I see that I’ve been posting once a month so far in 2026. I’ve lots to say but no time to say it. Some of it good, some of it bad, maybe sometime I’ll get around to it. No guarantees. On the good side, I’ve been working on a big project or two; may have something to say about those soon. I’ve also been meaning to write about the Planet 9 anomaly for months stretching into years now. Fascinating stuff related to MOND but not something I’ve worked on myself. On the bad side, I’ve been obliged to waste yet more time on my university administration’s insistence on merging our department into physics based on a snap decision made by a disinterested leader who employed all the forethought typically reserved for bombing a random country in the Middle East.
So I have had no time for novel posts lately, and today is no different. However, I thought readers of this blog would appreciate the post Paradigm Shifts in Modern Astrophysics: Applying Thomas Kuhn’s The Structure of Scientific Revolutions to Dark Matter at Heritage Diner that was pointed out to me by Moti Milgrom. Since I wouldn’t have seen it had he not mentioned it, perhaps that’s the case for you as well. I’m not gonna re-post it verbatim – you can read it there yourself – but I am going to offer a running commentary with a few observations, both personal and historical. So bring it up in a separate browser window and let’s read along…
This post riffs off of Kuhn’s The Structure of Scientific Revolutions as it pertains to dark matter and MOND. If you’re not familiar with it, Kuhn’s work on the philosophy of science is foundational to the way in which a lot of physical scientists approach their field (whether they realize it or not). Philosophers of science have done a lot more since then, but I’m not going to attempt to go there. I will look back to Popper* to note that I’ve heard Kuhn depicted as being some sort of antithesis to Popper. I don’t see it that way. To be pithy, Popper tells us how science should be done while Kuhn tells us how it is done. Who could have imagined that a human endeavor would be messy in practice and not always live up to its ideal?
I’m not sure how to do this; I guess I’ll excerpt relevant quotes and riff off those. The basic thesis is that dark matter is on the brink of a Kuhnian paradigm shift.
We are living through exactly that moment in modern astrophysics.
I certainly hope so! This moment in the history of science is taking a long damn time. A century ago, we went from “classical physics explains everything” to “quantum mechanics, WTF?’ in the space of about a decade. I’ve been working on matters related to MOND for over thirty years now, dark matter longer than that, and of course Milgrom started more than a decade before I did.
The essay discusses the “cartography of collapse,” which includes crisis and revolution:
The third stage is crisis — triggered when anomalies accumulate beyond the paradigm’s absorptive capacity. And the fourth is revolution, in which a new framework displaces the old not through incremental persuasion but through a gestalt shift, what Kuhn famously described as seeing the same duck-rabbit drawing and suddenly recognizing a rabbit where you had always seen a duck.
This resonated with me because I had exactly this experience. I started my career as much a believer in dark matter as anyone. I was barely aware that MOND existed (this seems to remain a common condition). But it reared its ugly head in my own data for low surface brightness galaxies. Try as I might – and I tried mighty hard, for a long time – I could not reconcile how the shapes of rotation curves depended on surface brightness as they should according to Newton while simultaneously lying exactly on the Tully-Fisher relation without any hint of dependence on surface brightness+. I could explain one or another, but not both simultaneously – at least, not without engaging in some form of tautology that made it so. I came up with a lot of those, and that has been a full-time occupation for many theorists ever since.
For me, this gradually became a genuine crisis. I pounded my head against the wall for months. Then, as I was wrestling with this problem, I happened to attend a talk by Milgrom. I almost didn’t go. I remember thinking “modified gravity? Who wants to hear about that?” But I did, and in a few short lines on the board, Milgrom derived from MOND exactly the result I found so confusing in terms of dark matter. This chance meeting in Middle Earth (Cambridge, UK) changed how I saw the universe. The change wasn’t immediate – it had to ferment a while – but ultimately I found myself asking myself over and over how this stupid theory could have its predictions come true when there was so much evidence for dark matter. Finally I realized that the evidence for dark matter assumes that gravity is normal; really it was just evidence of a discrepancy, and it could be that the assumption was at fault. That realization was sudden: where I’d always seen a duck, suddenly I could also see a rabbit.
Most scientists have not had this experience. What constitutes a crisis serious enough to contemplate a paradigm change is a highly personal matter of judgement. It happened in my data, so I took it seriously, but others didn’t care. So I made predictions for their data. Some of those came true, but they rejected the evidence of their own data. It just could not be so! At what point does a mere problem amount to a true anomaly?
Part of the sociological issue is that the dark matter paradigm has been in a constant state of crisis since its inception. The reasons vary over time. Sometimes valid solutions have been found to the crisis du jour, other times we’ve chosen to just live with it. It is much easier to live with a bad solution than to rethink one’s entire world view.
The problem with being in a constant state of crisis makes is that it seems like nothing can ever be a genuine crisis. Every foundational change is just another new normal. We complain, say it can’t be so, argue, offer bad ideas, reject them, get used to them, then eventually accept that one of them maybe isn’t so bad, so that must be what is going on. After a few years It is Known and people convince themselves that we expected just that all along.
It takes a lot of evidentiary weight for a paradigm to change, and it takes a lot of time for that to accumulate. But, as Kuhn recognized, mere facts are not enough. Humans and their attitudes matter. As Feyerabend noted,
The normal component [i.e. the accepted paradigm and its adherents] is large and well entrenched. Hence, a change of the normal component is very noticeable. So is the resistance of the normal component to change. This resistance becomes especially strong and noticeable in periods where a change seems to be imminent.
