If you’re sick of hearing about bogus wormholes, here are some other random topics:
- There’s a SCOAP3 for books initiative, lots of textbooks in the field of particle physics now made open access and available here.
- It’s completely mysterious to me why this is so, but Mochizuki’s failed proof of abc is now getting promoted at various places outside Kyoto, including Stony Brook, France (note what comes first in the suggested survey articles) and maybe even Oberwolfach.
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One thing I’ve never understood about the dark matter story is whether just having right-handed neutrino fields (which fit very well into the pattern of fields of the Standard Model) is enough to get fundamental physics to be consistent with astrophysical observations usually interpreted as evidence of dark matter. Stacy McGaugh despises this kind of idea, where you predict particles with only gravitational interactions, making fun of it here as The Angel Particle. He has a specific agenda (modified gravity), which I think explains the vehemence.
Ethan Siegel has an extensive discussion of gravity-only dark matter models, under the title Is dark matter’s “nightmare scenario” true?
John Baez has a blog post on Neutrino Dark Matter, based on talking to Neil Turok about this recent paper by Boyle, Finn and Turok.
- The National Academy has appointed an EPP-2024 committee, which has been holding various meetings. Taking a look at some of the videos, I was struck by what Lisa Randall (about 34:00-38:00 and 46:00-48:00) and Nima Arkani-Hamed (about 48:00-53:00) had to say in this one. They both emphasized the problem that it is hard for young theorists to make a career for themselves by trying to come up with a new idea, as opposed to a series of small improvements on ideas currently being pursued by many other people (often things that have been around for a long time). This I think has been a huge problem in the field for a very long time, possibly has gotten worse for some reason in recent years, and is part of the explanation for the moribund state of the subject. While they eloquently state the problem, unfortunately I don’t see much from them or anyone else about how to do something concrete to address it.
Thanks for the SCOAP link – what a fantastic resource – you have now expanded my list of unread theoretical physics books by 33%.
Did you notice that none other than Maria Spiropulu is co-chair of the EPP-2024 committee?
Frank,
Yes. Other fun fact: that video I watched and linked to was recorded on Nov. 30, the day of the wormhole announcement. From what I saw, I thought Spiropulu did an excellent job of chairing the meeting, especially considering that she likely had other things on her mind that day.
But, I was trying to make this posting wormhole-free…
E. Siegel’s ” nightmare scenario” article considers only one possibility ( the “wimpzillas”). But there are other candidates for gravitation – only interacting dark matter like primordial black holes, or proposals about quasi-stable black hole remnants ( assuming that Hawking evaporation stops near the end stage ), or Penrose’s proposal.
It goes without saying that most ( if not all ) of these scenarios are more or less speculative, but the same holds for many other proposals that have to do with this subject ( including modified gravity and the like).
Well, at least at Stony Brook the promotion comes with some caution « Dr. Fumiharu Kato (…) spent some time explaining the approach of professor Shinichi Morizuki to prove it. We want to caution those watching the video that at the time of the lecture there is no universal consensus in the mathematical community that the ABC conjecture is proven. Some details and references can be found in https://en.wikipedia.org/wiki/Abc_conjecture. »
Best
Stacy McGaugh despises this kind of idea, where you predict particles with only gravitational interactions, making fun of it here as The Angel Particle. He has a specific agenda (modified gravity), which I think explains the vehemence.
In reading various blog posts and a few papers of his, I have found Stacy to be knowledgeable, fair minded and intellectually honest. My understanding is his disagreements with the theoretical cosmology community center around the willingness of many theorists to overlook data that disagree with the standard cosmological model (LCDM), who propose ad hoc “solutions” when observations/data disagree with simulations, who introduce unmotivated particles and fields for dark matter when the best motivated candidates (e.g., WIMPs) are ruled out or are no longer plausible, and yet who willingly ignore or denigrate modified gravity proposals that actually predict a wide range of phenomena that LCDM cannot naturally explain or that conflict with dark matter simulations.
From his writings, Stacy sees dark matter theory as a moving target that can always be fixed by introducing new hypotheses or fine tuning existing parameters whenever conflicts with observations arise, essentially avoiding falsification of dark matter as an explanation of observations. This theoretical “approach” probably looks familiar to you from your own observations of many HEP/BSM theorists.
What Stacy calls “angel particles” — particles that interact gravitationally but in no other way, effectively rendering them unobservable and non-falsifiable — would be another example of avoiding conflict between observations and dark matter theory without abandoning currently held views.
