Astrophysicists have proposed a clever new way to shed light on the mystery of dark matter, the undiscovered stuff believed to make up most of the universe.
The irony is they want to try to pin down the nature of one unexplained phenomenon by using another, looking for dark matter with enigmatic cosmic emanations known as “fast radio bursts.”
Scientists argue that these brief but extremely bright flashes of radio-frequency radiation can help them determine if dark matter is really a particular kind of ancient black hole.
Fast radio bursts, or FRBs, provide a direct way of detecting these black holes, which have a specific mass, says Julian Muñoz, a graduate student at Johns Hopkins University and lead author of a new paper published in the journal Physical Review Letters.
Muñoz wrote the paper along with Ely D. Kovetz, a postdoctoral fellow; Marc Kamionkowski, a professor of physics and astronomy; and Liang Dai, who recently finished studying at Johns Hopkins and is now at the Institute for Advanced Study.
The paper builds on a hypothesis offered months ago that a gravity wave detected after a collision of black holes had actually unmasked dark matter, a substance not yet identified but believed to make up 85 percent of the mass of the universe.
The speculative study took as a point of departure the fact that the colliding objects detected by the CalTech/MIT LIGO experiment were roughly the predicted mass of “primordial” black holes. Unlike black holes from imploded stars, primordial black holes are believed to have formed in the collapse of huge expanses of gas during the birth of the universe.
The existence of primordial black holes has not been proved, but they have been suggested as a possible solution to the riddle of dark matter. With little evidence of them to examine, however, the hypothesis had not gained much traction.
The LIGO findings, however, raised the question again, especially as the black holes LIGO detected conform to the mass predicted for dark matter.
For the new study, scientists calculated how often primordial black holes would form binary pairs and collide. The team came up with a collision rate that fits LIGO data.
Key to the argument is that the black holes LIGO detected fall between 29 and 36 times the mass of the sun. The new paper considers the question of how to test the hypothesis that dark matter consists of black holes of roughly 30 solar masses.
That’s where the fast radio bursts come in. First observed only a few years ago, these powerful flashes last only fractions of a second. Their origins are unknown, but are believed to lie in galaxies outside the Milky Way.
If that’s true, Kamionkowski says, the radio waves would travel great distances before they’re observed on Earth. If a burst passed dark matter on the way, Einstein’s theory of general relativity says, it would be deflected. If it passed close enough, it could be split into two rays shooting off in the same direction—creating two images of one source.
The new study shows that if the dark matter is a black hole 30 times the mass of the sun, the two images will arrive a few milliseconds apart, one as an echo of the other.
“The echoing of FRBs is a very direct probe of dark matter,” Muñoz says. “While gravitational waves might ‘indicate’ that dark matter is made of black holes, there are other ways to produce very-massive black holes with regular astrophysics, so it would be hard to convince oneself that we are detecting dark matter. However, gravitational lensing of fast radio bursts has a very unique signature, with no other astrophysical phenomenon that could reproduce it.”
If primordial black holes are dark matter, “it is expected that several of the thousands of FRBs to be detected in the next few years will have such echoes,” Kamionkowski says.
So far, only about 20 fast radio bursts have been recorded since 2001. But a new Canadian telescope expected to begin operation this year seems promising for spotting radio bursts.
“Once the thing is working up to their planned specifications, they should collect enough FRBs to begin the tests we propose,” Kamionkowski says. Results could be available in three to five years. The team’s proposed methodology is published in the journal Physical Review Letters.