Researchers have discovered a fascinating connection between supermassive black holes and dark matter particles that might solve a longstanding puzzle in astronomy.
This new finding could explain how supermassive black holes (SMBHs) merge, helping us understand the mysterious “final parsec problem.”
The study, titled “Self-interacting dark matter solves the final parsec problem of supermassive black hole mergers,” was published in the journal Physical Review Letters.
It reveals that the behavior of dark matter particles, previously overlooked, plays a crucial role in enabling SMBHs to merge.
In 2023, astrophysicists detected a “hum” of gravitational waves throughout the universe, believed to come from millions of merging SMBHs, each billions of times heavier than our sun.
However, simulations showed that as these giant black holes spiral closer together, their approach stalls at about a parsec (roughly three light years) apart, preventing a merger.
This “final parsec problem” challenged the idea that merging SMBHs are the source of these gravitational waves and contradicted the theory that SMBHs grow by merging smaller black holes.
The new research offers a solution. “We show that including the previously overlooked effect of dark matter can help supermassive black holes overcome this final parsec of separation and coalesce,” says Gonzalo Alonso-Álvarez, a postdoctoral fellow at the University of Toronto and McGill University.
The study’s co-authors also include Professor James Cline from McGill University and CERN, and Caitlyn Dewar, a physics master’s student at McGill.
SMBHs are believed to reside at the centers of most galaxies. When two galaxies collide, their SMBHs fall into orbit around each other, gradually spiraling inward due to the gravitational pull of nearby stars. Previous models showed that as the SMBHs get closer, their interaction with the dark matter halo surrounding them becomes significant.
The gravity of the SMBHs was thought to expel dark matter particles, reducing their density and halting the black holes’ inward spiral.
However, Alonso-Álvarez and his team found that if dark matter particles interact with each other, they are not dispersed by the black holes. Instead, the dark matter density remains high enough to continue influencing the SMBHs, allowing their orbits to shrink and eventually merge.
“The possibility that dark matter particles interact with each other is an assumption that we made,” explains Alonso-Álvarez. “Our argument is that only models with that ingredient can solve the final parsec problem.”
The researchers’ model suggests that this dark matter interaction could explain the shapes of galactic dark matter halos and the gravitational waves detected by the Pulsar Timing Array. This array measures tiny variations in signals from pulsars—rapidly rotating neutron stars emitting strong radio pulses—to reveal gravitational waves.
“A prediction of our proposal is that the spectrum of gravitational waves observed by pulsar timing arrays should be softened at low frequencies,” says Cline. “The current data already hint at this behavior, and new data may be able to confirm it in the next few years.”
This study not only provides insights into SMBH mergers and gravitational waves but also offers a window into the nature of dark matter. “We found that the evolution of black hole orbits is very sensitive to the microphysics of dark matter,” says Alonso-Álvarez. “This means we can use observations of supermassive black hole mergers to better understand these particles.”
The researchers discovered that the interactions between dark matter particles in their model also explain the shapes of galactic dark matter halos. “We found that the final parsec problem can only be solved if dark matter particles interact at a rate that can alter the distribution of dark matter on galactic scales,” Alonso-Álvarez explains. “This was unexpected since the physical scales at which the processes occur are three or more orders of magnitude apart. That’s exciting.”
In summary, this groundbreaking research uncovers a crucial link between supermassive black holes and dark matter, offering a solution to the final parsec problem and advancing our understanding of the universe.