P. Feyerabend in Criticism and Growth of Knowledge
The post correctly points out that dark matter itself was an anomaly going back to Zwicky in 1933. This is often depicted^ as the first detection of dark matter, but it was also noted by Oort in 1932. Zwicky was aware of Oort’s work and cited him, but they’re very different results. Oort was worried about a factor of ~2 discrepancy in stellar dynamics in our local chunk of the Milky Way; Zwicky discovered a discrepancy of a factor of ~1000 in the Coma cluster of galaxies. These both imply the need for unseen mass, but the results are not at all the same. In retrospect, Oort’s discrepancy is a subtle detection of a flat rotation curve while Zwicky’s discrepancy was (at least) two distinct discrepancies: what we now consider the usual cosmic dark matter, but also missing baryons: most of the normal matter in clusters is in the hot, diffuse intracluster medium, not in the stars in the galaxies that Zwicky could see and account for. The modern discrepancy is only a factor of ~6, which is rather less than 1,000. (The distance scale also played a role in exaggerating Zwicky’s result.)
This all seemed crazy in the 1930s, even in the immediate aftermath of the quantum revolution. Consequently, Zwicky’s work was mostly ignored$. The subject of dark matter didn’t really take off until the 1970s. Considerable credit goes (rightly) to Vera Rubin, though many others made essential contributions – just on the subject of rotation curves, Albert Bosma, Mort Roberts, and Seth Shostak all made important contributions, the relative importance of which depends on who you ask.
An important aspect of scientific revolutions is persistence. Vera was persistent. She was fond of relating the story of showing her first (1970) flat rotation curve of Andromeda to Alan Sandage, only to have him dismiss it as “the effect of looking at a bright galaxy.” What the heck did that mean? Nothing, of course – it is the sort of stupid thing that smart people say when confronted with the inconceivable. So Vera persisted, and by the end of the decade had shown that flat rotation curves were the rule, not some strange exception. They became accepted as a de facto law of nature, and the dark matter interpretation was solidly in place by 1980.
The scientific community absorbed this anomaly not by questioning Newtonian gravity or Einstein’s general relativity, but by proposing an invisible scaffolding — a halo of non-luminous, non-interacting matter surrounding every galaxy. Dark matter became not a crisis but a patch.
Indeed, this seemed the most appropriate (scientifically conservative) course of action at the time, as summarized in this exchange (also from the early 1980s):
To emphasize the essence of what is said here:
Tohline: I might be so bold as to suggest that the validity of Newton’s law should now be seriously questioned.
Rubin: The point you raise is worth keeping in mind although I believe most of us would rather alter Newtonian gravitational theory only as a last resort.
This was a very reasonable attitude, at the time. But I’ve heard the phrase “only as a last resort” many times now over the course of many years from many different scientists. At what point have we reached the last resort? In the case of dark matter, once we’ve convinced ourselves that invisible mass has to exist, how can we possibly disabuse ourselves of that notion, should it happen to be wrong?
In Kuhnian terms the last resort is reached when the weight of anomalies in the standard paradigm become too great to sustain. But that point is never reached for many die-hard adherents. Whatever the right answer about dark matter turns out to be, I’m sure many brilliant people will go to their graves in denial. Hence the more cynical phrase
Science progresses one funeral at a time.%
But does it? What if the adherents of an ingrained but incorrect paradigm breed faster than they go away? I’ve seen True Believers train graduate students who’ve gone on to train students of their own. Each generation seems to accept without serious examination the inadequate explanations for the anomalies made by their antecedents, so the weight of the anomalies doesn’t accumulate; instead, each one gets swept separately under the proverbial rug and forgotten. Forgetting is important: when new anomalies come to light, hands are waved and new explanations are promulgated; no one chekcs if the new explanations contradict the previous generation of explanations. What passed before is a solved problem, and we need never speak of it again.
This is not a recipe for a scientific revolution, but for a thousand years of dark epicycles.
Returning to the post,
By the late 1980s and early 1990s, dark matter had been formally incorporated into the reigning cosmological framework. Lambda-CDM — where Lambda refers to the cosmological constant (a proxy for dark energy) and CDM stands for Cold Dark Matter — became the standard model of cosmology.
The essence of this statement is correct but some of the details are not. Dark matter was widely accepted by 1980. That’s still a little before my time, but my impression is that the magnitude of the discrepancy was at first a factor of two, so it could simply have been normal baryons that were hard to see. However, the discrepancy rapidly snowballed to an order of magnitude, so we needed something non-baryonic. This was happening simultaneously with talk of supersymmetry and grand unified theories in particle physics that could readily provide new particles to be candidates for the dark matter, leading to the shotgun marriage of particle physics and cosmology, two communities that had had little to do with each other before then, and which still make an odd couple. Cosmology as traditionally practiced by astronomers needed dark matter but didn’t much care what it was; particle physics was all about the possibility of new particles but didn’t care about the details of the astronomical evidence.
To rephrase the above quote, I think it is fair to say that “by the late 1980s and early 1990s, cold dark matter had been formally incorporated into the reigning cosmological framework.” But that framework was not yet LCDM, it was Ωm = 1 SCDM. The Lambda only came to prominence by the end of the 1990s, as I’ve related elsewhere. This process is depicted by many scientists as a revolution in itself, and in many regards it was. The cosmological constant had been very far out of favor; rehabilitating it was a grueling experience and no trivial matter. But it wasn’t really a scientific revolution in the sense that Kuhn meant: our picture didn’t fundamentally change, we just learned to accept a parameter& that was already there but that we didn’t like.
The post goes on to note the absence of dark matter detections:
This silence is itself an anomaly… as the silence deepens, the null result itself becomes harder to dismiss.
