This is an update to a post from a few years ago, which itself was an update to a webpage I wrote in 2008, with many updates in between. At that time, the goalposts for detecting WIMPs had already moved repeatedly. I felt some need then to write down a brief synopsis of the history of a beloved hypothesis (including by myself) that had obviously failed as the goalposts were in motion again. That was sixteen years ago.
It is important to remember where we started from, which is now ancient history lost in the myths of time to most who are now working in the field. Indeed, when I search for mention of the WIMP miracle, the theoretical argument that launched a thousand underground detection experiments, little comes up: this essential element of the field has been memory-holed after its failure. I suppose that’s to be expected, as the same thing happened with the decay of the B0 meson: once heralded as the “golden test” for supersymmetry, it simply stopped getting mentioned after it didn’t work out.
The original expectation for WIMPs was a particle of mass around 100 GeV/c2 with an interaction cross-section of about 10-39 cm2. While I remember this, it is getting rare to find this statement, so let me quote a particle physicist:
“The most appealing possibility – a weak scale dark matter particle interacting with matter via Z-boson exchange – leads to the cross section of order 10-39 cm2“
To translate a little bit, the Z-boson is a carrier of the weak nuclear force (as photons are for electromagnetism), so this envisions an otherwise normal interaction that involves a new particle, the WIMP. The weak force is, well, weak, so the interaction probability is small, as quantified by the tiny cross section of 10-39 cm2. That makes such interactions rare, but particle physicists are talented at detecting such phenomena. It helps to have a lot of target material in your detector in a place that is well-shielded from background interference, hence all the giant underground WIMP experiments. Consequently, to continue the quote above,
“the cross section of order 10-39 cm2 … was excluded back in the 80s by the first round of dark matter experiments.”
And so the goalposts were set in motion. There were many steps along this path, so I’ll highlight only one, circa 2008. To complete the quote from Resonaances,
“There exists another natural possibility for WIMP dark matter: a particle interacting via Higgs boson exchange. This would lead to the cross section in the 10-42 – 10-46 cm2 ballpark (depending on the Higgs mass and on the coupling of dark matter to the Higgs).”
So the interaction via the Z-boson had been excluded, but one can have other interactions, this one via the Higgs (which had not quite yet been detected: discovery was in 2012; the Resonaances quote is from 2011. Since then, the Higgs might be said to be “too normal” to make room for any of this.) The possibility of Higgs exchange leads to the blue-green predicted region of Trotta et al. (2008) in the exclusion diagram shown below. If one looks for such plots in the literature, one finds a natural tendency for their upper limits to migrate downwards along with the limits they portray. I thought it might be instructive to update the plot to show the full range of progress:
I call out the 2008 threshold because we had a conference here at CWRU in 2009 (while I was at the University of Maryland) at which the Trotta et al. prediction was presented. I had already become skeptical of the moving goalposts, so I wondered how much of the probability density was in the tail to low cross-section. A low-likelihood tail seems a lot more probable once the head is lopped off! I made this point at the time, and asked how important the tail was. The answer was about 2% or the probability. The speaker went on to express the usual overconfidence that WIMPs would be detected in the more likely region (marked by an X in the blue region with the handy arrow pointing to it).
The experimentalists have done a fabulous job in increasing the sensitivity of their experiments so that they can see to ever lower interaction cross section. Had WIMPs existed as predicted initially, or subsequently, they would have been detected by now. These experiments have succeeded in failing quite brilliantly. I had long before shown that the astronomical data did not add up for any flavor of dark matter. Maybe WIMPs don’t live in this universe?
While we’d be happy to detect dark matter anywhere in parameter space, the WIMP does have sweet spots: first 10-39 cm2 then 10-44 cm2. Now that those are gone, what’s next? From the particle physics perspective, I’ve heard it said that the next logical expectation for the cross-section is around 10-48 cm2. This apparently follows from “two-loop corrections.” I have only a vague idea of what that means, but in my practical experience it translates to “a difficult-to-compute effect so exotic that it likely has no bearing on reality, except maybe in the sixth place of decimals.”
More generally, this continual moving of the cross section goalpost is what I meant back in 2008 by the scientific version of the express elevator to hell. It just keeps going down, and can do so forever. I keep warning my colleagues about these things, and they keep not heeding the warnings. Being a scientific Cassandra is getting old.
