Some people have asked me to comment on the Scientific American article What if We Never Find Dark Matter? by Slatyer & Tait. For the most part, I find it unobjectionable – from a certain point of view. It is revealing to examine this point of view, starting with the title, which frames the subject in a way that gives us permission to believe in dark matter while never finding it. This framing is profoundly unscientific, as it invites a form of magical thinking that could usher in a thousand years of dark epicycles (feedback being the modern epicycle) on top of the decades it has already sustained.
The article does recognize that a modification of gravity is at least a logical possibility. The mere mention of this is progress, if grudging and slow. They can’t bring themselves to name a specific theory: they never say MOND and only allude obliquely to a single relativistic theory as if saying its name out loud would bring a curse% upon their house.
Of course, they mention modified gravity merely to dismiss it:
A universe without dark matter would require striking modifications to the laws of gravity… [which] seems exceptionally difficult.
Yes it is. But it has also proven exceptionally difficult to detect dark matter. That hasn’t stopped people from making valiant efforts to do so. So the argument is that we should try really hard to accomplish the exceptionally difficult task of detecting dark matter, but we shouldn’t bother trying to modify gravity because doing so would be exceptionally difficult.
This speaks to motivations – is one idea better motivated? In the 1980s, cold dark matter was motivated by both astronomical observations and physical theory. Absent the radical thought of modifying gravity, we had a clear need for unseen mass. Some of that unseen mass could simply have been undetected normal matter, but most of it needed to be some form of non-baryonic dark matter that exceeded the baryon density allowed by Big Bang Nucleosynthesis and did not interact directly with photons. That meant entirely new physics from beyond the Standard Model of particle physics: no particle in the known stable of particles suffices. This new physics was seen as a good thing, because particle physicists already had the feeling that there should be something more than the Standard Model. There was a desire for Grand Unified Theories (GUTs) and supersymmetry (SUSY). SUSY naturally provides a home for particles that could be the dark matter, in particular the Weakly Interacting Massive Particles (WIMPs) that are the prime target for the vast majority of experiments that are working to achieve the exceptionally difficult task of detecting them. So there was a confluence of reasons from very different perspectives to make the search for WIMPs very well motivated.
That was then. Fast forward a few decades, and the search for WIMPs has failed. Repeatedly. Continuing to pursue it is an example of the sunk cost fallacy. We keep doing it because we’ve already done so much of it that surely we should keep going. So I feel the need to comment on this seemingly innocuous remark:
although many versions of supersymmetry predict WIMP dark matter, the converse isn’t true; WIMPs are viable dark matter candidates even in a universe without supersymmetry.
Strictly speaking, this is correct. It is also weak sauce. The neutrino is an example of a weakly interacting particle that has some mass. We know neutrinos exist, and they reside in the Standard Model – no need for supersymmetry. We also know that they cannot be the dark matter, so it would be disingenuous to conflate the two. Beyond that, it is possible to imagine a practically infinite variety of particles that are weakly interacting by not part of supersymmetry. That’s just throwing mud at the wall. SUSY WIMPs were extraordinarily well motivated, with the WIMP miracle being the beautiful argument that launched a thousand experiments. But lacking SUSY – which seems practically dead at this juncture – WIMPS as originally motivated are dead along with it. The motivation for more generic WIMPs is lacking, so the above statement is nothing more than an assertion that runs interference for the fact that we no longer have good reason to expect WIMPs at all.
There is also an element of disciplinary-centric thinking: if you’re a particle physicist, you can build a dark matter detector and maybe make a major discovery or at least get great gobs of grants in the effort to do so. If instead what is going on is really a modification of gravity, then your expertise is irrelevant and there is no reason to keep shoveling money into your field. Worse, a career spent at the bottom of a mine shaft working on dark matter detectors is a waste of effort. I can understand why people don’t want to hear that message, but that just brings us back to the sunk cost fallacy.
Speaking of money, I occasionally get scientists who come up to me Big Mad that grant money gets spent on MOND research, as that would be a waste of taxpayer money. I can assure them that no government dollars have been harmed in the pursuit of MOND research. Certainly not in the U.S., at any rate. But lots and lots of tax dollars have been burned in the search for dark matter, and the article we’re discussing advocates spending a whole lot more to search for dark matter candidates that are nowhere near as well motivated as WIMPs were. That’s why I keep asking: how do we know when to stop? I don’t expect other scientists to agree to my interpretation of the data, but I do expect them to have a criterion whereby they would accede that dark matter is incorrect. If we lack any notion of how we could figure out that we are wrong, then we’ve made the leap from science to religion. So far, such criteria are sadly lacking, and I see precious little evidence of people rising to the challenge. Indeed, I frequently get the opposite, as other scientists have frequently asserted to me that they would only consider MOND as a last resort. OK, when does that happen? There’s always another particle we can think up, so the answer seems to be “never.”
I wrote long ago that “After WIMPs, the next obvious candidate is axions.” Sure enough, this article spills a lot of ink discussing axions. Rather than dwell on this different doomed idea for dark matter, let’s take a gander at the remarkable art made to accompany the article, because we are visual animals and graphical representations are important.
Where to start? Right in the center is a scroll of an old-timey star chart. On top of that are several depictions of what I guess are meant to be galaxies*. Around those is an ethereal dragon representing the unknown dark matter. The depiction of dark matter as an unfathomable monster is at once both spot on and weirdly anthropomorphic. Is this a fabled beast the adventurous hero is supposed to seek out and slay? or befriend? or maybe it is a tale in which he grows during the journey to realize he has been on the wrong path the whole time? I love the dragon as art, but as a representation of a scientific subject it imparts an aura of teleological biology to something that is literally out of this world, residing in a dark sector that is not part of our daily experience and may be entirely inaccessible to our terrestrial experimentation. Off the edge of the map and on into extra dimensions: here there be monsters.
