A problem called the final parsec problem details the theoretical difficulty of bringing supermassive black holes close enough for gravitational waves to rob them of momentum and cause them to eventually merge.
A screenshot from a simulation shows the merger of two supermassive black holes. Credit: NASA Goddard Space Flight Center/Scott Noble; simulation data, d'Ascoli et al. 2018
In the October 2023 issue, “The Great Buzz,” it is stated that astronomers were not sure that supermassive black holes in binary systems could get close to each other. I assume that objects with such massive gravity would attract each other over time and eventually merge. What am I missing?
Bill Ziegler
West Chicago, Illinois
All massive galaxies are thought to host supermassive black holes with millions or billions of times the mass of the Sun. When galaxies merge (and we know they do, and often), it seems like a foregone conclusion that their supermassive black holes (SMBHs) should merge, too. After all, we’ve seen smaller stellar-mass black holes merge. But the physics involved in how SMBHs eventually approach each other before merging gets a little tricky. This puzzle is often called the final parsec problem, and it has plagued astrophysicists since the 1980s.
Let’s start with two merging galaxies. Each has a SMBH. As the galaxies become entangled, they eventually form a single galaxy made up of the combined material (including stars, gas, and black holes) from the two progenitors. As things settle down, the two SMBHs, once in the middle of their respective galaxies, also begin to work their way toward the center of the final galaxy. They do this through a process called dynamical friction, also called gravitational drag. As black holes encounter nearby stars and gas, sometimes instead of falling into the SMBH, the star or gas cloud gets a gravitational boost. They basically slingshot themselves away from the SMBH, like a spacecraft using planetary gravitational assist to move through the solar system. Giving a star or gas some energy robs the SMBH of a tiny bit of its own, reducing its momentum.
Eventually, dynamical friction brings both SMBHs into the galactic center and they begin to orbit each other (point A in the diagram below). They continue to lose momentum through dynamical friction for several billion years, until they come within a distance of about 1 parsec (3.26 light years).
The process then stops, which brings us to the “last parsec” part of the problem. By this point, the black holes will have cleared the region of stars and gas, leaving nothing for further interactions (point B). The black holes would need an additional 10 billion years—basically the age of the universe—for enough stars and gas to reenter the region, refill it, and create enough drag through interactions to allow the black holes to cross the last parsec and merge.
What about gravitational waves? These ripples in spacetime carry energy away from orbiting objects so they can merge. But for gravitational waves to carry away enough energy for SMBHs to merge, those SMBHs must be at most 0.01 pc apart. So you see the problem: we know how to move SMBHs 1 pc apart, but we can't get them closer together.
In June 2023, the NANOGrav team of radio astronomers announced that they had detected a background hum of low-frequency gravitational waves of the kind we think would be generated by merging SMBHs. This suggests that there is a way to bring SMBHs close enough together so that they lose energy through gravitational waves and merge (point C). Astronomers have several ideas for how the universe might overcome the final parsec problem, in particular by throwing a third SMBH into the mix (which is then ejected from the system, depleting significant energy from the two remaining SMBHs). Or perhaps the calculations behind the SMBH approach are oversimplified and other factors need to be considered.
The recently detected background hum can't yet be separated into its components well enough to be able to say for sure whether we're seeing the merger of two super-astronomical complexes. But astronomers strongly suspect that we are, and now have an excellent starting point for finding that last piece of solid evidence that will show us the irrefutable merger of a pair of super-astronomical complexes.
Alison Klesman
Senior Editor