The problem with pushing detection limits to still lower cross-sections like 10-48 cm2 is that the universe is indeed full of weakly interacting particles with at least a little bit of mass: neutrinos. These are not as massive as WIMPs, and should not be confused with them: neutrinos are Standard Model particles that are known to exist and to have a very small mass (< 1 eV) while WIMPs are expected to be hundreds of GeV and require entirely new physics beyond the Standard Model. I shouldn’t need to say this, but WIMPs and neutrinos are very different beasts. However, they do both have mass and interact weakly, so I’ve noticed that some of the more rabid advocates of dark matter mix these two in order to claim that we know weakly interacting dark matter exists. That much is technically true, but in technical parlance it is also some bold bullshit. Hmmm, actually, I think it is worse than ordinary bullshit. It is willful scientific disinformation that intentionally sews confusion by conflating the unconfirmed existence of WIMPs with the known existence of neutrinos in order to lend an air of certainty to a failed hypothesis.
Meanwhile, experimental progress proceeds apace. The coming generation of WIMP detectors should be sensitive to the solar and atmospheric neutrino background. That is astrophysically interesting, as it can probe nuclear reactions in the sun and, in principle, those in every supernova that have ever exploded. This has bugger all to do with dark matter. However, since that’s what people are looking for, what they built these detectors to find, and they’re completely convinced dark matter exists, and a Nobel prize awaits whoever detects it first, I expect that the first neutrino detections will be misinterpreted as WIMP detections. There will be much arguing between groups, claims and counterclaims, and after a few years it will be recognized that these coming detections are neutrinos not WIMPs. First there will probably be many over-hyped claims that mislead the public into thinking dark matter has been detected.
But there I go being a scientific Cassandra again.
I am sure you discussed it before. Could you just provide the link to why neutrinos cannot be the dark matter?
No link, but they are too hot and thus moving too fast to be captured in the gravity of a galaxy.
Yes, this. Relativistic dark matter deters structure formation as the particles don’t move slowly enough to clump up on small (galaxy) scales.
Separately, neutrinos are fermions and subject to a close-packing limit imposed by the Pauli Exclusion Principle. A less well-known problem for neutrinos as dark matter is that there are some regions where the dark matter density is too high to be explained by fermions of neutrino mass. They can’t be packed in tightly enough.
Oh fun, so a neutrino of mass 1eV would have a Compton wavelength of (scribbles) ~ 10e-6 meters (one micron). I’d never thought about that. So as a semi-out-there idea what if the graviton has a tiny bit of mass. (I was listening to Claudia de Rham on Sean Carroll’s podcast where she talks about a graviton mass in the ~10e-30 to 10e-33 (eV) range*) A little graviton mass would be nice, first off because it gives a range limit to gravity and that worrisome logarithmic divergence in MoND now has a finite limit. And maybe MoND could then be some type of phase transition. Something like Bose-Einstein condensation maybe. k_B*T<(h_bar^2 / 2m) *(N/V)^2/3 (Thermal energy becomes less than ‘particle overlap energy’)
Where m is the graviton mass and N/V is the graviton density. One could maybe replace the thermal energy with the acceleration energy of the graviton m*a*L, where we hope to pop out a_0 (10e-10 m/s^2) in the end. But I have no idea what length scale (L) to use, nor what the graviton density is.
*See references 15 and 16 here… I didn’t get past the first few sentences. https://en.wikipedia.org/wiki/Claudia_de_Rham
Just as an aside, if I put numbers into the above I get a stupidly low number for the graviton density like 10e-75 m. Something like one graviton in the whole universe… so clearly this wrong. (or I made a math mistake.)
Yes, one of the ideas for inducing MOND-like behavior from dark matter is a Bose-Einstein condensate of particles so light they have astronomical Compton wavelengths. Superfluid DM is one example; there are a few others.
Yeah I read some Superfluid DM stuff. The condensate is in the galaxy and that makes the DM. This silly idea is much crazier than that. I think maybe there is no DM and gravity just changes at low acceleration. And it changes character because gravitons have some sort of phase transition at low acceleration, which is similar to low temperature. And the final maybe, maybe it’s a BE condensate. I think it has to be of primordial gravitons, like some left over background, CMB-esque. But this is all just me talking out of my a$$.
