Monday, September 07, 2015

Macro Dark Matter

[Some background info on the recent New Scientist feature I wrote with Naomi Lubick.]

I'm the half-face 2nd from the right.
In June I attended the “Continuation of the Space-time Odyssey,” a meeting dedicated to the “celebration of the remarkable advances in the fields of particle physics and cosmology from the turn of the millennium to the present day.” At least that’s what the website said. In reality the meeting was dedicated to celebrate Katie Freese’s arrival in Stockholm. She spared no expenses; pricy hotels, illustrious locations, and fancy food was involved. So of course I went.

It was an “invitation only” conference, but it wasn’t difficult to get on the list because, for reasons mysterious to me, the conference system listed me as a “chair” of the meeting, whatever that might mean. I assure you I didn’t have anything to do neither with the chairs nor with the organization, other than pointing out some organization might be beneficial. So please don’t blame me that there was no open registration.

So much about academia. Now let me say something about the science.
Two talks from the conference were particularly memorable for they stood in stark contrast to each other. The one talk was by Lisa Randall, the other by Glenn Starkman. Both spoke about dark matter, but that’s about where the similarities end.
Randall talked about her recent work on “Partially Interacting Dark Matter” (slides here). Her research in collaboration with JiJi Fan, Andrey Katz, and Matthew Reece is based on a slightly more complicated version of existing particle dark matter models. They consider some so-far undetected particles which are not in the Standard Model. But in contrast to most of the presently used models, these additional particles are not, as commonly assumed, unable to interact with each other. Instead, the particles exert forces among themselves, which has a several consequences.

First, it allows them to explain why dark matter hasn’t been seen in direct detection experiments: the stuff just isn’t as simple as assumed for the estimates of detection rates. Second, it means that dark matter has friction and thus at least partly forms rotation disks during galaxy formation, the “dark disks.” If they are right, our galaxy too should have a dark disk, and our solar system would traverse it periodically every 35 million years or so. If you trust Nature News, which maybe you shouldn’t, then this can explain the extinction of the dinosaurs. Right or wrong, it’s a story catchy enough to spread like measles, and equally spotty. Third, and most important, the partially interacting dark matter introduces additional parameters which you can use to fit unexplained data like the 130 GeV Fermi gamma ray line.

If the model they are using has any theoretical motivation, Randall didn’t mention it. Instead her motivation was that it’s a model which might be testable in the soon future. It’s a search under the lamp post, phenomenological model building at its worst. You do it because you can and you’ll get it published – it certainly won’t harm your peer review assessment if you are one of the best-cited particle physicists ever. But just exactly why this interaction model and not some other that won’t be observed until the return of the Boltzmann brains, I don’t know.

In other words, I wasn’t very convinced that partially interacting dark matter is anything more than something you can publish papers about.

 Now to Starkman (slides here).

He set out alluding to Odysseus’ odyssey, which lead to strange and distant countries that Starkman likened to the proposed dark matter particles like WIMPS and axions. In the end though, Starkman pointed out, Odysseus returned to where he started from. Maybe, he suggested, things have gotten strange enough and it is time to return to where we started from: the Standard Model.

His talk was about “macro dark matter,” a dark matter candidate that has received little if no attention. I had only become aware of it briefly before, through a paper by Starkman together with David Jacobs and Amanda Weltman. Unlike the commonly considered particle dark matter, macro dark matter isn’t composed of single particles, but of macroscopically heavy chunks of matter with masses that are a priori anywhere between a gram and the mass of the Sun.

It is often said that observations indicate dark matter must be made of weakly interacting particles, but that is only true if the matter is thinly dispersed into light, individual particles. What we really know isn’t that the particles are weakly interacting but that are rarely interacting; you never measure a cross-section without a flux. Dark matter could be rarely interacting because it is weakly interacting. That’s the standard assumption. Or it could be rarely interacting because it is clumped together to tiny and dense blobs that are unlikely to meet each other. That’s macro dark matter.

But what is macro dark matter made of? It might for example be a type of nuclear matter that hasn’t been discovered so far, blobs of quarks and gluons that were created in the early universe and lingered around ever since. These blobs would be incredibly dense; at this density the Great Pyramid of Giza would fit inside a raindrop!

If you think nuclear matter is last-century physics, think again. The phases and properties of nuclear matter are still badly understood and certainly can’t be calculated from first principles even today.

