At a certain level, everything in the Universe is in motion, from the electrons in our bodies’ atoms to, we now know, the vast filaments of the universe’s large-scale structure. That motion isn’t constant, however. 

We live in a large spiral galaxy that has a bar of stars spanning its core. Researchers Rimpei Chiba and Ralph Schönrich looked at data on the positions and composition of the Hercules stream of stars, which orbits in lockstep with the galactic bar. In a paper appearing in the Monthly Notices of the Royal Astronomical Society, they note that the composition of these stars is wrong for the part of the galaxy they are currently in: they have too many heavy metals and look more like stars from the galactic core. 

They are able to explain these misplaced stars if the bar’s rotation has slowed over time, causing these locked-in-place stars to migrate outward as they slowed and maintained their position relative to the bar. To slow the bar requires dark matter. If the over-density of stars and gas could gravitationally pull in slow-moving dark matter, the bar would be slowed, and they estimate that the bar has slowed at least 24% from its original speed. What is most fascinating to me about this result is that it requires dark matter to be stuff and not a modification to gravity. Now, we just have to find that stuff.

More Information

University College London press release

“Tree-ring structure of Galactic bar resonance,” Rimpei Chiba and Ralph Schönrich, 2021 April 19, Monthly Notices of the Royal Astronomical Society