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Dark matter is the cosmic holy grail. The elusive stuff is hypothesized to make up about 27% of the Universe (dark energy, which is something slightly different, makes up about 68%), but its existence has never been conclusively proven, largely because no one has ever actually seen the stuff. Scientists were hoping to change that with the Large Underground Xenon, LUX, which is the most sensitive, high-tech dark matter detector in existence. LUX completed its first run of experiments on Wednesday, but turned up nothing.
Before we get further into the LUX’s missoin, I want to address something I keep reading—that dark matter comprises the majority of matter in the Universe. This is where the confusion between dark energy and dark matter comes into play, and while they’re closely related, it’s not technically correct to conflate the two terms. Or at least, that’s what astrophysicists currently believe. Dark energy is a force that accounts, at least in part, for the accelerating expansion of the universe. Dark matter, on the other hand, isn’t a force—it’s, well, matter. We think it’s there and we think we’ve seen it, such as in the photo below of the Pandora’s Cluster. The areas noted by the blue and red circles indicate a separation between hot gas and dark matter. While colliding particles in gas clouds generate electrical energy that causes friction and slows them down, the dark matter remains unaffected by friction or other effects of the hot gas, leading to the separation between the two.
LUX includes a 6-foot titanium tank full of xenon cooled to a frosty temperature of approximately -150 degrees Fahrenheit. The device lives about 1 mile under the Black Hills of South Dakota, which shields it from everything other than the weakly interacting massive particles (WIMPs) thought to comprise dark matter. Theoretical physicists have postulated the existence of WIMPs, but they’re incredibly elusive because only gravity causes them to interact with ordinary matter. LUX attempts to detect these particles as they pass through Earth and bump into matter, which is pretty different than employing spectrometers in space to search for dark matter collision debris or using the Large Hadron Collider to smash particles together.
WIMPs can be high or low mass, and LUX’s search has focused primarily on the latter, which are even harder to detect. If one of these particles were to interact with a xenon atom it would produce light and electrons, which would then release photons. LUX would identify the photons and their signals, as well as measure their brightness, which would confirm or deny the presence of dark matter.
After three months of searching, LUX has come up empty, despite predictions that it’s sensitivity would yield positive results. Part of the reason scientists believed the new device would succeed is that other detectors—ones that are presumably less sensitive—have picked up some tentative dark matter indications. Scientists hoped that they could confirm these events, but that didn’t happen. In fact, LUX’s failure to find dark matter suggests that the other detectors didn’t actually find it either.
Still, scientists maintain that LUX is operating splendidly and they’re cautiously optimistic about the results of a 300-day run that begins in early 2014. It seems, for now, an issue of mind over matter.