Wednesday, April 15, 2015

Dark matter self-interaction detected?

Off-topic: My Facebook friend Vít Jedlička (SSO, Party of Free Citizens) established a new libertarian country, Liberland (To Live And Let Live), where the government doesn't get on your nerves. Before he elected himself the president, he had to carefully choose a territory where no one will bother him, where no one would ever start a war; he picked seven squared kilometers in between Serbia and Croatia because these two nations wouldn't dare to damage one another. ;-) There's a catch for new citizens, however: the official language is Czech.
Lots of mainstream writers including BBC, The Telegraph, IBTimes, and Science Daily promote a preprint claiming that they see the non-gravitational forces between the particles that dark matter is composed of:
The behaviour of dark matter associated with 4 bright cluster galaxies in the 10kpc core of Abell 3827 (published in MNRAS)
Richard Massey (Durham) and 22 co-authors have analyzed the galaxy cluster Abell 3827 – which is composed of four very similar galaxies (unusual: they probably got clumped recently) by the new Hubble Space Telescope imaging and by ESO's VLT/MUSE integral field spectroscopy.

The radius of the whole core – which is 1.3 billion light years from us – is 10 kpc. They show that each of the four galaxies has a dark matter halo. But at least one of those halos is offset by 1.62 kpc (plus minus 0.48 kpc, which includes all contributions to the errors, so that it's a 3.4 sigma "certainty").

Such offsets aren't seen in "free" galaxies but when galaxies collide, they may be expected due to the dark matter's collision with itself. With the most straightforward interpretation, the cross section is\[

\frac{\sigma}{m}=(1.7 \pm 0.7)\times 10^{-4}{\rm cm}^2/{\rm g} \times \left(\frac{t}{10^9\,{\rm yrs}}\right)^{-2},

\] where \(t\) is the infall duration. Well, if written in this way, it's only a 2.5-sigma certainty that \(\sigma\neq 0\), but that's probably considered enough for big claims by astrophysicists. (Astrophysicists apparently aren't cosmologists – only cosmology has turned into a hard science in recent 20 years.)

If that claim is right and dark matter interacts not only gravitationally (which is why it was introduced) but also by this additional interaction, it can not only rule out tons of models but also isolate some of the good ones (perhaps with WIMPs such as LSPs such as neutralinos).

The cross section cited above is safely below the recently published upper bound if \(t\) is at least comparable to billions (or tens of millions) years. The \(t\)-dependence of the new result makes it a bit vague – and one could say that similar parameter-, model-dependent claims about the cross section already exist in the literature.

Because of some recent thinking of mine, I should also mention that I think that it's also possible that an adequate MOND theory, with some specific form of nonlinear addition of the forces, could conceivably predict such an offset, too. A week ago, I independently rediscovered Milgrom's justification of MOND using the Unruh effect, after some exchanges with an enthusiastic young Czech female astrophysicist who liked some of my MOND/HOND remarks. For a while, my belief that "MOND and not dark matter" is basically right went above 10%, but it's dropped below 10% again when I was reminded that there are no MOND theories that are really successful with the clusters.

Another dark-matter topic. Today's AMS press conference didn't seem to change the picture much.

Off-topic: If you need to be reminded of the distances inside (our) galaxy, then be aware that the British rapper Stephen Hawking has recorded a cover version of the Monty Python Galaxy Song for you to learn from. Hawking has even hired someone to represent all the stupid, obnoxious, and daft people – you feel that you've had enough – namely Brian Cocks (I have to write it in this way to avoid the copyright traps).


  1. Very nice article Lubos.

    I think that Heisenberg was the most important figure in development of quantum mechanics, he isn't credited enough. He understood demise of realism which is the most important aspect of quantum mechanics. I agree that after his paper, the rest was inevitable.

    I just remembered that when I was 14, I bought a book by Penrose, The Emporer's New Mind I think. He has said that, why we just can't think of electron as distributed in space so that wave function directly describes it ? This was the first time when someone said to me wrong things about quantum mechanics (directly or indirectly). He and many other important people ,including some Nobel winners , confused me and I didn't understand quantum mechanics properly for some time. But I always knew that it was my problem, because quantum mechanics has worked for "every" experiment with quite different contents (I mean it works not just for electrons but for everything), so it was clear to me that there all physics and truth was already incorporated to equations (i.e. there shouldn't be a need for any hidden variable). I don't understand why people just can't see this simple point and try to understand what quantum mechanics actually say.

