Scientists discover how galaxies can exist without dark matter:
It started in 2018 when astrophysicists Shany Danieli and Pieter van Dokkum of Princeton University and Yale University observed two galaxies that seemed to exist without most of their dark matter.
"We were expecting large fractions of dark matter," said Danieli, who's a co-author on the latest study. "It was quite surprising, and a lot of luck, honestly."
The lucky find, which van Dokkum and Danieli reported on in a Nature paper in 2018 and in an Astrophysical Journal Letters paper in 2020, threw the galaxies-need-dark-matter paradigm into turmoil, potentially upending what astrophysicists had come to see as a standard model for how galaxies work.
"It's been established for the last 40 years that galaxies have dark matter," said Jorge Moreno, an astronomy professor at Pomona College, who's the lead author of the new paper. "In particular, low-mass galaxies tend to have significantly higher dark matter fractions, making Danieli's finding quite surprising. For many of us, this meant that our current understanding of how dark matter helps galaxies grow needed an urgent revision."
The team ran computer models that simulated the evolution of a chunk of the universe—one about 60 million light years across—starting soon after the Big Bang and running all the way to the present.
The team found seven galaxies devoid of dark matter. After several collisions with neighboring galaxies 1,000-times more massive, they were stripped of most of their material, leaving behind nothing but stars and some residual dark matter.
"It was pure serendipity," said Moreno. "The moment I made the first images, I shared them immediately with Danieli, and invited her to collaborate."
Robert Feldmann, a professor at the University of Zurich who designed the new simulation, said that "this theoretical work shows that dark matter-deficient galaxies should be very common, especially in the vicinity of massive galaxies."
Journal Reference:
Jorge Moreno, Shany Danieli, James S. Bullock, et al. Galaxies lacking dark matter produced by close encounters in a cosmological simulation, Nature Astronomy (DOI: 10.1038/s41550-021-01598-4)
(Score: 2, Interesting) by Anonymous Coward on Thursday February 17 2022, @06:46AM
We've know for a while that there are galaxies with little to no dark matter. What makes these interesting is that they are far away from any larger galaxies that might have stripped their dark matter away through a collision or near miss.
(Score: 5, Insightful) by darkfeline on Thursday February 17 2022, @10:32AM (9 children)
> our current understanding of how dark matter helps galaxies grow needed an urgent revision
More like, our current understanding of how gravity works needs an urgent revision. When you have to pad your equations to balance them, your first instinct shouldn't be to fix your padding scalars; fix your equations first.
Join the SDF Public Access UNIX System today!
(Score: 3, Informative) by HiThere on Thursday February 17 2022, @02:26PM (3 children)
"dark matter" is a place-holder. If you come up with a non-particulate theory that can actually produce the observed results it would be very interesting. So far no such theory has been seen. Several of them can handle many of the results, but so can Newton's, and that has the advantage of being relatively easy to calculate.
So far what "dark matter" means is "it's got mass, but we can't see it". There are a few other characteristics. It doesn't say what it is, though other theories hint that it can't be baryonic.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 3, Interesting) by DeathMonkey on Thursday February 17 2022, @05:22PM (2 children)
I don't know why people would assume that we can see everything that exists in the entire universe via electromagnetic radiation in the first place!
So it is a prerequisite that everything with mass must reflect EM and that all mass in the universe has a source of EM to reflect?
(Score: 2) by HiThere on Thursday February 17 2022, @10:54PM
Well, they also haven't found it by looking for interactions via the strong or weak force in atom smashers. So it is a bit of a puzzler. For awhile some folks thought it was a fourth neutrino oscillation, but that didn't pan out.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 0) by Anonymous Coward on Friday February 18 2022, @01:02AM
The problem with the term "dark matter" is that in different contexts it means different things. However, the casual science consumer and writer may not understand that. Dark matter in the most general use of the term absolutely exists. Because it is well established that some exists but not in quantities that add up to the total amount necessary, then more exotic forms have to make up at least some of the difference but those questions are even more involved. So when scientists use dark matter, they usually are referring to one of the subsets of dark matter. So you end up with amateurs seeing papers and articles using a bit of jargon but not understanding the context who end up using the term differently from how it was meant, which just adds to the confusion.
