Has Fermi Seen New Evidence for Dark Matter?

By Sean Carroll | October 28, 2009 1:22 pm

Speaking of successful NASA/DOE collaborations, there’s an interesting new paper on astro-ph claiming that the Fermi gamma-ray satellite has found evidence for a gamma-ray excess in the vicinity of the galactic center — similar to what you might expect from high-energy electrons produced by annihilations or decays of dark matter.

The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze
Authors: Gregory Dobler, Douglas P. Finkbeiner, Ilias Cholis, Tracy R. Slatyer, Neal Weiner

Abstract: The Fermi Gamma-Ray Space Telescope reveals a diffuse inverse Compton signal in the inner Galaxy with the same spatial morphology as the microwave haze observed by WMAP, confirming the synchrotron origin of the microwaves. Using spatial templates, we regress out pi0 gammas, as well as ICS and bremsstrahlung components associated with known soft-synchrotron counterparts. We find a significant gamma-ray excess towards the Galactic center with a spectrum that is significantly harder than other sky components and is most consistent with ICS from a hard population of electrons. The morphology and spectrum are consistent with it being the ICS counterpart to the electrons which generate the microwave haze seen at WMAP frequencies. In addition to confirming that the microwave haze is indeed synchrotron, the distinct spatial morphology and very hard spectrum of the ICS are evidence that the electrons responsible for the microwave and gamma-ray haze originate from a harder source than supernova shocks. We describe the full sky Fermi maps used in this analysis and make them available for download.

In English: if the dark matter is a weakly-interacting massive particle (WIMP), individual WIMPs should occsasionally annihilate with other WIMPs, giving off a bunch of particles, including electron/positron pairs as well as high-energy photons (gamma rays). Indeed, searching for such gamma rays was one of the primary motivations behind the Fermi mission (formerly GLAST). And it makes sense to look where the dark matter is most dense, in the center of the galaxy. But it’s a very hard problem, for a simple reason — there’s lots of radiation coming from the center of the galaxy, most of which has nothing to do with dark matter. Subtracting off these “backgrounds” (which would be very interesting in their own right to galactic astronomers) is the name of the game in this business.

But Doug Finkbeiner at Harvard has for a while now been suggesting that there was already evidence for something interesting going on near the galactic center — not in the form of high-energy photons, but in the form of low-energy photons. The so-called WMAP haze is alleged to be radiation emitted when high-energy electrons are being accelerated by magnetic fields, leading to low-energy photons (synchrotron radiation). And Finkbeiner and collaborators claim that a careful analysis of data from WMAP (whose primary mission was to observe the cosmic microwave background) reveals exactly the kind of radiation you would expect from annihilations near the galactic center.

If that model is right, it gives us some guidance about what to look for in the gamma rays themselves, which Fermi is now observing. And according to this new paper, this is what we see.

Excess gamma rays from the galactic center, from Dobler et al.

That’s one of many images, and has been extensively processed; see paper for details. The new paper claims that there is an excess of gamma rays, and that it has just the right properties to be arising from the same population of electrons that gave rise to the WMAP haze. These much higher-energy photons arise from inverse Compton scattering — electrons bumping into photons and pushing them to higher energies — rather than synchrotron emission. So we’re not talking about gammas that are produced by dark-matter annihilations, but ones that might arise from electrons and positrons that are produced by such annihilations. The authors pointedly do not claim that what we see must arise from dark matter, or even delve very deeply into that possibility.

There have been speculations that the microwave haze could indicate new physics, such as the decay or annihilation of dark matter, or new astrophysics. We do not speculate in this paper on the origin of the haze electrons, other than to make the general observation that the roughly spherical morphology of the haze makes it difficult to explain with any population of disk objects, such as pulsars. The search for new physics – or an improved understanding of conventional astrophysics – will be the topic of future work.

That’s as it should be; whether or not the gamma-ray haze is real is a separate question from whether dark matter is the culprit. But on a blog we can speculate just a bit. Therefore I’m going to go out on a limb and say: maybe it is! Or maybe not. But a wide variety of promising experimental techniques are attacking the problem of detecting the dark matter, and we’ll be hearing a lot more in the days to come.

  • Ellipsis

    Neal Weiner (with Nima Arkani-Hamed) was the author of the model that appeared right after the CDF multi-muons excess with a convienient theoretical model that would have explained their “new physics”. Not saying that this is the same, but seeing new physics in every corner typically leads to a lot of false positives.

  • onymous

    Neal Weiner (with Nima Arkani-Hamed) was the author of the model that appeared right after the CDF multi-muons excess with a convienient theoretical model that would have explained their “new physics”.

    You mean a model that appeared before the CDF multi-muons, with a model that did not explain it at all, as anyone who spent a couple of hours doing some estimates would have realized.

