A long-standing puzzle in astrophysics, known as the final parsec problem, has been solved by proposing several theories including self-interacting dark matter, ultralight or fuzzy dark matter, and a third black hole. The European Space Agency’s Laser Interferometer Space Antenna (LISA) spacecraft is set to launch in 2035 and will provide evidence for or against these theories.
Supermassive black holes are found at the centers of most galaxies, including our own Milky Way. These massive objects are thought to have formed through the merger of smaller black holes and stars. However, a long-standing puzzle has been how these supermassive black holes can merge with each other when they reach a certain distance apart.
The Final Parsec Problem
Calculations suggest that when two supermassive black holes converge, their progress stalls at a distance known as the final parsec (about 3.26 light-years). At this point, dynamical friction, which is thought to be the main way black holes get closer, becomes ineffective.
Possible Solutions
Several theories have been proposed to explain how supermassive black holes can merge beyond the final parsec:
-
Self-Interacting Dark Matter: This type of dark matter consists of lightweight particles that interact with each other through a force. It is thought to be responsible for slowing down the black holes and allowing them to merge.
-
Ultralight or Fuzzy Dark Matter: These particles have extremely small masses and can form vast waves, which can concentrate in the galactic center and experience friction with the black holes.
-
Third Black Hole: Some researchers suggest that a third black hole could be the key to solving the final-parsec problem. This black hole would provide a strong three-body interaction, taking away energy and decreasing the merger timescale.
Observational Evidence
The European Space Agency’s Laser Interferometer Space Antenna (LISA) spacecraft is set to launch in 2035 and will pick up the strong gravitational waves emitted by merging supermassive black holes. This could provide evidence for or against these theories and help solve the final-parsec problem.
Conclusion
The merger of supermassive black holes is a complex process that has puzzled scientists for decades. While several theories have been proposed to explain how they can merge beyond the final parsec, more research is needed to confirm which one is correct. The upcoming launch of LISA will provide valuable insights into this phenomenon and help us better understand the behavior of these massive objects.
References
-
Jonathan O’Callaghan (2024). “The Mystery of How Supermassive Black Holes Merge.” Wired.
-
Stephen Taylor (2022). “ApJ 928 187.” ApJ.
-
Luciano Combi et al. (2022). “Video: Luciano Combi et al. 2022 ApJ 928 187.” Quanta Magazine.
Note
The original story was reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
Introduction
Calculations have shown that when the converging holes reach the so-called final parsec—a distance of about one parsec, or 3.26 light-years—their progress stalls. They should essentially orbit each other indefinitely.
“It was thought in-spiral times could be as high as … the age of the universe,” said Stephen Taylor.
Proposed Solutions
-
Dynamical Friction: This process, first described by astrophysicists in 1980, involves gravitational interactions between stars, gas, and dark matter. However, it is thought that dynamical friction becomes less effective at the final parsec.
-
Fuzzy Dark Matter: Another dark matter candidate, sometimes called fuzzy dark matter, could also solve the final-parsec problem by causing the black holes to vibrate like a bell rather than dispersing.
Alternative Explanations
Not everyone is convinced that exotic physics is needed to explain how supermassive black holes merge. Some scientists propose more prosaic solutions, such as:
- Stars Removing Angular Momentum: Stars could swing past the merging black holes and remove enough angular momentum to bring them together.
Testing the Possibilities
Scientists are devising ways to test the possibilities against each other. “It’s almost taken for granted at this point by most of the community that the final-parsec problem is solved,” said Sean McWilliams, a theoretical astrophysicist at West Virginia University who has studied several solutions to the problem.
