Nearly a mile under the Black Hills of South Dakota sits a canister of the atomic element xenon, chilled cold enough to turn it to liquid. The canister is the Large Underground Xenon, or LUX, detector — the most sensitive dark matter detector in the world. But the results of a new analysis by the LUX Collaboration has left scientists perplexed about a substance that has guided the formation of the stars and galaxies since the cosmos began: dark matter.
Since the 1930s, scientists have known that there was something unexplained about the heavens. Swiss astronomer Fritz Zwicky studied the Coma Cluster, a group of about a thousand galaxies, held together by their mutual gravitational interactions. There was only one problem: The galaxies were moving so fast that gravity shouldn’t have been able to hold them together. The cluster should have been ripped apart. In the 1970s, astronomers Vera Rubin and her collaborator Kenneth Ford studied the rotation rates of individual galaxies and came to the same conclusion. There appeared to be no way the observed matter contained in galaxies would generate enough gravity to keep the stars locked in their stately orbits.
These observations, combined with many other independent lines of evidence, led scientists to consider several possible explanations. These explanations included the possibility that Newton’s familiar laws of motion might be wrong, or that our understanding of gravity needed to be modified. Both these proposals, though, have been largely ruled out.
Another idea was that there was somehow invisible matter that was generating more gravity. Initial ideas centered on the possibility of black holes, brown dwarf stars or rogue planets roaming the cosmos, but those explanations have also been dismissed. Using a ruthless process of elimination worthy of Sherlock Holmes, astronomers have come to believe the explanation for all of the gravitational anomalies is that there must be some sort of new and undiscovered type of matter in the universe, which Zwicky in 1933 named “dunkle materie,” or dark matter.
For decades, scientists have tried to work out the properties of dark matter and, while we don’t know everything, we know a lot. From astronomical observations, we know there is five times more dark matter in the universe than all the “billions and billions” of stars and galaxies mentioned in Carl Sagan’s oft-quoted phrase. We also know that dark matter cannot have electrical charge, otherwise it would interact with light and we would have seen it. In fact, by a process of elimination, we know that dark matter is not any known form of matter. It is something new. Of this, scientists are sure.
However, scientists are less sure about the details.
For decades now, the most popular theoretical idea was that dark matter was a WIMP, short for weakly interacting massive particle. A WIMP would have a mass in the range of 10 to perhaps 100 times heavier than the familiar proton. It was a particle like a heavy neutron (but definitely not a neutron), massive, electrically neutral, and stable on time scales long compared to the lifetime of the universe.
The WIMP was popular for two main reasons.
First, when cosmologists modeled the Big Bang and included WIMPs in the calculation, the WIMPs actively participated in the earliest phases of the birth of the universe but, as the universe expanded and cooled, the space between them grew large enough that they stopped interacting with one another. When scientists calculated how much mass should be tied up in the relic WIMPs, they found it was five times as much mass as ordinary matter, exactly the amount of dark matter seen by astronomers.
The second reason for the popularity of the WIMP idea is that it explained a mystery in particle physics. The recently discovered Higgs boson has a mass of about 130 times that of the proton. Theoretical considerations predicted a much larger mass, but if a WIMP exists, it is easy to reconcile the prediction and measurement. These two reasons account for the popularity of the WIMP idea and are called “the WIMP miracle.”
The LUX measurement is simply the most recent and most powerful of a long line of searches for dark matter. They found no evidence for the existence of dark matter and were able to rule out a significant range of possible WIMP properties and masses.
Now this doesn’t mean the WIMP idea is dead or that dark matter has been disproven. There remain WIMP masses that haven’t been ruled out, and there exist other possible dark matter candidates, including objects called sterile neutrinos, which are possible cousins of the well-known neutrinos generated in nuclear reactors and in the sun. Another recurring proposed dark matter particle is the axion, suggested in the 1970s to explain mysteries in the asymmetry of subatomic processes. (Although neither sterile neutrinos, nor axions, have been observed).
Nobody knows what the final answer will be. That’s why we do research. But there is no question that there is a mystery in the cosmos. Galaxies don’t act as we expect. The LUX measurement is a powerful new bit of information for astronomers to consider and has added to the general confusion, forcing scientists to take another look at ideas other than WIMPs.
All this reminds me of the old Buffalo Springfield song: “There’s something happening here. What it is ain’t exactly clear …”