  • Ellipsis

    No, after. Before CDF had released it to the arXiv, and after they had written the paper.

  • onymous

    So… before the CDF result was public. And completely unrelated to it. The paper you’re referring to was an attempt to fit Pamela, ATIC, DAMA, etc. into a coherent framework. If your point is that these people are chasing experimental signatures that might turn out not to be new physics, of course they are. You can make that point without distorting what the paper is about and repeating the stupid insinuations of the blogosphere that this was all about leaking information from CDF. The particle physics community collectively paid almost no attention to the multi-muon result, except to complain about the presentation or to use it as a joke. It’s only the blogosphere that characteristically overreacted and continues to think, inexplicably, that this is important for some reason.

  • Ellipsis

    onymous, I am a faculty member in particle physics. You are correct that they are indeed insinuations because no one knows (but the two authors), however no one thinks that they are wild insinuations. Anyway, that is not important (who cares if there was a leak — it is certainly not illegal or anything!), but what is good to note is the general history.

  • dmo

    This is the paper:


    It appeared before the CDF stuff was made public, and it was not influenced by the result. The authors are all outstanding physicists, and if they had wanted to build a model to explain the CDF findings they would have come up with something that actually came close to explaining the CDF data. There is absolutely no reason to think any of the authors had any sort of inside dirt.

  • Anne

    There was (is?) another puzzle around the Galactic center: there is a substantial 511 keV excess there – photons from electron/positron annihilation. I believe this was first noticed in EGRET data, and early INTEGRAL data confirmed it. Nobody had a good standard-physics explanation for where so many positrons were coming from, so people suggested it might be dark matter annihilation. That explanation is now looking a bit more dubious, since with more INTEGRAL data it looks like there’s some asymmetry there that’s hard to explain in terms of dark matter (the authors of the asymmetry paper suggested low-mass X-ray binaries as the origin of the positrons, and in fact there appears to be some correlation between the locations of known LMXBs and the 511 keV excess). But now I wonder how it relates to this result. Does the spatial distribution of the 511 keV excess line up with the WMAP/Fermi haze? The 511 keV excess comes from a population of cold electrons, so maybe you wouldn’t expect any relationship. But I’d be interested to hear whether there’s a relationship…

    The INTEGRAL paper: http://arxiv.org/abs/0810.3674

  • Christina Viering

    Interesting concept.

  • Andrew

    @Anne: the 511 keV excess was actually first noticed about 40 years ago, though I don’t know how well it was localized at the time. The INTEGRAL results show that the 511 keV excess is about 2/3 bulge component and the rest disk (which is I think where the asymmetry comes from). My understanding is that it’s pretty hard to get a big bulge component like that from conventional astrophysics, but I would expect INTEGRAL to push their data and analysis as far as they can to test these things.

    Having worked on the 511 keV excess and related dark matter theories, I can say that you don’t necessarily expect that signal to line up with the WMAP/Fermi haze. The reason is that the WMAP/Fermi haze comes from high energy e+e- created in DM annihilation, while 511 keV comes from low energy e+e- created by DM excitation (by collision) and subsequent decay to ground state. In these types of models, annihilation cross-sections are boosted at low velocities (by Sommerfeld enhancement), while the excitations require somewhat higher velocities (to provide the excitation energy).

    Since this is just a comment on a blog, let me go out on a limb (like Sean) and say that, if the data bears out the existence of spontaneously broken dark forces and this general type of dark matter model, this work will be a fine candidate for a Nobel prize. It’s certainly generated a lot of interesting research as far as I am concerned.

    Also, let me add that the Arkani-Hamed, Finkbeiner, Slatyer, & Weiner paper isn’t just about dark matter annihilation (or lepton jets), but it also unified some rough models of dark matter that Finkbeiner and Weiner (and others) have been talking about for years. So, while I heard the rumors too, I personally find them hard to believe.

  • straightbat

    Anne the 511 line at the Galactic center has been known since well before EGRET. Balloon experiments from the 70s. Chandra studies of the galactic center and bulge have uncovered a right royal mess of X-ray sources, CVs, LMXBs, both of the neutron star and black hole variety. For most of them there is no distance measurement so there is a fair bit of uncertainty in the luminosities and exact nature of the individual sources. As in the Integral paper you cite it seems to me that there may be ways to conjure up astronomical origins for various things with reasonable assumptions of n sources of xyz specific variety.

  • http://eskesthai.blogspot.com/2009/10/fermi-records-lighthouse-effect.html Plato

    Basically what they are saying in evidence is that Einstein has not been overthrown by any theoretical position stated by joining Electromagnetism with Gravity? So people will be defensible in their scientific position toward this and react?

    I draw attention to M87 just to support further contentions about the subject here knowing more about M87.