The representations here are fantastic. There is the coffee mug and the candle to represent the hard work of those of us who burn the candle at both ends wrestling with the dark matter problem. There’s a magnifying glass to represent how hard the experimentalists have looked for the dark matter. Scattered around are various totems, like the Polaroid-style picture at right depicting the gravitational lensing around a black hole. This is cool, but has squat to do with the missing mass problem. It’s more a nod to General Relativity and the Faith we have therein, albeit in a regime many orders of magnitude removed from the one that concerns us here. On the left is an old newspaper article about WIMPs, complete with a sketch of a Feynman diagram that depicts how we might detect them. And at the top, peeking out of a book, as it were a thought made long ago now seeking new relevance, a note saying Axions!
I can save everyone a lot of time, effort, and expense. It ain’t WIMPs and it ain’t axions. Nor is the dark matter any of the plethora of other ideas illustrated in the eye-watering depiction of the landscape of particle possibilities in the article. These simply add mass while providing no explanation of the observed MOND phenomenology. This phenomenology is fundamental to the problem, so any approach that ignores it is doomed to failure. I’m happy to consider explanations based on dark matter, but these need to have a direct connection to baryons baked-in to be viable. None of the ideas they discuss meet this minimum criterion.
Of course it could be that MOND – either as modified gravity or modified inertia, an important possibility that usually gets overlooked – is essentially correct and that’s why it keeps having predictions come true. That’s what motivates considering it now: repeated and sustained predictive success, particularly for phenomena that dark matter does not provide a satisfactory explanation for.
Of course, this article advocating dark matter is at pains to dismiss modified gravity as a possibility:
The changes [of modified gravity] would have to mimic the effects of dark matter in astrophysical systems ranging from giant clusters of galaxies to the Milky Way’s smallest satellite galaxies. In other words, they would need to apply across an enormous range of scales in distance and time, without contradicting the host of other precise measurements we’ve gathered about how gravity works. The modifications would also need to explain why, if dark matter is just a modification to gravity—which is universally associated with all matter—not all galaxies and clusters appear to contain dark matter. Moreover, the most sophisticated attempts to formulate self-consistent theories of modified gravity to explain away dark matter end up invoking a type of dark matter anyway, to match the ripples we observe in the cosmic microwave background, leftover light from the big bang.
That’s a lot, so let’s break it down. First, that modified gravity “would have to mimic the effects of dark matter” gets it exactly backwards. It is dark matter that has to mimic the effects of MOND. That’s an easy call: dark matter plus baryons could combine in a large variety of ways that might bear no resemblance to MOND. Indeed, they should do that: the obvious prediction of LCDM-like theories is an exponential disk in an NFW halo. In contrast, there is one and only one thing that can happen in MOND since there is a single effective force law that connects the dynamics to the observed distribution of baryons. Galaxies didn’t have to do that, shouldn’t do that, but remarkably they do. The uniqueness of this relation poses a problem for dark matter that has been known since the previous century:
This basic conclusion has not changed over the years, only gotten stronger. The equation coupling dark to luminous matter I wrote down in all generality in McGaugh (2004) and again in McGaugh et al. (2016). The latter paper is published in Physical Review Letters, arguably the most prominent physics journal, and is in the top percentile of citation rates, so it isn’t some minuscule detail buried in an obscure astronomical journal that might have eluded the attention of particle physicists. It is the implication that conclusion [1] could be correct that bounces off a protective shell of cognitive dissonance so hard that the necessary corollary [2] gets overlooked.
OK, that’s just the first sentence. Let’s carry on with “[the modification] would need to apply across an enormous range of scales in distance and time, without contradicting the host of other precise measurements we’ve gathered about how gravity works.” Well, duh. That’s the first thing I checked. Thoroughly and repeatedly. I’ve written many reviews on the subject. They’re either unaware of some well-established results, or choose to ignore them.
The reason MOND doesn’t contradict the host of other constraints about how gravity works is simple. It happens in the low acceleration regime, where the only test of gravity is provided by the data that evince the mass discrepancy. If we had posed galaxy observations as a test of GR, we would have concluded that it fails at low accelerations. Of course we didn’t do that; we observed galaxies because we were interested in how they worked, then inferred the need for dark matter when gravity as we currently know it failed to explain the data. Other tests, regardless how precise, are irrelevant if they probe accelerations higher than Milgrom’s constant (1.2 x 10-10 m/s/s).
Continuing on, there is the complaint that “modifications would also need to explain why… not all galaxies and clusters appear to contain dark matter.” Yep, you gotta explain all the data. That starts with the vast majority of the data that do follow the radial acceleration relation, which is not satisfactorily explained by dark matter. They skip+ past that part, preferring to ignore the forest in order to complain about a few outlying trees. There are some interesting cases, to be sure, but this complaint about objects lacking dark matter is misplaced for deeper reasons. It makes no sense in terms of dark matter that there are objects without dark matter. That shouldn’t happen in LCDM any more than in MOND$. One winds up invoking non-equilibrium effects, which we can do in MOND just as we do in dark matter. It is not satisfactory in either case, but it is weird to complain about it for one theory while not for the other. This line of argument is perilously close to the a priori fallacy.
The last line, “the most sophisticated attempts to formulate self-consistent theories of modified gravity to explain away dark matter end up invoking a type of dark matter anyway, to match the ripples we observe in the cosmic microwave background” actually has some merit. The theory they’re talking about is Aether-Scalar-Tensor (AeST) theory, which I guess earns the badge of “most sophisticated” because it fits the power spectrum of the cosmic microwave background (CMB).
I’ve discussed the CMB in detail before, so won’t belabor it here. I will note that the microwave background is only one piece of many lines of evidence, and the conclusion one reaches depends on how one chooses to weigh the various incommensurate evidence. That they choose to emphasize this one thing while entirely eliding the predictive successes of MOND is typical, but does not encourage me to take this as a serious argument, especially when I had more success predicting important aspects of the microwave background than did the entire community that persistently cites the microwave background to the exclusion of all else.