  2. Thanks, John, you must be right right about Heisenberg. He was already aware of the failure of realism. I think that he had some of the "matrix maths" behind observables, too. And that's a creative enough mixture that may be refined.

    And I also agree about your second point. When the equations are gotten so that they predict the right numbers which are measured in the experiments as well, obviously there must be the right interpretation that works. And this interpretation may be pretty much directly extracted from the experiment. I would say that one may *observe* that |psi|^2 is the probability density etc.

    Penrose's books were everywhere around and popular with other kids who liked such popular texts. I happened to avoid them as a teenager, perhaps because I viewed them as too humanities-oriented. ;-) Most likely, I still do LOL.

  3. Lubos

    With M-theory in mind, what is your best guess as to the composition of dark matter and dark energy. Do you favor models with graviphoton and graviscalar contributions?

  4. Dear kneemo, if you specifically talk about "M-theory", in the proper sense of the UV-complete theory approximated by 11-dimensional supergravity at low energies, then the most justifiable particle for dark matter would be an axion - or many axions - followed by WIMP, the lightest superpartner (general for all supersymmetric theories), probably a wino. But even within string/M-theory, there is no guarantee that the (11D) M-theory should be the legitimate description for our Universe.

    I don't think that the terms "graviphoton" and "graviscalar" are too meaningful at the level of string/M-theory. String theory unifies all matter and forces this unification contains gravity as well, so to one extent or another, all scalar fields that string/M-theory implies are "graviscalars" and all vectors are "graviphotons". Those coming from the extra dimensions - Kaluza-Klein theory - perhaps deserve these labels more than others. In this sense, some axions may be considered "graviscalars", like those deciding about "angles" of the hidden dimensions.

    Similarly, there are extra U(1) fields from the Kaluza-Klein theory - within M-theory - that may be called "graviphotons" but I don't think that those are viable dark matter candidates.

  5. But I like humanities :) . Speaking of humanity, Gell-man also likes humanity and this interview of him is I think related to recent discussion about Dyson:

    It may be possible that, somehow, actually Gell-man has said similar things and it is recorded like Dyson said it.

  6. kashyap vasavadaApr 15, 2015, 4:24:00 PM

    Interesting theory Lubos. How did they get equation for the cross section, especially the constant in front?

  7. Not very scientifically honest to totally ignore
    which is a larger study that suggests the cross section could be very small (if not zero).

  8. Yes, I had in mind toroidally compactified M-theory, such as that on T^7 yielding D=4, N=8 SUGRA, where the graviphoton and graviscalar lead to a modification of Newton's law at short distances. Such ideas were discussed by Joel Scherk, during the late 70's.

  9. Dear IMHO, your accusation is complete bullšit and only shows that you are a stupid troll.

    First, this very paper is linked to in this very blog post - search for "safely below".

    Second, the two papers are completely consistent with each other because the new paper is well below the upper bound from "your" paper, as I wrote.

    Third, the two papers are not only compatible with each other but they have some of the same authors!

  10. Perhaps I am misunderstanding the paper. Let's assume there are 100 job openings. What does this paper tell us about how may women will be hired, and how many should be hired?

  11. Dear kneemo, our Universe is surely not toroidally compactified M-theory, and it doesn't have N=8 SUGRA.

    Even N=4 SUSY is too much. Well, if one applied it to all of matter species, N=2 is still too much. Only N=1 SUSY applied to the whole Standard Model yields realistic theories, and N=1 compactifications are obtained by much more complicated, non-toroidal compactifications of string/M-theory.

    Heterotic string theory on Calabi-Yau manifolds; M-theory on G2 holonomy manifolds; and F-theory on Calabi-Yau 4-folds are 3 "most geometric and smooth" examples.

  12. "possible that an adequate MOND theory" Testable MoND sourcing....

