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The hypothesis of dark matter has proved incredibly successful in explaining the overall large scale structure of the universe and in interactions on the level of galactic clusters, which competing hypotheses such as modified gravity have failed to adequately explain. However, on the relatively smaller scales of individual galaxies, hypothesising dark matter shows some problems. In a paper recently accepted for publication in Physical Review Letters, astronomers Stacy McGaugh and Federico Lelli of Case Western Reserve University, and Jim Schombert of the University of Oregon, have made observations of 153 different galaxies with a wide variety of shapes, masses, sizes and amounts of gas. They have found a strong relationship between how quickly the galaxy rotates and the presence of normal (baryonic) matter alone. From an article on Case Western Reserve University's Daily:
[...] A team led by Case Western Reserve University researchers has found a significant new relationship in spiral and irregular galaxies: the acceleration observed in rotation curves tightly correlates with the gravitational acceleration expected from the visible mass only.
"If you measure the distribution of star light, you know the rotation curve, and vice versa," said Stacy McGaugh, chair of the Department of Astronomy at Case Western Reserve and lead author of the research.
The finding is consistent among 153 spiral and irregular galaxies, ranging from giant to dwarf, those with massive central bulges or none at all. It is also consistent among those galaxies comprised of mostly stars or mostly gas.
[...] "Galaxy rotation curves have traditionally been explained via an ad hoc hypothesis: that galaxies are surrounded by dark matter," said David Merritt, professor of physics and astronomy at the Rochester Institute of Technology, who was not involved in the research. "The relation discovered by McGaugh et al. is a serious, and possibly fatal, challenge to this hypothesis, since it shows that rotation curves are precisely determined by the distribution of the normal matter alone. Nothing in the standard cosmological model predicts this, and it is almost impossible to imagine how that model could be modified to explain it, without discarding the dark matter hypothesis completely."
[...] Arthur Kosowsky, professor of physics and astronomy at the University of Pittsburgh, was not involved but reviewed the research.
"The standard model of cosmology is remarkably successful at explaining just about everything we observe in the universe," Kosowsky said. "But if there is a single observation which keeps me awake at night worrying that we might have something essentially wrong, this is it."
Additional coverage and commentary by Ethan Siegel and Brian Koberlain. It seems that the universe has just thrown us yet another curve ball. This kind of correlation is just the sort of thing that modified gravity such as MOND and TeVeS predict. However, they fail miserably in explaining the large scale structure and evolution of the universe, which the dark matter explains admirably.
(Score: 5, Interesting) by Anonymous Coward on Friday September 30 2016, @05:41PM
Here is how I'm interpreting this. The rotation differentials between the inner and outer part of galaxies STILL don't appear to follow Newtonian physics: the outside rotates faster than "Newton" predicts. However, the differential rates are consistent with the volume and distribution of visible (normal) matter. Thus, distribution of normal matter appears to be a near perfect predictor of rotation rates even though it doesn't follow Newton. But, perfect prediction is not necessarily an explanation.
Thus, there's still a mystery of the "flat" rotation, but there's no evidence the deviation from Newton is caused by dark matter because variations in normal matter between galaxies match tightly with observed rotation rate curves. The "news" here is that dark matter doesn't explain the Newton deviation, and not that the mystery is solved.
But I wonder if dark matter is not attracted to and/or repelled by normal matter based on normal matter's mass. Thus, more normal matter in the outer part of a given galaxy could be proportional to the amount of dark matter. The original dark-matter assumption was its relation to normal matter with semi-independent. Maybe on a galactic scale they are tightly dependent such that if Galaxy B has 60% more normal matter in its outer edge than Galaxy A, then B also has 60% more dark matter in its outer edge, explaining the difference from Newton predictions for visible matter AND the tight relationship between rotation rates and visible matter. No?
(Score: 2, Insightful) by frojack on Friday September 30 2016, @06:00PM
So then you reject the finding that explains why dark matter might not exist at all, and you turn that finding inside out to explain an imaginary property of dark matter?
No, you are mistaken. I've always had this sig.
(Score: 3, Insightful) by Anonymous Coward on Friday September 30 2016, @09:48PM
Dark matter "explains" other things besides galaxy rotation. And we still haven't solved the flat-rotational-curve mystery itself, only de-linked it directly from dark matter as previously modeled.
Either way, there is something odd about gravity over longer distances. Whether it's dark matter, gravity being an inconsistent force, entanglement, and/or something else, we don't know yet.
(Score: 0) by Anonymous Coward on Friday September 30 2016, @06:46PM
What about dark matter attached to strings? I posit there is as much evidence for this as the current theory!
Throw a few strings in there for tensioning and I am sure the theory will appear to match the data!
After all, nothing but quality science comes from just adding on ethereal bits and pieces to your theory until it by chance matches a set of data!
As long as the maths works out, you will get a paper out of it!
(Score: 2) by Gaaark on Friday September 30 2016, @09:59PM
but if you attach dark matter to strings, you need over 40 new imaginary dimensions that ARE there, we just have to find them. Yeah.
But they ARE there! Yep yep yep!
Because they solve problems!
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 0) by Anonymous Coward on Friday September 30 2016, @10:27PM
I think we are just missing an important piece in our theories of gravity. That's it, no invisible stuff necessary.
(Score: 0) by Anonymous Coward on Saturday October 01 2016, @12:29AM
As a thought experiment, what kind of force-by-distance curve of gravity would match observations for the three "categories" of distances: solar-systems (traditional Newtonian), galactic level (explaining the "flat" spin curve), and the inter-galactic distances, such as Hubble's gravity lenses? And maybe even the accelerating expansion of the universe.
Rather than looking at many simple forces, we can explore a complicated single force (gravity with a "weird" curve).