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Some of the strongest evidence for dark matter to date has been discovered – and ironically, that's thanks to its absence. In a pair of studies published this week, astronomers have shed new light on dark matter through close observation of a galaxy previously found to have very little of the stuff, while the same team found a new example of a similar oddball galaxy.
It's generally believed that galaxies are held together through the gravitational influence of clumps of dark matter, so to find a galaxy with little to no dark matter was a surprise. And while it might sound like a strike against the theory, it actually ends up supporting it.
A Second Galaxy Missing Dark Matter in the NGC 1052 Group (DOI: 10.3847/2041-8213/ab0d92) (DX)
Still Missing Dark Matter: KCWI High-resolution Stellar Kinematics of NGC1052-DF2 (DOI: 10.3847/2041-8213/ab0e8c) (DX)
(Score: 2) by melikamp on Tuesday April 02 2019, @02:24AM (31 children)
No I mean, would it hurt the submitters to post links to full text? Here's the other one:
https://arxiv.org/pdf/1901.05973.pdf [arxiv.org]
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @02:39AM
Have to wait for one of the MOND guys to work out a value for that one. I am no expert.
But it is another "ultra-diffuse galaxy", which (afaik) means there is less deviation between MOND and Newtonian predictions: https://physics.stackexchange.com/questions/373555/mass-discrepancy-acceleration-relation-in-%CE%9Bcdm-paradigm [stackexchange.com]
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @03:08AM (29 children)
Someone already published a mond prediction for this one:
From Table 2:
https://arxiv.org/abs/1901.02679 [arxiv.org]
In your paper they say:
So it is on the low end but keep in mind what these intervals mean. If you check the MOND predictions for 100 galaxies you would expect about 10 to be outside the 90% CI if MOND was correct.
(Score: 1) by khallow on Tuesday April 02 2019, @04:31AM (28 children)
it's a classic epicycle theory even more so than the dark matter hack.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @04:41AM (22 children)
It's not even a theory, think about it more like Keplers laws.
MOND is a quantitative relationship with amazing predictive skill (and thus usefulness) that needs to be explained. It seems very unlikely "dark matter" (arbitrary sized and shaped spheriods of some exotic substance surrounding each galaxy) can explain why MOND works. This is precisely because the deviations from newtonian dynamics should not be so lawlike in that case.
(Score: 1) by khallow on Tuesday April 02 2019, @08:46AM (21 children)
Except of course, that would explain it. That's why the dark matter is proposed in the first place.
Predictive? Like what? None of the things mentioned so far have been predicted by MOND.
We already know general relativity is a massive deviation from Newtonian dynamics at cosmological scales.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @09:22AM (3 children)
Sorry, I thought you had some idea of what you are talking about. MOND has predicted every galactic rotation curve discovered since 1983. Dark matter has been post hoc fit to every single one, just exactly enough to match the MOND prediction.
(Score: 1) by khallow on Tuesday April 02 2019, @10:14AM (2 children)
So has dark matter based theories. Epicycles are a wonderful thing, until you get too many of them.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @10:23AM (1 child)
No they have not. They have never predicted a single one because it is impossible. All black matter explanations are post-hoc. Every single one.
And MOND doesn't have epicycles. It is a simple relationship where you plug in values for the surrounding mass. The only trouble is getting good estimates of those values. There is nothing similar to epicycles that are added ad hoc.
Dark matter on the other hand, is literally 3D epicycles. It is invisible spheroids of stuff added in after the fact to make the predictions work.
(Score: 1) by khallow on Tuesday April 02 2019, @10:44AM
To the contrary, small object near a large object (where the weakly interacting matter can be drawn off by gravitation interference) is one such place. It predicts the existence of galaxies without much in the way of dark matter. The mechanism of creation conveniently falls under the external field effect (EFE) assumption.
Two have already been mentioned. The initial weak gravity deviation from Newtonian mechanics. And the EFE assumption. Neither has physical explanation. It just fits data, just like the dark matter assumption does.
In other words, "simple relationship where you plug in values for the surrounding mass". We already know the universe has priors since distribution of matter follows no simple relation on any scale we can observe. So dark matter priors is not even a little bit of a stretch.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @09:24AM (12 children)
I mean you are responding in a thread discussing how MOND predicted one of these galaxies curves too. Someone worked out the prediction before this paper came out (+ a dozen more). Are you even comprehending any of this?
(Score: 1) by khallow on Tuesday April 02 2019, @10:35AM (11 children)
The data preceded the prediction (keep in mind that the theory had to be modified in the first place to explain the sort of system which is adjacent to a large mass, and hence, would be mostly likely to be stripped of dark matter in the dark matter theories). And you still neglect the big issue. MOND has all sorts of adaptations to fit observation. But we continue to find things it needs to adapt to. For a short list:
Seriously, why is the presence of dark matter considered a big problem even though we still have much we don't understand about such things, but the presence of aphysical characteristics not considered a problem? My view is that dark matter is simply stuff that we haven't seen yet - sorry, that's not that big a stretch despite all that has gone on, and MOND is a massive ad hoc with a number of justifications based only on explaining incomplete data.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @10:56AM (3 children)
You seem incapable of even recognizing that in this very case someone published a MOND-based prediction, then later another group published rotation curve data that matched the prediction.