    M87 is the nearest example of an active galactic nucleus with a bright optical jet. The jet appears as a string of knots within a widening cone extending out from the core of M87. The FOC image reveals unprecedented detail in these knots, resolving some features as small as ten light-years across. According to one theory, the jet is most likely powered by a 3 billion solar mass black hole at the nucleus of M87.See: Hubble Site

    I would think Susskind’s thought experiment using elephants applies here?


  • piscator

    Have to agree with ellipsis. when people with a track record of seeing new physics everywhere they look announce, again, evidence for new physics, one shouldn’t get too excited. If FERMI themselves find evidence for something, that will be more significant.

    The INTEGRAL 511keV line is just one of the many things that make this situation smell for me. INTEGRAL themselves said that they found LMXBs correlated well with the origin of the 511keV emission. This came out way earlier than the ‘Theory of Dark Matter’ hubris. But the fact that the probable astrophysical origin of this line had already been identified didn’t seem to matter – this was another ‘anomaly’ to be ‘explained’ – while also as in comment 9 ignoring the original work on it.

    The Nobel Prize for this can be awarded simultaneously with the Nobel Prize for split supersymmetry, mm-size extra dimensions and 17keV neutrinos :)

  • Sili

    I don’t know if it’s too ‘simple’ for the regulars, but I really liked Ethan Siegel’s recent dissection of all things Dark Matter.

  • Andrew

    piscator, I don’t think anyone is ignoring possible astrophysical explanations; I’m certainly not. But I think the “probable astrophysical origin” is not as clear-cut as you imply for the 511 keV line. In 2005, the INTEGRAL collaboration (astro-ph/0506026) listed novae, SN Ia, LMXBs, and light dark matter annihilation as suitable candidate explanations. The paper Anne linked makes a case for LMXBs. This past spring, Lingenfelter et al argued rather that positrons from SN Ia just happen to propagate farther than we’d thought (ESA had a press release on this). So which probable astrophysical origin is your favorite? Meanwhile, the “7 years of INTEGRAL” workshop two weeks ago tried to get Neal Weiner to talk about exciting dark matter explanations (though he couldn’t come, so the invitation on their page lists the speaker as TBD).

    Look, it’s quite possible that the correct explanation for the 511 keV line (or the higher energy e+e- seen by PAMELA, Fermi, etc) is astrophysical in nature, even if it’s not previously expected astrophysics. And if that turns out to be the case, then I’ll learn something, be happy, and move on. Nonetheless, it’s easy to see from the recent literature that this is a heated topic, and I think it’s fair to say that people from both the astrophysics and particle physics side of the discussion have their biases, as is often the case. I’m not sure it’s fair to label one idea or another as “hubris,” though.

  • Wayfinder

    The paper in question’s title is “A Theory of Dark Matter”. I think the “A” is important in terms of evaluating the hubris involved.

    I’m a little surprised that Ellipsis (despite being a faculty in particle theory, oh my!) hasn’t conceded the point about his or her insinuations: that paper does not explain the CDF multi-muons. If the authors had really been after that signal, they would have done a simple estimate, and jacked up the coupling by an order of magnitude or so. If they were trying to fit the CDF data that was about to be released, they were pretty incompetent in doing so.

    That group of authors does sometimes go for flashy or superficial, but it has no history of incompetence.

  • onymous

    The paper in question’s title is “A Theory of Dark Matter”. I think the “A” is important in terms of evaluating the hubris involved.

    Well, there is an implicit allusion to Weinberg in 1967….

  • Doug Finkbeiner


    Thanks for mentioning the Fermi haze on Cosmic Variance; we were delighted to see a hard-spectrum inverse Compton signal right where we expected it, and with roughly the right amplitude. We welcome debate about this claim, and have made the maps used in our analysis available for anyone to examine (here). We still do not know where these hard electrons come from, but if there is even a small chance they come from dark matter, I anticipate a lively discussion over the next few years.

    I’m not sure what motivates someone to bring up the CDF multi-muon claim, which is unrelated and far from compelling. True, after their result appeared on the arXiv, we did (briefly) attempt to fit it into our framework of WIMPs and dark forces, but initial attempts missed by orders of magnitude on rates, timescales, or both. There appears to be no plausible connection. The suggestion that our dark matter ideas were influenced by CDF is, well, laughable. This did not prevent the blogosphere from manufacturing a controversy where there was none (see here, especially comment 6). So if D0 confirms multi-muon events, we will revisit it, but for now this does not rise to the level of “actionable intelligence.”

    What is more interesting to me is the INTEGRAL 511 keV line. As mentioned above, the basic observation has been around nearly 40 years, and is still not understood. Neal and I were fairly conservative in our 2007 paper on exciting DM (astro-ph/0702587). The approach of the paper is clearly “if you had make the 511 keV signal with weak-scale WIMPs, what would you have to resort to?” We tried a bunch of things, and one of them seemed appealing enough to publish.

