A few posts back, the issue of the Bullet Cluster came up in the comments. I was still working my way up to addressing that in a full post, so I limited myself to making a sociological observation based on my experience:
People who invoke the bullet cluster in response to queries about MOND are usually doing so to deflect from the need to engage with it.
That is a general observation that was not aimed at anyone in particular, but it was made in response to a comment citing Don Lincoln saying this, so Dr. Lincoln hopped into the comments to reply:
Or…and bear with me here…some people actually find the Bullet Cluster (and the DF2/DF4 situation) to be persuasive.
Don Lincoln, posting as Science Guy, 4 June 2026
FWIW, I’m a physicist, working in the field. And my views have changed over the years. In the early 1990s, I was pretty sure that dark matter was MACHOs. When MACHO, OGLE and all them disproved that conjecture, I then strongly favored MOND (broadly defined…not necessarily Milgrom’s conjecture, but rather the much vaguer paradigm that inertia or gravity needed some improved understanding). However, in the modern day, Bullet and Dragon Fly have again changed my leaning.
Yes, yes, the Bullet Cluster is moving awfully fast. Yes, yes, it’s a problem for LCDM. But this isn’t the same level of problem as it poses for modified physics. For LCDM, it is simply an unusual entry on the tail of a known velocity distribution, while the problems it poses for MOND-ish issues is more central.
Now, this is your page and you are allowed to have an echo chamber and sycophants…no problem. But to characterize those who disagree with you as somehow not being thoughtful is just…sloppy. And dismissive. And patronizing.
The broad community might be wrong. But, you know what? So could you.
FWIW, it will be difficult to convince me of >>ANY<< solution without DM being produced in particle accelerators. Indirect measurements are background-prone. And direct measurements, while super helpful, will tell us where to look in accelerators. (Think the DAMA debacle.)
My fear is that dark matter is real, but it only interacts gravitationally or far weaker than the weak force. If that’s the case, our grandkids will be having this argument.
In my experience, Dr. Lincoln is one of the more reasonable people connected to this debate. He says some things that are fair, and also some things that are revealing of the current sociology, so it is worth exploring point by point.
Or…and bear with me here…some people actually find the Bullet Cluster (and the DF2/DF4 situation) to be persuasive.
The sentence starts with sociology: the “bear with me” trope is an assertion of reasonableness before it is demonstrated, which it may or may not be – often it is employed by people who think they’re being reasonable when really not so much. But in this case, yes: lots of people find the Bullet Cluster to be persuasive. My complaint is not that. It is that they cite the Bullet Cluster as an excuse to not think further about MOND and stifle debate. That has been my lived experience.
The Bullet Cluster achieved the status of a totem object long ago:
I myself find clusters persuasive. As I’ve written repeatedly, they pose a real problem for MOND. The Bullet Cluster is just one example, and an extremely weird case at that. The universe is big, so there’s always a unicorn somewhere: astronomers long ago learned not rely too much on the weirdest object in a category. So I’m more impressed when people cite clusters in general as a problem for MOND, both because that’s true, and because it evinces awareness of the subject beyond the totem that has become the go-to code word for dismissing a predictively successful paradigm without understanding it.
Dr. Lincoln also cites “the DF2/DF4 situation.” I’m not sure why that comes up if the Bullet Cluster by itself is entirely persuasive. This is a topic very much in my expertise, and I do not find it all that persuasive. I spent some time reading up on this situation – the data keep changing – in hopes of saying something quantitative here, and that left me feeling it was even less persuasive than I had given it credit for.
The DF2/DF4 (and now DF9) galaxies have become prominent examples of galaxies that appear to lack dark matter. Such objects are a problem for MOND, because you can imagine stripping away dark matter (though it is hard to do) but you can’t switch off the force law. However, the prominence of these particular objects has more to do with advertising (a ridiculous amount of attention has been devoted to these few weird objects) than with how convincing they are. That’s not to say they aren’t important, just that their importance is exaggerated.
These DF galaxies are a good example of cognitive dissonance in action. People give more weight to evidence that supports what they already believe, and less to that which supports things that don’t. In this case, the other evidence is all the other galaxies in the universe. This is a classic case of missing the forest for a few outlying trees.