There is no room for ad hoc adjustments to MOND, it is set in stone since 1983. The only thing is getting good estimates of the parameters to plug in. So every time you repeat that it is false. You can go look at the original papers to see it (but I know you won't).
It is pointless to discuss something with someone who just repeats falsehoods that are easily disproved by just reading a few sentences in a journal article they refuse to look at. That is messed up dude.
(Score: 1) by khallow on Tuesday April 02 2019, @12:56PM (2 children)
Open cluster data predates 1983. And I don't buy that current MOND-based predictions are based on that theory.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @01:12PM (1 child)
You just "don't buy it", because data mentioned in a paper existed before the paper was written. That is your argument now...
This is so sad, on other topics you seemed to have an actual thought process.
(Score: 1) by khallow on Tuesday April 02 2019, @01:51PM
Yes. That's correct. Please recall the key problem. There is no physical explanation for any part of MOND, then or now. It's instead an adjustment to existing theory to explain discrepancies. That's exactly the role of dark matter as well. Both are epicycles.
Further, we have the problem of the gravity environment. Somehow MOND claims that these disperse galaxies are close enough higher mass to have these mechanics, but not the galaxy, NGC 1052 which is generating the external gravitational field in question. NGC 1052, instead has a dark matter halo [hawaii.edu] (possibly with two axes, "Some galaxies (e.g.NGC 1052, M32)are well-fit by two-integral models"). Think about that. Something outside of a galaxy has no MOND effect allegedly due to the external gravitational field of the galaxy, but the galaxy itself does. How did the gravitational field grow that much stronger outside NGC 1052 that it eliminated the MOND effect?
(Score: 2) by Gaaark on Tuesday April 02 2019, @02:02PM (6 children)
"Seriously, why is the presence of dark matter considered a big problem even though we still have much we don't understand about such things,"
And yet you have big problem with QI, even though it is a better, more scientific model.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 1) by khallow on Wednesday April 03 2019, @04:16AM (5 children)
The catch is that MOND at least explains most galaxies with "no" dark matter effects as being embedded in a stronger gravitational field. QI seems to predict that dark matter effects should be stronger because of the weak internal gravitational field of the galaxy rather than the opposite which is actually observed.
(Score: 2) by Gaaark on Wednesday April 03 2019, @10:21AM (4 children)
Uhhhh,...QI seems to predict there IS NO dark matter.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 1) by khallow on Wednesday April 03 2019, @12:51PM (3 children)
(Score: 2) by Gaaark on Wednesday April 03 2019, @10:45PM (2 children)
Okay, I guess I don't understand where you are going.
QI only deals with motion and speed: galaxies don't fly apart due to inertia.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 1) by khallow on Wednesday April 03 2019, @11:12PM (1 child)
The problem here was motion and speed of rare sorts of galaxies that the other two theories have figured out how to shoehorn explanations for. The QI explanation is lacking since the galaxies in question are low density satellite galaxies, most of their mass got robbed by NGC 1052 in some long ago collisions. So they're precisely the sort of low local gravity objects that QI should be describing. Unless QI has the MOND issue where external gravitation fields (here from NGC 1052) cause the physics to somehow change.
(Score: 2) by Gaaark on Thursday April 04 2019, @01:37AM
run the numbers against his formula.
I'm betting you get the numbers you're looking for.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 2) by FatPhil on Tuesday April 02 2019, @07:35PM (3 children)
> We already know general relativity is a massive deviation from Newtonian dynamics at cosmological scales.
The bulk of what we can see out there that is interacting gravitationally with each other isn't travelling anything like relativistic speeds in any of their reference frames, so the deviations from Newton are pretty tiny - smaller than the error bars around any measurements we make. The things receding from us with enormous relative velocities are not interacting with us gravitationally at all, so the deviations are effectively zero.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 1) by khallow on Wednesday April 03 2019, @12:54PM (2 children)
(Score: 2) by FatPhil on Wednesday April 03 2019, @08:44PM (1 child)
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 1) by khallow on Wednesday April 03 2019, @11:16PM
Sorry, not with respect to mass that's far away. Newtonian dynamics, for example, would have instantaneous interactions between widely separated superclusters of galaxies not billions of years of lag. Cosmological inflation and the current observation of "negative energy" (which really is just observed current stretching of the universe).
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @04:50AM
And look how the dark matter proponents are misusing statistics. What they did is something like publish 100 different papers about one galaxy each, wherein they compare 90% confidence intervals to MOND predicted values for every galaxy.
So they made 100 different comparisons and didn't adjust their significance level for that. It is actually very likely a few will be outside the 90% CI if MOND is correct. In fact, we would predict 10% of the predictions are outside the interval.
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @05:10AM (3 children)
Also, the external field effect is crucial to MOND, it isn't something added on later. It is just negligible in most cases so they ignore it.
https://arxiv.org/abs/1010.1349 [arxiv.org]
(Score: 1) by khallow on Tuesday April 02 2019, @10:45AM (2 children)
(Score: 0) by Anonymous Coward on Tuesday April 02 2019, @11:00AM (1 child)
Very first paper about MOND, section III, # 2.
http://adsabs.harvard.edu/doi/10.1086/161130 [harvard.edu]
There is nothing ad hoc about this.
(Score: 1) by khallow on Tuesday April 02 2019, @12:54PM