This is correct, and yet… Physicists have built many experiments that have achieved extraordinary sensitivities. If cold dark matter was composed of WIMPs as originally hypothesized, we would have detected them long ago. Initially, the reaction was to modify WIMPs. Did we say the cross-section would be 10-39 cm2? We meant 10-44 cm2. When that was excluded, we slid the cross section still lower, but people also started giving themselves permission to think the unthinkable. By unthinkable I mean a particle that can’t be detected, not modified gravity. That’s more unthinkable. So the anomaly isn’t dismissed, but it is treated with less gravity than it should be, and certainly with less import than a positive detection would have been granted. Did we say WIMPs? We didn’t mean just WIMPs. It could be anything. (They damn well meant WIMPs and only WIMPs#. Anyone who tells you otherwise is gaslighting*% you, and probably themselves.)
The post goes on to talk about MOND. It gives me too much credit for the gravitational lensing work. This was done by Tobias Mistele, and our work is based on that of Brouwer et al. But it is correct to note that these data are a problem for the dark matter paradigm. Rotation curves remain flat beyond where dark matter halos should end. If correct, this is a genuine anomaly. Perhaps in some distant future it will be recognized** as such in retrospect; at present it seems mostly to be ignored.
It goes on to talk about the JWST observations. Yeah, that part is correct. The community seems to be in the usual process of gaslighting itself into denial of the anomaly. For the first two years after JWST started returning images of the deep universe, people were aghast. How can this be so? It was all anyone could talk about. But then the unexpected became the new normal. Hands were waved, star formation was accepted to be absurdly efficient, and people accepted the impossible. I no longer hear the talk of how problematic the JWST observations are; this chatter simply stopped.
Anomalies don’t weigh a paradigm down if we don’t accept that they’re anomalies. But I’ve lived through the revolution, it’s hard to see a positive outcome while it is still ongoing. For it is certainly true that
What waits on the other side of the dark matter revolution — if that is what is coming — we cannot yet know.
The future is the unknown territory. We don’t know, and can never know, if dark matter doesn’t exist – it is impossible to prove the negative. But we do know MOND works much better than it should in a universe made of dark matter. That demands a scientific explanation that is still wanting. But MOND by itself is not a complete answer, so we are like the parable of the blind men and the elephant, each sensing a different part of reality but as yet unable to see the whole.
Still, there is reason for optimism. The article closes by noting that
Kuhn’s deepest insight was not that science changes. It is that the change, when it comes, is never merely technical. It is a reorganization of the world itself — the universe seen suddenly whole in a configuration it has always had, but that we had simply lacked the paradigm to perceive.
Not knowing how things ultimately work out is good, actually. One way or the other, there is still fundamental science to be done. We have not reached the stage of looking for our discoveries in the sixth place of decimals.
*Trivia I just learned looking at Popper’s wikipedia page: he was spending his last days in London around the same time I was a postdoc in Cambridge just starting to struggle with the scientific and philosophical implications of the dark matter-MOND miasma.
Unrelated trivia: I was at a workshop in Jerusalem early in the century but missed the opportunity to meet Jacob Bekenstein because I was too shy to bother the great man.
+If you do not find this confusing, you are not thinking clearly.
^A nice, brief summary of this early history is related by Einasto. This is the first place I’ve seen the citation to Opik (1915) written out. I’ve only heard mentioned verbally before, so I’ll have to try looking that up later.
The full story is way more complicated than this sounds, and still gets debated off and on. The amplitude of the Oort discrepancy is much smaller today. Locally, the 3D density of mass seems to be accounted for by known stars, gas, and stellar remnants (which were still a new thing in the 1930s). So this Oort limit shows no discrepancy. There remains a modest discrepancy in the 2D dynamical surface density. It appears to me to boil down to the vertical restoring force having a (sometimes ignored) term that depends on the gradient of the rotation curve. Were that falling in a normal Newtonian way, there would be no discrepancy. But it isn’t; this deviation from Newton in the radial direction leads to the Oort discrepancy in the vertical direction. Instead of being as negative as Newton predicts, dV/dR is close to zero, hence my description of this as an indirect detection of a[n almost] flat rotation curve. (dV/dR = -1.7 km/s/kpc, so not exactly zero, but a lot closer to zero than Newton without dark matter would have it be.) The vertical discrepancy is nevertheless much reduced, now being well below a factor of two.
$To his apparently great embitterment. He had some choice things to say about astronomers of his time. I am inclined to suspect that those who praise Zwicky the loudest today would have been among those he had reason to complain about had they been contemporaries.
%This is attributed to Planck, but he had a lot more nuanced things to say about it in his Nobel Prize lecture.
&Einstein disavowed the cosmological constant as his “greatest blunder,” so one argument against it was (for a long time) that it should never have been a part of the theory of General Relativity in the first place. I wonder how things might have gone had that been the case – that he had never introduced Lambda. Perhaps then the data that led to us accepting Lambda would have required a genuine revolution, but it isn’t obvious that we would have accepted it (we might still be debating it), nor is it apparent that LCDM is what comes out of such a revolution. But we don’t get to do that experiment: the Great Man had suggested Lambda, so it was OK to bring it back: we weren’t wrecking his theory by introducing a crazy new entity, we were just admitting an unlikely (antigravity-like) component thereof.