I used to spend a lot of time fact-checking claims# to falsify MOND. So when DF2 was first announced as a problem for MOND with great fanfare, I went to check. It was not. Indeed, had I known of this object’s in advance, I could have used MOND to correctly predict its velocity dispersion as I did with Crater 2 and the 30+ dwarf satellites of Andromeda. This rather quenched my enthusiasm for fact-checking every claim, so I vowed* not to spend more time doing so.
This is a damned-if-you-do, damned-if-you-don’t situation. If incorrect claims are left uncontested, the community seems to assume they are correct. If one spends the time to write a paper, you are diverted from your own research. Once published, much of the community remains unaware of the rebuttal, or chooses to believe their preferred narrative (another example of cognitive dissonance).
So before writing this post, I found myself breaking my vow. These are interesting objects, and there is now a new one (DF9). All three of these dwarfs have unusually large diameters and low velocity dispersions, about 8 km/s. That’s pretty much what we expect for the stars we see. No dark matter, no MOND. Even though it is really weird to find galaxies without dark matter in a universe made of dark matter that requires dark matter to make galaxies, it is even worse for MOND, if true.
There are devils in the details. The data for DF2 have changed repeatedly, both its distance and velocity dispersion. Which version to believe? Working my way through the literature, I found the statement “We find an instrumental resolution σinst = 0.375 Å (13.0 km s−1)” and decided to stop right there. A rule of thumb in this business is that you shouldn’t try to measure a velocity dispersion smaller than your instrumental resolution because, well, you can’t resolve it. 8 km/s is smaller than 13 km/s. Now, in principle, you can tease more information out of the data, but that’s hard to do. In the best case, the two dispersions add in quadrature, so to infer 8 km/s, what you’ve really observed is √(82+132) = 15 km/s. That’s not much different from the instrumental resolution, and I’ve seen plenty of claims where that obscured a correct MOND prediction (e.g., Cetus). For DF2, we expect an intrinsic velocity dispersion of ~13 km/s, which is sensitive to the distance that keeps changing and to the EFE of the rough neighborhood in which these dwarfs find themselves. That corresponds to observing √(2*132) = 18 km/s. So we have to be able to distinguish between 15 and 18 km/s. That can be done, but it is a lot less clean than the difference between 8 and 13 km/s sounds.
There are other corrections for broadening and binaries, so the above is the sanitized version. Binaries are hard to correct for in these unresolved objects. In nearby ultrafaint dwarfs where we measure velocity dispersions one star (or unresolved binary) at a time, the correction can be dramatic. Boötes III provides a rececnt example, having: “a velocity dispersion of σv=1.69+1.03−0.85 km s−1, about six times smaller than the previously reported 10.7±3.5 km s−1.” That takes it from having lots of dark matter, completely inconsistent with MOND, to bang on what MOND predicts. So you can perhaps appreciate my relutance to put too much credence in every claim made about measurements of these ultrafaint/ultradiffuse galaxies. This is the hardest place to work, and my experience has been that as the data improve, so too does agreement with MOND.
Is DF2 even a significant problem? Accepting the updated numbers as stated, corrected (and perhaps overcorrected) to reflect all the above effects, Keim et al. report two independent measurements for DF2 that give 6.3+3.1-3.7 and 9.2+3.8-4.5 km/s. In our paper we found that MOND predicts 13.4+4.8-3.7 km/s. Those are all consistent within the stated uncertainties: the one-sigma error bar of the lower measurement overlaps with that of the prediction,& and the higher measurement is in as good agreement with MOND as could be expected given the uncertainty on both measurement and prediction. Dark matter advocates would be lauding such agreement to high heaven if they could% make this prediction.
That’s a long segue for a parenthetical comment by Dr. Lincoln. It takes a lot of words to address five, and it isn’t for me to judge if the interpretation he seems to take for granted is better or worse than the alternative I describe. Even so, there’s a saying about this that might apply.
OK, let’s return to Dr. Lincoln’s comments. In case you’d forgotten – which I almost did, having spent so much time trying to track down DF2 data – this is a post about the sociology illustrated by Dr. Lincoln’s comments.
my views have changed over the years. In the early 1990s, I was pretty sure that dark matter was MACHOs. When MACHO, OGLE and all them disproved that conjecture, I then strongly favored MOND (broadly defined…)
My views have changed over the years too. In the early 1990s, I was completely sure that the dark matter was WIMPs. Had to be. I remember joking with other astronomers about how futile the search for MACHOs would be. As Rob Kennicutt put it in 1995 at IAU 171: “What are the MACHO people doing? Have they never heard of Big Bang Nucleosynthesis?” I recall shrugging and thinking that these projects wouldn’t detect dark matter, but they would provide a great variable star database. And so it came to pass.
