Throughout cosmic history, galaxies have engaged in a continuous process of merging into larger structures. Central to these galaxies are supermassive black holes, which ultimately must merge as well, resulting in the formation of even larger black holes. However, a longstanding question has puzzled astrophysicists: how do these supermassive black holes manage to get close enough to each other to spiral in and merge? In theoretical calculations, when these black holes approach the final parsec--approximately 3.26 light-years--their progression tends to halt, leading to a situation where they appear to orbit indefinitely.

Astrophysicist Stephen Taylor from Vanderbilt University pointed out that it was previously believed that the time required for black holes to merge could be comparable to the age of the universe itself, raising concerns about the possibility of black hole mergers occurring at all. Nonetheless, recent observations have provided evidence that these mergers do take place. Data gathered from pulsar timing arrays last year revealed a consistent background of gravitational waves, which are ripples in the fabric of spacetime. These waves are likely generated by supermassive black holes that are in close orbits and nearing merger.

To address the mystery of how these black holes overcome the final parsec barrier, a new hypothesis has emerged: dark matter may play a role in reducing angular momentum between the black holes, facilitating their closer approach. Gonzalo Alonso-Álvarez, a physicist from the University of Toronto, suggests that a viscous form of dark matter might be the key to solving this issue.

Dark matter constitutes approximately 85% of the universe's mass, yet its exact nature remains elusive. While the simplest models of dark matter particles do not aid in black hole mergers, a recent study from a team of Canadian physicists proposed that a more complex variant known as self-interacting dark matter could provide a solution. Such particles could interact sufficiently with the supermassive black holes to draw them closer together. If this theory holds, it would imply that our understanding of dark matter is more intricate than previously thought.

Other hypotheses have also surfaced, including the concept of fuzzy dark matter, which could similarly assist in merging processes. Throughout this discourse, scientists have been exploring various potential explanations to understand which conditions effectively facilitate these cosmic mergers.

When galaxies merge, gravitational interactions prompt their respective supermassive black holes to gradually drift toward one another. This process, known as dynamical friction, was first articulated in 1980. However, as the black holes near the final parsec, the efficiency of dynamical friction diminishes. The black holes consume surrounding material, creating a void that reduces the density of stars and gas in their vicinity. Consequently, they find themselves in a relatively empty space, allowing them to orbit each other indefinitely.

Alonso-Álvarez emphasizes that, similar to how Earth orbits the sun without falling into it, two black holes should also maintain their orbital paths unless an external force extracts their energy. This is where self-interacting dark matter may be crucial, as it could generate friction that encourages closer proximity between the black holes, potentially leading to a merger within a time frame of around 100 million years.

Meanwhile, fuzzy dark matter, composed of ultralight particles, may also concentrate around galactic centers and produce friction with the black holes, aiding in the loss of angular momentum necessary for a merger.

While various exotic scenarios have been discussed, some scientists argue that more straightforward explanations could suffice. For instance, the influence of stars passing close to the merging black holes may help extract enough angular momentum to facilitate their merger. Additionally, each black hole could possess an accompanying disk of gas that draws in material from a broader disk, further aiding in energy loss necessary for merging.

As the scientific community continues to investigate these intriguing possibilities, researchers are now focused on determining which mechanisms are at play in the merging of supermassive black holes. Alonso-Álvarez intends to validate his self-interacting dark matter hypothesis through upcoming pulsar timing array data. In the later stages of merger, black holes primarily lose angular momentum through gravitational waves. If self-interacting dark matter influences this process, scientists should observe a reduction in energy levels at the critical distances.

The European Space Agency's Laser Interferometer Space Antenna (LISA), scheduled for launch in 2035, is anticipated to provide valuable insights by detecting the strong gravitational waves produced during the final moments of black hole mergers. Ultimately, the characteristics of these signals could shed light on the complexities of the merger process and help resolve the final parsec mystery.