How dancing black holes get close enough to merge

The numerous black hole mergers discovered by gravitational wave observatories show that black holes collide far more often than previously believed. Black holes can now merge swiftly, according to new study, if they are trapped in the accretion disk of a supermassive partner.

Astrophysically speaking, it is not difficult to bring two black holes together. They either randomly come across one another in the depths of interstellar space, or they are born that way as the children of a binary star system. They can continue to circle each other indefinitely once in orbit.

Astronomers were aware that binary orbiting black holes inevitably met their demise even before the gravitational waves from merging black holes were discovered. We are aware that practically every galaxy in the cosmos has a massive black hole at its heart and that enormous galaxies form as a result of several smaller galaxies joining together. However, most galaxies only have one massive black hole, which suggests that if galaxies merge, then their black holes must as well.

However, getting black holes to collide is rather difficult. You must remove energy and angular momentum from the system in order to bring two circling objects closer together. This is a difficult task since space is relatively frictionless. Planetary systems frequently engage in this activity, although it typically entails the presence of gas or the emission of radiation. Black holes don't have this choice.                                                                                                              

Because gravitational waves are so weak, they can only effectively remove energy and momentum from binary systems when the two black holes are already quite near to one another. The "final parsec challenge" refers to the difficulty of bringing two black holes near enough together for gravitational waves to complete the process of driving their merger.

Astronomers have devised a number of solutions to the last parsec puzzle throughout the years. These methods often include the existence of a third object that is located far outside the binary pair's common orbit. The third companion may be able to gently pull on the binary, lengthening their circle, if the circumstances are ideal, with precisely the proper alignments and speeds. The double black hole orbit becomes more elliptical as a result of this stretching. The black holes begin to spend more and more time together as the ellipticity increases. The space between the black holes is eventually closed by gravitational waves once they have traveled a certain distance apart.

But in that case, the third companion must be configured precisely, and that configuration might not be sufficient. Astronomers calculate that there are between 15 and 38 black hole mergers annually within each cubic gigaparsec of the universe's volume (roughly 1/12000th of the total volume of the observable universe) based on all gravitational wave observations, and mechanisms that depend on a third companion produce less than half that number.

Researchers have proposed a novel method for black holes to merge in a recent work that was published in the preprint database arXiv(opens in new tab). Although it makes use of a similar arrangement, it is considerably more general than the employment of a distant third companion.

It all begins close to a gigantic black hole. Since supermassive black holes are found at the cores of galaxies, which are packed with stars and therefore numerous smaller black holes, this isn't a crazy theory. We can probably detect several binary black holes lazily circling the primary supermassive one, as we can with most objects around the galactic center.

The study's authors found that there is a probability for a merger to occur if the pair is inclined relative to its orbit around the supermassive black hole. The orbital rotation axis of the binary pair must first precess, or wobble, over time. The core supermassive black hole's powerful gravity will routinely pull on the binary, increasing its eccentricity and raising the possibility of a final merger if the pace of that precession matches the orbital period around it.

—A unbalanced merger may have created a black hole that has gone wild.

—Smaller black holes clash in unusual ways around a monstrous black hole.

It must be a really fortunate coincidence for the binary's orbit around the center supermassive black hole to be just correct for that type of matching to occur. Fortunately, the authors discovered that nature can handle that. Regardless matter where the binary black holes are created, if they are in the supermassive black hole's disk, they will gradually move inward.

They will ultimately locate a distance where their orbital period coincides with their precession period, regardless of their initial orbit. The center supermassive black hole's gravity will have enough time to raise the eccentricity if they remain in that orbit for a substantial amount of time.

The authors concluded that this situation is very typical. They found that binary black holes frequently merge by performing several simulations of black hole characteristics and initial circumstances.

It's not yet apparent if this is the main mechanism for mergers or if there are other methods as well. In any case, the art demonstrates how intricate black holes' lives may be and the various ways they might dance in the dark.                      

 at @Spacedotcom.