It is also a bit strange to complain that AeST “explain[s] away dark matter [but] end[s] up invoking a type of dark matter.” I think what they mean here is true at the level of quantum field theory where all particles are fields and all fields are particles, but beyond that, they aren’t the same thing at all. It is common for modified gravity theories to invoke scalar fields#, and this is an important degree of freedom that enables AeST to fit the CMB. TeVeS also added a scalar and tensor field, but could not fit the CMB, so this approach isn’t guaranteed to work. But are these a type of dark matter? Or are our ideas of dark matter mimicking a scalar field? It seems like this argument could cut either way, and we’re just granting dark matter priority as a concept because we thought of it first. I don’t think nature cares about the order of our thoughts.
None of this addresses the question of the year. Why does MOND get any predictions right? Just saying “dark matter does it” is not sufficient. Until scientists engage seriously with this question, they’re doomed to chasing phantoms that aren’t there to catch.
%From what I’ve seen, they’re probably right to fear the curses of their colleagues for such blasphemy. Very objective, very scientific.
*Galaxies are nature’s artwork; human imitations never seem adequate. These look more like fried eggs to me. On the whole, this art is exceptionally well informed by science, or at least by particle physics, but not so much by astronomy. And therein lies the greater problem: there is a whole field of physics devoted to dark matter that is entirely motivated by astronomical observations yet its practitioners are, by and large, remarkably ignorant of anything more than the most rudimentary aspects of the data that motivate their field’s existence.
+There seems to be a common misconception that anything we observe is automatically explained by dark matter. That’s only true at the level of inference: any excess gravity is attributable to unseen mass. That’s why a hypothesis is only as good as its prior; a mere inference isn’t science, you have to make a prediction. Once you do that, you find dark matter might do lots of things that are not at all like the MONDian phenomenology that we observe. While I would hope the need for predictions is obvious, many scientists seem to conflate observation with prediction – if we observe it, that’s what dark matter must predict!
$The discrepancy should only appear below the critical acceleration scale in MOND. So strictly speaking, MOND does predict that there should be objects without dark matter: systems that are high acceleration. The central regions of globular clusters and elliptical galaxies are such regions, and MOND fares well there. In contrast, it is rather hard to build a sensible dark matter model that is as baryon dominated as observed. So this is an example of MOND explaining the absence of dark matter better than dark matter theory. This is related to the observation that the apparent need for dark matter only appears at low accelerations, at a scale that dark matter knows nothing about.
#I, personally, am skeptical of this approach, as it seems too generic (let’s add some new freedom!) when it feels like we’re missing something fundamental, perhaps along the lines of Mach’s Principle. However, I also recognize that this is a feeling on my part; it is outside my training to have a meaningful opinion.
Hi Stacy, lovely post, as always. Regarding the artwork in the SA article which
you reproduced, I think there was probably a sub-conscious science history force
working at the artist there, as the dark matter dragon -and the whole setting for
that matter- bear an uncanny resemblance to the appearance of the salamander of the Alchemists dancing around their hottest fires. My guess is the artist has researched the dark matter problem enough to notice the striking similarities to other undetectable dominant causal entities of the past: Phlogiston, Aether, Cartesian Epicycles driving the planets round the sun, etc.
If you gaze intently enough into the brightest fire, you will see the crowned salamander dancing, or the new dark matter candidate for that….
Oh wow. I did not know about the Alchemist’s salamander. I guess these things just keep coming around.
I thought that a more appropriate salamander-shape, since this is the USA, is the gerrymander (see the cartoon in the etymology section of the Wikipedia article https://en.wikipedia.org/wiki/Gerrymandering). It is clearly a contrived shape, specifically designed to produce the required outcome.
“It’s exactly the same crap as with string theory, and supersymmetry, and inflation, and dark sectors, and many other research bubbles in the foundations of physics. It is mathematical fiction; it’s nothing to do with reality any more.”
https://www.youtube.com/watch?v=cBIvSGLkwJY&
(Sabine Hossenfelder)
“A universe without dark matter would require striking modifications to the laws of gravity..”
Obviously they are assuming that Reality “must” follow their cherished “laws of gravity”, this is obviously a dogmatic approach and far from being objective and scientific.
This sounds a lot similar to the oxymoron of “post empirical science”.
Modern mainstream scientific thinking is getting closer and closer to renounce objectivity as science foundation giving priority to wishful thinking and dogmatism.
But at the end of the day objectivity will prevail if science survives the current social degradation.
Speaking of SciAm articles;
https://www.scientificamerican.com/article/jwsts-little-red-dots-offer-astronomers-the-universes-weirdest-puzzle/
It does seem the Webb is finding those ever further galaxies, that just can’t be shoehorned into the current model.
I’ve pointed out previously the expanding universe model fails on the most rudimentary level, given that if intergalactic space were expanding, the speed of the light crossing it should increase proportionally, in order to remain consent.
Not presume two metrics based on the speed and spectrum of the same light.
Which I’m using as a plug for my own idea of why there isn’t enough mass to explain the entire spectrum of gravity.
The premise is backwards. The properties our tactile, object oriented minds refer to as mass/matter, are intermediate effects of the centripetal dynamic of wave action, versus the inherent radiant tendency of light.
Galaxies are energy radiating out, as structure coalesces in.
This ties into the problem with time. As mobile organisms, this sentient interface our bodies have with their situation functions as a sequence of perceptions, in order to navigate, so our sense of time is the present going past to future. Though the evident reality is that activity and the resulting change turns future to past. Tomorrow becomes yesterday, because the earth turns.
There is no dimension of time, because the past is consumed by the present, to inform and drive it. Causality and conservation of energy. Cause becomes effect.
Energy is conserved, because it manifests this presence, creating time, temperature, pressure, color and sound, as frequencies and amplitudes, rates and degrees.
So this presence goes past to future, because the patterns generated come and go, future to past.
Energy drives the wave, the fluctuations rise and fall. No tiny strings necessary.