Physicists were stunned for example when the quark gluon plasma turned out to have lower viscosity than anybody expected. They still argue about the equations governing the behavior of matter in neutron stars. Nobody has any idea how to calculate lifetimes of unstable isotopes. I recently talked to a nuclear physicist who told me that the state-of-the-art for composites is 20 nucleons. Twenty. This brings you just about up to neon in the periodic table. And that is, needless to say, using an effective model, not quarks and gluons. The Standard Model interactions are well-understood at LHC energies or higher, yes. But once quarks start binding together physicists are back to comparing models with data, rather than making calculations in the full theory.

So matter of nuclear density containing some of the heavier quarks is a possibility. But Starkman and his collaborators prefer to not make specific assumptions and keep their search as model-independent as possible. They were simply looking for constraints on this type of dark matter which are summarized in the figure below

Constraints on macro dark matter. Fig 3 from arxiv:1410.2236

On the vertical axis you have the cross-section, on the horizontal axis the mass of the macros. The grey and green diagonal lines are useful references marking atomic and nuclear density. In general the macro could be made up of a mixture, and so they wanted to keep the density a variable to be constrained by experiment. The shaded regions are excluded by various experiments.

To arrive at the experimental constraints one takes into account two properties of the macros that can be inferred from existing data. The one is the total amount of dark matter which we know from a number of observations, for example gravitational lensing and the CMB power spectrum. This means if we look at a particular mass of the macro, we know how many of them there must be. The other property is the macros’ average velocity which can be estimated from the mass and the strength of the gravitational potential that the particles move in. From the mass and the density one gets the size, and together with the velocity one can then estimate how often these things hit each other – or us.

The grey-shaded left upper region is excluded because the stuff would interact too much, causing it to clump too efficiently, which runs into conflict with the observed large scale structures.

The red regions are excluded by gravitational lensing data. These would be the macros that are so heavy they’d result in frequent strong gravitational lensing which hasn’t been observed. These constraints are also the reason why neutron stars, brown dwarfs, and black holes have long been excluded as possible explanations for dark matter. There are two types of lensing constraints from two different lensing methods, and right now there is a gap between them, but it will probably close in the soon future.

The yellow shaded region excludes macros of small mass, which is possible because these would be hitting Earth quite frequently. A macro with mass 109g for example would pass through Earth about once per year, the lighter ones more frequently. Searches for such particles are similar to searches for magnetic monopoles. One makes use of natural particle detectors, such as the sediment mica that forms neatly ordered layers in which a passing heavy particle would leave a mark. No such marks have been found, which rules out the lighter macros.

What about that open region in the middle? Could macros hide there? Starkman and his collaborators have some pretty cool ideas how to look for macros in that regime, and that’s what my New Scientist piece with Naomi is about. (Want me to keep interesting stories on this blog? Please use the donate button that’s in your face in the top right corner, thank you.)

Macro dark matter of course leaves many open questions. As long as we don’t really know what it’s made of, we have no knowing whether it can form in sufficient amounts or is stable enough. But its big advantage is that it doesn’t necessarily require us to construe up new particles.

Do I like this idea? Holy shit, no, I hate it! Like almost all particle physicists, I prefer my interactions safely in the perturbative regime where I can calculate cross-sections the way I learned it in kindergarten. I fled from the place where I made my PhD because everybody there was doing nuclear physics and I wanted nothing of that. I wanted elementary particles, grand unification and fundamental truths. I would be deeply disappointed if dark matter wasn’t a hint for physics beyond the standard model, but instead would drift into the realm of lattice qcd.

But then I was thinking. If everybody feels this way we might be missing the solution to a 80 year old puzzle because we focus on answer that we like and answers that are simple, not answers that are in our face yet complicated. Yes, macros are the most conservative and in a sense most depressing dark matter model around. But at least they didn’t kill the dinosaurs.


  1. "In other words, I wasn’t very convinced that partially interacting dark matter is anything more than something you can publish papers about."

    I'm not so sure. That dark matter is simple (a WIMP, say) is just that, a simple assumption. Other matter self-interacts. Why should dark matter be any different?

  2. Phillip,

    That's right. I have no problem with dark matter self-interacting in general. My issue is that this specific model is obviously construed to be "just about" detectable. There isn't any particular reason why dark matter should have this interaction with these interaction strenghts. It's just the simplest thing you can do that puts you in the ballpark for dinosaur extinction. It's too bad that the talk wasn't recorded (or at least I can't find the recording) because Lisa said this pretty much literally, that the motivation to look at this particular model is that it can be ruled out soon (to be fair, I think she didn't say anything about dinosaurs).

