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Darth matter. Awful pun is awful. Credit: 20th Century Fox/Getty Images

The dark force secretly slowing down our galaxy could be dark matter

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Jun 22, 2021, 8:25 PM EDT

When something went wrong in Star Wars, it usually had to do with the Dark Side of the Force, but an unexplained phenomenon in our galaxy is probably connected to dark matter.

Something suspicious is going on in a galaxy not so far, far away. In the core of the Milky Way lies the Hercules stream of stars. Its spin is mysteriously slowing down, and what exactly is going on has been predicted for decades with no luck. Now it has finally been measured and possibly proven. Astrophysicists Rimpei Chiba from the University of Oxford and Ralph Schoenrich of University College London think they have figured out what dark force is doing this — and it isn’t the Sith. Dark matter has seemingly been countering the spin and slowing it down.

Billions of stars totaling trillions of solar masses are trapped by a spinning bar at the center of the Milky Way. Chiba and Schoenrich, who recently published a study in Monthly Notices of the Royal Astronomical Society, found that the spin of that bar has slowed down to around 75% of what it was when it first came into existence.

“The finding was not expected,” Chiba, who led the study, and Schoenrich, who was a co-author, told SYFY WIRE via email. “Previous constraints on dark matter have mostly concentrated on mapping the gravitational potential, but the bar's slowdown that we have quantified links to the inertial mass (dynamical response) of dark matter taking up the angular momentum from the galactic bar.”

Dark matter has never before been measured by its inertial mass rather than its gravitational energy. Inertial mass is how much an object resists forces working against it, as opposed to gravitational mass, or the strength of the gravitational force between objects. Dark matter has been slowing the spin of the galactic bar through dynamical friction, or the drag that objects experience as they orbit through dark matter. How much of this drag holds a moving object back depends on where clumps of dark matter are and the amount found in any given region (spatial distribution), as well as how fast any of those particles are moving, anywhere (velocity distribution).

The galactic core (right) of the Milky Way (left). Credit: NASA


Stars in the Hercules stream are gravitationally trapped by the spinning bar, and will move outward as the bar’s spin grows slower and slower. The proof that Hercules stars migrated away from the bar while maintaining their orbits is in their chemistry. Stars that started out in the galactic core are full of heavier elements, while the core is ten times richer in these elements than the halo. These stars are trapped in orbit around the resonance, which occurs when a consistent gravitational influence is exerted by one orbiting body on another, but can still move outward.

“Since our measurement quantifies the loss of the bar's angular momentum, the finding is in tension with alternative gravity theories without dark matter, which has to take up the angular momentum lost by the bar,” Chiba and Schoenrich noted. “We cannot see a different solution for explaining this angular momentum loss.”

The team has run into opposition even though their explanation is the only one that completely makes sense. Though dark matter is still just about as dark and mysterious as it sounds, there are ways to infer it until technology advances enough to be able to detect it otherwise. Some scientists have used models excluding dark matter to show why the spin of the galactic bar is slowing down, but the problem with these is that they end up with little or no slowdown as a result. Others who have proposed models that involve alternative gravity and only some dark matter have not seen the same results that Chiba, Schoenrich, and their research team have.

“You can see it this way — finding this new type of evidence for dark matter is like finding a big island in the ocean,” they said. “Knowing it is there is great, but to explore and use it for further study, you need new tools. At the end of the road, you get new constraints on galactic history and the unique opportunity to differentiate between different dark matter models.”

Sorry Vader, but even the power of the Dark Side probably has nothing on the strength of all the dark matter in the universe.