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MOND or Dark Matter?

FattyCatty

Picky V. Nitty
Joined
Aug 22, 2010
Messages
2,338
I saw this article in Science and wondered how common belief in MOND is as opposed to belief in dark matter. Also, does belief in MOND rule out belief in dark matter?
Thousands of physicists, astrophysicists, and astronomers are searching for dark matter, mysterious stuff whose gravity seems to hold the galaxies together. However, an old and highly controversial theory that simply changes the law of gravity can explain a key property of galaxies better than the standard dark-matter theory, one astronomer reports. That claim isn't likely to win over many skeptics, but even some theorists who favor the standard theory say the analysis hands them a homework problem they should solve.

"The standard theory should explain this, and it doesn't yet. That's fair to say," says Simon White, a cosmologist at the Max Planck Institute for Astrophysics in Garching, Germany, who was not involved in the current analysis.
<snip>
For the past 28 years, Milgrom's idea, known as Modified Newtonian Dynamics (MOND) has generated a long-simmering debate. Many researchers argue that ever more evidence from clusters of galaxies, the largest scale structure of the universe, and the afterglow of the big bang points to the existence of dark matter. Still, a few researchers counter that when they look at the details, MOND does a better job—at least on the galactic scale.

Now, in the latest shot from the MOND side, Stacy McGaugh, an astronomer at the University of Maryland, College Park, reports that MOND can explain an observed correlation between the mass and the rotation speed of galaxies—that is, the speed of those outer stars—called the baryonic Tully-Fisher relation. MOND researchers had tried to do this before, but for their models to work, they had to make an untested assumption about the relationship between a star's mass and the amount of light it puts out. That assumption introduces a large uncertainty, weakening the argument.

To avoid that problem, McGaugh gathered data from various sources on 47 galaxies that contain more hydrogen gas than stars. The mass of the gas can then be estimated directly. McGaugh made a plot of visible mass versus rotation speed for the galaxies. He then plotted the prediction that comes straight out of MOND in a few lines of algebra. The MOND line went right through the data. "You draw the line and the data fall right on it," McGaugh says. "No muss, no fuss." He reports the result in a paper in press at Physical Review Letters.


The paper by Stacy McGaugh referred to seems, from what little I could understand, to be more ambiguous and less either or on the existence of dark matter than the Science article implied.

[QUOTE
]A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies

Stacy S. McGaugh
Department of Astronomy, University of Maryland, College Park, MD 20742-2421
(Dated: February 22, 2011)​

The current cosmological paradigm, ΛCDM, requires that the mass-energy of the universe be dominated by invisible components: dark matter and dark energy. An alternative to these dark components is that the law of gravity be modified on the relevant scales. A test of these ideas is provided by the Baryonic Tully-Fisher Relation (BTFR), an empirical relation between the observed mass of a galaxy and its rotation velocity. Here I report a test using gas rich galaxies for which both axes of the BTFR can be measured independently of the theories being tested and without the systematic uncertainty in stellar mass that affects the same test with star dominated spirals. The data fall precisely where predicted a priori by the modified Newtonian dynamics (MOND). The scatter in the BTFR is attributable entirely to observational uncertainty. This is consistent with the action of a single effective force law but poses a serious fine-tuning problem for ΛCDM.
<snip>
If MOND is essentially correct in that it is pointing towards an extension of gravitational theory, then the experiments seeking to detect dark matter will find null results. This would also be the case if dark matter has a different nature than currently presumed. If dark matter is detected, then the MOND formula is still useful as a phenomenological constraint on the effective force law in spiral galaxies. In any case, the predictive power of the simple formula proposed by Milgrom [2] is telling us something profound about Nature.[/QUOTE]
 
I think it would be fair to say that belief amongst astronomers in MOND is uncommon. However, pretty much everyone takes it seriously as a challenger.
The paper in question is on my reading list so I won't say anything about it yet.