There are qualitative differences between a graviton with zero mass and a graviton with non-zero mass however slight that could pose a problem.
But the fact that dark matter particle candidates like Fuzzy Dark Matter are increasingly favoring a dark matter particle that is a boson with a mass comparable to the mass-energy of a vanilla graviton is pretty notable (and of course, the energy of a graviton gravitates) and shows some theory convergence.
So, is it true in general that fermionic dark matter particles with particle masses of less than a neutrino (i.e. probably on the order of under 1 meV) are excluded as dark matter candidates?
I’m not aware of a particle physics reason against them.
Stacy, if WIMPs are excluded, there remain three options to explain rotation cirves: measurement issues, MOND and Verlinde’s emergent gravity. Can the data distinguish between the last two cases? (Excuse me if you answered this already.)
Oops – four – as black holes is still another possibility for black matter.
There are an endless number of possibilities for dark matter. WIMPs have long been the leading candidate, but if they are excluded, one can always make up something else.
Verlinde’s emergent gravity, as currently understood, predicts an additional force around a0 that is above what MOND predicts, so they can in principle be distinguished, and the data sit less well with emergent gravity. See, e.g., https://arxiv.org/abs/1702.04355
Presumably these experiments leave open the possibility that the ‘dark matter particle’ only interacts gravitationally. Awkward for the experimenters but seems possible.
Yes, one can only experimentally exclude particles that interact with some force other than gravity.
This possibility was considered so absurd 20 years ago that it went unmentioned. I heard people start to whisper about it after that. About a decade ago, a colleague who is a leader in WIMP detection experiments mentioned it in a talk as “the nightmare scenario” and discussed it no further. Since then, people have been talking it up more and more. I call these Angel particles (https://tritonstation.com/2022/11/29/the-angel-particle/) because this hypothesis, while logically valid, removes us entirely from the regime of physical testing: we might as well count the number of angels dancing on the head of a pin.
Would this be distinguishable from a modified gravity theory?
Yes. Modified gravity theories predict a unique correspondence between dynamics and the visible source of the gravitational potential. Dark matter particles that only interact through gravity need do no such thing.
You can still observationally exclude particles that don’t interact by any means other than gravity, generically, by showing that the correlations between inferred dark matter distributions and observable ordinary matter distributions are too tight to be produced solely by gravitational interactions. It is a very different and indirect and somewhat model dependent proof but can be robust enough if one works at it. Some researchers have reached that conclusion.
Yes, I am one of those people. The issue is getting other scientists to understand the arguments against particle dark matter.
I’m not sure what these correlations between (hypothetical) dark matter distributions and baryonic matter are. Are you thinking of the ones that illustrate Renzo’s rule? Thank you for your attention.
Yes, Renzo’s Rule, along with the Baryonic Tully-Fisher Relation, the Central Density Relation, and the Radial Acceleration Relation. If you get that last one, the others follow.
Though I neglected to link to it, I give a brief synopsis of what we require for dark matter at https://tritonstation.com/2023/08/17/required-dark-matter-properties/. Dynamically cold and non-baryonic are the usual suspects, but at this point we also need something that automatically leads to MOND-like behavior in galaxies. This is not something that LCDM galaxy simulations provide an adequate explanation for, nor do I see how they can ever legitimately hope to do so without fine-tuning.
Sorry to hijack the thread but it looks like I can’t leave a top comment?
Off topic with the post but too important not too mention,
As you must be aware, a Z = 14.32 massive and hyperluminous galaxy has been spectroscopically confirmed.
https://github.com/orgs/a-cosmology-group/discussions/225
With the hubble constant of 73 km (1% error) it means the galaxy is only 254 millions years old.
If we do a basic extrapolation from this paper
https://arxiv.org/abs/2405.12665
It means the galaxy is at least between 650 to 2150 millions years older than the big bang.
There is one paper that attempt to “explain” the galaxy https://arxiv.org/abs/2405.20370
using hypothetical mechanisms of unclear (to me) plausibility
IMO this galaxy fully falsify LCDM but would be nice to quantify how unplausible it is.
Working on it. I have a paper about these high-z galaxies that has been a long time in prep. It doesn’t make it easier that we keep discovering these things!
