So What's Going on With That 'Hurricane of Dark Matter?'

By Ryan F. Mandelbaum on at

It’s the perfect science-fiction device: a hurricane of dark matter. Recent, real-life research has demonstrated that our Sun is currently engulfed in a so-called a stellar stream. Some publications have seized on this ominous-sounding idea, reporting that Earth is about to be walloped by a dark matter storm—but in fact, if it exists, we’re already inside the storm. The reality of the situation isn’t quite so dire, but it’s interesting nonetheless.

Stellar streams are populations of related stars that could have once been pieces of a dwarf galaxy or globular cluster, but have now been shredded apart by gravitational forces and pass through parts of our galaxy. Since astronomers are pretty sure that stuff called dark matter serves as the gravitational scaffolding for dwarf galaxies, then presumably a stellar stream should also contain some percentage of dark matter. Maybe a dark matter experiment could detect dark matter particles from the recently discovered stream running through our cosmic neighbourhood.

“There’s supposed to be a stream that we’re inside of right now,” Ciaran O’Hare, postdoctoral researcher in the Department of Theoretical Physics at the University of Zaragoza, told Gizmodo. “It immediately made me think about the consequences for dark matter.”

The most successful models to describe the universe are based on experiments showing that that the cosmos is made up of 4 per cent regular matter, perhaps 70 per cent “dark energy” that is driving the universe apart, and the rest, around 25 per cent, “dark matter.” Scientists have reason to believe that dark matter consists of particles, the same way that everything we can see is made of particles, and that it’s the scaffolding for the universe’s large-scale structure. But this dark matter has only ever been observed based on its gravitational effects.

Scientists are in the midst of hunting for a dark matter particle. But meanwhile, surveys such as the European Space Agency’s Gaia telescope and the Sloan Digital Sky Survey have created enormous maps of the sky, complete with positions, velocity, and other data points for our region of the Milky Way galaxy. The data revealed “substantial structures,” such as this stream and accompanying dark matter hurricane, called S1, hitting the Solar System head-on.

Perhaps this dark matter hurricane could be detectable by today’s existing dark matter detection experiments, and would have a distinguishable signal from the Milky Way’s background dark matter. It would move much faster—a “hurricane,” compared to the background dark matter “wind,” according to the paper published last week in Physical Review D. The researchers then analysed whether they’d be able to spot the stream with upcoming dark matter detectors.

If dark matter comprised of a “weakly interacting massive particle” or WIMP, a particle with a mass similar to the other particles but that interacts weakly (imagine a light breeze interacting with a skyscraper), then the hurricane would only be detectable if the particles had a specific mass range, according to the paper. Experiments like the XENON NT detector in Italy wouldn’t be able to pick out the direction of a potential dark matter particle, explained Laura Baudis, physics professor at the University of Zürich who was not involved in the new paper. Doing so would take a much larger detector, given the kinds of WIMPs that have already been ruled out.

And if dark matter is instead comprised of much lighter particles called axions, the hurricane’s effects would perhaps be more noticeable, according to the paper’s calculations.

“I think it’s quite an interesting idea,” Sownak Bose, postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, told Gizmodo. “All these experiments trying to detect dark matter have a possibility to have their signals boosted by this extra flux.”

Bose pointed out that the amount of dark matter in the stream might vary based on the specific dwarf galaxy that created the stream—or if the dark matter is something more exotic than the theoretical WIMPs and axions. But he was excited about what the incredible amount of Gaia data could do for studies of these streams.

So, if there is a dark matter hurricane, we’re literally inside of it right now. But we’re safe (from the dark matter, at least). This so-called storm is most exciting for its scientific prospects. Said O’Hare: “We know so little about dark matter that any sort of better knowledge about its structure adds up to help us understand it.” [PRD]