Stacy seems well aware that the key reason modified gravity is so disagreeable to cosmologists (beyond its conflicts with the approach they have devoted much of their careers to) is that it does not fit nicely with GR. And since GR is a very well tested theory that forms a backbone of fundamental physics, it is really problematic to propose without very strong evidence that GR is wrong in a domain where it is expected to be valid. He freely acknowledges in his blog posts that if one starts with GR as a governing fundamental theory for galactic scale structure and beyond, the need for dark matter is unavoidable. He says he, too, was strongly in the dark matter camp during the first part of his career, but repeated conflicts between observational data and dark matter phenomenology caused him to gradually change his thinking.
So I’m not sure it’s really fair to say “He has a specific agenda (modified gravity).”
One thing I’ve never understood about the dark matter story is whether just having right-handed neutrino fields (which fit very well into the pattern of fields of the Standard Model) is enough to get fundamental physics to be consistent with astrophysical observations usually interpreted as evidence of dark matter.
I’d be surprised if this is an unexplored idea. Why not ask Stacy if he is aware of observations that strongly disfavor or rule out the idea, assuming you have an idea/proposal for the neutrino mass? How would you test the idea observationally so that it is more than metaphysics, especially since we don’t currently know if right-handed neutrinos actually exist?
Marty Tysanner
The same objections you mentioned about DM are also equally valid for modified gravity proposals:
Those alternative theories that incorporate additional fields/ forces are obviously associated with unknown particles that need to be detected for any of these proposals to be tested and confirmed. So we can’t avoid the need for detection of unknown particles even in that case…
Another alternative ( for those who try to avoid this ) is theories with massive graviton(s) that are already on the verge of falsification.
Moreover, the so far proposed MOND -oriented models need also DM as an addition, to explain observations, ( I think that most MOND proponents admit that) so they’re just complicating things further.
Also, I’m not sure I’m following the reasoning about the “falsifiability” of only gravitationally-interacting DM:
Only detection via any of the three interactions of the standard model of particle physics is ” legitimate ” for the confirmation of existence of DM?
Gravity does not count as an interaction? What about alternative hypothetical “purely gravitational” theories without additional fields? Are they unfalsifiable too? Gravitational effects ( lensing etc ) do not count?
Ok, exactly because gravity is so “weak” there’s a need for additional observations, nobody disagrees with that.
But these criticisms seem to me a bit over the top.
As a dark matter phenomenologist, let me just remind everyone that the _primary reason_ to believe in the phenomenon of cold dark matter is the _very high precision_ with which we measure the CMB power spectrum, especially modes beyond the second acoustic peak. There is a stone-cold, qualitative, crystal clear prediction of CDM about the relative sizes of the second and third peaks that modified gravity _profoundly and irremediably_ gets wrong: it thinks the third peak should be relatively larger than the second (due to the difference between compression and rarefaction — see Wayne Hu’s tutorials for pictures), whereas CDM thinks they should be about the same or the third peak should be lower. They are, in fact, at _very high precision_ about the same. All the thousands of other modes confirm this picture (now up to ell of 2000 which hardly any deviations), but this _simple, qualitative fact_ is to me the most convincing way to see that we need a new matter-like field. Any talk I’ve ever seen by McGaugh (or more exotic modified gravity people like Verlinde) elides this fact, and they evade the questions when I put my hand up to ask. I have invited McGaugh to a conference before specifically to discuss this point, and he just doesn’t want to.
In fairness to McGaugh, he pointed out some very interesting features of galactic DM distributions that do deserve answers. But it turns out that there are a plurality of possibilities, from complex DM physics (self interactions) to unmodelable SM physics (stellar feedback, galaxy-galaxy interactions). There are no such alternatives to CDM to explain the CMB power spectrum.
As to Peter’s question of whether right-handed neutrinos could be the DM particle — sure, but you still have to explain why they’re generated in the quantity we measure from the CMB (this isn’t trivial). But yes, this is a credible possibility, the details of which are elaborated every day on the hep-ph arXiv. And interestingly, such models can have similar features to the self-interacting CDM mentioned above.
Marty Tysanner,
I have followed Stacy McGaugh’s blog for a while, much of his critique of dark matter models seems well-argued. This particular argument though seemed to me very unconvincing, partly because the ratio of sneering polemic to serious content was high, partly because right-handed neutrinos seem to be an extremely well-motivated theoretical idea, and particles with only gravitational interactions are not at all unobservable (they may very well be exactly what explains galaxy rotation curves, CMB peaks, etc).
If you want to argue against a massive right-handed neutrino model of dark matter, to me you need a much more serious argument.
clayton,
Thanks for your comments. I’ve always assumed this was a well-studied topic in the field, just never have had the time or energy to look into this. Two problems I wonder about are, given just the SM with right-handed neutrinos, and what we currently know about mass terms (from Higgs couplings)
1. can you get the right amount of dark matter production in very early universe models?
