Our Galaxy's Spin Is Slowing. And Dark Matter Is to Blame

The galaxy you live in is losing steam. And that's okay.

Connecting the mind-numbingly large spiral arms of the Milky Way is an enormous spinning galactic bar composed of billions of clustered stars, but it's slowed down by roughly one-quarter speed since it came into being, according to a recent study published in the journal Monthly Notices of the Royal Astronomical Society.

It's not enough for human vertigo, but it's okay to feel dizzy, because dark matter is to blame for this slowing motion of galactic spin.

A dense star cluster moved away from the galactic core

Besides, in the last 30 years, astrophysicists predicted this would happen. But before now, no one had actually seen and measured it in action. And, most crucially, the observations provided new insight into the nature of dark matter, which functions as a cosmic counterweight to the galactic spin, pulling it to a slower rate. The researchers analyzed observations taken by the Gaia space telescope of a large star cluster called the Hercules stream, which moves in tandem with the bar, roughly matching its radial velocity.

This phenomenon also happens to the planet Jupiter's Greek and Trojan asteroids, which orbit behind and ahead of Jupiter, in its Lagrange positions. When the galactic bar's spin loses speed, astrophysicists would expect the Hercules stream stars to move farther out in the galaxy, since their orbital period would change to match that of the bar. But the researchers discovered a chemical fingerprint in these stars: They are abundant in heavier, "metal" elements, which means they came from a place closer to the galactic core, where stars form from gas consisting of roughly 10 times the metal content of comparable gas in the outer rim of the galaxy.

Dark matter slowed down the galactic bar by at least 24%

With this insight, the research team was led to the conclusion that the galactic bar, which contains billions of stars equating to trillions of times the mass of our sun, had lost nearly a quarter of its radial speed, or at least 24%, since it came into being. "Astrophysicists have long suspected that the spinning bar at the center of our galaxy is slowing down, but we have found the first evidence of this happening," said Ralph Schoenrich of UCL Physics & Astronomy, who is also co-author of the study, in a ScienceDaily report. "The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter."

"Our research provides a new type of measurement of dark matter — not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar's spin," added Schoenrich, in the report. Since dark matter has so far remained undetected by all known scientific instruments, it has remained a mystery in modern physics, despite scientists' knowing that there could be roughly five times as much dark matter as there is ordinary, everyday matter. With this discovery, we've come to know the characteristics and behavior of dark matter a little better. And whatever its ultimate nature, it wields immense power in the universe, slowing down the spin of our galaxy's entire bar.