Dark matter might be the driving force behind black hole mergers

Researchers have discovered a significant connection between supermassive black holes (SMBHs) and dark matter particles, potentially resolving the longstanding “final parsec problem” in astronomy. 

This discovery, detailed in an article published in the journal Physical Review Letters, suggests that previously overlooked behaviors of dark matter particles facilitate the merging of SMBH pairs into a single larger black hole.

Final parsec problem 

In 2023, astrophysicists detected a pervasive “hum” of gravitational waves throughout the universe. They hypothesized that this background signal emanated from millions of merging SMBH pairs, each billions of times more massive than our Sun. 

However, theoretical simulations indicated a stall in the approach of these colossal objects when they were approximately a parsec apart (about three light years), preventing a merger. 

This “final parsec problem” conflicted with the theory that merging SMBHs were the source of the gravitational wave background and that SMBHs grow from the merger of less massive black holes.

Interaction of dark matter particles 

Study co-author Gonzalo Alonso-Álvarez is a postdoctoral fellow in the Department of Physics at the University of Toronto and the Department of Physics and Trottier Space Institute at McGill University. 

“We show that including the previously overlooked effect of dark matter can help supermassive black holes overcome this final parsec of separation and coalesce,” said Alonso-Álvarez. “Our calculations explain how that can occur, in contrast to what was previously thought.”

The research was led by Professor James Cline from McGill University and the European Organization for Nuclear Research (CERN) Theoretical Physics Department in Switzerland and Caitlyn Dewar, a master of science student in physics at McGill.  

The team developed a new model showing that dark matter particles interact with each other in such a way that they are not dispersed.

This interaction maintains the density of the dark matter halo, allowing the SMBHs to continue spiraling inward and eventually merge.

Merger of supermassive black holes 

Supermassive black holes are thought to reside at the centers of most galaxies. When two galaxies collide, the SMBHs fall into orbit around each other.

As they revolve, the gravitational pull of nearby stars tugs at them, slowing them down and causing them to spiral inward. 

Previous models showed that when the supermassive black holes approached within a parsec, they began interacting with the dark matter cloud or halo, dispersing the dark matter particles and halting the energy draw from the pair, thereby stalling their mutual orbits.

Contrary to these models, the new findings reveal that dark matter particles’ interactions prevent their dispersion, maintaining the halo’s density and enabling the SMBHs’ orbits to degrade further. 

Modeling the behavior of dark matter particles 

“The possibility that dark matter particles interact with each other is an assumption that we made, an extra ingredient that not all dark matter models contain,” noted Alonso-Álvarez. “Our argument is that only models with that ingredient can solve the final parsec problem.”

The background hum detected by the Pulsar Timing Array, which measures minute variations in signals from pulsars, provides indirect evidence supporting this new model. 

“A prediction of our proposal is that the spectrum of gravitational waves observed by pulsar timing arrays should be softened at low frequencies,” explained Cline.

He noted that the current data already hint at this behavior, and new data may be able to confirm it in the next few years.

Mysterious nature of dark matter 

The research not only offers insights into SMBH mergers and the gravitational wave background but also sheds light on dark matter’s nature.

“Our work is a new way to help us understand the particle nature of dark matter,” said Alonso-Álvarez. 

The experts found that the evolution of black hole orbits is highly sensitive to dark matter’s microphysics, suggesting that observations of supermassive black hole mergers can enhance our understanding of dark matter particles.

For instance, the interactions modeled between dark matter particles also explain the shapes of galactic dark matter halos.

Distribution of dark matter on galactic scales 

According to Alonso-Álvarez, the team 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.

The discovery is particularly exciting as it bridges processes occurring on vastly different physical scales, providing a new perspective on the interplay between dark matter and cosmic structures.

This breakthrough offers a profound understanding of the microphysics of dark matter and its role in cosmic phenomena. 

“We found that the evolution of black hole orbits is very sensitive to the microphysics of dark matter, and that means we can use observations of supermassive black hole mergers to better understand these particles,” said Alonso-Álvarez.

This research provides a significant step forward in resolving the complexities of dark matter interactions and their impact on the cosmos.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–