Guest Post: Katherine Freese on Dark Matter Developments

Katherine Freese The hunt for dark matter has been heating up once again, driven (as usual) by tantalizing experimental hints. This time the hints are coming mainly from outer space rather than underground laboratories, which makes them harder to check independently, but there’s a chance something real is going on. We need more data to be sure, as scientists have been saying since the time Eratosthenes measured the circumference of the Earth.

As I mentioned briefly last week, Katherine Freese of the University of Michigan has a new book coming out, The Cosmic Cocktail, that deals precisely with the mysteries of dark matter. Katie was also recently at the UCLA Dark Matter Meeting, and has agreed to share some of her impressions with us. (She also insisted on using the photo on the right, as a way of reminding us that this is supposed to be fun.)


Dark Matter Everywhere (at the biannual UCLA Dark Matter Meeting)

The UCLA Dark Matter Meeting is my favorite meeting, period. It takes place every other year, usually at the Marriott Marina del Rey right near Venice Beach, but this year on UCLA campus. Last week almost two hundred people congregated, both theorists and experimentalists, to discuss our latest attempts to solve the dark matter problem. Most of the mass in galaxies, including our Milky Way, is not comprised of ordinary atomic material, but instead of as yet unidentified dark matter. The goal of dark matter hunters is to resolve this puzzle. Experimentalist Dave Cline of the UCLA Physics Department runs the dark matter meeting, with talks often running from dawn till midnight. Every session goes way over, but somehow the disorganization leads everybody to have lots of discussion, interaction between theorists and experimentalists, and even more cocktails. It is, quite simply, the best meeting. I am usually on the organizing committee, and cannot resist sending in lots of names of people who will give great talks and add to the fun.

Last week at the meeting we were treated to multiple hints of potential dark matter signals. To me the most interesting were the talks by Dan Hooper and Tim Linden on the observations of excess high-energy photons — gamma-rays — coming from the Central Milky Way, possibly produced by annihilating WIMP dark matter particles. (See this arxiv paper.) Weakly Interacting Massive Particles (WIMPs) are to my mind the best dark matter candidates. Since they are their own antiparticles, they annihilate among themselves whenever they encounter one another. The Center of the Milky Way has a large concentration of dark matter, so that a lot of this annihilation could be going on. The end products of the annihilation would include exactly the gamma-rays found by Hooper and his collaborators. They searched the data from the FERMI satellite, the premier gamma-ray mission (funded by NASA and DoE as well as various European agencies), for hints of excess gamma-rays. They found a clear excess extending to about 10 angular degrees from the Galactic Center. This excess could be caused by WIMPs weighing about 30 GeV, or 30 proton masses. Their paper called these results “a compelling case for annihilating dark matter.” After the talk, Dave Cline decided to put out a press release from the meeting, and asked the opinion of us organizers. Most significantly, Elliott Bloom, a leader of the FERMI satellite that obtained the data, had no objection, though the FERMI team itself has as yet issued no statement.

Many putative dark matter signals have come and gone, and we will have to see if this one holds up. Two years ago the 130 GeV line was all the rage — gamma-rays of 130 GeV energy that were tentatively observed in the FERMI data towards the Galactic Center. (Slides from Andrea Albert’s talk.) This line, originally proposed by Stockholm’s Lars Bergstrom, would have been the expectation if two WIMPs annihilated directly to photons. People puzzled over some anomalies of the data, but with improved statistics there isn’t much evidence left for the line. The question is, will the 30 GeV WIMP suffer the same fate? As further data come in from the FERMI satellite we will find out.

What about direct detection of WIMPs? Laboratory experiments deep underground, in abandoned mines or underneath mountains, have been searching for direct signals of astrophysical WIMPs striking nuclei in the detectors. At the meeting the SuperCDMS experiment hammered on light WIMP dark matter with negative results. The possibility of light dark matter, that was so popular recently, remains puzzling. 10 GeV dark matter seemed to be detected in many underground laboratory experiments: DAMA, CoGeNT, CRESST, and in April 2013 even CDMS in their silicon detectors. Yet other experiments, XENON and LUX, saw no events, in drastic tension with the positive signals. (I told Rick Gaitskell, a leader of the LUX experiment, that I was very unhappy with him for these results, but as he pointed out, we can’t argue with nature.) Last week at the conference, SuperCMDS, the most recent incarnation of the CDMS experiment, looked to much lower energies and again saw nothing. (Slides from Lauren Hsu’s talk.) The question remains: are we comparing apples and oranges? These detectors are made of a wide variety of types of nuclei and we don’t know how to relate the results. Wick Haxton’s talk surprised me by discussion of nuclear physics uncertainties I hadn’t been aware of, that in principle could reconcile all the disagreements between experiments, even DAMA and LUX. Most people think that the experimental claims of 10 GeV dark matter are wrong, but I am taking a wait and see attitude.