    Observed anomalies are instead diagnostics: Massless boson photons detect no vacuum refraction, dispersion, dissipation, dichroism, or gyrotropy. Postulate contingent vacuum symmetries are exactly true for fermionic matter (quarks, hadrons). Parity violations, symmetry breakings, chiral anomalies, baryogenesis, Chern-Simons repair of Einstein-Hilbert action are vacuum trace chiral anisotropy acting only upon hadrons. Bosons and fermions have non-identical rules.

    Diagnostics have contingencies: Noether's theorems couple exact vacuum isotropy to conservation of angular momentum. Vacuum trace chiral anisotropy selective to hadronic matter leaks conservation as MoND's 1.2×10^(-10) m/s^2 Milgrom acceleration.

    Contingencies are testable: Existing bench top apparatus (University of Washington, CENPA, Eöt-Wash Group) using commercial materials (enantiomorphs of single crystal cultured alpha-quartz): DOI: 10.5281/zenodo.15107. Test spacetime geometry with geometry, not composition or field. Supportive theory is Einstein-Cartan-Kibble-Sciama gravitation. A geometric Eötvös experiment falsifies dark matter by empirical measurement while remaining consistent with all prior observations. Do it.

  13. Perhaps the economics departments are choosing applicants randomly because they realize the law of diminishing returns and figure it is a more efficient use of department resources than to actually evaluate the candidates.

  14. Yes, you are correct on all counts... I didn't read your post carefully enough.

    Although it should be mentioned that the results are consistent with (and creeping towards) a cross section of zero. They also mention that they can not rule out astrophysical effects as the cause of their observations.

    Personally, I would bet on a small, but non-zero cross-section. I don't think this is evidence one way or another for super partners.

  15. This must be some misunderstanding. I never assumed 50-50 to be natural, and it is not realized in the world, either.

    By the overrepresentation, I simply mean that there is a group of people who should be "male STEM faculty" and those who are "female STEM faculty". These two groups aren't equally large, their ratio is N

    In the Academia, the ratio is N/2, so there are twice as many "female STEM faculty" in the Academia than in the general population.

  16. Dear Lubos,
    I am a bit confused that while in the bullet cluster case the dark matter is supposed to drag behind the visible matter, it shall be the opposite here. While the dark matter may interact with itself, so for sure does the visible matter.
    I always thought that MOND theories are just ad hoc constructions without a solid theoretical justification. If you work on such models you must see it somewhat differently.

  17. It is fair to say, I think, that Max Born had very deep insight and, perhaps, the most thorough grasp of quantum mechanics of anyone at or near its beginning. But even he did not fully grasp the universality of the most profound advance in physics.

  18. This is not the first time I see that troll trolling here, I recognized his name immediately ... ;-)

    Its always such situations I really curse Disqus for abolishing the possibility to downvote comments ...

  19. Having read (and re-read) several books from Penrose I must agree that he is oftentimes a great confuser, rather than a great explainer.

  20. According to the article in Nature:

    this is not the only 'step towards gender equality' in politically incorrect STEM.

    Nature is turning into such junk, I think I'll cancel my subscription.

  21. Up vote!

    Heh, heh, nobody beats Lubos in telling these creeps, who pretend to have an opinion, what they deserve.

  22. It's amazing, Tony. This obvious bias, a Nazi-style inequality for the candidates is termed "a sign of progress" by Nature.

  23. Ting's talk is online now ( One extra bin and a claimed fit for a peak at 275+-32GeV. Probably more details in here ( once they upload the video.

  24. It would be great. A neutralino at 700 GeV. Looking at the video, 48|44 seconds slide. I don't see how with this huge error margin of the last bin, one can have any confidence to speak of about the peak.

  25. I thinl that the Gaia satellite is what is going to teach us the most about dark matter in the next decade.

  26. Everybody is jumping on this profitable bandwagon. Big title on MarketWatch (feed the poor 'journalist'):

    No, no, we can't be happy as the society until all the numbers, in every aspect of life are exactly 50%-50% gender-wise. Of course, then we have to work also on heterosexual oppressors-gay/lesbian split, then on big nose-small nose split, until everything is perfectly balanced.

