How quickly is the dark matter close to Earth zipping around? The speed of dark matter is known to have far-reaching consequences for contemporary astrophysical research, but this basic property has indeed eluded researchers for years.
In a paper featured Jan. 22 in the journal Physical Review Letters, a global team of astrophysicists provided the first clue: it turns out that the solution to this mystery is present among a few of the oldest stars existing in the galaxy.
Essentially, these old stars act as visible speedometers for the invisible dark matter, measuring its speed distribution near Earth. You can think of the oldest stars as a luminous tracer for the dark matter. The dark matter itself we'll never see, because it's not emitting light to any observable degree - it's just invisible to us, which is why it's been so hard to say anything concrete about it.
Mariangela Lisanti, Assistant Professor of Physics
In order to decide which stars act like the undetectable and invisible dark matter particles, Lisanti and her colleagues focused on a computer simulation, Eris, which employs supercomputers for replicating the physics of the Milky Way galaxy; including dark matter.
"Our hypothesis was that there's some subset of stars that, for some reason, will match the movements of the dark matter," said Jonah Herzog-Arbeitman, an undergraduate and a co-author on the paper. His work with Lisanti and her colleagues the summer after his first year at Princeton contributed to this journal article besides turning into one of his junior papers
Herzog-Arbeitman and Lina Necib at the California Institute of Technology, another co-author on the paper, developed a number of plots from Eris data that compared different properties of dark matter to properties of varied subsets of stars.
Their huge innovation came when they compared the velocity of dark matter to that of stars with varied "metallicities," or ratios of lighter elements to heavy metals.
The curve representing dark matter exquisitely matched up with the stars that comprise of the least heavy metals: "We saw everything line up," Lisanti said.
"It was one of those great examples of a pretty reasonable idea working pretty darn well," Herzog-Arbeitman said.
For decades, astronomers have known that metallicity can function as a proxy for a star's age, since metals and various other heavy elements are developed in the mergers of neutron stars and supernovas. The small galaxies that blend with the Milky Way usually have moderately less of these heavy elements.
In retrospect, the association between the oldest stars and dark matter should not be surprising, said Necib. "The dark matter and these old stars have the same initial conditions: they started in the same place and they have the same properties ... so at the end of the day, it makes sense that they're both acted on only through gravity," she said.
Why It Matters
Since 2009, researchers have been making attempts to directly observe dark matter, by putting extremely dense material - frequently xenon - deep underground and then waiting for the dark matter that flows via the planet to interact with it.
These "direct detection" experiments were compared to a game of billiards by Lisanti: "When a dark matter particle scatters off a nucleus in an atom, the collision is similar to two billiard balls hitting each other. If the dark matter particle is much less massive than the nucleus, then the nucleus won't move much after the collision, which makes it really hard to notice that anything happened."
That is why constraining the speed of dark matter is so essential, she explained. If dark matter particles are both light and slow, they may not have sufficient kinetic energy to also shift the nuclear "billiard balls", even if they smack right into one of the balls.
"But if the dark matter comes in moving faster, it's going to have more kinetic energy. That can increase the chance that in that collision, the recoil of the nucleus is going to be greater, so you'd be able to see it," Lisanti said.
Scientists had originally expected to see enough particle interactions -- adequately moving billiard balls -- to be capable deriving the velocity and mass of the dark matter particles. However, Lisanti stated, "we haven't seen anything yet."
Thus, instead of employing the interactions for determining the speed, researchers like Lisanti and her colleagues are expecting to flip the script, and make use of the speed for explaining why the direct detection experiments have still not detected anything.
The failure -- at least so far -- of the direct detection experiments results in two questions, Lisanti said. "How am I ever going to figure out what the speeds of these things are?" and "Have we not seen anything because there's something different in the speed distribution than we expected?"
Having a totally independent method to work out the speed of dark matter could allow us to shed light on that, she said. However, it is only theoretical so far. Real-world astronomy has not caught up to the wealth of data generated by the Eris simulation, thus Lisanti and her colleagues are still not aware of how fast the oldest stars in the galaxy are moving.
Fortunately, that information is being assembled presently by the European Space Agency's Gaia telescope, which has been responsible for scanning the Milky Way since July 2014. So far, details on just a small subset of stars have been released, but the complete dataset will include a lot more data on almost a billion stars.
The wealth of data on the horizon from current and upcoming stellar surveys will provide a unique opportunity to understand this fundamental property of dark matter.
Mariangela Lisanti, Assistant Professor of Physics