I seem to recall, in answer to your last question, that to fit cluster results MOND needs some dark matter - although it can be hot, so neutrinos if heavy enough can do the job. Otherwise it doesn't exactly force you not to believe in other dark matter but it'd surprise you a lot more if it turned up.
 
Thank you for your replies. The link was interesting. I wondered if you saw the reply to the linked article by Stacy McGaugh, author of the article in the OP? If so, what did you think of it?

17. Stacy McGaugh Says:
February 27th, 2011 at 10:20 am
Hi Sean,
Thanks for noticing.

I agree with much of what you have to say, but certainly not all.

For example, I agree about the third peak of the CMB power spectrum. But you neglect to mention that I was the only person to correctly predict the amplitude of the second peak. That the third peak is high simply means ΛCDM survives this test, not that MOND fails it. For as you say, we need a proper covariant theory to do that, and it is not yet clear what that is, or indeed, if it is even possible to construct.

The success of ΛCDM for the CMB hardly guarantees its success in galaxies. It too has its problems in dwarf spheroidals. The data in the particular paper you cite were really rather ambiguous. MOND does poorly in two cases (Draco and Ursa Minor) but does pretty well in the other five cases. So which is the forest, and which are the trees?

More recently, I’ve looked into this very subject with Joe Wolf. The new “ultrafaint” dwarfs are much worse for MOND than Draco and Ursa Minor. I was ready to declare MOND dead myself because of this. Then I thought I should check the literature one last time, and discovered that Brada & Milgrom had in fact discussed how tidal effects (which are stronger in MOND) could plausibly cause the failing Joe and I were seeing. Indeed, they quantified the point at which the apparent failure should set in, and we found that is exactly where it does set in. So what I initially thought was a clear falsification is not.

As for the press, it is a curious spin. In my initial submission to PRL all I said was “here is a specific prediction of a hypothesis, MOND, and geez, it works surprisingly well.” It wasn’t just the reporters who asked about the implications for dark matter, it was also (quite reasonably) the referees. As Ben notes above, that MOND works even to the extent it does poses a fine-tuning problem for dark matter models.

Much of the evidence that we often point to for dark matter is ambiguous. More properly, we should say there is clear evidence for mass discrepancies. What you see plus Newton does not explain the data. So either there is dark matter, or we need to modify Newton. Like yourself and many other scientists, I am more comfortable with the former approach. I have noticed, however, that the universe does not seem too concerned about my personal comfort zone. If it were, it would stop handing me results that have MOND written all over them just as clearly as you say the CMB has dark matter written all over it.

If we’re right about dark matter, why does MOND get any prediction right at all?
I’m open to an explanation for this in the conventional context. Indeed, I have spent an enormous amount of my research time trying to come up with one myself. So far I have failed. That doesn’t mean it is impossible, but I can say it is a lot harder than most scientists seem to realize. Usually people invoke “feedback,” but this has become a code word for excusing anything we don’t understand. (Remember step 2 of that old cartoon “then a miracle occurs”?) Maybe it will. But please forgive me if I require a somewhat higher standard of proof than faith that what works at cosmological scales must inevitably work for galaxies.

Indeed, you mention that it is not admirable to “hang onto a theory because [you] want to believe it.” I could not agree more! We must hold ourselves to very high standards of intellectual honesty. That’s how I got involved in this issue in the first place. I believed in cold dark matter as much as you obviously do. But MOND cropped up in my data. Should I deny that? Should I not report it?

What I should do is consider the other evidence. I was already familiar with dark matter, but I had to learn a lot about MOND. I have. I’ve written lengthy reviews about it, long ago addressing many of the points above – though not always favorably to either theory. I can tell the story from either side, both positive and negative, warts and all. Indeed, I decided that to be fair, I had to work as hard on behalf of MOND as I work on behalf of ΛCDM.

Have you tried that?
 
I saw this article in Science and wondered how common belief in MOND is as opposed to belief in dark matter.