To answer your question directly, not everything with mass must reflect or emit electromagnetic radiation. Black holes are the most obvious example. But even if all objects must reflect or emit such radiation, that doesn't mean we are guaranteed the ability to detect that radiation even with advances akin to science fiction.
(Score: 2) by bloodnok on Thursday February 17 2022, @07:04PM (4 children)
I agree. I am of the opinion that gravity does not even exist as a fundamental force and that all of the effects of gravity, including weight, arise solely from the curvature of spacetime.
I came to this idea not because I think I am smarter than thousands of physicists, but because the smartest minds in the world for the last century have been unable to find a compelling theory for gravity as a force.
So I concluded that if the smartest people in the world cannot find what they are looking for, then maybe it is not there.
In fact, all we need is:
1) a theory for how curved spacetime leads to the observed effects of gravity on matter;
2) a theory for how matter affects spacetime.
I have weak hypotheses, and no equations, for the first, and nothing for the second so this is hardly world shattering, but in spite of that it seems that I am no further from the answer than the physics community at large (and I still believe they are the smartest minds in the world).
__
The Major
(Score: 1) by khallow on Thursday February 17 2022, @09:02PM (3 children)
(Score: 2) by bloodnok on Friday February 18 2022, @04:14AM (2 children)
As far as I understand it, most physicists believe that the curvature of spacetime is the result of gravity, whereas I believe that, what we perceive as, gravity is the result of the curvature of spacetime.
I believe we can eliminate gravity as a fundamental force and replace it simply with curved spacetime.
__
The major
(Score: 1) by khallow on Friday February 18 2022, @04:34AM (1 child)
I'll just note that it's really not that different a viewpoint if you look at gravity as a manifestation of curved spacetime rather than the other way around. Also, I'm wrong about the name of the tensor I mentioned earlier, it's the "stress-energy tensor".
Either way, you still have explain why the structure is there, be it gravity or curvature. Quantum mechanics is not much help. I wouldn't be surprised that spacetime is emergent phenomena of some more basic system. It's possible that time doesn't exist at all normally, unless you happen to have event horizons or an observer with memory.
(Score: 2) by bloodnok on Friday February 18 2022, @07:15PM
Agreed. This is just a change of viewpoint, except that gravity is no longer a fundamental force and we can stop looking for it.
Instead we can concentrate on the interaction between matter and spacetime, which of course is no easier.
Thanks for engaging.
__
The Major
(Score: 3, Interesting) by Gaaark on Thursday February 17 2022, @04:08PM (4 children)
They can't even prove DM exists, so I can conclusively say "The team has found EVERY galaxy devoid of dark matter". Prove me wrong.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. I have always been here. ---Gaaark 2.0 --
(Score: 4, Informative) by maxwell demon on Thursday February 17 2022, @05:06PM (3 children)
Easy: A “galaxy devoid of dark matter” is a galaxy whose stars follow Newton's laws when assuming the observed matter is all the matter there is in that galaxy. This clearly is not the case for all galaxies, therefore you're wrong.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by Gaaark on Thursday February 17 2022, @05:31PM (2 children)
No:
I'm saying EVERY galaxy is devoid of dark matter: prove me wrong.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. I have always been here. ---Gaaark 2.0 --
(Score: 3, Interesting) by khallow on Thursday February 17 2022, @06:56PM (1 child)
(Score: 1, Interesting) by Anonymous Coward on Thursday February 17 2022, @11:22PM
Yup, Dark Matter is a placeholder for "Something needs to account for what we are seeing, and we need a name for it"
Basically, the speed at which galaxies tend to spin doesn't jive with how much mass they seem to have; that is, they need more mass to spin that fast and not fall apart.
Nothing we are aware of has mass but doesn't interact with EM to the degree that this matter would have to.
Modified theories don't tend to work because the observed effect differs from galaxy to galaxy, with some having no observed difference (that is observations of a few galaxies don't require the introduction of dark matter to make sense, even though most do).
Nobody likes this situation as we have a name for a thing, but we can only observe it indirectly from an incredible distance in highly complex situations, but as far as I have heard, there hasn't been any galaxies observed that required a negative amount of DM to make sense, which is one of the few things that would totally disprove it.