints of dark matter to explain the parts of the universe we just haven't been able to figure out. Dark matter would neatly explain the strange behaviours of galaxies and oddly bent light in our universe. But a new paper may have snuffed out dark matter as a candidate for a mystery at the centre of our galaxy.

The X-shaped bulge at the galaxy's centre. Image: NASA/JPL-Caltech

Things get exciting when physicists find unexplained bumps in their data that don't agree with their theories. One such bump, an excess of gamma rays shining from the centre of the Milky Way, has led physicists to draft plenty of theories, some involving the elusive dark matter - a natural go-to for astronomical things that the laws of physics can't cleanly explain. New research from an international team of physicists implies that whatever's causing the gamma rays could be a little less dark than they thought.

"We still know that dark matter exists," study author Martin Pohl from the Institute of Physics and Astronomy at the University of Potsdam in Germany told Gizmodo. "But we don't really have any sort of credible indication that we might have seen the signature of it other than through gravity."

Astronomers and physicists are hunting for the source of the "galactic centre excess". An Earth-orbiting telescope, the Fermi-Large Area Telescope, has detected a confusing emission of gamma rays from the galaxy's centre. Some have proposed that dark matter would give rise to this observed behaviour by releasing gamma rays during interactions with regular matter. As a reminder, matter makes up only about four per cent of the stuff in the universe. Around 25 per cent is dark matter, mysterious stuff that only appears to interact with regular matter through its gravity but seems to form the universe's backbone. The rest would be dark energy, the undetectable source causing the universe's expansion to accelerate.

Pohl and the other researchers on his team compared the data to models they'd produced. These models take into account that stars seem to arrange themselves in an X-shape in the galactic centre. They found that the gamma-ray excess could better be explained by these models than by models involving dark matter - dark matter would arrange itself in a sphere regardless of the shape of the galactic centre, according to the paper published today in Nature Astronomy.

The researchers don't know for sure what would cause the excess, but perhaps it could be a kind of dense gamma-ray emitting neutron star, called a millisecond pulsar.

At least one other researcher agreed that the new models seem to weaken dark matter as the culprit behind this gamma-ray mystery. "I think it's not the final proof that dark matter is not there, but it's on a good track to do that for the first time," Francesca Calore, astrophysicist at the Laboratoire d'Annecy-le-Vieux de Physique Théorique of The National Center for Scientific Research France, told Gizmodo. She (and others) are working on similar modelling to find astrophysical sources of this excess.

Others aren't convinced yet. "The correlation identified in this paper might be real, but could equally well be a spurious feature resulting from, for example, the over-subtraction of some of the background templates," Dan Hooper, head of the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory, told Gizmodo in a Twitter direct message. In other words, Pohl and his team might be messing something up in their modelling.

And even if these results did rule out the gamma-ray excess as a potential hint of dark matter, there are plenty of other confusing things in space with potentially dark explanations. Those include recent measurements of distant hydrogen, strange excesses of antimatter, and too few high-energy electrons.

The story is far from over. As we've previously written, many physicists spend their time looking for the things that aren't there so the truth can emerge. Little by little, they use their data to chip away at a boring rock so the gems inside, the real discoveries, can emerge. And maybe dark matter is part of, but not all of the story, said Calore.

"It's not a simple game," she said. "Ruling out something is more complicated than saying something is there."

[Nature Astronomy]

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