Well, "belief" probably isn't the right word, but aside from that, not many really. The basic point is that while laypeople often seem to think of dark matter as some mysterious made up thing, it's not actually all that strange in terms of the physics. We've been finding new particles all the time ever since we invented particles, and we already have things like neutrinos that are pretty similar to how we expect most dark matter to be. Postulating one more kind of matter that has perfectly sensible reasons for not being seen before is a very simple explanation that can solve a whole pile of questions.

MOND, on the other hand, not so much. To start with, as far as I'm aware there's never actually been a complete MOND theory. Plenty of ideas, but none that can actually explain everything we see. A lot of the attitude towards MOND is basically that it looks somewhat promising as a mathematical theory, but not worth paying much attention to until it's had a lot more work. As Stacy McGaugh in the post you quote:
For as you say, we need a proper covariant theory to do that, and it is not yet clear what that is, or indeed, if it is even possible to construct.
Not only is it not complete, they don't even know what the complete theory might look like yet. While the same could be said for LCDM theories, proposing a few extra particles is, dare I say it, rather less extraordinary, so MOND will have to look a lot more convincing before it can become the preferred approach.

Also, does belief in MOND rule out belief in dark matter?

No. Dark matter is literally just matter that we can't see. MOND may require less of it to explain certain observations, but if it ruled out dark matter entirely then no-one would ever take it seriously since we obviously can't see everything. In addition, as Edd notes, neutrinos are dark matter and we already know they exist. A theory that didn't allow them to exist would never get off the ground.
 
Well, "belief" probably isn't the right word, but aside from that, not many really. The basic point is that while laypeople often seem to think of dark matter as some mysterious made up thing, it's not actually all that strange in terms of the physics. We've been finding new particles all the time ever since we invented particles, and we already have things like neutrinos that are pretty similar to how we expect most dark matter to be. Postulating one more kind of matter that has perfectly sensible reasons for not being seen before is a very simple explanation that can solve a whole pile of questions.

MOND, on the other hand, not so much. To start with, as far as I'm aware there's never actually been a complete MOND theory. Plenty of ideas, but none that can actually explain everything we see. A lot of the attitude towards MOND is basically that it looks somewhat promising as a mathematical theory, but not worth paying much attention to until it's had a lot more work. As Stacy McGaugh in the post you quote:

Not only is it not complete, they don't even know what the complete theory might look like yet. While the same could be said for LCDM theories, proposing a few extra particles is, dare I say it, rather less extraordinary, so MOND will have to look a lot more convincing before it can become the preferred approach.



No. Dark matter is literally just matter that we can't see. MOND may require less of it to explain certain observations, but if it ruled out dark matter entirely then no-one would ever take it seriously since we obviously can't see everything. In addition, as Edd notes, neutrinos are dark matter and we already know they exist. A theory that didn't allow them to exist would never get off the ground.
Thanks for your reply. So, if I'm understanding correctly, MOND isn't accepted as an alternate theory to LDCM theories. How interesting and worthy of study are its predictive abilities, then? Shouldn't there be an explanation for why it can make correct predictions? Or is that not really important?
 
Thanks for your reply. So, if I'm understanding correctly, MOND isn't accepted as an alternate theory to LDCM theories.

It's accepted as a potential alternative. It's just not accepted as a complete, correct alternative, because even the people working on it acknowledge that it's nowhere near complete.

How interesting and worthy of study are its predictive abilities, then?

As interesting and worthy of study as any other theory in the field. It may turn out to be completely wrong, but we won't know that until people actually work through all the maths and compare their predictions to real measurements.

Shouldn't there be an explanation for why it can make correct predictions?

It makes some reasonable predictions, and totally fails to make others. No explanation is needed for that beyond simple coincidence, or the fact that it's been specifically designed to explain existing observations. If it can manage to consistently make better predictions than established theory across the whole of cosmology and not just a few specific cases, things will start getting really interesting. Until then, it will remain just a somewhat interesting idea that has yet to be written off.
 
