Something's in the air

By Daniel Holz | November 25, 2008 2:21 am

And it might even be dark matter.

There’s been a rash of slightly odd and suggestive results as of late. There was the observation last month by the PAMELA satellite of an anomalous positron excess at ~50 GeV. This week the balloon-borne ATIC experiment reports seeing a bump in electrons and positrons (they can’t tell the difference) at 500 GeV. MILAGRO has recently seen some weird gamma ray hotspots at 10 TeV. As if all this isn’t enough, Doug Finkbeiner has been warning us that there is an unexplained CMB “haze”.

The reason we care about all of these observations is that they may be pointing to dark matter! Yes, dark matter is dark, so it’s awfully hard to see directly. But some of the favored dark matter particle candidates happen to annihilate when they smack into each other (which happens at sufficiently high density), producing “conventional” stuff (such as electron-positron pairs and gamma rays). And that stuff you can see! So if there are little clumps of dark matter floating around in the Milky Way (which are indeed expected from galaxy formation models, in some cases down to fractions of an Earth mass), then it is conceivable that the dark matter at the center of these clumps annihilates, and produces a visible signal. Needless to say, this would be insanely exciting; hence all the fuss about these recent anomalous results. Yesterday an article on the current dark matter zeitgeist even made it to the top of the front “page” of the New York Times website, right up there with the latest bailout (Citigroup) and some football results. So you know it must be important.

Nobody is claiming a smoking-gun detection of dark matter annihilation as of yet. It may be around the corner. Or not. All we can do is keep looking.

GLAST/Fermi rocket launch

PAMELA and ATIC will refine their results. GLAST (now called Fermi) is airborne, and actively collecting data, and will make a gorgeous map of the gamma-ray sky. And data from the Large Hadron Collider will soon be pouring in. There’s a chance that it will directly detect the dark matter, but regardless, we can’t wait to find out what it does (or doesn’t) see.

There’s a general excitement in the air. In the coming year or two we may discover the dark matter. Then again, it is entirely conceivable that throughout the entire course of human existence the dark matter is never identified. It is precisely this uncertainty which keeps science interesting.

CATEGORIZED UNDER: Science
MORE ABOUT: dark matter, LHC
  • http://mirror2image.wordpress.com Serge

    I did short trip to wikipedia and read that most likely dark matter candidates “raise naturally” in supersymmetry model. So are there any connection/hints in the latest data for or against it ?

  • Jason Dick

    Serge,

    It’s far too early to say much of anything about any particular dark matter candidates. Right now all we have are unexplained excesses at certain energy levels in some experiments. I suspect that most of what we’re seeing right now is actually some currently not-understood astrophysical source (something like a neutron star), and not dark matter. It will take time to nail down whether or not any of these signals are dark matter at all, let alone what type.

    But it is exciting that we’re finally getting signals that may well be from dark matter. I expect we’ll probably have a fair idea of what makes up dark matter within ten years.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    It is provocative that a number of anomalies are popping up in a set of different experiments. (We could add DAMA to the list as well.) My casual impression is that none of them are precisely what you would expect if you asked what should be predicted by WIMP dark matter, but it’s possible to find models that work (of course).

  • http://danielholz.com daniel

    It’s hard to know if these are a bunch of cracks in the standard model, and we’re about to shatter everything and revolutionize our understanding of dark matter. Or if these are just conventional deviations (e.g., a nearby pulsar, statistical “bad luck”), not signifying anything particularly profound (as far as dark matter is concerned). Theorists are certainly hard at work; this morning’s arXiv list included a model accounting for everything (and falsifiable by GLAST/Fermi): http://arxiv.org/abs/0811.3641

  • http://blogtopciz.com Zenny

    something in the air, eh? sniff. cosmic ray ozone maybe?

  • Lawrence Crowell

    Dark matter rules! Oops, sorry there is dark energy as well. Never mind.

    This all looks really exciting. As the PAMELA results suggest DM at the TeV range it will be interesting if some channel production turns up in the LHC.

    Lawrence B. Crowell

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    I *guarantee* you that there is at least a little bit of dark matter in air.

    * guarantee void if falsified.

  • Eric

    I’d find their explanation in terms of Kaluza-Klein DM more convincing if they made some attempt to explain the apparent uptick in the flux just below 100 GeV, and if they had enough data above in the >1TeV range to convince me that the spectrum actually returns to the GALPROP prediction. Eyeballing figure 4 in that letter to Nature, the fit doesn’t look nearly good enough to take the claim that “the solid curve … reproduces all of the measurements from 20 GeV to 2 TeV” at face value.