We also heard about the hints of detection of a completely different dark matter candidate: sterile neutrinos. (Slides from George Fuller’s talk.) In addition to the three known neutrinos of the Standard Model of Particle Physics, there could be another one that doesn’t interact with the standard model. Yet its decay could lead to x-ray lines. Two separate groups found indications of lines in data from the Chandra and XMM-Newton space satellites that would be consistent with a 7 keV neutrino (7 millionths of a proton mass). Could it be that there is more than one type of dark matter particle? Sure, why not?

On the last evening of the meeting, a number of us went to the Baja Cantina, our favorite spot for margaritas. Rick Gaitskell was smart: he talked us into the $60.00 pitchers, high enough quality that the 6AM alarm clocks the next day (that got many of us out of bed and headed to flights leaving from LAX) didn’t kill us completely. We have such a fun community of dark matter enthusiasts. May we find the stuff soon!

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23 Responses to Guest Post: Katherine Freese on Dark Matter Developments

  1. Wonderful post, thank you. What are the present results on MACHO searches reporting? Or have these been eliminated?

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  2. DEL says:

    Hidden due to low comment rating. Click here to see.

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  3. universal_jet says:

    Hidden due to low comment rating. Click here to see.

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  4. Pierre Schmidt says:

    Remember, Jetto, there’s no such thing as the so-called “aether.” It’s a myth whose existence was disproved long ago. Anybody who believes in the ether is completely nuts!

    Well-loved. Like or Dislike: Thumb up 14 Thumb down 1

  5. Katherine Freese says:

    HI Jerry
    Yup, MACHOs are dead. Long live WIMPs. I wrote a series of papers in the 90s arguing that the numbers of MACHOs were limited for a variety of reasons. The answer to the dark matter might have been lots of faint stars and substellar objects, in the category of MACHOs. But by looking at Hubble Space Telescope and other data, we concluded these could make up at most 3% of the Milky Way. Then we studied stellar remnants, particularly white dwarfs, as MACHO candidates. When the Sun runs out of nuclear fuel it will become a white dwarf. Since these white dwarfs came from ordinary stars, they would have produced a ton of detectable signals, including infrared light, carbon and other elements — and these are just not seen. Microlensing experiments named MACHO, EROS, and OGLE eventually confirmed that we were right. Bottom line: 15% of the dark matter in the Milky Way could still be made of white dwarfs. But that’s it. MACHOs cannot completely solve the dark matter problem.

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  6. universal_jet says:

    Hidden due to low comment rating. Click here to see.

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  7. Joan Hendricks says:

    When will the book “The Cosmic Cocktail” be coming out? I am so wanting to learn what dark matter is in my lifetime!

    Like or Dislike: Thumb up 1 Thumb down 0

  8. DEL says:

    People, you have voted me down 13:0 (as of now) and I really am puzzled. Is it the rough style I used? Is it the religious-like text I included? Or is it the scientific content of my comment? If it’s one of the former, well, maybe there’s a cultural rift across which my particular sense of humor doesn’t work, or maybe I really deserve it. But if it’s my scientific disposition, then I want to know: would the voters be indeed disappointed if dark matter and/or dark energy turn out superfluous? Are they so in love with as-is physics that they won’t even see what this game is all about? (There were many distinguished scientists in the early 20th century that resisted relativity as long as they lived. One smart guy, I don’t remember who, said that paradigms shift slowly—funeral by funeral.) I’d like to understand what kind of community I’m communicating with.

