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The Sciences

Something's in the air

Cosmic VarianceBy Daniel HolzNovember 25, 2008 1:21 PM

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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.

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