Dark Atoms

Almost a year ago we talked about dark photons — the idea that there was a new force, almost exactly like ordinary electromagnetism, except that it coupled only to dark matter and not to ordinary matter. It turns out to be surprisingly hard to rule such a proposal out on the basis of known astrophysical data, although I suspect that it could be tightly constrained if people did high-precision simulations of the evolution of structure in such a model.

In fact our original idea wasn’t merely the idea of dark photons, it was dark atoms — having dark matter bear a close family resemblance to ordinary matter, all the way to having most of its mass be in the form of composite objects consisting of one positively-charged dark particle (a “dark proton”) and one negatively-charged dark particle (a “dark electron”). We thought about it a very tiny bit, but didn’t pursue the idea and only mentioned it in passing at the very end of our paper. There is an informal rule in theoretical physics that you should only invoke the tooth fairy (propose an extremely speculative idea or hope for some possible but unprovable result) once per paper, so we stuck with only a single kind of charged dark particle.

But once someone invokes the tooth fairy in their paper, anyone who writes another paper gets to invoke the tooth fairy for themselves. (That’s just how the rule works.) And the good news is that it’s now been done:

Atomic Dark Matter
Authors: David E. Kaplan, Gordan Z. Krnjaic, Keith R. Rehermann, Christopher M. Wells

Abstract: We propose that dark matter is dominantly comprised of atomic bound states. We build a simple model and map the parameter space that results in the early universe formation of hydrogen-like dark atoms. We find that atomic dark matter has interesting implications for cosmology as well as direct detection: Protohalo formation can be suppressed below $M_{proto} sim 10^3 – 10^6 M_{odot}$ for weak scale dark matter due to Ion-Radiation interactions in the dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine splittings of order $100 kev$, consistent with the inelastic dark matter interpretation of the DAMA data while naturally evading direct detection bounds.

(Note that one of the authors has been a guest-blogger here at CV.) It looks like a great paper, and they seem to have done a careful job at chasing down some of the interesting implications of dark atoms. In fact the idea might be more robust than that of the one in our paper; the fact that dark atoms are neutral lets you slip loose of some of the more inconvenient observational bounds. And the last sentence of the abstract points to an intriguing consequence: by giving the dark matter particles some structure, you might be able to explain the intriguing DAMA results while remaining consistent with other (thus far negative) direct searches for dark matter. Stay tuned; that dark sector may turn out to be a pretty exciting place after all.

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25 Responses to Dark Atoms

  1. NewEnglandBob says:

    Ummm….what???

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  2. King Cynic says:

    Presumably the tooth fairy is made of dark atoms.
    As are Boltzmann brains.

    When are we going to declare war on civilizations that don’t carry Standard Model quantum numbers?

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  3. A major aspect of Stephen Baxter’s “Xeelee Sequence” series is a war between species composed of standard matter and species composed of dark matter. The level of interaction is primarily gravitational which leads to very interesting tactics. He wrote the first book in the series in the early 90s so at least among science fiction writers the idea has been floating around for a while.

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  4. Joseph Smidt says:

    Here is the problem, as I see it, where this is going: dark photons -> dark atoms -> dark etc… It isn’t that I would have a hard time believing such things, it’s just that doesn’t this violate observations?

    Don’t the best models for dark matter assume it is collisionless, not just with the standard model particles, but with itself? I haven’t worked out the details, but if dark matter could interact with each other with some U(1) interaction say, this would make them no longer collisionless right? And therefore behave different than collisionless models predict?

    And where does it end? If there is an entire dark standard model then dark matter should clump up into “dark stars” and “dark galaxies” right? Doesn’t this violate observations?

    Anyways, maybe this stuff is real, but for whatever reason, to me, these ideas seem to violate the collisionless nature of dark matter, not only with the SM but with itself. Unless the couplings were almost zero.

    Am I completely wrong here?

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  5. King Cynic says:

    Not having read the paper, I suppose that the self-interactions are avoided by pairing off particles into neutral “atoms”. When people talk about dark matter not having self-interactions, they’re usually imagining “charged” particles interacting with each other. Forming atomic states would perhaps screen the particles from interacting too much, except through much weaker residual (Van der Waals?) interactions.

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  6. Schenk says:

    The ramifications of this subject are far more serious than any one man could imagine in the present !

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  7. BlackGriffen says:

    The problem with that proposal lies in the original evidence for dark matter – galactic dynamics. The evidence from studying lensing around galaxies and the rotation curves suggests that the dark matter is in a spherical halo around the galaxies. Furthermore, the bullet cluster shows that dark matter interacts with other dark matter far more weakly than the interstellar medium. So, unless you can explain a temperature imbalance between light and dark sectors, dark matter capable of producing dark photons should collapse into compact objects as light matter must have before reionization. Under that collapse, if nothing else, the dark matter would have produced black holes, and those black holes should become dark photon bright whenever they consume any dark matter, ionizing the surrounding dark matter. That would present a problem for the bullet cluster because it means that a fair amount of the dark matter should have interacted in a fashion similar to the ISM… Depending, I guess, on how rarefied and ionized the intergalactic dark matter is.

