The Sciences


Cosmic VarianceBy Sean CarrollApr 20, 2012 1:30 PM


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Science keeps advancing, in fits and starts. It was a good week for intriguing results from experiments. The first bit of news, which has been the subject of the most internet buzz, is a new paper by Chilean astronomers C. Moni Bidin, G. Carraro, R. A. Mendez, and R. Smith, which claims that there's no evidence for dark matter in the dynamics of stars near the Sun. If this were true, it would imply something funny going on with the distribution of nearby dark matter, which could have significant implications for direct searches here on Earth (see below). It wouldn't really be much of a threat to the idea of dark matter itself, since there's plenty of evidence for dark matter elsewhere. But it might mean that the distribution in the Milky Way was very different from the kinds of models we like to use, for example by being much lumpier. We just heard a great physics colloquium here at Caltech by Katie Freese, who talked about this result very briefly. Her opinion matched those of the skeptics in Ron Cowen's article linked above: this paper makes a lot of assumptions, some of the a bit dubious, and we would need to see something much more solid before we become convinced. The biggest issue is that they don't actually measure the DM distribution near the Sun; they try to measure it in a region between 1500 and 4000 parsecs below the galactic plane (which is actually pretty far away), and then fit to a model and extrapolate to what we should have nearby. This kind of procedure relies on our understanding of the vertical structure of the galactic disk, which isn't all that great. So it's definitely an intriguing result, one that should be taken seriously and followed up by other surveys, but nothing to lose sleep over just yet. The second bit of news is another puzzling absence: a lack of neutrinos that were predicted to be produced by gamma-ray bursts. These bursts are among the most energetic events in the post-Big-Bang universe, and for a long time were a major mystery to astrophysicists. More recently, a consensus had grown up that GRB's (as they are called) are associated with intense beams of particles created by newly-born supernovae. That's a model that seems to fit most of the data, anyway, and it also makes a pretty good prediction for the production of associated neutrinos. But a new paper by the marvelous IceCube experiment has thrown a spanner into the works: they should have been able to see those neutrinos, and they don't. IceCube consists of a series of thousands of detectors arrayed within a cubic kilometer of Antarctic ice, and looks for flashes of light associated with high-energy particles passing through. Recently they have been keeping an eye out for signs of neutrinos that should be associated with GRB's that have been detected by the Swift and Fermi satellites -- but no luck. It's a puzzle that will send GRB theorists back to the drawing board. One of the funny aspects of the story is that particles from GRB's are a leading candidate to serve as the origin of high-energy cosmic rays, but that seems to be out the window now. It's still possible that the cosmic rays come from active galactic nuclei, but there's another group of theorists who have something new to chew on. The final bit of news is even dicier, and hasn't received any internet buzz at all yet -- I only heard about it through Katie Freese's talk. Maybe because there was no press release and the shocking claim is hidden within the guts of a technical paper with a boring title. The paper is by our friend Juan Collar and Nicole Fields. Recall that the DAMA dark matter experiment looks for an annual modulation due to the fact that the Earth moves through the dark-matter "wind" at different velocities during different times of the year. And they see a signal -- very strongly -- but many people have questioned whether what they are seeing is really due to dark matter. Juan has been leading another experiment, CoGeNT, which has been trying to check DAMA's results -- and has found a very tentative signal that seems to agree with them (which wasn't what most people were expecting). One of the reasons for the skepticism is that there are other experiments, which aren't tuned specifically to look for annual modulations, but nevertheless should be sensitive to dark matter at the level implied by DAMA and CoGeNT -- and they see nothing. More recently, some of these experiments have started looking for annual modulations -- and they see nothing. Here for example is a recent paper by the CDMS experiment that says exactly that. But the new paper by Collar and Fields claims that CDMS have analyzed their own data incorrectly. They argue that (1) CDMS isn't really sensitive to the kind of annual modulation purportedly seen by CoGeNT, and (2) if you look carefully there is actually a statistically significant (more than 5 sigma!) bump at low energies, consistent with the kind of low-mass dark matter particle you would need to explain the annual modulations. My impression is that the CDMS folks are unmoved by this argument. It's certainly always very hard to analyze the data from somebody else's experiment. This kind of controversy comes down to very particular aspects of data collection, analysis, and sources of systematic error. It's way over my head, so I have no professional opinion about who is right. But at the very least it's a reminder (as if we needed one) that the dark-matter-detection game is heating up, and big news might be creeping up on us. The universe loves puzzles.

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