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  • Anon

    There is something I don’t quite understand here. I thought that DM models are ‘colisionless’ and yet here we are talking about DM anihilation? how do they find each other? The interaction is pretty damn short range…

  • Jason Dick

    Anon,

    Their collisionless property is only an approximation. It is, however, exactly because this is a very good approximation that we expect there to still be lots of anti-particles remaining for dark matter (whereas the much stronger-interacting normal matter we have today is entirely devoid of anti-particles, except those created at later dates). So, they don’t interact much, but every once in a while you still get an annihilation when a dark matter particle manages to smack into its anti-particle. It’s not going to happen often, naturally, as they don’t experience much of any attraction (mostly just gravitational, and that’s extremely weak between individual particles). But there’s a lot of dark matter out there, so we expect the signal should be measurable.

  • Brody Facoum

    Anon – let me make a mess of wrongness trying to answer your question. :-)

    Self gravitating globules could lead to DM particles in the centre of a sufficiently massive DM globule to be squashed into a short interaction range, analogous to degenerate phases in sufficiently dense collections of normal matter (as in degenerate dwarfs or neutron stars).

    The problem is that the gravitational collapse of the DM globule should do work, but the DM should not radiate photons (and in CDM, it should not result in highly accelerated DM particles in a way that causes problems at the epoch of matter-radiation equality). Where does the energy go?

    Alternatively, there could be interactions analogous to normal matter nuclear fusion, such as p-p fusion chains in stars, again with most of the energy dissipating through some mechanism other than photon radiation.

    Either of those bug me a bit — wouldn’t we expect some ordinary visible matter to be swept up into the self-gravitating clump of DM? That matter would heat up and radiate photons, right? (And also absorb and reflect light from elsewhere).

    Alternatively alternatively, these high energy gamma rays could be because of DM particle decay rather than annihilation; I think this would be allowed by WIMPy LCDM models.

    Alternatively^3, this could be ordinary-matter pulsar physics, such as primary production of e+ within the pulsar or secondary production in nearby ordinary matter (in collsion with high energy nuclei ejected from the pulsar) with the pulsar’s magnetic field accelerating those secondary positrons into high energy jets.

    I could of course be completely out to lunch, and welcome corrections from people who aren’t.

  • Jason Dick

    Brody,

    Self gravitating globules could lead to DM particles in the centre of a sufficiently massive DM globule to be squashed into a short interaction range, analogous to degenerate phases in sufficiently dense collections of normal matter (as in degenerate dwarfs or neutron stars).

    The problem is that the gravitational collapse of the DM globule should do work, but the DM should not radiate photons (and in CDM, it should not result in highly accelerated DM particles in a way that causes problems at the epoch of matter-radiation equality). Where does the energy go?

    Right, the dark matter doesn’t collapse much on its own, because it is almost collisionless. Once you get a halo of dark matter, unless it interacts with something external to it, it largely just stays as it is for then after. Much of the annihilation, then, comes as a result of the normal matter, as though there’s much less of it around, it is far, far clumpier. The greater density of the normal matter, then, causes the dark matter to fall into the normal matter’s potential wells, producing more annihilations. This is why, for example, there have been some suggestions that perhaps there’s some annihilation of dark matter going on near the galactic core.

    Bear in mind that even though there is no net work being done on the dark matter (to a very good approximation), the dark matter is still more likely to be found in wells where normal matter has condensed into compact objects.

  • http://coraifeartaigh.wordpress.com Cormac O’ Raifeartaigh

    I don’t find DM speculative at all. Why should we demand of matter that it be visible? Bets anyone?

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  • Brody Facoum

    Jason -

    Thanks. I have a couple dumb, unrigorously framed questions:

    Right, the dark matter doesn’t collapse much
    on its own, because it is almost collisionless. Once you
    get a halo of dark matter, unless it interacts with
    something external to it, it largely just stays as it is
    for then after.

    A DM halo obviously interacts with lots of things external
    to it, at least gravitationally: the rest of the mass in
    the structure the halo is part of, and mass external to
    the structure.

    Must CDM be self-collisionless or could it simply be
    reluctant to form small scale dark matter structures?
    Obviously it is much less dense than normal matter,
    because it went unnoticed by classical astronomers like
    Kepler and Newton (and pretty much everyone else until the
    20th century). Perhaps there is a dark charge which works
    against gravitational attraction analogously to electron
    degeneracy pressure.

    dark matter is still more likely to be found
    in wells where normal matter has condensed into compact
    objects

    A large pool of dark matter would not only slowly accrete
    more dark matter, it would also sweep up normal matter,
    which has probably been denser at small scales than DM
    since BBNS. The denser material would sink into the pool.
    If DM is allowed to become too dense, the normal matter
    simply would sink into the centre of mass forming layers
    of increasingly heavy atoms and nuclei. In the early days
    of initial structure formation, that would mean isotopes
    of H to Be and the products of huge pressures upon the
    corewards layers (heavier elements, and heavy isotopes
    that are stabilized by pressure). Conversely, if DM is
    forbidden from becoming dense enough, we have something
    similar to the hot dark matter smearing problem of
    attracting mass out of the centre of galaxy scale
    structures. However at a “Goldilocks” temperature, the
    outer layers of the structure would retain a lot of mass,
    counteracting the infall of mass to the centre of the
    overall structure, and would thereby enable the formation
    of stable stars and star clusters.