#Or axions! Or warm or self-interacting dark matter. Or macros nee strange nuggets! Or or or… Sure, there have been lots of ideas for what the dark matter could be. But when we say that “by the late 1980s and early 1990s, cold dark matter had been formally incorporated into the reigning cosmological framework” what the vast majority of scientists working on the topic (including myself) meant was that CDM == WIMPs. We were aggressively derisive of other ideas, and these are only dredged up again now because of the experimental non-detection of WIMPs. WIMPs are still a better dark matter candidate than the others for the same reasons that we were derisive of the others back in the day. We haven’t been looking as hard for the others, so comparable experimental limits do not yet exist. To quote myself,
The concept of dark matter is not falsifiable. If we exclude one candidate, we are free to make up another one. After WIMPs, the next obvious candidate is axions. Should those be falsified, we invent something else. (Particle physicists love to do this. The literature is littered with half-baked dark matter candidates invented for dubious reasons, often to explain phenomena with obvious astrophysical causes. The ludicrous uproar over the ATIC and PAMELA cosmic ray experiments is a good example.)
McGaugh (2008)
*%An easy way to deflate such gaslighting is to ask why so many experiments have been built to search for WIMPs but not all these other allegedly great dark matter candidates. After a pause and dismayed stare, you’ll probably get an answer about “looking under the lamp post” because that’s where it is possible to make detections. That’s sorta true, but it isn’t the real reason. The real reason is that we all drank the Kool-Aid of the WIMP miracle, so genuinely believed that the dark matter had to be WIMPS, not merely that they were a convenient experimental target. (I did not chug the kool-aid as hard as the people who based entire careers on building WIMP detection experiments, but I did buy into the idea to the exclusion of other possibilities for dark matter – as did most everyone else.)
**In retrospect, Galileo’s observations of the angular size and phases of Venus were utterly fatal to the geocentric paradigm. That’s easy to say now; at the time it was just another piece of evidence.
The JWST alarm was predictable the moment you recognize that a model built on multiple free parameters tuned to existing data was never actually being tested, it was being fitted. JWST is just an honest sample test and the results showed exactly what rigorous testing of an assumed model looks like. At this point even the CMB blackbody spectrum isn’t a unique signature of metric expansion anymore either. The field needs to be honest about what is “proof” and what is an “assumption”.
While Thomas S Kuhn’s ” The Structure of Scientific Revolutions” is his most famous book it is not actually the best illustration of his thesis. There is a point in science where people tend to stop reading the original papers and begin to rely on textbooks, or survey articles. Kuhn explored this for the beginnings of Quantum Theory in his book “Black-Body Theory and the Quantum Discontinuity 1894-1912”. For this book Kuhn went back and re-read Planck’s original work on Black-Body Theory and realised that it did not require quantised oscillators, something that was pointed out first by Ehrenfest, then by Einstein, and later by Lorenz, all of whom had realised that quantised oscillators were essential to derive the correct form of the Black-Body distribution. It is only in the second edition of Planck’s lectures which date from 1911-12 that he addresses these criticisms and we see the derivation of Planck’s Black-Body distribution in the form we now recognise. Because of the rapid development of Quantum Theory in the first two decades of the 20th Century, this initial mis-step by Planck was not widely appreciated until Kuhn uncovered it. Sometimes, as in Planck’s case, an incorrect derivation can give the right outcome and so one gets the argument that the derivation must be right because it agrees with observations.
That exchange between Tohline and Rubin is a gem. It’s concerning how much that basic idea (that V^2r = M assumes Newtonian gravity) has been swept under the rug. I studied physics and consume enormous amounts of content about physics and astronomy, and yet for so many years I didn’t even know this, because every talk I saw assumed dark matter and acted so assuredly that it must exist that no alternative need even be mentioned in passing.
Perhaps of course dark matter will somehow end up being correct in the end, but even that in no way would justify the misrepresentation of its current evidentiary support.
“So the anomaly isn’t dismissed, but it is treated with less gravity than it should be” — what an excellent pun.
😁
@tritonstation It's neither a duck nor a rabbit.
It's neither DM nor MOND.
It's an optical illusion.
It’s perhaps good to remember how succesful string theory has been theoretically; the amount of seemingly unsolvable problems that miraculously got solved just in time strengthened the belief in its significance.
The problem is not the high esteem for theoretical success, but the complete absence of any esteem for good old realism. You want a theory explaining reality? Current modern physics gives highly accurate predictions from a theory of which many physicists believe it doesn’t actually describe REAL entities (the wavefunction), so we end up with a theory that it’s all due to imaginary mechanics with fantasy worlds. We need more dumb people to step in, who just bluntly say “are you seriously suggesting real things emerge from non-real things? That’s impossible and idiotic!”.
It’s the same postmodern paradigm plagueing current politics: the post-truth mentality. I am tempted to quote Pontius Pilate. Actual facts and science (although I would call String theory science as well) are considered worthless. Authority is now considered merely “some annoying opinion of some guy wanting to impose his views on me”. This is very related to the demise of respectful treatment of your neighbour, not even mentioning the demise of trying to do your neighbour good. The problem is that since it’s woven into our societies, also me and probably you too are displaying some of the same behaviour.
The Dark Matter crisis is just a symptom of the scientifically incoherent Expanding Universe paradigm. Like DM, Dark Energy, Expanding Spacetime, The Big Bang, Inflation, and whatever the Babble du Jour cover story is for the Singularity, all constitute either a physical consequence of the model or some form of fitting mechanism invoked to reconcile the EU with the Cosmos we actually observe. All of those physical consequences or fitting mechanisms are undetected or undetectable – none are supported by any direct empirical evidence; they only exist in the model that requires them.
The Expanding Universe model is based on the FLRW equations which are solutions to the field equations of General Relativity. Those solutions rest on the erroneous assumption that it is reasonable to solve the GR equations for a non-relativistic universal framework (the FLRW metric) despite the fact that Relativity Theory abjures such a framework. The result speaks for itself in the models lack of any empirical basis.