2. in the later universe, is the way this dark matter evolves consistent with everything you observe?
Hi. This is an interesting discussion. I think it goes a long way to illustrate how much we take for granted in our own experience that is not shared with others.
My distaste for the angel particle has nothing to do with right-handed neutrinos or any position on modified gravity. It has everything to do with my lived experience with the search for WIMP dark matter, in which the goalposts have been moved time and time again – chiefly to lower interaction cross-section. So, I reasoned (long ago – see this 2008 page: http://astroweb.case.edu/ssm/darkmatter/WIMPexperiments.html), that the obvious culmination of the failure to detect dark matter would be to come up with a particle that was utterly undetectable. This might be true – who could tell? – but it is also the logical (and predicted) extrapolation of a failed hypothesis. I do indeed sneer at the latter.
Right-handed neutrinos are a specific kind of particle that does make a certain kind of sense, as described here. I’ve nothing against that. There is, however, no guarantee that, should they exist, they have anything to do with the dark matter problem.
I take considerable umbrage at the presumption that everything I say can be attributed to an agenda in favor of modified gravity. That is simply not correct. It isn’t my theory.
It may surprise readers to learn that I was an advocate of dark matter before I ever heard of MOND. What changed, for me, was that MOND made predictions that came true in my data. So, I changed my mind: perhaps I had been wrong to be so sure that the answer had to be dark matter and couldn’t possibly be modified gravity. This was an excruciatingly painful admission, but isn’t that what we’re supposed to do as scientists?
OK, now to answer Clayton. I don’t recall ever refusing to discuss the CMB at a conference. I have done so many times. Nor have I done as he says, and “elided” the phenomenology he describes in the CMB. Quite the contrary; I agree with most of what he says above and have said so on many occasions. I have lots else to say, so I can’t fit into every talk something I don’t have much to say about.
One thing Clayton elides is that I did make a prediction for the CMB peaks, both with and without dark matter, and it was the prediction without dark matter that came true. This has remained true in every data set as the CMB data have improved (see https://tritonstation.com/2020/07/31/second-peak-bang-on/). But it is also true that the ansatz I used to make the correct prediction for the second peak also predicts a smaller third peak. This is not observed, as he says, so maybe it was just a huge coincidence – or maybe that’s just how far the ansatz could take us.
The presumption for a long time has been that no modified theory of gravity could get the CMB right. I have a lot of sympathy for that attitude. I have had just as much difficulty trying to get dark matter to explain observed rotation curves. It only does so at the level of “we need more mass.” It does not do what MOND does in predicting what we get from what we see.
It is no longer true that it is impossible to explain the CMB and galaxy power spectra with modified gravity. Skordis and Zlosnik have done just that (arXiv:2007.00082). I don’t know if that is the right theory, but it is at least a demonstration that what was thought to be impossible can be done. It is simply not correct to say that there are no alternatives to CDM to explain the CMB power spectrum.
Stacy McGaugh,
Thanks for your comment and apologies for misrepresentation of your position. I had been following some of the same goalpost movement in the context of SUSY models. Over the years one can find endless plots of “SUSY predictions” on plots of dark matter limits, always extending below the current limits, but within reach of the next generation experiment. This kept going over many generations of experiments, and many orders of magnitude improved limits. In recent years I’ve stopped looking at these things, since I think most people have now given up on this.
On dark matter research in general, it’s a huge and confusing topic. I’m aware of the time and effort it would take to master the subject and have well-informed views, and it’s time I don’t have. I’ve followed your blog with curiosity, the idea that there’s a problem with the conventional wisdom is one I’m not unwilling to believe, given my experience in the rest of HEP theory. At this point not much I can do except to hope that better work and better data lead to a consensus. The reason for bringing this up here was mainly to ask for more information about what to me looks like the most theoretically attractive explanation.
Hi Dimitris Papadimitriou,
Those alternative theories that incorporate additional fields/ forces are obviously associated with unknown particles that need to be detected for any of these proposals to be tested and confirmed.
Logically speaking, is this an example of “proof by assertion”? Modified gravity proposals (like MOND) modify the form of the gravitational interaction without adding new particles/fields to the already-known set. That doesn’t mean that modified gravity proposals account for every observational fact, or even “make sense” from the standpoint of our current theoretical understanding, but those are different issues.
Only detection via any of the three interactions of the standard model of particle physics is ” legitimate ” for the confirmation of existence of DM? Gravity does not count as an interaction?