    What an insane society


    SHOCK HORROR! New report out finds wimmin underrepresented in international rugby. 100% of players discovered to be male. Affirmative-action blitz called for.

    "How else are we to get the opportunity to acquire decent cauliflower ears if they won't even let us out of the men's shower room?" explained Clarissa Grungethuttock, leading report author and relief-massage trainer. "It's an absolute travesty," she said.

  28. I had read this some time ago based on you linking it in another of your posts. I found it very helpful to gain insight into how the matrices are book-keeping tools for the energy states. Sometimes it is useful to return to the earliest work where the logic tends to be most exposed, and not obscured by later refinements that sometimes hide the initial motivations to solve a problem. Born's article/talk was a watershed insight for me to understand what QM is doing.

    The idea about letting go of realism seems to have not at all lost its relevance today. I am amazed at how some people need a literal or realistic interpretation of the formalism. From what I have seen generally, these are the people who don't use the theory in a practical way and are more interested in philosophical interpretations. People who are actually using the math to do real experiments, and/or to manipulate physical systems, however, seem perfectly fine with how QM works, and are not off chasing the rainbow of a realistic interpretation.

    Thank you for sharing your reflections on this very interesting piece of writing, Dr. Motl.

    Best wishes


  29. Dear Lubos,
    I read "Der Teil und das Ganze" (English "Physics and beyond" from Heisenberg himself. I recommend it to you if you did not read it so far. The book makes it very clear that Heisenberg understood that realism had to go very early, even before he left school.) E.g. there are conversations how to imagine chemical bonds in a realistic theory and how atoms could always assume the same forms even after heavily disturbing them.
    By the way I am still looking forward to see the double slit experiment treated in the operator formalism as you once promised. :-)

  30. I would be very interested to see what Born had to say in the article footnoted at 26. One infers from this paper that he "clung" to some form of realism. I think it presumptuous to assume he failed to understand what he and his colleagues had wrought; rather I prefer to think that having been through a revolution (or maybe two) he was more keenly aware of the danger of lapsing into dogma than those who have lived with "settled" principle for many years.

    A great piece from a great man. Thanks for posting.

  31. Thanks for reminding me of the Heisenberg picture double slit experiment explanation, too.

  32. Dear Richard, I think that the page 265 of the Nobel lecture, written at almost the same time as the reference 26, makes it clear that he agreed that quantum mechanics forbids the usual ideas about the reality.

    So I guess that the reference 26 discusses that despite this fact, one may still perfectly well use the same *language* as we did in times of realism to talk about the external world, like the concept of the particle, even though realism doesn't hold.

  33. Dirac has said that (when he was very old) he believes quantum mechanics is not the the final theory/framework and it will be replaced by a deterministic theory. He then said that, this theory will require even more radical changes than quantum mechanics brought as we can't return old classical ideas. I don't have any idea how.

    The only possible problem with quantum mechanics seems to be in context of cosmology and even in that case it works with anti de sitter space. De sitter space is seems to problematic, but I am not sure.

  34. Hardly shocking. It's been clear for some time that the goal of feminism is not ending "patriarchy", but creating matriarchy. A goal which they're terrifyingly well along in realizing.

  35. Well, since the 'qualifications' of the applicants in the study were generated to be equally distributed.... If the hiring were merit based, it should be 50. But the study finds it instead to be roughly 67.

  36. No it would be merit based 50 only for the sample provided in the study. We can't determine what the actual percentage is, which Lubos would say is 33.

  37. kashyap vasavadaApr 18, 2015, 4:08:00 PM

    In a way I am very glad that QM (and its non real meaning) have proved to be a good theory! That is the reason I am not embarrassed to be a physics professor and believing in religion at the same time!!

  38. Lee Smolin at page 336 wrote that women are disadvantage because of "blatant discrimination". This has now been proven false, just feminist misandric ideology. So now I am more inclined to think that his whole book had no value


    By Nigel Short
    This is about men and women in chess, and very well written by Nigel Short, who is a really bright guy, even though Kasparov is trashing him again these days...