So first, this is exactly the kind of thing MOND predicts, as discussed in https://tritonstation.com/2022/01/03/what-jwst-will-see/
Secondly, the age of the galaxy depends on both the Hubble constant and the shape of the expansion history. That we need a bit more cosmic time is one of the reasons we brought back Lambda: https://tritonstation.com/2019/01/28/a-personal-recollection-of-how-we-learned-to-stop-worrying-and-love-the-lambda/. Long story short, I don’t think the galaxy can be adjudicated to be older than the universe in this fashion, though it is conceivable that the parameters required to accommodate it would be inconsistent with LCDM. The bigger problem is that it appears to be massive so soon after the Big Bang.
Perhaps we should all take a closer look at the big bang?
That’s quite a funny statement actually 😀
Good luck getting closer to 13.8 billion years past & lightyears away and both increasing with the speed of light! It’s the reason that with all the tough work, large collaborations and sophisticated data analysis that went into the Lambda-CDM version of the big bang, I still doubt we can be sure of the main idea of what truly happened. Inflation is a good idea, but highly unfalsifiable, and the extremely short time it is said to have applied to the universe raises good questions.
Still, it’s of course the current best scientific theory on the origin of the universe.
The big bang is not very “scientific”. A lot of scientific data and research has been selected to comply with a desire to support any origin story.
The Big Bang is a very unscientific model based on 100 year old assumptions that contradict well established physics. None of the defining elements of the model are observables.
Unfortunately, in the scientific community it is detrimental to one’s career to question the underlying paradigm of an “expanding universe.” Scientific evaluation is not allowed wrt the standard cosmological model – it is de facto forbidden.
To put things in perspective it should be noted that the only motivation for the failed DM hypothesis was (is) the hubristic conceit, now falsified, that our gravitational theories (Newton-Einstein), derived without any real knowledge of the Cosmos, are “universal laws”.
This point was made by Tohline forty years ago at IAU Symposium 100 in a question posed to Vera Rubin (see the very end). Her response concluded with
“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.”
I think that was the correct attitude at the time. It persists to this day as a widely held attitude; I’ve even heard the term “last resort” come up repeatedly. Which begs the question, what exactly would be the last resort?
Exactly, but naive reductionism is deeply ingrained in mainstream thinking. Theories always have a limited range of applicability, at least for two reasons: first assumptions are impossible to validate at unlimited range and second complexity is a boundary for theories that always have been conceived and tested with very simple conditions.
The great PW Anderson is always there to remain us that “More is different” but dogmatic mindsets never listen.
Dark Matter is a Ricci soliton. https://iopscience.iop.org/article/10.1088/1402-4896/ad1d45
could coming generation of WIMP detectors also included sterile neutrino?
if they exist what do they imply about MOND
No – sterile neutrinos are called sterile because they don’t interact through the weak force, so WIMP detectors are blind to them.
Sterile neutrinos most interesting potential impact on the problem would be if they exist with a cosmic mass density in excess of the limits on structure formation. Too much mass density in a neutrino (normal or sterile*) would falsify the standard structure formation paradigm, and LCDM with it.
For MOND, sterile neutrinos would just be another component of mass. They are sometimes invoked to fit the CMB, but I don’t think that really works. A more interesting effect could be to contribute to the mass budget in clusters of galaxies, perhaps explaining the residual discrepancy MOND suffers there.
*This statement assumes a mass for sterile neutrinos comparable to that of normal neutrinos (sub-eV). This need not be true since we are in the realm of made-up particles; we can make up all sorts of things. A sterile neutrino might be a form of warm dark matter if heavy enough (~ keV).
hi
thanks for the reply
if Sterile neutrinos existed and explain all the missing mass, is there any need for MOND? couldn’t GR + Sterile neutrinos explains the observations ?
how could they be detected
Sterile neutrinos could be the dark matter, but just having dark matter (of any flavor) does not suffice to explain the observations. We need to understand why MONDian behavior happens, why MOND has any predictive power at all. Just having dark matter doesn’t do that.
https://tritonstation.com/2023/01/05/question-of-the-year-and-a-challenge/
is both sterile neutrinos and MOND compatible together with all observations including third peak of CMB and galaxies clusters
No, not really. When Angus (2009) hypothesized sterile neutrinos to explain the CMB in a MONDian universe, it was a reasonable match to the WMAP data that were then available. It comes close but doesn’t really fit the Planck data. So while an extra form of mass like this would help with the cluster problem, I think one needs a proper relativistic theory that incorporates MOND to even begin to address the CMB (e.g., the Aether-Scalar-Tensor theory of Skordis & Zlosnik).
