Giant black holes and dark matter may share a hot origin story
At the heart of the mystery is why some black holes grew so large so quickly.
Space is home to mysterious giants—black holes that have grown larger and faster than scientists expect. The formation of these behemoths may share a common root with another cosmic mystery, dark matter, according to a recent study.
Astrophysicists know that violent deaths of stars can form black holes typically up to tens of times the mass of the sun. Yet astronomers have seen black holes that are billions of solar masses, says Hooman Davoudiasl, a theoretical particle physicist at Brookhaven National Laboratory in New York and author of the study published last month in Physical Review Letters. What’s more, there’s evidence that these “giants” have existed since the first billion years of the universe’s history.
Although black holes grow over time by absorbing stars or combining with one another, scientists aren’t sure how some of the biggest black holes could have eaten enough stars to reach their size so swiftly, roughly within the first 10 percent of the universe’s history.
One possible explanation is that these black holes are primordial, meaning they were formed in the early universe and predate the first stars, Davoudiasl says. Back then, the universe was much hotter and denser–about the temperature of the sun’s core.
That the early universe underwent a rapid change is key to the group’s theory, says Julia Gehrlein, a physicist at Brookhaven National Laboratory and another author of the study.
This change, which they call a phase transition, made it more likely for the densest regions in the early universe to collapse into black holes. It’s a little like the phase transitions we see in states of matter–for example, liquid water becoming a gas, says Gehrlein. “Water behaves differently before and after the phase transition,” she says, and the same goes for particles in the early universe.
That transition would have been a “turbulent, violent event” that could create gravitational waves, Davoudiasl says.
It would have happened in what scientists call the “dark sector,” that is, among dark matter and other particles that we can’t observe directly. Dark sector means anything outside of “the standard model particles we know,” which make up atoms, Gehrlein says.
[Related: We’re still in the dark about a key black hole paradox]
If and how primordial black hole formation occurred is somewhat contentious among scientists. A phase transition would have allowed some dense pockets of the universe to collapse into primordial supermassive black holes, Gehrlein says. Other groups have studied the implications of this kind of transition, but this study is the first to link supermassive black holes and dark matter together in this way, she says.
“We could connect the mass of our dark matter candidate and the mass of these black holes to each other because both of them depend on the temperature at which this transition happens,” Gehrlein says. A temperature on par with our sun’s core happens to work well with a theory of ultra-light dark matter’s formation and that of primordial black holes.
Meanwhile, the phase transition could also have led to the existence of ultra-light dark matter particles–particles that would be only a tiny fraction of the mass of a neutrino, which is the lightest known particle.
It’s possible that the mystery surrounding black hole growth and why we have dark matter are “two sides of the same coin,” Davoudiasl says. If “this event, this phase transition that happened at that temperature, gave rise to these supermassive black holes,” he says, it would have “provided just the right ingredients” to form ultra-light dark matter particles, too.
Both the formation of supermassive black holes and the nature of dark matter “belong to the most pressing open questions in contemporary astronomy and physics,” says Tanja Rindler-Daller, an astronomer at the University of Vienna who was not involved in the study.
“Any model that can convincingly explain both of them at a stroke would certainly be revolutionary,” given it pans out in reality, Rindler-Daller says.
Researchers can look for signs to prove or disprove the theory in two main ways, Gehrlein says. They can try to figure out whether ultra-light dark matter exists, by studying the particles’ effect on the astrophysics of galaxies. It’s also possible researchers could measure gravitational waves produced during the early universe. Using future detectors–at different frequencies than those detected by the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO–researchers might glimpse ripples from these turbulent times of the early universe.