https://phys.org/news/2024-01-stars-slowly-milky-edge-galaxy.html
By clocking the speed of stars throughout the Milky Way galaxy, MIT physicists have found that stars further out in the galactic disk are traveling more slowly than expected compared to stars that are closer to the galaxy's center. The findings raise a surprising possibility: The Milky Way's gravitational core may be lighter in mass, and contain less dark matter, than previously thought.
The new results are based on the team's analysis of data taken by the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise location, distance, and motion of more than 1 billion stars throughout the Milky Way galaxy, while APOGEE is a ground-based survey.
The physicists analyzed Gaia's measurements of more than 33,000 stars, including some of the farthest stars in the galaxy, and determined each star's "circular velocity," or how fast a star is circling in the galactic disk, given the star's distance from the galaxy's center.
[...] The team translated the new rotation curve into a distribution of dark matter that could explain the outer stars' slow-down, and found the resulting map produced a lighter galactic core than expected. That is, the center of the Milky Way may be less dense, with less dark matter, than scientists have thought.
(Score: 3, Interesting) by weirsbaski on Thursday February 01 2024, @07:12AM
IANAAP (Astro-Physicist), but there's something I've been wondering. Maybe someone can offer some insight why I'm wrong.
While a figure skater is twirling, she brings her arms in. To conserve angular momentum, this causes her rotational speed to increase. Likewise, as matter spirals in to a black hole to meet its fate, its angular speed increases. But matter can't accelerate to C (speed of light), so regardless of how close the matter gets to the black hole, there's only so much angular momentum it can provide (because it'd have to move faster than C to keep the momentum constant as it draws extremely near).
What if the galaxy's faster-than-expected rotation is frame-dragging caused by the falling matter? Ie- the matter can't accelerate far enough to keep the equations balanced, so spacetime itself absorbs some of the momentum by circling itself faster around the black hole?
Again, I'm curious but IANAAP.