So what if the most basic tendency of waves is to synchronize? The effect would be centripetal.
Consider anything actually falling into black holes and not broken up and radiated out before getting there, is shot out the poles as quasars. Which are like giant lasers and lasers are synchronized light waves.
Sometimes simple does work. Just look at those tiny red dots.
“We know neutrinos exist, and they reside in the Standard Model – no need for supersymmetry. We also know that they cannot be the dark matter, so it would be disingenuous to conflate the two.”
You mentioned in a comment on a previous blog post that depending on the mass of the neutrinos, the neutrinos can be the residual dark matter in galaxy clusters after taking into account MOND.
Yes, conceivably, though I think the window for that to be viable is closing as the experimental limit from KATRIN drops below 1 eV. As the mass lowers, they remain closer to relativistic and don’t stick to cluster potentials. But they remain a potentially important component of the mass density of the universe.
To sum up those bounds: The old KATRIN bound on the lightest neutrino mass was 0.8 eV. Now it is down to 0.45 eV. This pushes the limit on the sum of the three neutrino masses to 1.41 eV in a normal hierarchy and 1.46 eV in an inverted hierarchy.
After a full run of data collection, KATRIN is expected to lower that bound to 0.2 eV. This would push the limit on the sum of the three neutrino masses to 0.66 eV in a normal hierarchy and 0.71 eV in an inverted hierarchy.
Thanks for the update; I hadn’t realized that KATRIN had gotten that far below 0.8 eV. The new, lowered sum of the three is near the lower limit of the interesting range to help with clusters (as I recall at any rate; it has been a very long time since I looked into that in detail).
I certainly hope they measure something, and don’t just wind up with a limit of 0.2 eV. Right now the upper limit from cosmology on the sum of the three neutrinos is 0.12 eV, which barely allows an inverted hierarchy. A mass bigger than this from KATRIN would constitute a falsification of structure formation as we understand it in LCDM and provide a clear indication of the need for something like MOND.
a thousand years of dark epicycles (feedback being the modern epicycle) on top of the decades it has already sustained.
does feedback apply to MOND, are there epicycles in MOND
There are many real feedback effects, most of which operate on stellar scales (AGN excepted). Those are all real. But “feedback” is invoked as a generic solution to all problems dark matter theories encounter on galaxy scales, and it is not at all clear that this hypothetical galaxy-scale feedback is related to the real, physical feedback of stars in the ways that are conjectured. So it is used as a proxy to get from a legitimate prediction that is not observed (e.g., NFW halos) to something more like (but often still unlike) reality. It is the new epicycle in the philosophical sense.
Feedback can happen in MOND, but it is not important to the problem. I believe I’ve written about this before, so try using the search feature. The place where I would caution people about the danger of epicyclic thinking is in clusters of galaxies, where it may be tempting to invoke mechanisms to make things work out for MOND.
Thanks for the post, which is great as always. I know your arguments are very much for physicists, and in those terms it’s as it should be said. But if you’re talking to the world of science journalism, and in a way you are here (or if you were, and they should give you an article in SciAm) it would probably be a distillation of key points.
It comes down to a list of evidence that something is at work that cannot possibly be DM – not in any of its forms that we’ve imagined so far, or that we can imagine imagining in the future. You’d have that down a lot better than me, but for one point, to paraphrase a bit I’ve quoted from another post, ‘the connection with the visible matter means DM would need to know intimately about what the baryons are doing, without interacting with them’.
How could it do that? That needs a lateral solution – to me, as I’ve said, the visible matter emits what is taken to be DM, and at larger scales an excess builds up, and its origin is by then far from obvious. But the basic point unanswered can cut through prejudice. I know you’ve had trouble doing that (personally I’ve seen it go further – you can even prove something to people, and if it’s not what they want to hear, they’ll ignore it). But I also understand the general frustration about the way they ignore points that show that even if we find DM tomorrow, we still haven’t solved the puzzle.
Indeed. How could it be so? If one takes the time to rub two brain cells together, it is obvious that the observed intimate connection should not occur. But we, as a community, don’t permit this thought, so we’re on into epicycles and psychology, not science.
I thought that your work with Tobias Mistele on weak lensing (arxiv 2406.09685) made the modified inertia approach much less likely. Am I wrong?
That depends on how lensing works in the deeper theory; I don’t think we can exclude modified inertia this way, merely say we don’t know what such a theory might predict for lensing.
Do you have any opinion on Mike McCulloch’s Quantized Inertia program? https://physicsfromtheedge.blogspot.com/
It seems like a good idea, but, like Verlinde’s emergent gravity, doesn’t quite work right, at least the last I checked. These are both in the category of “close but no cigar” along with a number of other ideas like superfluid dark matter that seem to introduce an excess amount of force around a0 before asymptoting to something like the right behavior.
McCulloch has just published a book released :July 15, 2024
“Quantised Accelerations: From Anomalies to New Physics”
by Michael E McCulloch.
It spells out his quantised inertia approach and tests it against a large number of anomalous measurements in the prevailing physics models. It appears to completely eliminate the need for dark “anything”.
McCulloch’s programme is very interesting, and I have corresponded with him for some years, but for me the problem with it is that it quantises gravity at the Planck mass, not at the elementary particle mass. That is not to say that it is “wrong” in any sense – the Planck mass clearly has something important to tell us about physics (in general) and gravity (in particular), and what he is doing may well be completely correct. But for me, it doesn’t get to the heart of the matter. (Passive) gravity has been tested at the scale of individual atoms, on a scale where Newton’s Third Law is not disrupted by signalling delays, so that it can be assumed to apply equally to active gravity. That means individual atoms are emitting individual gravitons, and QI doesn’t deal with that scenario.