In light of our current cosmological knowledge and well-established basic physics, the Expanding Universe model is scientifically unsustainable. The maximum speed of light is such that it takes light 2.5 million years to reach us from Andromeda, our nearest galactic neighbor, and more than 10 billion years to reach us from the most distant galaxies we observe.
Those straightforward physical facts constitute a direct falsification of the Expanding Universe model which is built upon the premise that the Cosmos can be modeled with a simultaneous framework, one that does not and cannot have any physical relevance according to the foregoing basic physical considerations.
That foundational assumption of a universal framework is as fundamentally wrong in its conceptual account of physical reality as Ptolemy’s geocentrism was. The Expanding Universe of the standard model does not and cannot exist in light of our current cosmological knowledge.
Setting aside the Expanding Universe model has the salutary effect of eliminating the need for a belief in Dark Matter, Dark Energy, Expanding Spacetime, the Big Bang, Inflation and the Singularity. That in turn means we can begin to consider the Cosmos as it actually appears to us, rather than having to gaze at it through the foggy glasses of a shaggy mathematical misconception.
Well, no. I don’t agree. The expansion of the universe is observed; it isn’t contingent of FLRW.
The expansion of the universe is not an observation, it is only an assumption of the model. The only thing that is empirically observed with regard to a “universal expansion” is a correlation between the cosmological redshift and distance. The recessional velocity interpretation of that correlation has always been an assumption.
That assumption has always had difficulties with basic physics ever since the inferred velocities began to approach light speed which happened early on. That’s why Hubble always doubted the recessional velocity interpretation.
The RV interpretation is also downstream of the “Universe” assumption that is directly contradicted by the basic physical considerations mentioned. The maximum speed of light being what it is, does not allow for the possibility that the Cosmos on the scale we observe it can be a simultaneously interconnected entity as the standard model assumes. Simply put there ain’t no such animal. It is not observable and to borrow from John Bell, it is not beable.
Since the Universe of the standard model is precluded by basic physics, it’s expansion is a moot point as far as empirical reality is concerned. The redshift-distance relationship being an observed phenomenon, it must have a real physical cause. It cannot be explained by the imaginary expansion of an imaginary Universe.
Are distant spiral galaxies not rotating then? We measure their rotation (and rotation curves) via redshift. Is that illusory?
I did not say or imply that the cosmological redshift is illusory. What is measured with regard to rotational velocity is a differential between the redshift on the side of the galaxy rotating towards us and the opposite side of the galaxy rotating away. That redshift differential is caused by a local (to the remote galaxy) Doppler effect. No matter how you slice it that purely local effect has nothing to do with the overall cosmological redshift which is dependent on a galaxy’s distance from us.
If you want to argue against the idea of the expansion, in terms of observational evidence, you can try to do that. But you haven’t started – you’d have a lot more to dismiss than just the recession v approach. The idea that the only thing suggesting expansion is the recession velocity interpretation is way out of date.
There is no empirical evidence for the expansion of the Universe assumed by the model. If you want to interpret the observed redshift-distance relation as the result of the expansion of a model Universe that is impervious to direct observation according to the known laws of physics you can certainly do that.
But what you cannot do is produce any empirical evidence for any element of the Big Bang model – including the expansion. If you think you can, cite something specific that you think constitutes direct empirical evidence – of any element of the standard model. I’ll be happy to pick it apart for you and demonstrate that all you have are model-dependent inferences.
The *most obvious* interpretation of the redshift-distance relation is an expanding universe. One can come up with other explanations, sure.
The *most obvious* interpretation of the measured cosmic abundances of the isotopes of the light elements is nucleosynthesis in the early universe. This only works out if the expansion rate is correct (in the radiation dominated era). I’m sure other explanations could be made, but this is a very difficult one to get around.
The *most obvious* interpretation of the microwave background is as the relic radiation field from the early, hot universe, now cooled by expansion. There are measurements T(z) at intermediate redshifts that track the expected evolution of the temperature of the radiation field. This is hard to do otherwise, though people have made lots of suggestions.
As convenient as it would be for me to simply disbelieve these three empirical pillars of the hot big bang, I don’t. Are there model-dependent inferences involved? Sure, those are impossible to avoid at some level. Can we explain it all away? I think not. Maybe I’m wrong, but please do not try. At least not here; it sounds like you have plenty of material for your own publication. I myself am not hosting an intergalactic kegger for big bang deniers.
Well, I said there are other things ‘suggesting expansion’, and there sure are, I’ll point out an indirect one. Stacy is much better equipped than me to answer about observation.
However you interpret the redshift-distance relation, the more distant galaxies are seen as they were longer ago. From that we know galaxies are going through changes over time. They’re going through suspiciously well-coordinated changes over time…. bursts of star formation across many galaxies at the same point in history, and all kinds of other changes.
In your picture the redshifts are perhaps due to some mechanism not unlike tired light that we don’t yet know about. So the galaxies just sit there, not moving, but changing in unison over time for some reason. You get a kind of ultra-horizon problem, where it’s not clear how they affect each other to stay synced up, or how they got synced up in the first place.
I don’t disagree on one aspect of this, I think there could be at least one redshift effect that we don’t yet know about. But it doesn’t alter the result on this particular question – I’m sorry to say that it seems the universe has exploded 🙂
Our friendly local librarian tracked down a translation of Opik 1915. It is on the arxiv as https://arxiv.org/abs/2203.14871. There are some interesting (and more modern) comments added at the end, but my reading is that Opik is arguing against there being much unseen mass.
Aside from astronomers and astrophysicists, most people don’t truly grasp how insanely small the MOND acceleration threshold is. If a marble resting on your living room carpet began rising vertically from a standing start at 10^-10 g’s, it would take over one hundred years to reach the height of a coffee table. Curiously, this incredibly small acceleration rate is similar in magnitude to the expansion rate of the universe where a0 is approximately equal the speed of light times the Hubble Constant.