    This is why, for example, there have been some
    suggestions that perhaps there’s some annihilation of dark
    matter going on near the galactic core.

    Could the annihilation question also be put as: if a dark
    matter structure arose in a part of the universe with a
    very small amount of visible matter, would it eventually
    accrete sufficient dark mass to trigger ongoing
    annihilations in the core of what by analogy could be
    called a “dark matter star”?

    dark matter is still more likely to be found
    in wells where normal matter has condensed into compact
    objects

    Sure, but not at very high densities in wells like our
    solar system, or surely we would have noticed it affecting
    planetary orbits and the sun’s physics. There may be a
    gentle density gradient within the solar system, but there
    are tight limits on that from observations of
    trans-Neptunian objects, right?

    So, couldn’t one also say that hot, dense normal matter is
    more likely to be found in wells where dark matter has
    condensed into relatively dense structures?

  • Jason Dick

    Must CDM be self-collisionless or could it simply be
    reluctant to form small scale dark matter structures?

    I’m pretty sure those are one and the same question. In order to collapse, it must emit energy in some way. And in order to do that, it has to collide with things. Obviously there are some limits as to just how collisionless it must be based upon our current observations of dark matter both in the early universe and today. For the possibility of a dark charge, you may want to check out Sean’s relatively recent blog post on dark photons.

    A large pool of dark matter would not only slowly accrete
    more dark matter, it would also sweep up normal matter,
    which has probably been denser at small scales than DM
    since BBNS. The denser material would sink into the pool.
    If DM is allowed to become too dense, the normal matter
    simply would sink into the centre of mass forming layers
    of increasingly heavy atoms and nuclei. In the early days
    of initial structure formation, that would mean isotopes
    of H to Be and the products of huge pressures upon the
    corewards layers (heavier elements, and heavy isotopes
    that are stabilized by pressure).

    Well, not quite. Big bang nucleosynthesis (BBNS) is pretty insensitive to the nature of dark matter as the universe just had hardly clumped at all back then. BBNS was driven almost entirely by the overall temperature and density, not the minuscule fluctuations thereof. What places the most stringent limits upon dark matter’s properties in the early universe is the CMB itself, which basically places an upper limit upon the temperature of dark matter (too warm, and you don’t get any structure, but it can be as cold as you like).

    Could the annihilation question also be put as: if a dark
    matter structure arose in a part of the universe with a
    very small amount of visible matter, would it eventually
    accrete sufficient dark mass to trigger ongoing
    annihilations in the core of what by analogy could be
    called a “dark matter star”?

    Not sure. Depends upon the details of how quickly it loses energy to its environment. For example, if annihilations are sufficiently common that more energy is lost through annihilations than collisions, I suspect that the dark matter halo may never gain in density significantly except by the addition of new matter, such that it won’t ever achieve the densities required. If the collisions are common enough that they dump more energy into their environment than annihilations, then sure, I might imagine something like this happening, but it’d take a very long time.

    Sure, but not at very high densities in wells like our
    solar system, or surely we would have noticed it affecting
    planetary orbits and the sun’s physics.

    Right, the effect is pretty small, and you need very large amounts of normal matter for the overdensity of normal matter to count significantly. And yes, the converse is also true, that normal matter is more likely to be found in the more dense regions of dark matter. But the point is that the normal matter interacts much more strongly and tends to fall inward into the potential well, whereas the dark matter stays relatively stable.

  • Anon

    “Their collisionless property is only an approximation. It is, however, exactly because this is a very good approximation that we expect there to still be lots of anti-particles remaining for dark matter So, they don’t interact much, but every once in a while you still get an annihilation when a dark matter particle manages to smack into its anti-particle. It’s not going to happen often, naturally, as they don’t experience much of any attraction. But there’s a lot of dark matter out there, so we expect the signal should be measurable”

    Ok, that’s exactly the point, what about the bounds about collisionless DM versus the pamela results that require large cross sections (Somerfeld enhanced?). We are not in a particularly hot spot in the galaxy, so this is happening everywhere, how can it remain ‘colisionless’? can you throw out some numbers?

  • ivy privy
  • Jason Dick

    Anon,

    Well, I think the only way to explain an anomalously large annihilation signal from dark matter is to assume that we actually just happen to be in an unusual region of the galaxy with a good amount of extra dark matter nearby. So I think the more likely explanation is that any such signals are just due to some poorly-understood astrophysical process.

  • Anon

    Jason, they argue that’s a possibility, but they also concentrate on enhanced cross sections (boost factors) from new forces with an average DM density of ~ 0.35 GeV/cm^3 I think, which is what comes out from relic abundance. So that’s my puzzle, how come we get DM annhiliation if we are no living in any special place!

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