From a general knowledge of physics perspective, the thorn in the side of every theory to explain inertia is how to get information to transit from a given particle in the Universe to any other particle in the Universe, or from a test particle to distant boundaries, instantaneously. Special Relativity forbids it. Presumably, the only way around this is to have each particle, even as it transforms into other types of matter-energy through the course of the Universe’s evolution, to keep track of its inertial status vis-à-vis every other particle from the instant of creation when the Universe was just a microscopic point. This would effectively correspond to an inertial navigation system that needs no external inputs. But that would seem to result in a deterministic Universe – no free will.
From the current observations I wonder if there is the need for that information to travel faster than the speed of light. But if it does, nonlocality isn’t that much an issue IMO. It wasn’t for Newton, and that brought him a lot farther than others. A leap of faith is sometimes necessary, and the explanations that make it more acceptable to your stomach may very well follow (much) later.
Quatized inertia makes a clear prediction. Since the minimum acceleration depends on the scale of the Universe, that acceleration depends on redshift. I believe in the past was larger. Anyway, that means that rotation curves should depend on z, and as far as I know, dr. Stacy McGaugh said this prediction is not consistent with observations.
One thing that puzzles me when people talk about neutrino mass is that they talk about it as though they know what mass is, whereas if MOND teaches us anything, it is that we don’t know what mass is. Neutrino mass is a hypothesis, like dark matter. It is not a fact. It is crucially model dependent, and crucially dependent on knowing what mass is. It is crucially dependent on mass being an intrinsic property of elementary particles, invariant and universal. MOND surely teaches us that that is not how mass actually behaves in the real universe.
The meaning of mass is a deep issue that we usually don’t delve into!
What people mean by neutrino mass is the amount of energy between states required to understand oscillations between neutrino types, as observed coming from both the sun and the atmosphere via cosmic ray showers. That only tells us the difference in energy between the mass eigenstates; we still don’t know the absolute scale – hence experiments like KATRIN.
Hopefully that will give us a number to work with, if not an answer to the deeper question you pose.
Well, yes, but it is also relevant to ask where does the energy for neutrino oscillation come from? Since the energy required is so small, it could come from almost anywhere – including quantum fluctuations in the gravitational field. It doesn’t have to come from intrinsic energy of the neutrinos themselves.
Sci Am was once a great publication. Alas that is no longer true. I let my subscription lapse some time in the late 1980’s. Please don’t waste too much of your energy in critiques for we who are (already) members of the choir. I’m holding out hope for a massive >10eV sterile (or otherwise) neutrino. There have been a few experiments that hinted at such, but follow-ups have not panned out from what I can tell. Still there was something in the earlier measurements that has not been explained, AFAIK. Understanding neutrino physics/ mass seems like a big hole in our model of the universe. It would be nice if more people were working on that.
On a different topic, I’ve become fascinated by galaxy clusters. Do you know of any good books/ papers/ review articles that talk about our understanding of these beasts? As always thankyou so much for your blog and research work.
Sorry to bother you. I bought “Galaxies in the Universe: An Introduction” Spark and Gallagher.
That is an excellent text.
does the visible mass of a galaxy agree with mass inferred from gravitational lensing ?
does MOND or dark matter agree with mass of galaxies from gravitational lensing?
what does MOND have to say about gravitational lensing?
https://tritonstation.com/2021/06/28/the-rar-extended-by-weak-lensing/
https://tritonstation.com/2024/07/10/the-radial-acceleration-relation-to-very-low-accelerations/
https://tritonstation.com/2024/06/27/tully-fisher-from-gravitational-lensing/
what about the bullet cluster where there is significant gravitational lensing not centered on the visible mass, implied that there is invisible mass ?
I believe we’ve had this conversation before.
There needs to be additional mass in clusters in MOND. That additional mass is a problem in itself, but if we accept it is there, then there is no reason to expect the lensing to be exactly centered on the stuff we can see when we know we can’t see some of the stuff. This is simply not a valid argument.
Clusters don’t make sense in any theory: https://tritonstation.com/2024/02/06/clusters-of-galaxies-ruin-everything/
Once again the obvious hierarchical structure is behind theories failures.
Dark matter genesis is the underlying assumption that General Relativity has an unlimited complexity range of applicability, this assumption (naive reductionism) is invalid in any theory, from quantum mechanics to GR.
Complexity is a source of irreducibility, this has been showed to be true in formal mathematics and there are tons of empirical evidence showing that physical Reality also follows that principle.
As Hegel mused quantitative changes eventually leads to a qualitative shift, succinctly put by P.W Anderson as More is Different.
Reality hierarchical structure can’t be ignored otherwise we’ll keep introducing fictitious things like dark matter or dark energy trying to extend the applicability range of a theory beyond its natural hierarchical level.
Nice post! Generally speaking, I gather that the MOND regime is where the local influence of mass on an object is nearly removed – since you found that the data for flat rotation curves extends nearly indefinitely. What is the Machian interpretation of this regime? Should the object’s inertia change to a value that depends only on the global distribution of matter? Or does the observer now become more involved in determining the object’s inertia?
Something like the former, that a particle’s effective inertial mass goes towards zero when it is not accelerated relative to everything else. One can even derive something that looks like the MOND interpolation function from this ansatz, though a mechanism remains lacking. See https://ui.adsabs.harvard.edu/abs/1994AnPhy.229..384M/abstract
Yep – we’re all just whistling Dixie at this point.
My (ignorant) 2c: Machs principle being a coincidense (eg meaning nothing) is a really stupid thought. It follows that inertia/mass is related somehow to the cosmic structure. And if its related to cosmic strucrure it is not crazy to thing its actually related to cosmic mass structure. Then it will be also related to local mass…simple 🙂
** Really thank you for this blog, I enjoy it.
Yes, Einstein had similar thoughts in a paper published in 1919. Not enough data at that time to pursue it properly. The idea was forgotten. But now we have excellent data, and we can revisit the idea. Local masses of electron, proton, neutron, etc, determined via Mach’s Principle from the rotation of Solar System bodies? Obviously crackpot, unless you approach the idea scientifically. MOND suggests that local masses of electron, proton, neutron etc determined by Mach’s Principle from the rotation of a galaxy are different. Why not explore this idea scientifically, mathematically, statistically, even historically? If it is wrong, it should be easy to falsify, surely? I’ve tried hard for ten years, and I can’t falsify it.