    So there are scenarios to test your model, and they aren’t even all that complicated. They just aren’t cosmological ones.

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  8. Sean says:

    Joseph and BlackGriffen– Yes, there are obvious questions to be raised when you suggest that the dark matter couples to a long-range field. That’s why you do the research and write papers about it! These are exactly what the papers investigate.

    Obviously we know enough about dark-matter dynamics to say that it’s not simply a copy of ordinary matter. But if you’re inventing new particles and forces, you have various parameters to play with, including the masses and the strengths of the interaction. There is a limit in which the particles become very heavy and/or the forces become very weak, and in that limit you just recover ordinary non-interacting dark matter. The question is, where exactly is the dividing line between allowed and ruled out?

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  9. John R Ramsden says:

    @4 “If there is an entire dark standard model then dark matter should clump up into “dark stars” and “dark galaxies” right? Doesn’t this violate observations?

    I thought the idea was that dark matter *does* clump into dark galaxies, overlapping with visible matter galaxies, to account for the otherwise anomalous rotation rates of stars in the latter.

    Also, given their elusive nature and apparent reluctance to “clump up”, or interact at all with normal matter, is it easy to exclude tachyons of some kind as candidates for dark matter? After all, strictly speaking, relativity only prohibits travel *at* the speed of light doesn’t it?

    Mind you, I suppose that might be ruled out by the Bullet galaxy, where a collision has apparently separated the visible matter from the dark matter – If the latter comprised tachyons then presumably, attracted by gravity to sluggish visible matter, it would have no trouble keeping up, like a cloud of flies buzzing round an ambling cow.

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  10. Joseph Smidt says:

    @John (#9)

    Dark Matter does clump up into a cob-web structure. (You may have seen pictures) But it does not clump the same way ordinary matter does. This was my point, adding charges and atoms, and etc seems like dark matter is becoming like another copy or ordinary matter which is ruled out.

    But, Sean says these details have been worked out in these papers so I need to read them before I say much more.

    I would love to think dark matter has all these properties, it just seems like the data fits best if you assume it doesn’t. (So it seems to me. But I admit I need to read the papers.)

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  11. Sili says:

    Stay tuned; that dark sector may turn out to be a pretty exciting place after all.

    I see what you did there.

    What sorta observations will help you decide one way or another? Can it be done with any of the gizmos up there now (I really want for something to find a good use of Kepler)? And if not what thingummajig do you most want to see built/launched? (The Astronomy Cast kindly informed that the Ares V rocket is gonna be an unparallelled workhouse (assuming they actually build the damn thing). Apparently it could launch the Gemini Observatory, as is, to make our biggest (and most useless) spacebased observatory yet.)

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  12. What about dark antimatter?

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  13. Pingback: Tales from the Tubes — 12/​09/​09 | Young Australian Skeptics

  14. Arrow says:

    “But if you’re inventing new particles and forces, you have various parameters to play with, including the masses and the strengths of the interaction.”

    If you are inventing particles and forces you can make them fit any data you want.

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  15. Pingback: This Week in Science | RQDC

  16. Antoni Jaume says:

    “Under that collapse, if nothing else, the dark matter would have produced black holes, and those black holes should become dark photon bright whenever they consume any dark matter, ionizing the surrounding dark matter.”

    Since dark matter interacts gravitationally with standard matter, dark matter black holes capture normal matter, so they should be visible.

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  17. g says:

    I’d suggest that any dark analogues to non-dark particles be named with an -ark suffix – protark, electrark, neutrinark – but quarkark defeats that idea.

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  18. Dennis says:

    This all seems to be dark logic, driven by the dark scientific method.

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  19. Peter Fred says:

    It is almost impossible for scientist to think “out of the box” and yet they do not want to listen to amateurs who can think out of the box. Due to the dark sector problem astrophysics is in a “Kuhnian crisis.” In past Kuhnian crises it was the amateur’s Darwin and Copenicus who came up with the ideas that saved the day for those who believed in every word written in the textbook.

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  20. ObsessiveMathsFreak says:

    The adjective “dark” must be the worst thing to happen to physics since the luminiferous aether.

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  21. Sili says:

    Then please get to doing some Dark Michelson-Morley measurements, OMF.

    —–

    I rather like quarkark – I think you’re on to something.

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  22. DM says:

    Actually, the idea of “dark atoms” is rather old. The idea was proposed within the “mirror world” models a long time ago. See, e.g. arXiv:0804.4518, and references therein.

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  23. Sili says:

    Come to think of it, wouldn’t the presumably much higher mass of protarks and electrarks make the corresponding ‘orbit’ too small to be stable?

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  24. Gene says:

    In the Atomic Dark Matter Abstract, the first sentence is: We propose that dark matter is dominantly comprised of atomic bound states. The Authors should edit the “comprised of” to ‘composed of’ since ‘comprised of’ is incorrect English usage.

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  25. If this web page needed subscription or password to access, it would be part of the “Dark Web” — which accounts for a majority of pages on the Web!

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