Much speculation has gone into attempting to explain this connection. I’ve had my own ideas that are wildly speculative. One idea is that ionized matter in stars and gas clouds are emitting equal numbers of both positive energy and negative energy gravitons. For this to happen gravity-like forces would need to exist within atoms that are at least equal in strength to electromagnetism. No such forces exist in the Standard Mode (SM). But I had an idea that extends the SM in the sense that an already existing parameter is actually a very short-range, gravity-like force 39 magnitudes larger than Newtonian gravity. This finite range, gravity-like field would be bi-energy, inducing extremely strong curvatures of the metric that would stimulate the emission of both positive and negative energy gravitons from agitated (ionized) atoms.
At astronomical scales geometry would be critical to the effect these (putative) gravitons would have. The more spherically symmetric a system is the greater the cancellation of the effect. Since the bi-energy graviton emission would be isotropic from individual stars it would not affect their gravitational potential. On larger scales, the non-spherical geometry of spiral galaxies, should promote the sequestering of these opposing emissions: negative-energy gravitons accelerating the expansion of the Universe as dark energy, while the positive-energy gravitons would augment the Newtonian potential in galactic and extra-galactic structures.
Good point. Even most astrophysicists don’t grasp how tiny a0 is.
“The basic idea is that dark matter is on the brink of a Kuhnian paradigm shift.” What is really happening near the Planck scale? Let us assume that gravitational energy is conserved & all gravitons have spin 2. Even if the preceding assumption is wrong, it might be nearly impossible to refute by empirical evidence. What are some opinions about the following 4 predictions?
(1) Within 2 years from March 2026, Mordehai Milgrom & Eduardo Guendelman will share a Nobel Prize.
(2) Within 2 years from March 2026, Nissan Itzhaki, Uri Peleg, & Paul Steinhardt will share a Breakthrough Prize.
(3) Within 3 years from March 2026, Stacy McGaugh & Pavel Kroupa will win Breakthrough Prizes.
(4) Within 6 years from March 2026, Edward Witten will become a Nobel Laureate.
Hi Stacy.
Thank you for this post on “Paradigm Shifts in Modern Astrophysics: Dark Matter”.
The end of the second paragraph of the Heritage Diner article contains (for me) the key sentence “Only when a viable alternative emerges, to explain both the old data and the new, does the paradigm finally break”. The current paradigm of LCDM is a theory of cosmology. The alternative MOND is a theory of galaxies. So a bit of a mismatch to begin with. Personally, I don’t see MOND as the “viable alternative”; I’m looking for something different.
For me the key test of any “viable alternative” is the explanation of the height & spacing of the acoustic peaks in the power spectrum of the CMB (cosmic microwave spectrum). This observation is not mentioned in the Heritage Diner article. Unfortunately, the calculation of the acoustic peaks is a complicated & messy business and several of the publicly available computer codes (e.g. CLASS) cannot be used as they assume the existence of dark matter. So, it is going to be difficult for a mere mortal to break the current paradigm (but I am giving it a go!).
Indeed, it is apples and oranges to start. MOND, by itself, pretend to be a theory of cosmology. But to rephrase what you say, LCDM is not a viable alternative to MOND as a theory of galaxies. It’s not, no matter how many times people assert it to be so. So why should cosmology be an imperative? We’ve never ever been right about it before.
Nevertheless, I agree – as I’ve written about many times – that it would be good to explain the acoustic power spectrum of the CMB. (I merely reject the assertion that fitting the CMB is *more* important than explaining galaxy dynamics.) Skordis & Zlosnik’s have shown that it is possible to build a theory that fits the CMB data.
We need lots of us mortals to work on the problem. I think a key hold up is the lack of critical mass. No one wants to work on breaking the paradigm until it is broken, but the paradigm never breaks until enough people have worked on alternative to show that they’re better.
That’s the essence of Kuhn’s distinction between incremental and revolutionary science. Once the paradigm flips, the incrementalists will come piling over. But they’ll resist doing so until the dam breaks – however that happens!
As you say, MOND can’t completely ‘replace’ DM, they do well in different areas, at different scales. Those who dare look beyond the deadening ‘either/or’ approach may find a solution, perhaps a hybrid theory, but new concepts will be needed. This pattern with a false duality has existed before, where both competing ideas turn out to be right in one way, wrong in another. For one example, absolute motion versus relative motion, which started with the Greeks arguing it out, has turned out to need increasingly complex mixtures of the two.
One of Kuhn’s ingredients for revolution is new data that contradict the present paradigm beyond repair, and the JWST has given us that now. I’ve found that proving a new paradigm isn’t enough, without the data demanding it. Even showing that one can pick any two points on any orbit around any spherical mass, and find them to be connected by the law of refraction to 16 decimal places doesn’t do it, making ‘DM’ likely to be a very small-scale emitted medium, graded because it dissipates, as (talking of persistence) I’ve shown here: https://tritonstation.com/2024/06/18/rotation-curves-still-flat-after-a-million-light-years/#comment-22413
But it’s a lot easier to replace LCDM than GR, particularly because of the data on the ‘impossible’ era in the early universe, with 7 or 8 main puzzles at different redshifts. You might remember I mentioned I’ve been looking for an equation for some years that has several quantities starting high and descending in proportion (anyone who might want to work on a joint paper let me know). It was found a few weeks ago, and comes out a very solid conceptual picture that’s already published. It solves quite a list of puzzles, with different quantities that land very neatly at different z values. Maybe if there’s both a solution and a real need for it, Kuhn’s ideas might start to apply – either on its own isn’t enough.