Same.
I’ve also had other physicists say something to the effect of “this must violate something fundamental.” Those who bother to check come back and say they can’t identify any such thing.
Yes. When people react without thought, they are apt to think of something as fundamental, which they may later realise is not quite as fundamental as they thought. The one that really gets me is the assumption in particle physics that forces are mediated by bosons. Apparently this is fundamental, but if you ask why it is fundamental, they have no answer. They “know” that forces are mediated by bosons, and not by fermions. But then they can’t find a (bosonic) graviton to mediate gravity. So what is going on? What does the experimental evidence say? Never mind the theory, look at the experimental evidence.
Look at MOND. Look at the MOND radius of a galaxy, the universe, the Solar System, a binary star system, a proton (why not? Milgrom did, in 1983). The MOND radius is proportional to the square root of the mass (Milgrom, 1983). In conformal theories, mass is a component of a vector. The square root of mass is therefore a property of a spinor. Massless (or near enough) spinors are neutrinos. So how do two astronomical bodies exchange information about their masses, in order to decide who is going to orbit around whom? It has to be the neutrinos, because nobody else is available to do the job. Never mind that they are fermions and there is a “law” that says fermions are not allowed to exert force. The law is an ass.
Ok then more shameless offtopic speculation : IF mass is related to the distance to other mass (and IF again that does not break something fundamental) then in an expanding universe mass is decreasing (?!) . An accelerated decrease . Can the energy equivalent of this decrease in mass somehow be the dark energy?
There have always been problems with Mach’s principle – at a conference in ’93 there turned out to be various incompatible versions. That’s not a good sign. And there’s reason to think inertia and mass are in the relationship of change and time – one is the concept/quality, the other’s the quantifiable aspect. Nevertheless, at present we officially take one to be a small-scale phenomenon, and the other to be a very large scale one. But it may be that inertia has got behind mass: several concepts have been moving towards small-scale physics, they may all end up there. Since the ’90s attempts to re-describe inertia, such as being due the quantum vacuum, haven’t done well enough to replace Mach, but they have shown that inertia may end up small-scale, just as the Higgs field, with a lot more certainty, has done with mass. And gravity may need to go the same way.
Indeed, Mach’s Principle by itself is inadequate. One generic result to emerge from attempts to modify inertia is that such a theory has to be inherently nonlocal. That causes a negative gut reaction, as it has lots of unpleasant consequences – e.g., one needs to know a particle’s past trajectory to calculate the force it experiences now, not just the current location in a gravitational potential. I didn’t want to face up to that (and simulators absolutely do not want to attempt it, for obvious reasons), but I haven’t found any fundamental objection and there is no guarantee that the universe has to make this easy for us.
I don’t know what applies when trying to modify inertia, but if we assume inertia is a long-range effect as in Mach’s principle, there’s the problem of how matter ‘knows which way to move’ as if instantly. If there’s a lightspeed delay, matter responds to an out-of-date version of the universe, which brings problems – if not, there are other problems. Suppose we say inertia is unexplained, but it may be a local effect (I’ve just submitted a paper that includes a definition for inertia, it’s not on a preprint site at present). Why does a modified inertia theory have to be inherently non-local – is that assumptions related to Mach, Milgrom or something else – is it needed for the result to interpret MOND?
Modified inertial theories have to be nonlocal to conserve momentum.
https://ui.adsabs.harvard.edu/abs/1994AnPhy.229..384M/abstract
Thanks. So it seems that only if MOND arises from a change to inertia, is it difficult to conserve momentum – otherwise the problem doesn’t arise. Perhaps the only other possible need for a non-local view of inertia is if we’re still trying to make something like Mach’s principle work – even though the Higgs field has now shown mass (inertia’s partner-concept), to be very short range. So it looks like there’s room for inertia to be a so-far-unexplained local effect, which would remove a few problems.
I think that’s the point – the evidence is that matter actually does respond to an out of date version of the universe. That’s how I interpret Yasher Ahalom’s theory of retarded gravity, anyway, and that seems to be very successful at explaining a lot of things. If signals cannot travel faster than light, then it is impossible for matter to respond instantaneously to something that happens at a distance. “Non-locality” seems to be the current buzzword to describe this phenomenon, but I think “non-locality” is an illusion caused by not having a consistent Machian definition of “locality”.
Retarded gravity has only been found to work in the solar system a short distance from Earth, and more recently via a probe a short distance from Jupiter. Two different theories (one by Hafele, of Hafele and Keating) found a lightspeed delay to gravity solves the flyby anomaly, near a rotating planet. The effect was found by the NASA team to relate to latitude (in my picture the emitted medium behaves differently when the mass is rotating). Anywhere else in the solar system, retarded gravity would make it fall apart. The immediate effect worried Newton, GR has a way out of it, which makes gravity behave more or less as if instantaneous.
Mach’s principle is like thermodynamics for the arrow of time. It’s not very good, but it’s the best thing we’ve got, so some put it the hole in the jigsaw. But if you leave holes, with room for new ideas, and admit we don’t know yet, progress is quicker.
Both Mach’s Principle and the Holographic Principle are inherently nonlocal, but the latter may be simpler and requires one fewer dimension.
Yes, perhaps that is a better way of looking at the problem. All physical processes happen at a time that is called (locally) “now”, as a result of influences that have arrived from the rest of the universe, but have taken arbitrary lengths of time to arrive. Here and now we cannot tell how long they took, or how far they have travelled, which reduces the effective dimension by one.
Mach’s principle is only non-local if you think there is a universal concept of “now” and that we need to measure rotations with respect to how the universe is “now”. But a physical realisation of Mach’s Principle requires a signal, in which case you might as well draw a large sphere, and consider the signal crossing that sphere as the only information we have about the outside universe.