We have here two different recollections of the same time period. Both are valid, but which is a fair representation of the community? Could be both. Gradually I’ve come to recognize that there were [at least] two distinct communities working on this issue, one in physics and one in astronomy. They don’t need to be echo chambers to develop mutually exclusive attitudes; the networks of communication can be broad and yet largely distinct. There is also a temporal aspect. I’m told by astronomers a few years older than myself that baryonic dark matter (particularly brown dwarfs) was favored in the late 1970s; indeed, that it seemed pretty obvious at that time. This changed quickly and was mostly (though never totally) supplanted by non-baryonic dark matter by the mid-1980s. Even that depended on the sub-community – those of us more concerned with cosmology rejected baryonic dark matter as nonviable while it remained reasonable to those of us more concerned with the dynamics of individual galaxies. I had a foot in both camps, and it metaphorically tore me apart.
In the mid-1990s, I was wrestling with our new data for low surface brightness galaxies. It did not make sense in terms of dark matter. Any kind of dark matter. I began to fear that the entire dark matter paradigm was no longer viable, a conclusion I fought tooth and nail to avoid. I worked much harder trying to save dark matter than I ever did subsequently working on MOND. More importantly, I think I was only receptive to MOND^ because I was already deeply concerned for the viability of dark matter. There lies the great schism: to me, dark matter was already practically falsified. No one else had that horrible, visceral experience – it was like losing a dear friend – so most of the community glibly ignored the surprising successes of MOND.
Maybe I was wrong to doubt dark matter? This is why I challenge other scientists to state their own criteria for its falsification. I don’t expect them to accept something so important just because I say so. But I do say so, and for good reasons – reasons most of them seem to be unaware of, so we’re starting from very different places. But if dark matter is a scientifically valid physical hypothesis, then it should be falsifiable. How can we tell if it is wrong?
I’m old enough to remember when cuspy dark matter halos were an absolute prediction of cold dark matter. That prediction failed, yet we find ways for CDM to persist. If we gave up on CDM as easily as we give up on MOND, we would have stopped talking about it thirty years ago.
Right. Where were we?
Yes, yes, the Bullet Cluster is moving awfully fast. Yes, yes, it’s a problem for LCDM. But this isn’t the same level of problem as it poses for modified physics. For LCDM, it is simply an unusual entry on the tail of a known velocity distribution, while the problems it poses for MOND-ish issues is more central.
I agree with this and I don’t. The issue is more central for MOND because a theory that seeks to supplant dark matter appears to need dark matter. So why bother?
It’s a good point, and as I’ve said over and over and over again, I find the situation in clusters profoundly dissatisfactory for MOND. Perhaps it even falsifies it. But a loss for MOND isn’t an automatic win for non-baryonic dark matter (which also requires new physics), so we bother because of everything else MOND does right that dark matter does not.
A common misconception here is that the unseen mass in MOND is necessarily non-baryonic. That’s a logical fallacy that stems from the sloppiness of the term “dark matter” which many of us equate with non-baryonic dark matter. Non-baryonic dark matter requires new physics. MOND requires new physics. So it sounds like we need double-new physics here when in fact we “just” need some undetected baryons$ – something we need in both paradigms:
What about the collision velocity? How bad a problem this is for LCDM depends on how unusual an entry it is in the tail of a known velocity distribution. There are many assessments of this; they range from “pretty unlikely” to “oh hell no.” For example, Lee & Komatsu found “the probability of finding 3000 km s-1 in (2-3)R 200 is between 3.3 × 10-11 and 3.6 × 10-9.” That’s worse than an unusual entry, that’s “oh hell no.” Various other workers hedge this way and that so it comes across as less improbable, but the most optimistic assessment I saw recently (and cannot now relocate) is that there is only a 10% chance of the Bullet Cluster existing in the volume of the universe that contains it. That’s over the whole sky, not just the 5% of the sky that has been surveyed in a way that could find it. So pretty unlikely, but not necessarily fatal. Ascencio et al. attempt to quantify this, finding that the “Bullet Cluster is in 2.78σ tension” with LCDM – that’s bad, but not fatal – but that including El Gordo results in a “combined tension” “estimated as 6.43σ”. That exceeds the usual 5σ threshold for fatal.