    See, I've been working on phenomenological models myself for 10 years or so. There's no shortage of what you can do. You can always add something to the known physics and then hide it just beyond current experimental bounds. And if you run out of ideas, then take two unmotivated models and combine them to a new unmotivated model. I assure you chances are good it'll get published.

    The important thing for pheno models is that you have a good theoretical motivation for why this is a good model. In quantum gravity pheno you take models that are motivated by findings in theoretical approaches to qg (extra dimensions, dimensional reduction, minimal length, discreteness, LIV, space-time fluctuations etc). Without that motivation, what's the point? I am missing that motivation in the partically interacting model. WIMPs (or some of them) and axions each have their motivation, which you might or might not buy, but at least they have one.

    Sure, that model might be right. But think of it from a probabilistic perspective. In the space of all possible interacting models you pick one that you like because it's just around the corner. What are the chances of that being correct? I'd say, they're tiny, and that's why I'm totally not excited about this development. Best,


  3. Bee, are the videos of this meeting on the web?

  4. Shantanu,

    There don't seem to be videos online. I'm a little puzzled about this because I thought I recalled it being recorded. I'll be back in Stockholm Friday and will inquire about this.

  5. "The important thing for pheno models is that you have a good theoretical motivation for why this is a good model."

    OK, we agree here. I have the same beef about f(R) theories. (See arXiv:0805.1726 for a nice review of this field (pun, as always, intended).)

  6. Dark matter: Tully-Fisher relation (spiral galaxies at all redshifts), gravitational lensing. Dark matter distribution is not perturbed (cooling, collisional ejection) or purged (scavenging by stars, black holes, galactic central back hole). Lensing includes filaments unstable over redshift given only isotropic attractive interaction.

    Tully-Fisher can be MoND's Milgrom acceleration. 10^(-10) m/s^2 non-conservation of angular momentum suggests Noether's theorems plus observed trace vacuum chiral anisotropy toward hadrons. Existing bench top apparatus decides, healing physics quantum gravitation to SUSY. Theory cannot judge itself to be empirically true. Look.
    Where are solar axions? doi:10.1103/PhysRevLett.112.091302

  7. If the dark matter is exotic quark & gluon stuff, as far as I can guess, it would affect cosmological nucleosynthesis, but maybe the constraints are weaker than the other ones?

  8. Bee,
    All known non-dark matter we know comes from atoms and eventually from fundamental particles like quarks, gluons and leptons. Why would dark matter be only clumps of material without any constituents? So I think, looking for just clumps will not solve the problem.

  9. Dark matter consists of epicycles :)

  10. Unsolicited typo patrol says:

    "She shied no expenses" - this could be a usage which I am not familiar; I am familiar with "She shied away from no expenses" or "She spared no expenses" or "She shied at no expenses" (as when a horse shies at an obstacle which it is asked to jump).

    "causing it to clump to efficiently" - the second "to" should be "too".

    "it’s a story catchy enough to spread like measles, and equally spotty" - perfect.

  11. "The phases and properties of nuclear matter are still badly understood and certainly can’t be calculated from first principles even today."

    Does this mean that the computations are intractable, or that we wouldn't know how to calculate them even if we had unlimited computing power?

  12. Hi JimV,

    Thanks, I've fixed that. I'm afraid you're right, "shying no expenses" (keine Ausgaben scheuen) is a German expression, not an English one... Best,


  13. Kevin,

    Hmm, an interesting question. I'd say they are intractable since the underlying theory is known, but I'm not entirely sure you can prove that with the methods currently known on computers that presently exist they could be done in finite time. So is that intractable or not? Best,


  14. kashyap,

    I don't know what you mean with "clumps of material without any constituents". Of course the macros have constituents. The constituents are standard model particles, quarks and gluons, but really the exact composition isn't known. Best,


  15. I'm inclined to agree on theoretical motivation (or lack of) but the thing about lamposts is that one looks pretty silly if one didn't check and the keys were later found there (however unlikely the event). If science is about ruling things out, as Popper claimed, then there is a sense in which perhaps lampost investigations have their place!
    Regards, Cormac

  16. "(Want me to keep interesting stories on this blog? Please use the donate button in the top right corner, thank you.)"