At least, that’s how I understand the holographic principle. Perhaps I am wrong.
Yes, and it may just be that the cosmological observations that we can make here and now are what is consistent with the holographic principle. For example, the often referenced Nariai solution in Schwarzschild de Sitter spacetime admits a cosmic horizon that transforms into a smaller black hole. Maybe this is what evolves through our observation as we attempt to retrace the universe’s evolution.
Mond is nonlocal too. Isn’t it. The EFE and all.
The EFE isn’t non-local, it’s about what happens in a particular place – at the MOND radius of the internal field. The question of whether gravity is now weak enough for MOND to kick in, becomes the question of whether the combined fields are, rather than just the internal field.
The EFE includes the acceleration from non-local sources, so does break the principle of location invariance. That is, if one imagines a gravitational experiment (e.g., the dynamics of a dwarf galaxy) that is performed in the depths of intergalactic space and then again in the presence of an external field less stronger than the internal field of the experiment, one would get a different result. That is, if you take a dwarf that is isolated and ask what its velocity dispersion is, you get one answer; if you then put that dwarf in orbit around a massive galaxy like the Milky Way, you get a different answer. There is some evidence for this in the dwarfs of Andromeda, where there are pairs of photometrically indistinguishable dwarfs where one is more in the isolated regime and the other more in the EFE regime, and one does see the difference – https://iopscience.iop.org/article/10.1088/0004-637X/775/2/139
This is why MOND breaks the Strong Equivalence Principle (but not the Weak or Einstein EP). If the SEP held, there would be no difference between identical dwarfs located in different places.
Won’t having different local masses of particles within a single galaxy depending on their location within the galaxy cause changes to their atomic spectra. We certainly don’t see this for spectra of stars in our own galaxy, or in absorption by clouds of interstellar gas.
It doesn’t have to. It all depends on the model. At present mass is assumed to be the cause of lots of things, like gravity and atomic spectra, but if it turns out to be more like an effect than a cause, the models will have to be re-written. Clearly it is not possible just to put a variable mass into the current standard models, because that is easily falsified by observations.
It’s hard to imagine a way for the EFE not to be about field combining. Measurements show you have to take both fields into account. Whatever one thinks a gravitational field is, even without an interpretation for MOND, the EFE seems to give us some clues about how fields combine. It shows that they do, and that it make a difference when they do – to whatever is going on in MOND. That narrows the possibilities down, both about MOND and about gravity in general.
I agree. If we think of the gravitational field as a signal (as we do for the electromagnetic field, using real or virtual photons), then there comes a point where these signals interfere with each other. If there is a weak signal in a strong background, or a strong signal in a weak background, then we don’t have to worry too much. But if the signal and the background (noise) are comparable, then we do.
Off topic. Is it really the case that if we compare all the possible versions of MOND we have/know, including the relativistic ones and superfluid DM etc., that the first Milgrom model from 1980s fits the data best?
Yes, at least insofar as that simple formula captures a very broad range of phenomena. The difficulty comes when you try to build a theory that does just that. Modified Poisson theories like AQUAL and QUMOND make distinct predictions, but they are subtle enough that the “yes” is fairly qualified, but still: just using the original formula gets one a very long way.
Milgrom discussed tests to distinguish them in https://ui.adsabs.harvard.edu/abs/2012PhRvL.109y1103M/abstract I tried applying this but gave up, because it was hard to know the asymptotic Vflat (very far away) independently from the baryonic mass. It was also attempted in https://ui.adsabs.harvard.edu/abs/2020A%26A…636A..56P/abstract
Some people have asked me to comment on the Scientific American article What if We Never Find Dark Matter? by Slatyer & Tait.
what is your thoughts on sterile or right hand neutrinos, the likelihood of discovery-that succeeds as Dark Matter? its probably the best motivated explanation for neutrino mass via seesaw and experiments are under way
I’ve seen the evidence for these things seesaw so often that I have no strong opinion. Sometimes there seems to be good evidence for sterile neutrinos, sometimes not. They seem unlikely to me a priori, but then so does a finite neutrino mass.
At the core of all these models is the problem of reconciling the Einstein picture of spacetime with the Dirac picture. If you just assume these pictures are the same, you get contradictions coming out of your ears, and then you start adding more and more epicycles to your theory to get rid of these contradictions, and you end up tying yourself in knots.
Some thoughts that come to mind regarding all this:
First, that we theoretical computer scientists really ought to get ourselves some space probes — ones that can peer directly into the face of God and report back to us on whether P=BPP, whether BQP is in AM, and so on. What the astronomers do feels like cheating to me, like peeking at the answers in the back of the book.
Second, that space is where the excitement is. Of the four big discoveries of the past decades — dark matter, dark energy, neutrino mass, and gravitational waves — the three were not predicted by theorists, who’ve been busily trying to explain them post hoc.
The third conclusion is that it’s time for a new religion: one that would celebrate the release of new astronomical data as an event roughly analogous to Moses descending from Sinai with new tablets in hand. Seriously — am I the only person who sees measuring e.g. the CMB fluctuations as a religious obligation?
I do not think the material origin of everything if explained by a mechanism has any meaning for religion. Ethics are the most notorious and essential aspect of religion. There is no room for etthics without a sense of teleology – what is the purpose of life?
Anyhow, belief is better than religion. I find no difficulty in believing in the use of MI theory as Milgrom introduces it, even if it doesn’t give satisfactory explanations yet for how or why inertia depends nonlocally on the environment.
I do agree however that astronomical observations have religious meaning due to the sheer size and detailedness of everything in the universe.
By the way, when Moses came with the stone tablets the people of Israel weren’t really celebrating, because they had instead chosen to rever an idol. For who believes that story, dark matter theorists were far from the first ones deluded into structurally ignoring evidence because that’s the habit and the new evidence breaks their preferred way of working.
Cosmology is where science, philosophy, and religion all overlap. It is very hard to disentangle the science of cosmology from our preconceived beliefs about how it should be – even if those initial beliefs are scientifically informed.
