A gravity wave passing by Earth may have left a clue to solving a major mystery: the nature of the unseen “dark matter” that makes up most of the mass of the universe.
Primordial black holes have in the past been suggested as a possible solution to the dark matter mystery.
As scientists worldwide celebrated that confirmation of Albert Einstein’s prediction that such waves exist, a team from Johns Hopkins University was diving into calculations based on data from LIGO, the Laser Interferometer Gravitational-Wave Observatory.
Their results, published recently in Physical Review Letters, suggest that it’s conceivable that dark matter–the nature of which has long been a mystery–might consist of black holes of a special type.
“We consider the possibility that the black hole binary detected by LIGO may be a signature of dark matter,” write the scientists in their summary, referring to the black hole pair as a “binary.” What follows are five pages of annotated mathematical equations showing how the researchers took the mass of the two objects LIGO detected as a point of departure.
Their bottom line: these objects could be part of the mysterious substance known to make up about 85 percent of the mass of the universe.
The astrophysicists are cautious, however.
“We are not proposing this is the dark matter,” says one of the authors, Marc Kamionkowski, a professor of physics and astronomy. “We’re not going to bet the house. It’s a plausibility argument.”
Primordial black holes
A matter of scientific speculation since the 1930s, dark matter has recently been studied with greater precision; more evidence has emerged since the 1970s, though always indirectly. While dark matter itself cannot yet be seen, its gravitational effects can be.
For example, the influence of dark matter is believed to explain inconsistencies in the rotation of nearby visible matter in galaxies.
The Johns Hopkins team, led by postdoctoral fellow Simeon Bird, was struck by the mass of the black holes detected by LIGO, an observatory that consists of two expansive L-shaped detection systems anchored to the ground. One is in Louisiana and the other in Washington State.
Black hole masses are measured in terms of multiples of our sun. The colliding objects that generated the gravity wave detected by LIGO–a joint project of the California Institute of Technology and the Massachusetts Institute of Technology–were 36 and 29 solar masses. Those are too large to fit predictions of the size of most stellar black holes, the ultra-dense structures that form when stars collapse. But they are also too small to fit predictions for the size of supermassive black holes at the center of galaxies.
The two LIGO-detected objects do, however, fit within the expected range of mass of a postulated third type called “primordial” black holes.
Primordial black holes are believed to have formed not from dying stars but from the collapse of large expanses of gas during the birth of the universe. While their existence has not been established with certainty, primordial black holes have in the past been suggested as a possible solution to the dark matter mystery.
Birth of the universe
Because there’s so little evidence of them, though, the “dark matter is primordial black holes” hypothesis has not gained a large following among scientists.
LIGO’s findings, however, raise the prospect anew, especially as the objects detected in that experiment conform to the mass predicted for dark matter.
Scientists in the past have suggested that conditions at the birth of the universe would have produced lots of primordial black holes distributed roughly evenly in the universe, clustering in halos around galaxies. All this would make them good candidates to be dark matter.
“If you have a lot of 30-mass events, that begs an explanation.”
The team calculated how often these primordial black holes would form binary pairs and, eventually, collide. Taking into account the size and elongated shape believed to characterize primordial black hole binary orbits, the team came up with a collision rate that conforms to the LIGO findings.
More observations from LIGO and other evidence would be needed to support the hypothesis, including further detections like the one announced in February. That could suggest greater abundance of objects of that signature mass.
“If you have a lot of 30-mass events, that begs an explanation,” says coauthor Ely D. Kovetz, a postdoctoral fellow in physics and astronomy. “That the discovery of gravitational waves could be connected to dark matter is creating lots of excitement among astrophysicists,” he adds.
“It’s got a lot of potential,” Kamionkowski says.
Source: Johns Hopkins University