Thanks for your reply. So, if I'm understanding correctly, MOND isn't accepted as an alternate theory to LDCM theories. How interesting and worthy of study are its predictive abilities, then? Shouldn't there be an explanation for why it can make correct predictions? Or is that not really important?

MOND isn't really a theory - it's just an incomplete idea. For example, it doesn't predict by how much light should bend as it passes near a galaxy or galaxy cluster.

There have been attempts to turn it into a full theory, but at least the attempts I know of are failures. So in my view it's not possible to compare dark matter theories to MOND.

On the other hand it is true that there is a striking observed relation between luminosity and rotation speed which has no known explanation in dark matter theories. One possibility is that the observed relation is a coincidence. Another is that dark matter does explain it for as-yet unknown reasons having to do with the way galaxies form. The third possibility is that something like MOND is correct. I think it's fair to say that no one knows which is the case, but most astrophysicists would lean towards the second option.
 
Same author, new paper

'The bottom line is, I told you so': JWST observations upend standard model of how galaxies form, new study claims

The standard model of galaxy formation predicts that only dim light should be seen from the primitive galaxies that took shape in the first billion years after the Big Bang. The unusually large and bright galaxies detected by JWST bolster predictions made by a rival theory known as modified Newtonian dynamics (MOND). The researchers published their findings Nov. 12 in The Astrophysical Journal.


"What the theory of dark matter predicted is not what we see," study lead author Stacy McGaugh, an astrophysicist at Case Western Reserve University in Ohio, said in a statement. "The bottom line is, 'I told you so.' I was raised to think that saying that was rude, but that's the whole point of the scientific method: Make predictions and then check which come true."
But are supporters of MOND on the record as predicting this (unusually large and bright galaxies in the early universe) beforehand?
 
The whole Dark Matter vs MOND thing might not even really be a thing. A few somewhat recent developments worth noting:

1. A Keplerian decline in the Milky Way's rotation curve shows no need for dark matter or MOND.


2. Reanalysis of the MACHO constraints on PBH in the light of Gaia DR3 data

"together with the rate of expected events according to the new density profile, we find that the Dark Matter halo could be composed up to 100% of massive compact halo objects"


We're finding enough rouge planets and the like that there's isn't as great a need for exotic dark matter.

Check out this video at 7:18, Euclid is detecting MACHOs:


3. Vanishing stars could be transformations to black dwarfs


If you're pretty confident that the universe itself is only three times older than the Earth, you could be pretty certain there hasn't been enough time for stars to form and effectively burn out, becoming what's called a black dwarf.

That could be wrong though, and galaxies could be filled with dead stars.
 
If you're pretty confident that the universe itself is only three times older than the Earth, you could be pretty certain there hasn't been enough time for stars to form and effectively burn out, becoming what's called a black dwarf.

That could be wrong though, and galaxies could be filled with dead stars.
The 'problem' with that idea is that it doesn't match up with the Big Bang theory and what we know about the life cycles of stars. But I think you may be right, and the theories are wrong. I suspect the Universe is actually a lot older than 13.5 billion years, which would give plenty of time for normal 'dark' matter to accumulate. I also suspect that the CMB wasn't caused by a hot singularity, but developed over much a longer timeframe too.

IOW, current scientific thought is the equivalent of Young Earth Creationism. The Big Bang theory was attractive because it posited a dramatic 'beginning' to the Universe that wasn't 'too' far into the past, very much like the biblical creation story. Then we had to develop wacky theories to explain stuff that doesn't fit the paleological data, much like creationists do. This suggests the influence of religion on science is stronger than most of us realise.

To get closer to the truth we need more and better observations, and a willingness to accept what they tell us. And that means going in with an open mind, not just looking for stuff to confirm your preconceptions. The so-called 'crisis in cosmology' is real. Both MOND and Dark Matter just throw in an arbitrary exotic effect to fit the data. Then pressure is put on to 'discover' a new particle or properties of gravity that probably don't exist. Meanwhile actual obsevations are finding surprising things that don't support either theory. There is so much we don't know. It seems foolish to assume some supposed exotic effect, when the correct reponse should be 'we need more data'.
 