Since I’m not aware of suggestions that gravity can’t be treated as an interaction, or that we can’t infer physical facts from gravitational effects, I don’t know how to respond other than “of course gravitation counts as an interaction.” But that’s beside the point.
If all you see are gravitational effects with no signature from any of the other three known interactions, what can you infer from it? If you put a thermally and electromagnetically shielded box on a sensitive torsion balance and measure the mass of its contents gravitationally, can you say, in a practical sense, whether the box contains a ham sandwich, a glass of water or a lump of an exotic new metallic alloy? Can you falsify the experimentalist’s claim that the box holds an exotic alloy without opening up the box and examining its contents via a non-gravitational interaction?
Likewise, how can you meaningfully claim observation of any dark matter particle if said particle interacts only gravitationally? How can you empirically distinguish two hypothetical dark matter candidates, both of which only interact gravitationally?
Hi Peter,
[…] right-handed neutrinos seem to be an extremely well-motivated theoretical idea, and particles with only gravitational interactions are not at all unobservable (they may very well be exactly what explains galaxy rotation curves, CMB peaks, etc).
Even with good theoretical motivation, how would one test the idea that right-handed neutrinos meaningfully contribute to dark matter phenomena if they only interact gravitationally? How can we empirically distinguish them from other possibilities, so that the idea can become more than a story we tell ourselves?
On a different note… Since I haven’t studied the right-handed neutrino idea at all, please forgive me if the following question is naive. Presumably right-handed neutrinos would be included as a term in an expanded Lagrangian that includes the SM plus gravity (gravitons), and thus would be governed by a QFT. Hence there should be at least one interaction vertex for them, one that describes the necessary coupling to gravity. The question I have about the idea is the following: Is there an additional coupling whereby right-handed neutrinos are created in the early universe but not today, and if so, what might that coupling look like? Or does the neutrino/graviton coupling alone mediate the right-handed neutrino creation, with the required energy to create them only being available in the early universe?
Marty Tysanner,
I’m not a cosmology or dark matter expert, maybe one can answer your questions. I have plenty myself, just know that the SM with neutrino masses to me most naturally has right-handed neutrino fields. About contact with observation, I’m assuming your cosmological model produces gravitation-only particles, they evolve and you get structure formation. Does this match what you are seeing now? I have no idea what the problems here are.
Hi Peter,
Thanks. I have bent over backwards to remain objective and address the data that people say I don’t address (e.g., arXiv:1404.7525 ). There is just too much to know, and ultimately, which way we lean depends on how we weigh rather disparate lines of evidence.
Much of the discussion in the context of particle physics seems not to do this, and skip directly to “what could the dark matter be?” This is indeed an interesting question, but its phrasing also presupposes the answer. We’ve invented dark matter for good reason, but now that we have it wedged into our consciousness, how could we tell if we were wrong?
I see Marty is posing a similar question about distinguishing different types of non-interacting particles. That’s a good question. My concern is that, and also more general: is the dark matter paradigm falsifiable?
The statement by clayton that stellar feedback and galaxy-galaxy interactions are ‘unmodelable SM physics’ might come as a surprise to all the people who’ve spent their careers modelling those things
to respond to Stacy (to whom I do want to reiterate my genuine admiration for pointing out many interesting features of galactic rotation curves) and Felix and anyone else — the CMB is a linear system that we are able to model at _extremely high precision_, and the predictions without CDM are _qualitatively wrong_. The CMB allows a precise measurement of the CDM and baryon densities independently because it is such a simple linear system.
Galaxies are not! Star-formation physics is incredibly complex and nonlinear. So perhaps it’s a bit imprecise to say that galaxies are “unmodelable” — they are unsolvable in the same sense that the Navier-Stokes equations are unsolvable. Obviously, all such systems are modelable. But we simply do not have sufficiently good models at this time to make high-precision measurements of fundamental physics from them.
As for the Skordis and Zlotnik papers — they require a new field that carries a substantial energy density that redshifts like a^-3. It walks and quacks like CDM (even though it doesn’t have perturbations like CDM, but they also don’t solve for the matter power spectrum — another linear quantity that CDM gets right).
Some resources:
https://chrisnorth.github.io/planckapps/Simulator/# (click on the “Toggle power spectrum plot” button on the top right, the one that looks like a squiggle, and see how different the power spectra and the CMB maps look with and without CDM!)
http://background.uchicago.edu
https://arxiv.org/abs/1112.1320
Dear Clayton,
Maybe you could try to enter the discussion with Dr McGaugh here https://tritonstation.com/2023/01/13/what-we-have-here-is-a-failure-to-communicate/
If not, well, silence is better than an answer, a confession.
Best,
Maurice