For many practicing cosmologists, the field has become largely synonymous with large scale structure, in which galaxies are mere tracer particles. Hence my paraphrase of Tolkein’s Ainulindalë:
“A single galaxy might seem a little thing to those who consider only the immeasurable vastness of the universe, and not the minute precision to which all things therein are shaped.”
You could add a fourth: Theoretical Math.
It is helpful if one has had an ‘ah ha!’ moment; for example realizing a religious conviction is as quaint as a flat earth, in order to reshape the thought processes in our own minds in a way lets the air out of disproven convictions.
Today in archives: arXiv:2410.14773
“We rule out cosmic variance at the 3-σ level, providing spectroscopic confirmation that galaxy formation models do not match observations at z>3. ”
The “little red dots” are not angels dancing on pins. They are full-up galaxies, some severely aged and eerily similar to our own neighborhood if you ignore the scaling.
They are likely not little either, but dynamically scaled wrongly within this failed model that we try to interpret them in.
The data keep showing that there are massive galaxies with comparatively old stellar populations in place at high redshift. Their linear sizes are weird, as you say, which depends on the metric. It’s almost as if FLRW cosmology is breaking down. Some more. Again.
Stacy, this paper is intriguing: https://arxiv.org/abs/2408.00358 It explains rotation curves without any modification to general relativity. What do you think about it?
I think attempts along these lines have consistently fallen flat. Whether that is the case here or not I cannot immediately say. It seems unlikely, as relativistic corrections are typically of order (v/c)^2, which is one part in a million for giant galaxies and much less for dwarfs which are more dark matter dominated. This does sound different, but one does have to explain a lot of things besides rotation curves.
There is no spin 2 graviton. What experiment actually sees, when you look far enough out in the galaxy, is spin 1 (Newtonian gravity) plus spin 1/2 plus spin 0. That corresponds to conformal covariance (i.e. Lorentz covariance plus scale covariance). But if you still believe in general covariance, you have to add spin 1/2 plus spin 0 by hand.
Spin 1/2 is a spinor, so it behaves like matter, so you call it “dark matter”, because you can’t see it. The reason you can’t see it is because it isn’t matter, it is part of the gravitational field. You can’t “see” it, any more than you can “see” a photon. Spin 0 is a scalar, so it behaves like energy, so you call it “dark energy”, because it is very weak, so you don’t feel it.
Tritonstation I have a lot of admiration for you and your works.
I am aware of the various empirical evidence regarding MOND and how e.g. GAIA DR4 will solve the matter once and for all.
There are at least two kinds of omissions from your blog posts, that would be insightful to talk about:
1) There are many empirical studies showing gravitational anomalies (independently of MOND), such as mossboer rotors excess redshift, etc
https://www.scirp.org/journal/paperinformation?paperid=115246
https://www.researchgate.net/publication/353090350_Simple_experiment_to_demonstrate_that_light_acts_on_gravity
Those gravitational anomalies can give insights on the nature of gravity, e.g. wether it is lesagian cf the allais effect.
2) The idea that particle physics experiments on WIMPs have mostly ruled out the search space, the existence of dark matter is a popular misconception.
In principle what you are saying, such as sunk cost fallaxy, conflicts of interests and the huge non parsimonous epistemological cost of invoking new particles (ockam razor), is reasonable.
There are contrary to popular belief, countless experimental evidence for new physics in particle physics.
Here is a very non exhaustive summary
https://arxiv.org/abs/2309.03870
Most anomalous signals are direct empirical measures of a weak interaction caused by new particles. Wether those new particles can be numerous and or massive enough to reproduce dark matter cosmological observations is an open question but the existence of new particles is almost proven.
Contrary to the misconception, larger, higher energy colliders are not that useful as most anomalies found are far below TEVs hence the limiting factor is accuracy, which will soon be improved very significantly by luminosity upgrades.
NICA and FAIR are almost ready and will revolutionize particle physics.
They will allow the current anomalies to be confirmed at >5 sigmas hence proving once and for all the existence of new particles.
Now as to which particles are the best dark matter candidates?
A main one would be sterile neutrinos.
The other main candidate is strikingly underappreciated.
It is maximally ironic that the best candidate for dark matter happen to be a particle composite already predicted by the standard model and known to be abundant: the glueballs.
https://arxiv.org/abs/2306.09510
https://www.arxiv.org/abs/2408.14245
I personally believe that neither dark matter nor MOND explain the rotation of galaxies and that quantized inertia which has zero free parameter should be tested more extensively.
What everyone is missing is that the physics of rotation are anomalous and therefore that a physical effect that is rotation dependent is the most direct solution.
When galaxies rotations are sorted in their direction, it is shown that the hubble tension disappears. This cannot be ignored, it has to be asked, why does it works so well?
https://arxiv.org/abs/2407.20487
I am not an expert on all things pertaining to 1, so I refrain from expressing opinions on them.
Yes and no on 2.
Yes, there are anomalies, like the mass of neutrinos, that need to be sorted out and may well indicate physics beyond the Standard Model. That’s fine, but whatever that new physics is may have nothing to do with the missing mass problem.
No, it is a misconception to think that the objection to dark matter is based on non-detection of WIMPs. That’s not it at all. The objection is to the need for epicyclic fine-tuning in any DM-based model that merely invokes new particles to be extra mass. See, e.g., https://tritonstation.com/2024/08/12/whyd-it-have-to-be-mond/ This problem was established in the previous century (http://astroweb.case.edu/ssm/mond/mondvsDM.html), long before people got worried about WIMP non-detections.
There is a common misconception in the field, that all we need to do is come up with a new particle that can be the dark matter. That does not suffice. Whatever it is must also naturally explain the observed phenomenology, which is not restricted to rotational motion – https://tritonstation.com/2018/09/14/dwarf-satellite-galaxies-iii-the-dwarfs-of-andromeda/