IOW, current scientific thought is the equivalent of Young Earth Creationism.
While I somewhat agree with the rest of your post, this is way off base. Current theories may well turn out to be wrong. But not like Young Earth Creationism is wrong. If you want to overthrow the current leading theory, you need to come up with a better theory to replace it. That already happened a long time ago for YEC proponents.
To get closer to the truth we need more and better observations, and a willingness to accept what they tell us. And that means going in with an open mind, not just looking for stuff to confirm your preconceptions. The so-called 'crisis in cosmology' is real. Both MOND and Dark Matter just throw in an arbitrary exotic effect to fit the data. Then pressure is put on to 'discover' a new particle or properties of gravity that probably don't exist. Meanwhile actual obsevations are finding surprising things that don't support either theory. There is so much we don't know. It seems foolish to assume some supposed exotic effect, when the correct reponse should be 'we need more data'.
I agree. Although trying out different hypotheses is part of the process to arrive at a better theory. But more data and better data is even more important. Eventually someone will come along (hopefully) with a new testable hypothesis that works better.
 
Puppycow posted about this story in the JWST thread:


If you look at the edges, they are dark, and blocking the light.

Galaxy's are supposed to be more "primordial" near the edges, because the supernovae that enrich galaxies with heavier elements than hydrogen and helium take place closer to the center.

What's all that crud around the edge? Hydrogen and helium?
 
Puppycow posted about this story in the JWST thread:


If you look at the edges, they are dark, and blocking the light.

Galaxy's are supposed to be more "primordial" near the edges, because the supernovae that enrich galaxies with heavier elements than hydrogen and helium take place closer to the center.

What's all that crud around the edge? Hydrogen and helium?
It was previously thought that the Sombrero was a single galaxy, but this picture has changed the view to being a picture of a larger dust-free elliptical galaxy that recently had a collision with a smaller dust-filled spiral. This explains the gap between the core and the dust.
 
Puppycow posted about this story in the JWST thread:


If you look at the edges, they are dark, and blocking the light.

Galaxy's are supposed to be more "primordial" near the edges, because the supernovae that enrich galaxies with heavier elements than hydrogen and helium take place closer to the center.

What's all that crud around the edge? Hydrogen and helium?
Is your galaxy suffering from Galactic Crud? "Galactic Turtle Wax" will make it all bright and new again! Be the envy of the other galaxies in your cluster, use "Galactic Turtle Wax" today!
 
Ok, maybe someone can tell me this.

If dark matter exists and interacts with normal matter via gravity only, what would happen to dark matter in the vicinity of a black hole? Would dark matter contribute to an accretion disk around the black hole, and if so, would it create any visible effects? And would hawking radiation be a way to convert dark matter absorbed by the BH to something non-dark?
 
If the Sombrero galaxy were viewed face-on instead of edge-on, would we even notice the encircling ring of dust?
And if the 'dust' was relatively large lumps of rock, would they radiate the same? I suspect most or all of this 'dark' matter is just normal matter that we can't see because our instruments aren't good enough.
 
While I somewhat agree with the rest of your post, this is way off base. Current theories may well turn out to be wrong. But not like Young Earth Creationism is wrong. If you want to overthrow the current leading theory, you need to come up with a better theory to replace it. That already happened a long time ago for YEC proponents.
There was a time when we didn't have the tools to date the actual age of the Earth, let alone the Universe. Tectonic plates weren't accepted until the late 1960's, and all the books I read before then didn't mention it. The prevailing theory was that migration between continents occurred over 'land bridges'.

Yes, the religiously motivated 'Young Earth' theory was disproved years ago - because we now have solid evidence that it's much older. But that doesn't apply to the Universe. The Big Bang theory is largely fitted to the data we do have, with a speculative rapid expansion at the 'beginning'. But now we are beginning to get data that's not fitting the theory, much like stuff was discovered about the Earth (fossils, geographical features etc.) that cast doubt on its supposed age.