    We are also obviously interested in astrophysical explanations. The idea that LMXBs could explain the signal had some traction for a while, but the previously claimed asymmetry of the INTEGRAL signal has gone away with additional data. Furthermore, a detailed study (not yet published) by Pietro Ubertini and his student (with IBIS) showed no evidence that LMXBs were the source. For those of you who don’t know him, Pietro is the PI of INTEGRAL. So I think it is safe to say that the origin of the 511 keV line is still a mystery.

    Let me close by reassuring piscator that I share his suspicion of those who claim to see new physics everywhere they look. One of the greatest challenges of theoretical physics is to simultaneously entertain multiple visions of how nature could be — even if some of them are radical or mutually incompatible — and keep track of what we actually know and how we know it. We will inevitably wander down speculative paths, pointing out the logical consequences of this or that assumption,
    and often be surprised by the connections that arise. When such connections are remarkable enough, we publish them. So while accusations of bombast for an over-the-top title may be justified, accusations of hubris are not. Indeed, humility is required to appreciate the myriad possibilities before us and not limit them for the wrong reasons. Anyone who thinks that we have claimed discovery of “new physics” has seriously misunderstood our approach and the motivation for these papers. If, at some point in the future, we do decide we have found “new physics,” we will not be nearly so subtle about it, and we will provide the extraordinary evidence required by extraordinary claims.

    Until then, particle physicists and astrophysicists can learn a lot from each other in the kind of friendly and fruitful collaboration I have had with Neal and Nima and our students. There’s likely a long road ahead, and we intend to have a lot of fun along the way. –DPF

  • King Cynic

    Whenever a theorist analyzes an experimentalist’s data and claims to see evidence for X, and said experimentalist has not herself claimed evidence for X, everyone except the experimentalist should studiously ignore the claim.

    It is almost a theorem that theorists making claims on experimental data that the experiments haven’t made themselves are wrong.

    In this particular claim, we should just wait for Fermi to have their say.

  • John R Ramsden

    @18 “Whenever a theorist analyzes an experimentalist’s data and claims to see evidence for X, and said experimentalist has not herself claimed evidence for X, everyone except the experimentalist should studiously ignore the claim.”

    What doctrinaire nonsense! Apart from anything else, the experimentalist might be completely unaware of the theory in question, or of all its implications. Nobody can know everything, especially these days.

    Anyway, isn’t science supposed to be collaborative, with scientists specializing on what best reflects their interests and aptitudes? Or do you claim each scientist must be a self-contained jack or jill of all trades, having to think of concepts, verify them by experiments, *and* work up a complete theoretical model?

    I wouldn’t go into the hotel business if I were you (or any business for that matter), or you’ll have to greet guests at the entrance, check them in, rush into the kitchen to cook their meals, and manage room service, all on your own!

  • King Cynic

    @19: That specialization is exactly the issue here. It requires significant training and detailed inside knowledge to properly fit anything but the simplest experimental data. Details of detector response and backgrounds are paramount. Generally only the people who did the experiment are qualified to properly do the kind of fits that are at issue in this paper. When theorists attempt to do this, without really understanding the detector, they almost always botch it.

    The proper way to do this is for a theorist with a clever idea to convince the experimentalists that their idea is worth following up on. I’m sure that the Fermi team will do exactly this, but for the time being the rest of us should be extremely skeptical of anyone trying to analyze another group’s data and making a claim that the experimentalists themselves aren’t endorsing yet.

  • http://doyouknow.yo2.cn eagle1879

    Looking forward to further research on it.

  • Greg

    very compelling and exciting! one of the greatest pursuits of our time

  • Gordon

    Yes, John, it is nonsense–consider Penzias and Wilson’s discovery of the cosmic background radiation proposed by theorists Gamow et al. It took theorists Dicke, Peebles, and Wilkinson to
    tell them what they had stumbled onto. The latter should have shared the Nobel.

  • Peter

    @17: One of the greatest challenges of theoretical physics is to simultaneously entertain multiple visions of how nature could be — even if some of them are radical or mutually incompatible — and keep track of what we actually know and how we know it.

    Nicely put

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  • http://sites.google.com/site/simpatico29x/Home?pli=1 Thomas Neil Neubert

    Perhaps I’m being dense but I fail to understand how this particular observation supports in any way a dark matter hypothesis. I think the authors are correct in not delving too deeply into that possibility. What is most important is the possibility of A NEW phenomenon; a correct explanation may take a long time. Descriptive science is enough; we don’t need a theory, especially if the theory is technical jargon (e.g. “dark matter” ) that just means “we don’t have any idea what kind of matter is causing this”; then just say we don’t know, we just don’t have a theory.

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  • Aaron Sheldon

    Remind me again, exactly what are the estimates for super nova rates in the galactic center?


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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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