For the CMB, what happens if you ignore dark matter but also ignore MOND? I’m really into putting all MOND effects on modifying inertia, IMO with good reason – that’s why I suggest this. If the explanation why inertia is modified sounds OK to most people, at least this would unify MOND with whatever quantum gravity theory turns out to be right (by preserving the inverse square law). That gives a unified theory on dynamics at least.
The modified inertia approach is a lot more promising than most people seem to realize. It seems inconsistent with GR in principle, but in practice it is not obvious what would change.
Ignoring both MOND and dark matter is exactly how I made the prediction for the fist-to-second peak ratio. The ansatz was that, whatever the grander theory may be, the early universe was adequately approximated by GR alone (w/o DM).
This works for the amplitudes of the first two peaks, but not beyond. In the no-DM, pure GR limit, we predict a baryonic damping spectrum in which each peak is lower than the one before it. WMAP and then Planck observe the third peak to be about the same amplitude as the second, so the simple ansatz fails around L = 600.
It is not surprising that the ansatz fails at some point, but I see no reason for it to be this point (or if that’s really the right way to frame it. The peaks, even the first two, are more narrow than they would be in a purely baryonic, GR-only universe.)
This is all in my 2004 paper https://arxiv.org/abs/astro-ph/0312570
I’m having a hard time understanding why modified inertia could be promising.
It may seem that for some mathematical reason modified inertia could lead us more easily than modified gravity to the deep, fundamental law that is at the root of the observed MOND behavior. However, in the meantime we can look for predictions from both approaches that can let us distinguish between them.
Either we have some, and then we can rule out one of the two with data, or we don’t and in this case the fact that MOND, as the effective rule that we know so far, works the same for both massive and massless particles is a strong indication that it is gravity that is modified. Or something else, but inertia is only a contender.
Why believe in something complicated, when observation tells us to look for something simpler first?
To answer your last question first, modifying inertia is simpler than modifying gravity. Modified gravity theories tend to invoke additional scalar, vector, and/or tensor fields, turning our one parameter extension of Newtonian theory into another two or three (or more) parameter madhouse. In modified inertia, we imagine that the effective inertial mass depends on acceleration. It already depends on velocity, increasing as v approaches c, the extra mass being that of the kinetic energy via E = mc^2. If it depends on dx/dt, why not d^2x/dt^2?
And yes, there are tests that can, at least in principle, distinguish between modified inertia and modified gravity. I believe Milgrom discussed one in his original paper. Another one is https://scixplorer.org/abs/2012PhRvL.109y1103M/abstract
A problem with modifying gravity is locality: MOND gives a modification that somehow works, from the distance on where acceleration gets small enough. The standard model of particle physics, which is highly realistic and well-established, transfers force with bosons. It would be logical if locality also works with bosons, the graviton.
But these gravitons must then somehow change at low total force strength – there might be many gravitational forces from many directions – which we know pass through each other – but when on an object they give a total force with a < a0 suddenly the gravitons must have changed to produce a higher force. And the next object in their path might have its total force higher giving just Newton, so there they must have changed back. That's why instead of just gravitons the extra fields Stacy mentions are required.
But if MOND is due to modified inertia, F = ma or a = F/m is modified to a^2 = F/m for acceleration small enough. This means that resulting acceleration from some force/kg is much larger (it's the square root of the tiny Newtonian value) and around a0 transitions smoothly to F/(a0 m) = a. It's just that the "effort" required for an acceleration scales with an extra factor linear in a but maximized at a0.
According to Nobel Laureate Frank Wilczek, axions “have the right properties to make the dark matter of the universe.”
https://www.quantamagazine.org/frank-wilczek-on-the-strong-force-quarks-and-dark-matter-20210315/
My guess is that Professor Wilczek has never spent a significant amount of time studying MOND’s successful predictions. Can a debate between McGaugh & Wilczek be arranged?
Yes, that would be my guess – most advocates of dark matter are ignorant of MOND. That was true of me originally; it is more true of [most] particle theorists who really really really want dark matter to be a new particle so that there is something new beyond the Standard Model so their field doesn’t continue to become ever more moribund.
And thanks, but I’m not interested in debating Wilczek. You would basically get a repeat of https://tritonstation.com/2025/04/14/neiu-debate-dark-matter-or-modified-gravity/
With no clear guidance on where to go next, one thing is patently clear in your essay: The choice made in the late 1990s to reinstate the cosmology constant was a poor one. We have proceeded for a century now with a papal infallibility: Mr. Einstein said it, and we cannot deviate from his sermons. For those of us who have been rising our hands and stating, “this is a poor set of canons”, it is a small victory.
If anything has been established by AI, it is the ultimate ‘garbage in, garbage out’ engine; aka a complete set of epicyclic French curves: Models with poor predictive power.
There is, in some physics circles, an air of papal infallibility about Einstein. That is one of the human attitudes that makes science work in practice differently than it ought to, as Kuhn documented.
While I agree with your first paragraph, I’m not sure what you mean by the second. My first reading was that AI is a is the ultimate ‘garbage in, garbage out’ engine. That I agree with. Reading it again, you have that critical comma, so perhaps you mean that Einstein’s GR has been debunked by AI. That is not true. GR works at high accelerations; what we need is a more general theory that encompasses those successes as well as those of MOND. That’s the elephant the blind men have yet to perceive.
As of March 13, 2026, Bosma and Roberts have Wikidata entries, but no Wikipedia entries in any language.
Albert Bosma, Wikidata
https://www.wikidata.org/wiki/Q102314293
Morton S. Roberts
https://www.wikidata.org/wiki/Q136218786
It might be a good idea to create Wikipedia entries for both of them.