So what I'm saying is that the situation is like back in the 1800's, before we had radiometric dating, Darwin's theory of evolution etc. In 1862 Lord Kelvin used recently developed heat theory to estimate that the Earth was between between 20 million and 400 million years years old. But there were a lot of factors he didn't know about, making it little more than a guess. This allowed Young Earth proponents to keep their religious-based 6-10,000 thousand years old theory alive for another century - helped of course by the pervasiveness of Christianity at the time. Not that much different from the Big Bang theory today, which is believed by most on 'faith' alone.
 
Ok, maybe someone can tell me this.

If dark matter exists and interacts with normal matter via gravity only, what would happen to dark matter in the vicinity of a black hole? Would dark matter contribute to an accretion disk around the black hole, and if so, would it create any visible effects? And would hawking radiation be a way to convert dark matter absorbed by the BH to something non-dark?
Yes, dark matter will contribute to accretion discs, but there is no sign that it heats with friction, or that it gives off photons like a blackbody.

Whether Hawking radiation could convert dark matter to other matter depends on the old question whether information is lost in a black hole. The simple explanation (on my level of understanding) is that Hawking radiation only draws energy from the black hole, with no information about what conditions is inside, or what went into the black hole.
 
And if the 'dust' was relatively large lumps of rock, would they radiate the same? I suspect most or all of this 'dark' matter is just normal matter that we can't see because our instruments aren't good enough.
I believe that normal matter in the form of lumps of rock can be ruled out. The huge amounts that we are talking about would be visible through its radiation that is dependent on temperature, and they would be heated by collisions and radiation.
 
Yes, dark matter will contribute to accretion discs,
No, it would not.

Accretion disks form because different bits of ordinary matter orbiting a black hole in non-coplanar orbits are going to collide. This leads to a loss of angular momentum until their orbits become coplanar about the average. But those collisional interactions aren't gravitational, they're primarily electromagnetic.

Dark matter doesn't interact electromagnetically, so orbiting dark matter particles won't collide with each other or with ordinary matter. So they won't coalesce into the accretion disk. They will remain a halo around the black hole, just as dark matter forms a halo around galaxies rather than collapsing into the galactic disk.
 
No, it would not.

Accretion disks form because different bits of ordinary matter orbiting a black hole in non-coplanar orbits are going to collide. This leads to a loss of angular momentum until their orbits become coplanar about the average. But those collisional interactions aren't gravitational, they're primarily electromagnetic.

Dark matter doesn't interact electromagnetically, so orbiting dark matter particles won't collide with each other or with ordinary matter. So they won't coalesce into the accretion disk. They will remain a halo around the black hole, just as dark matter forms a halo around galaxies rather than collapsing into the galactic disk.
Thanks for clarifying.
 
Got any good examples of these rings in face-on galaxies?
Just search for “ring galaxies”. Hoag’s Object is a particularly beautiful one:
06DB0400-1A72-48BC-BA9A-990071166132.jpeg
Notice that you can see another ring galaxy inside the ring at top.

This could be how the Sombrero would look from above the ring.
 
Just search for “ring galaxies”. Hoag’s Object is a particularly beautiful one:
View attachment 57951
Notice that you can see another ring galaxy inside the ring at top.

This could be how the Sombrero would look from above the ring.
That ring is made of stars though. It's giving off light.

You don't see the ring of dust like you do edge on, because in that case the dust is blocking light.
 
That ring is made of stars though. It's giving off light.

You don't see the ring of dust like you do edge on, because in that case the dust is blocking light.
Nevertheless, this is how the Sombrero would look from that angle. All galaxies with dust have stars mixed up within the dust. Just look at spiral galaxies seen from the side, and you can see the stars in the Sombrero too, even if the dust is thicker.
 
Ok, but if the Sombrero galaxy looked that way face-on, its outer right of dust would be invisible.
 