    Unlike Mr. Universal_jet, I don’t have a better theory of gravity to propose, neither a different interpretation of the observations—if I had such, I would have submitted them to Nature, not to this blog. I do accept the current Lambda-CDM view as inescapable under the circumstances. And if that exonerates me, I do believe that Kathy and the other dark matter researchers are doing the best that can be currently done to solve the dark matter problem. I was just expressing a dream, maybe a pipe dream, that the solution won’t take the form of a new particle, since I believe that the greatest problem about dark matter is not its nature but the fact that it has to exist at all.

    So I’d like to address my question to Kathy herself: Would you, too, be disappointed if a “good” modification to the theory of gravity is discovered that renders dark matter superfluous?

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  9. John says:

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  10. BobC says:

    News release from CERN:

    “Inspired by the guest post by cosmologist Katherine Freese on the Preposterous Universe blog, the CERN Scientific Policy Committee has mandated that cosmologists at all CERN facilities shall be required to wear feather boas to more clearly distinguish them from the experimental physicists.”

    Please note that CERN failed to specify the color of the feather boas. So it may be possible to distinguish groups within the cosmological community by the tint of their plumage.

    However, multicolored plumage should be discouraged: If you can’t make up your mind, please either wear gray or own multiple boas (worn one at a time, please). It is also permissible to use complex tints, where spectral analysis may be used to identify fine gradations of community affiliation.

    Only those who agree in all things with Katherine Freese will get to wear violet boas.

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  14. Luiza says:

    No doubt that dark matter remains a mystery, and there is a lot of theory around this issue, I believe that soon Researchers discover its origin

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  17. Brett says:

    I have absolutely nothing to contribute to this. Thank you for the guest post on what I consider to be the most fascinating mystery in science at the moment. Each energy spectrum ruled out makes it that much more exciting.

    Like or Dislike: Thumb up 1 Thumb down 0

  18. JimmyDP says:

    Great post! But a couple of questions:

    1 You say dark matter is its own antiparticle i.e. it’s majorana/real scalar, why can’t it be dirac/complex scalar?

    2 If dark matter annihilates to quarks it can surly be produced by them too ( XX->qq vs qq->XX). So what does the LHC have to say about ~30 GeV DM?

    And then some comments:

    1 Your arxiv link is not to the paper, it think
    http://arxiv-web3.library.cornell.edu/pdf/1402.6703.pdf
    is what you’re looking for.

    2 This excess is far greater than the 130 GeV excess it’s something around 40 sigma… it’s not going to “go way” with more statistics. However we may find a non-DM source for it.

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  19. “For example, I heard a while back ago about a women that developed a theory that showed how gravity could leak from higher dimensions that worked close to Brian Greene.”

    What about higher dimensions that work close to Sean Carroll? Or Max Tegmark? What is so special about Brian Greene? :-)

    Like or Dislike: Thumb up 1 Thumb down 0

  20. “People, you have voted me down 13:0 (as of now) and I really am puzzled. Is it the rough style I used? Is it the religious-like text I included? Or is it the scientific content of my comment?”

    Presumably it is because a) you lump several otherwise dissimilar ideas together, b) imply that dark matter is similar to these, and c) imply that your personal preference is more important than what we discover about the universe.

    Although, as you point out, you don’t go that extra step, almost all rants which start off like this lead to the author’s own untestable, unrefereed theory of everything which claims to explain everything but actually was experimentally ruled out before the author was born.

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  21. DEL says:

    Hidden due to low comment rating. Click here to see.

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  22. Shantanu says:

    Katie,
    What about other recent possibilites which have recently become popular such as soliton stars , bose-einstein condensates, or even a new degree of freedom from GR as advocated by Slava (http://arxiv.org/abs/1308.5410)?

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  23. Tom Breton says:

    It is unfortunate that in many people’s minds, dark matter has become indistinguishable from some hypothetical new particle to be discovered.

    These aren’t developments in dark matter, these are potential developments in particle physics. These developments haven’t even found a particle yet, much less provide any reason to suppose that this undiscovered particle accounts for 5/6 of the ordinary matter in the universe. The history of the search for such a particle gives little reason to hope.

    Is it for lack of another candidate? We’ve ruled out MOND and MACHOs and neutrinos, so we grind away at an increasingly improbable search for a particle that will explain it all? Parallel Dark Matter is a more reasonable candidate. It has at least the brown dwarf observations to its credit.

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