There is the obvious temptation to believe the assessment we prefer. There are analyses in which the Bullet Cluster collision speed doesn’t seem all that bad, though it never looks right. I’ve worked on this too, and we found it was pretty much impossible, and it looks to me like the more favorable analyses may be squeezing the toothpaste tube to make the collision lees improbable by shifting that to improbability in other parameters. One I recall requiring a ridiculously perfect, head-on bullseye collision.
In contrast, the high collision speed is entirely natural in MOND. That’s what happens as a consequence of the enhanced long-range force law. It just falls out, no muss, no fuss. So it isn’t just improbable in LCDM; it happens because of MOND, just like every other surprising result for the past forty years. This is why I say galaxy clusters ruin everything. We can’t say they favor one paradigm or the other without under-weighting some aspect of the data.
I guess that was enough science, because now we get to sociology:
to characterize those who disagree with you as somehow not being thoughtful is just…sloppy.
I may be wrong, but I am pretty much the antithesis of sloppy. Sloppy is describing me of characterizing those who disagree with me as not being thoughtful. I do not doubt the intellectual energy of people working on dark matter. I do doubt how deeply most of them have allowed themselves to think about MOND. I know this problem well, as I suffered it myself early on. MOND is too horrible to contemplate. Surely we can’t be that wrong! So we find some reason not to do so. The most common reason such people cite is the Bullet Cluster. It is my lived experience that lots of scientists (though certainly not all) fall into that category. If anybody finds that insulting, then they should do better to not be that person. I find it strange if not surprising that a scientist is offended at being challenged to think more about a topic about which he is, to use his word, dismissive.
FWIW, it will be difficult to convince me of >>ANY<< solution without DM being produced in particle accelerators. Indirect measurements are background-prone. And direct measurements, while super helpful, will tell us where to look in accelerators. (Think the DAMA debacle.)
My fear is that dark matter is real, but it only interacts gravitationally or far weaker than the weak force. If that’s the case, our grandkids will be having this argument.
OK, so this makes sense, but just let me note that it assumes that the solution to the mass discrepancy problem is a novel form of dark matter that can be created in an accelerator. That would be great if it could be done. All plausible dark matter particle candidates for which such a detection is plausible are pretty much excluded at this point, and building a bigger accelerator promises zero surety of success.
More generally, I don’t accept that the solution is a novel form of particle dark matter. That’s very much not in evidence! But I do share his fear of a non-interacting particle. I call that the Angel Particle because, indeed, we can argue about how many angels can dance in the core of a neutron star forever and ever. I have a greater fear, that by attending only to the failings of MOND while ignoring its successes – and a lot of scientists are guilty of that – we help to usher in a new age of dark epicycles.
I will not be available to respond to comments for a while, so I have deactivated them for this post.
#Initially it came as a surprise how often claims to falsify MOND did not hold water. That has since become my experience with the vast majority of such claims. There is a powerful temptation to see in the data what one wants to see.
*For the most part, I’ve kept to this vow. It is hard not to check what MOND predicts when one hears claims that “it can’t do this!” when my experience, over and over again, is that usually it can. Not always, but usually.
&I decline the opportunity to refine the prediction by chasing the continually changing distance estimate. Regardless of how close the current numbers are to the underlying truth, this is certainly not a five sigma exclusion of MOND.
%The prediction cannot be made with dark matter. I’ve tried. Indeed, I’ve tried many things – there are multiple paths by which one might attempt to do this, and they do not yield a consistent answer. Worse, the one thing that is clear is that for low mass galaxies like these, there is a lot of scatter in DM halo mass at a given luminosity. The natural prediction from this is that galaxies of the same (low) luminosity have very different velocity dispersions. Yet the opposite is observed; luminosity is strongly predictive of kinematics. So we can fault MOND for predicting 3 km/s for Antlia 2 when 6 km/s is observed because it makes a prediction. We don’t fault LCDM because it does not make a comparably precise prediction to test.
^Unlike Dr. Lincoln, I mean MOND specifically. I considered other modified gravity theories, but they fail. It is very obvious when the force law is wrong. Only MOND worked at the time and continues to work today. That it even comes close is telling us something profound, a lesson the community steadfastly refuses to learn.
$I can hear from across the ocean Dr. Kroupa shouting that they’ve solved this problem.