    Of course I do; and I imagine your donation income is not huge. But, I expect to know something about the finances of charities I give to. Mainly, I like to know approx. how much they receive and how they spend it. In your case, it's not about whether you deserve compensation. It's more like, will my small amount make a difference? Do you already receive enough that I can make a bigger difference to my neighborhood's homeless? There's no doubt I want people to keep seeing articles like your "I wasn't born a scientist". Would it be OK to inquire through your public staff address (full name@org)?

  17. Mike,

    You can email me if you want, but I'm happy to answer your question publicly. I've only put up this donate button a month ago, so I can't give you any statistics, sorry. As to how I use the money. My employment contract is running out in November. I don't yet have a new contract. I have some grant money coming in after a gap, some time in December, and that is supposed to be worked into a contract which hasn't yet appeared. But roughly speaking the way it looks right now it won't be enough to cover employment benefits. Which means basically that all money I manage to make on the side will go into pension plan and health insurance.

    Having said that though, I would prefer you think of this differently. Writing this blog is working over time. I work on my research 40 hours per week (on paper), I have two kids to raise. Writing a piece like this takes me roughly 2-3 hours. What do you think my time is worth?



  18. coraifeartaigh,

    By all means one should look under the lamp post. The question is though, how much should one expect to find something? And if you don't expect to find something, does it make sense to construe elaborated theories about how that something might look like? It just strikes me as a huge waste of time (and, in the end, of money). Best,


  19. Thanks, B, the cause is just & that's more than enough info. If 500 of us make the same donation, that will be ample. If 5000, that would be a windfall. It's more than I was thinking of. Not that I wouldn't be happy for you. :)

  20. Lamp posts: A streetlight not radiating in the visible brightly baths lamp posts in darkness. Look to falsify, not to confirm. Euclid: maps; Newton: Mercury, Maxwell; Dirac equation: Otto Stern; particle theory: Yang and Lee. Macroeconomics models scientific socialism to Darwinian capitalism, all mocked by "guided" central interventions.

    Theory elegantly demands vacuum mirror symmetric toward boson photons "must" be mirror symmetric toward fermion quarks (hadrons). Attempting falsification of established theory is silly (exceptions noted). Look to falsify, not to confirm.

  21. George,

    Is it? I looked at the paper, but it seems to be merely about comets. Besides this, I know I've said it before, but do they really have to jump on every piece of nonsense just because the author once wrote a well-cited paper? It is exceedingly annoying for people in the field, and I think it sets wrong incentives all over the place. Best,


  22. "In reality the meeting was dedicated to celebrate Katie Freese’s arrival in Stockholm."

    Maybe the relaxed Scandinavian atmosphere will allow her to find time to write her own papers rather than copying them from another review by someone else from several years ago.

  23. Let me take this opportunity to publicly apologize for a false accusation. I hope that it will be accepted.

    I relied too much on automatic data from arXiv (something we should all be careful about). More extensive comments on this, by myself, Freese, and others (showing that I was not the only one to jump to conclusions) include this comment from me: "A variant of the common-source scenario is that Riotto got his text from an even earlier text from Freese (bad) while Freese merely re-used her own text (OK).". It turns out that this is what happened. Riotto has admitted to plagiarism, though in the view of most or all, not going far enough to make amends.

    On the positive side, this should be a warning to arXiv and to those who use automatically generated data to be very careful with such accusations. It would have been possible to make the correct accusation in the first place, as Freese's paper which Riotto plagiarized is on arXiv, but arXiv goofed here. Freese comes out looking at least as good as if there had been no accusation at all, and perhaps this will serve as a warning to people not to commit plagiarism. Of course, just reformulating something is not enough; if one uses a source at all, it should be cited.

    Of course, considering that Freese had been plagiarized and not vice versa, my original comment here is in the completely wrong tone. For this as well, I apologize.

    (It's not a defense, but I once had an entire paper plagiarized. Not just text, but also new conclusions, etc. Fortunately, I was able to stop it from being published and get the offending paper withdrawn from arXiv. Again, not a defense, but it explains why I am rather sensitive on this issue. At least I hope that, as mentioned above, something positive comes out of the resolution. I do have a defense for not adding this comment and the last one at the URL above until today, as I was in hospital yesterday. No internet access during full anesthesia.)


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