Since 2015, researchers have had a new tool for studying our universe: gravitational waves. These warps in the space-time continuum are created by high-mass objects closely interacting and giving off energy through these space-changing waves. Thanks to relativity and powerful computers, researchers are able to digitally listen to the ring of these soundwaves and determine the geometry of the merging objects, including both rough estimates of the masses and the shape of their orbit prior to the merger.

Prior to actually being able to measure black hole mergers, researchers assumed that the orbits of merging objects would circularize as they emitted energy in gravitational waves. This is what all the models say should happen and what our observations of other kinds of binaries have demonstrated does happen.

But Nature had other ideas. 

In 2019, an object cataloged as GW190521 didn’t look as we expected. Researcher Imre Varos explains: The gravitational wave event GW190521 is the most surprising discovery to date. The black holes’ masses and spins were already surprising, but even more surprising was that they appeared not to have a circular orbit leading up to the merger.

Mysteries are the kind of thing astronomers are compelled to poke at until they are no longer mysteries, and for the past several years, researchers have been working to brainstorm how this could happen. The answer appears to be that you need external forces driving the system.

In our old model, black holes in high-density regions like the cores of galaxies moved in a three-dimensional sphere with a myriad of orbits that rarely interacted, but since we know black holes do interact to have weird mergers, researchers reconfigured galaxies to confine black holes to a disk of material, causing them to move more like billiard balls than juggler’s balls. This forced the black holes to interact more often and for pairs to get pulled into elliptical orbits through interactions with a third object. Lead researcher Johan Samsing adds: We have now shown that there can be a huge difference in the signals emitted from black holes that merge in flat, two-dimensional disks, versus those we often consider in three-dimensional stellar systems,  which tells us that we now have an extra tool that we can use to learn about how black holes are created and merge in our Universe.

These results appear in the journal Nature.

More Information

University of Copenhagen press release

“AGN as potential factories for eccentric black hole mergers,” J. Samsing et al., 2022 March 9, Nature