Dark matter is interesting in its own right, a mysterious ‘something’ that according to WMAP data must account for 23 percent of the universe (the breakdown now thought to be 72 percent dark energy, 23 percent dark matter, 4.6 percent atoms and less than 1 percent neutrinos). From a propulsion standpoint, dark matter intrigues us because it may represent a reaction mass conceivably useful for future space flight. It’s also Nobel Prize territory for the team that identifies it, which is why so many teams are looking, with one team’s provocative results drawing criticism.
The Italian and Chinese physicists on the DAMA Project have held out since 2000 for their claim that they are detecting dark matter beneath the Gran Sasso mountain in Italy. The modulation is yearly and could represent the Earth’s motion through a dark matter stream as it orbits the Sun. The larger DAMA/LIBRA experiment now reaffirms the phenomenon, which appears as flashes in the team’s sodium iodide detector. With the rate of flashes highest in June and lowest in December, the findings are provocative.
Such flashes could signal WIMPs (weakly interacting massive particles), which pass through most matter as if it were not there. But are the flashes DAMA/LIBRA is seeing enough to claim a detection of dark matter, or do they merely open up a range of possibilities? Cosmic Variance recently let Juan Collar (University of Chicago), himself a member of a dark matter team, loose on the question. Collar believes the modulation the team is finding is unmistakable, but disputes the team’s conclusions:
…to conclude from something this mundane that the experiment “confirms evidence of Dark Matter particles in the galactic halo with high confidence level” or that there is “an evidence for the presence of dark matter particles in the galactic halo at 8.2 sigma confidence level” is simply delusional. There is evidence for a modulation in the data at 8.2 sigma, stop. Compatible with what would be expected from some dark matter particles in some galactic halo models, full stop. Anything beyond this is wanting to believe, and it smears on the rest of us in the field. Of course, of course… there is no other observed process in nature that peaks in the summer and goes through a low in winter, so this must be dark matter, right? (Occam is turning in his grave, rusty razor still in hand. He is thinking a remake of that opening scene in “Un chien andalou”, with help from this little lady. I am channeling him loud and clear).
Collar is obviously a lively guy with eclectic tastes. I had to look up Un chien andalou, which turns out to be a surrealist film by Luis Buñuel and Salvador Dalí, one whose Wikipedia summation leaves me inclined to avoid it. He may be right about Occam, though. What about the possibility of experimental error in the DAMA/LIBRA experiments? Collar thinks we can’t rule it out, and is critical of the team’s attempts thus far to do so.
But this is not a slash and burn job on a particular experiment. Collar calls the work of DAMA/LIBRA ‘phenomenal’ on many fronts, and finds much to admire in it. But his appraisal is leavened with deep skepticism, and it’s one I wanted to call to your attention because of Collar’s high visibility in this area (he’s quoted in this New York Times article on the difficulty of dark matter detection) and because his lengthy post is a great example of what weblogs can do to spread good science from the source.
Centauri Dreams takes no position at all on DAMA/LIBRA other than to report the ongoing controversy, which all dark matter watchers will follow with interest. As I say, whoever does confirm a dark matter detection is easily in Nobel range. On that score, be aware as well of the Large Underground Xenon detector (LUX), which will look for evidence of dark matter in an abandoned South Dakota goldmine. Here we’re talking about 600 pounds of liquid xenon suspended in a huge water tank, some 4800 feet underground in the Homestake mine near Lead, South Dakota. The signature of dark matter would again be the flashes given off by WIMPs as they hit xenon atoms. Expect this experiment to start coming together later in the year as the mine is prepared to receive its equipment.