Ok, but if the Sombrero galaxy looked that way face-on, its outer right of dust would be invisible.
Obviously you see more dust with James Webb, which is the point of having an infrared telescope. But it doesn’t change the fact that we now see the Sombrero as a ring galaxy, and we already know many ring galaxies.
 
But are supporters of MOND on the record as predicting this (unusually large and bright galaxies in the early universe) beforehand?
It depends very much on what you consider a prediction. What is commonly said was that they expect galaxies to form “faster than CDM”. But that is pretty useless, especially given different galaxy formation models within LCDM disagree by over an order of magnitude. There are no luminosity functions, or detailed galaxy formation models to turn to. There are some early papers by Sanders (e.g. 1999, 2007) who estimated “galaxies with masses above 10^11 solar masses collapsing by z=10”. But this doesn’t mean much. Firstly, there is no estimate of the number density of such galaxies, are these everywhere or is it one per universe? Not just that, but these models are incredibly simplistic. For a start they are only simulating gravity, they are essentially treating the matter as collisionless dark matter. They do not include pressure, or real physical processes like star formation and feedback. So the masses quoted are total masses, not the stellar masses (or UV magnitudes) estimated in observations. They aren’t realistic, and they aren’t even comparable to the data. You might ask why the modelling side hasn’t progressed in 20 years, it shows the level of interest from proponents. One has to remember that these same calculations fail to match lots of other things, and you cannot self-consistently model cosmology in MOND.

Also, when looking back at the history of these claims, one can also see another “successful prediction” by the same MOND papers. That reionization happened very early. LCDM estimated the redshift to be around 7. The year 1 WMAP results placed it about z=17. McGaugh touted this as evidence for MOND. But what happened after that is that the measurements improved dramatically, early reionization went away, and its redshift steadily decreased with time. Planck puts the value at z=7.67 ± 0.73. People now talk about reionization ending late.

https://arxiv.org/pdf/astro-ph/0312570

This was heralded by McGaugh as a major test, and yet today it has been deleted from their discussion. The same happened with the CMB. He wrote a paper on how the CMB will “clearly distinguish” between LCDM and MOND, and interpreted early results in MOND's favour with much jubilation. But then the data got better, MOND failed the test and LCDM passed. McGaugh has done this “I told you so” thing three times, and the previous two times he turned out to be wrong. In this case, there are some vague predictions, which can't really be compared to anything measured.
The whole Dark Matter vs MOND thing might not even really be a thing. A few somewhat recent developments worth noting:

1. A Keplerian decline in the Milky Way's rotation curve shows no need for dark matter or MOND.
Which is not what the paper says at all. If the observations are taken at face value, it reduces the mass of DM needed, but it’s still non-zero. The bigger problem is these results conflict with other measurements of the Milky Way’s rotation curve, follow-up papers have pointed out that in the method the outer-radii are highly correlated. It has been shown that such correlation can lead to underestimating the mass by a factor of 2, and that the modelling needed to calculate the rotation curve may not be robust.

Galaxy's are supposed to be more "primordial" near the edges, because the supernovae that enrich galaxies with heavier elements than hydrogen and helium take place closer to the center.
This is not the case. Anywhere you see stars has been polluted by supernovae, the stars you see formed with massive siblings which are long dead. The metallicities are lower further out, but still significantly enriched.
 
For somebody who isn't anti-science, Hossenfelder sure does pump out a lot science hostile bovine excrement.

How about some examples?

If anything, she may be a bit more forth-coming on calling BS on other research than others.

Do you think calling BS on, eg, quantum computing claims, is the same as pumping out BS?

FWIW, here's Dr. Becky's take:

 
Obviously you see more dust with James Webb, which is the point of having an infrared telescope. But it doesn’t change the fact that we now see the Sombrero as a ring galaxy, and we already know many ring galaxies.
A, that's not a fact. Some interpret the Sombrero as a ring galaxy.
B. total non-sequitur: the type of galaxy is irrelevant. It's that we see the crud better from edge-on which is basically invisible face-on.
 
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