Try Higgs charge universe phase locked at CMB temperature as “warpable” structure of space as dark matter. And dislocations in structure as standard model matter.
Dark matter is the real either.
As the possibilities for DM steadily get ruled out, this contradiction you point out becomes more important. It might seem depressing, but if one really wants to know what’s out there, limiting the possibilities down is good, not bad:
‘We have two very different requirements on the dark matter. From a cosmological perspective, we need it to be dynamically cold. Something non baryonic that does not interact with photons or easily with baryons. From a galactic perspective, we need something that knows intimately about what the baryons are doing […] So that’s where we’re at right now. These two requirements are both imperative – and contradictory’[3].
As they’re imperative, that might tell us something. I quoted this in the introduction of a paper that was submitted to a good journal (if anyone wants to advise about posting it on the arXiv, I’d be grateful), and followed it with this:
———–
So it seems the need is to explain how DM can be both unconnected to the visible matter, and also closely connected to it. According to PSG[4], the visible matter emits what is taken to be DM. It is emitted consistently, in a way related to mass distribution, and this explains the connection. But once emitted, it hardly interacts with baryons, and this explains the non-connection. At larger scales an excess builds up, and the original link is less apparent – the contradiction is partly about the relationship DM has with visible matter at two different scales.
—————-
Would you comment on this possibility, lateral though it is? It’s arguable that lateral conceptual ideas are needed now. Could it fit with elements of the far more detailed picture that you have?
Attempts to get people to listen, what with the near-proof, sometimes feel like an irresistible force meeting an immovable weight – but I know you’ve experienced something similar in other areas.
I guess the important thing here is the mechanism. How does the required behavior follow from this hypothesis?
Thanks, I’m trying to understand what you mean by the required behaviour. I know all about the hypothesis (it’s a theory, it makes predictions for experiments, comparatively minor funding required for that). I guess one question is, as cosmology is less my field, if you look at a large-scale map, is it possible to say if the DM behaves as if it was emitted?
I know in the standard picture it starts with DM, then clusters form at the intersections. There are a lot of problems with that, but my picture is sketchy too. In my picture it’s the other way round, the clusters come first, then they emit ‘DM’, which flows to make the filaments. It is constantly dissipating and being replaced by more behind it.
I guess a general question is, can you see any way to check the theory in relation to data? I have equations for quite a few aspects of the medium at any point in a Newtonian field, not all. One particular rate of change related to the medium leads to the inverse square law. (Also some for the MOND regime, where the field effectively gets deformed out of shape. That aspect of MOND is there in the mathematics, but far more relevant in PSG than in most theories, as the field is made of radially travelling waves.)
Light is a good tracer of mass on large scales, to the extent we can discern, so I guess they could go together in that sense. In an expanding universe, one does need something extra, be it dark matter or modified gravity, to give structure formation a boost. So in that regard, a definitive feature of the DM paradigm is that dark halos form first and normal matter follows later. So that’s an issue to consider if you want to invert things.
There’s something in my picture I won’t go into that boosts structure formation – I know MOND provides something like that, with higher effective masses etc. Anyway, I’ll send you the final draft, might be of interest, sorry I sent a version before it was ready. You might find something that can be compared with data, in the clear cut areas of it, which are not the early universe. Thanks
Try this one on:
Space is massive, and not particulate – the tail ends of baryons – linking its density to distributions by baryonic Tully-Fisher.
Explains how matter determines space.
a₀² is proportional to a limiting energy density due to all matter in the universe – RAR.
Space is massive and “locked” in place.
Slightly off topic: you haven’t written much about axions as dark matter candidates. What do you think about that? Is this a similar story to WIMPs ( an unfalsifiable hypothesis), or are there compelling reasons why axions should be dark matter?
Axions are the next most candidate after WIMPs in terms of logical possibilities we thought up early on. From the perspective of particle physics, it is a reasonable thing to consider next, and there are plenty of axion detection experiments going on or ramping up. From the perspective of the astronomical data that didn’t go into inventing cold dark matter – the need to explain the close coupling of dynamics with normal matter – axions don’t help; they’re just another flavor of extra mass that lacks this critical feature.