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
We see evidence of supermassive black holes merging everywhere. There's one problem: we're not sure exactly how they do it. Recently, a team of astronomers has proposed that a particular form of dark matter may hold the key to unraveling this cosmic mystery.
Supermassive black holes are the largest black holes in the universe. Their mass ranges from a few hundred thousand to a few hundred billion times that of the Sun. Almost every galaxy in the universe has a giant black hole of this type at its core.
Astronomers believe these black holes grew to their enormous proportions over billions of years, from a combination of feeding on any material that came too close and merging with other giant black holes. Last year, a team used an observing technique known as a pulsar timing array to pick up a “background hum” of gravitational waves consistent with a large population of merging supermassive black holes. These gravitational waves are emitted when black holes spiral closer together during the merger event.
But between those two scales, astronomers don't know how black holes lose energy. When they're separated by about a parsec (3.26 light years), there isn't enough material to serve as an energy reservoir, and gravitational waves aren't strong enough to do it on their own. Hence the problem.
However, astronomers are not exactly sure. as The merger process plays out as it progresses. What's especially tricky is a problem known as the final parsec problem. For two black holes to merge, they have to get close to each other. And to get close, they have to lose orbital energy. At large distances, black holes can lose orbital energy through a variety of means, such as by interacting with gas or stars in a galaxy. When black holes are close together — just a handful of light-years — all the material has been stripped away, but by then the emission of gravitational waves can strip energy from the system, allowing the black holes to collide.
Dark matter interacting with itself
A new article published on July 9 in Physical examination letters offers a possible solution to this problem. The authors' solution involves dark matterthe invisible matter that appears to dominate the mass of galaxies. While there is a substantial amount of circumstantial evidence for the existence of dark matter, such as the speed at which galaxies rotate and the growth of large structures in the universe, we still do not know the identity of the dark matter particle. All we know is that dark matter does not interact with light. It only interacts with the rest of the universe through gravity.
The simplest model for dark matter assumes that there are no collisions, meaning that dark matter does not interact with itself. But there are some models of dark matter in which it can interact weakly with itself. These models are known, appropriately, as self-interacting dark matterAstronomers proposed this dark matter model decades ago as a way to address some of the shortcomings of the collisionless dark matter idea. Most importantly, collisionless dark matter has a hard time producing the observed number of dwarf satellite galaxies and matching the densities observed in galaxy cores.
“The possibility that dark matter particles interact with each other is an assumption we made, an additional ingredient that not all dark matter models contain,” said study co-author Gonzalo Alonso-Álvarez, a postdoctoral researcher in the Department of Physics at the University of Toronto and the Department of Physics and Trottier Space Institute at McGill University, in a Press release“Our argument is that only models with that ingredient can solve the final parsec problem.”
The astronomers behind the study found that when they replaced collisionless dark matter with self-interacting dark matter in their models, the final parsec problem was no longer an issue. Supermassive black holes are thought to merge as the last stage of a merger between two galaxies. As supermassive black holes enter the core of the resulting galaxy, they encounter higher densities of dark matter. The black holes interact with the dark matter through gravity (much like they do with gas and stars when they are farther apart), which absorbs energy from the black hole's momentum into the dark matter particles. With collisionless dark matter, the dark matter particles absorb this extra energy and then simply leave. But with self-interacting dark matter, the extra energy added to the particles simply goes into more interactions.
This allows the dark matter to act as a reservoir that can absorb the kinetic energy of the black holes as they approach. With this additional reservoir available, the supermassive black holes quickly close the last parsec and find themselves in their final gravitational embrace.
Seeking confirmation
Researchers predict that in such a scenario, the resulting gravitational waves should be slightly different than those expected in the collisionless dark matter case. And this is exactly what astronomers have seen with pulsar timing arrays.
“One prediction of our proposal is that the spectrum of gravitational waves observed by pulsar timing arrays should be smoothed (lower in power) at low frequencies,” said James Cline, co-author of the study and a professor at McGill University and the Department of Theoretical Physics at CERN in Switzerland. “Current data already hint at this behaviour, and new data could confirm it in the coming years.”
While not a confirmed detection of dark matter or a confirmed solution to the final parsec problem, this study does show that dark matter may be more complicated than we naively assume and may play an important role in the evolution of black holes.
And in turn, black holes can help us reveal the true nature of dark matter.
“Our work is a new way to help us understand the nature of dark matter particles,” says Alonso-Álvarez. “We have discovered that the evolution of black hole orbits is very sensitive to the microphysics of dark matter and that means we can use observations of supermassive black hole mergers to better understand these particles.”
