Astrophysicists have created a new model for the formation of supermassive black holes, which arise from smaller black holes that are able to merge due to the internal interactions of dark matter.

A new study has offered a solution to the long-standing mystery of how the heaviest objects in the universe—supermassive black holes—formed. In 2023, astrophysicists discovered the background noise that fills space—a hum of nanohertz gravitational waves coming from every direction—and it was hypothesized that this signal came from millions of pairs of merging black holes, each billions of times more massive than our Sun.

But theoretical models have shown that as these giant space objects spiral closer together, their motion stops at about a parsec apart. That is, a distance of just over three light-years prevents unification. This phenomenon has been called the “last parsec problem.”

The problem is not only a contradiction with the theory that merging black holes are the source of the gravitational background, but also with the theory that supermassive black holes grow by merging with less massive black holes.

In current representations, the gravity of such massive objects disperses the dark matter particles, reducing their density. Therefore, the friction between the black holes and the dark matter prevents them from getting closer.

They simply describe a dance in spiral orbits around each other at the centers of their galaxies, and freeze at the “last step,” “bound” to dark matter before merging.

Dark matter is the invisible matter that makes up about 95 percent of the matter in the universe. We can’t see it directly, but its gravitational influence on visible matter is undeniable.

New research published in the journal Physical Review Lettersproposed a solution to this mystery. The authors showed that the previously overlooked effect of dark matter interacting with itself helps black holes overcome this final parsec of separation and merging.

According to one such model, the internal interactions of dark matter maintain its density around a pair of merging black holes. This causes the attraction between the black holes and dark matter to continue acting, shrinking their orbits and causing them to merge. The discovery also explains the origin of the nanohertz gravitational wave background.

Astrophysicists also predicted that the spectrum of gravitational waves observed by pulsar transients (a method for detecting gravitational waves from pulsars) should be weakened at low frequencies. This is consistent with existing data, and new observations in the coming years may confirm it.

In addition to solving the last parsec problem, this approach is also becoming a tool for studying the properties of the darkest matter. Since the motion of supermassive black holes is sensitive to the properties of dark matter, observing their mergers will help us better understand the nature of these mysterious particles.

So the researchers have already discovered that the interactions they modeled between dark matter particles also explain the shape of the galactic halo – a spherical formation made up mainly of dark matter into which each galaxy is submerged.

Thus, the new model not only solves the last parsec problem of explaining the nature of the emergence of supermassive black holes, but also sheds light on the most mysterious phenomenon in the modern universe: dark matter.

There is another perspective on nanohertz gravitational waves. In 2021, physicist Mykola Gorkavy predicted that the universe should be full of relic nanohertz gravitational waves. But they were created not by the merger of ordinary black holes, but by the merger of the mass of particularly massive black holes during the collapse of the past universe.