To y’all still hollering at me about the Big Bang:
The Big Bang is a scientific theory subject to test and falsification like any other. Unfortunately, it is also a subject that impinges on religion and engenders reactions (both positive and negative) that have more to do with faith than science.
I know of far too many scientists who are guilty of treating the BB as a religion, and that is a Bad thing. Someone mentioned Sean Carroll in this context; you can see a conversation I had with him here: https://astroweb.case.edu/ssm/mond/carrollcorrespondence.html
At the other extreme are people who hate the idea. Far too many of these people think they are making scientific arguments when they are not. That is also Bad.
I am not here to sort this out for you. I have neither the time nor the inclination to wade through every claim in the hope of finding one that might hold a drop of water. Got enough on my plate as it is, thanks.
Also gonna remind everyone that this is not the place to publish your theories. I’m getting dozens of submissions a day. Many, thought not all, are AI generated. If there are any valid ideas in any of that, I will never be able to discern the signal amidst all the noise.
I find the modified inertia variant of MOND very appealing, with respect to the toy model I touched on upthread. The thinking was that inertia for any given particle arose from bi-energy graviton emission along the axis of (net) acceleration. When a particle becomes truly inertial, devoid of acceleration, the bi-energy graviton emission ceases, and this would mark the transition to the MOND regime. I even wondered if 10^-5 g. acceleration signals detected from rapidly spun-up niobium ring superconductors at the Austrian Research Center might have their origin from this hypothetical process. A serious drawback to this idea is that negative energy density is forbidden by various energy conditions. But I hoped that perhaps it didn’t apply to gravitons. Since the inertia of any body is precisely a function of the body’s mass, I worried that what amounts to a local effect could be at all viable without invoking some kind of non—local interaction to satisfy Mach’s Principle. Had some ideas to solve that involving compactified extra dimensions (along the lines of the original ADD paper in 1998 and later Randall-Sundrum models, going from memory), which of course adds complexity and unseen ‘stuff’.
One nice feature of this speculative, bi-energy graviton emission process (which I hoped might be the physical mechanism underlying MOND) is that it would bootstrap structure formation in the early Universe due to the expansion-contraction dynamic of opposite energy graviton emissions. Additionally, with bi-energy graviton emission largely originating from the ionization of matter it provides a causal connection between the Tully-Fisher relation and the surface brightness of spiral galaxies. Also thought there might be a way to bring the Bullet Cluster into the MOND fold by appeal to bi-energy graviton emissions. While the emissions would be strong along the collision axis, as per the model, I couldn’t see how to account for the two gravitational wells of putative Dark Matter that gravitational lensing indicated.
I know you have too much to do and a full plate. Still I’m interested in these ‘platypus galaxies’. Single pixel images in JWST, but with a tighter spectrum than quasars. https://science.nasa.gov/missions/webb/scientists-identify-astronomys-platypus-with-nasas-webb-telescope/
These seem to fit with the predictions of MoND, but I hear no one trumpeting that fit.
Love your blog, thanks.
Quick answer: I looked at those, and didn’t know what to make of them. Still don’t.
In terms of explaining MOND’s successful predictions, there might be two basic possibilities: FUNDAMOND string theory or FUNDAMOND non-string theory. My opinion is that Professor Eduardo Guendelman of Ben-Gurion University of the Negev has already provided a highly plausible way of establishing FUNDAMOND string theory by providing a mathematical and conceptual approach to 2 different types of string tension — allowing Newton-Einstein inertia & FUNDAMOND inertia. I recently received the following email reply:
“… thank you for your comments, I wish that prediction concerning the Nobel prize would be true, but I am not so optimistic as you. The connection between my work and that of Milgrom has yet to be studied and verified, will look at your suggestion,
best,
Eduardo”
Can a McGaugh versus Guendelman debate be arranged?
On the philosophical side, what I wonder about is this (from a theoretical standpoint), when we look into the extremely distant universe, should we see ourselves?
And if so, what if the view that we get instead is just the product of all the corrections that we make to remove ourselves from our own observations?
There are people who have looked to observational signatures of the universe repeating itself, as might happen if it had a finite boundary and light traveled ’round so we see a duplicate of our own universe. There are conceivable if unlikely-seeming topologies that might do that. If so, there is no sign of it yet.
It is possible that the signature is a rabbit/duck scenario, and we are staring right at it. Not that it looks any different, but our perspective defines how we see it.
I was stuck at the laundromat yesterday for several hours, so naturally I brought along reading material that interested me. In this case it was a paper by a group of theorists (arXiv:2007.07920v2) dated 16 Dec., 2020. The title was “Right-handed Neutrino Dark Matter, Neutrino Masses, and non-Standard Cosmology in a 2HDM”. After making it about one third of the way through the paper I realized that what these physicists were doing was equivalent to being in a dark room with an unknown animal in the middle, representing the mystery they are trying to solve. As they touched the animal at different spots (equivalent to various detectors taking data) the most logical and sensible thing they could do was to organize the data in a way that it would fit into the current knowledge base. Not knowing what the beastie in the room was, these and other scientists, over the decades, have created a phenomenological model of enormous complexity with more ‘effects’ and ‘mechanisms’ than Carter has little liver pills. The mathematical modeling has become so complex that it makes the Ptolemaic geocentric model of the solar system look simple by comparison.
All in all, I think the electroweak sector of the Standard Model is ripe for a major discovery that may well usher in the next paradigm shift, as the unknown animal, or animals become fully illuminated. It will be as revolutionary as Nicolaus Copernicus’s revolutionary heliocentric model of the solar system.