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Supermassive black holes (SMBHs) are the churning engines of galaxies, but astrophysicists aren’t exactly sure how two of them merge.
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While evidence suggests this occurs, we’ve never seen a merger take place or detected its resulting gravitational waves. Furthermore, our best mathematical modeling of black holes coalesence suggests that SMBHs actually stall roughly three light-years apart, a conundrum known as the “final parsec problem.”
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A new study now suggests that previously overlooked interactions among dark matter particles could explain how SMBHs eventually overcome this final parsec and join together.
Although supermassive black holes (SMBH) are the most exotic, gargantuan, galaxy-shaping objects in the universe, we still know little about them. Considering the physics-breaking nature of their existence, some lingering mysteries are to be expected, but even some basic questions remain. For example, can two SMBHs merge together?
Astrophysicists predict that these kinds of mergers are possible. Evidence throughout the universe suggests as much, and supermassive black holes’ smaller cousins—stellar-mass black holes—are known to merge because their ripples through spacetime have been detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO). However, scientists have never seen a SMBH merger in progress, and attempting to theorize one runs into a big problem—the math doesn’t quite line up.
Because of some wonky physics surrounding these spacetime conundrums located in the hearts of galaxies, calculating the dynamical friction of these two merging bodies leaves a seemingly unbridgeable parsec (roughly 3 light years) between them. In other words, the these bodies just orbit around each other indefinitely and never completely merge.
This is what’s known as the “final parsec problem,” and it stands in direct conflict with a 2023 study that discerned that the “clamor” of gravitational waves produced by SMBH mergers was heard for the first time. So either our science is wrong, or our math is wrong. But a new study from the University of Toronto, McGill University, and the European Council for Nuclear Research (CERN) posits that it’s our math that might be missing a few variables.
The team developed a new model that shows how dark matter interactions could explain how this seemingly impassable parsec can be traversed. The results of the study were published in the journal Physical Review Letters.
“We show that including the previously overlooked effect of dark matter can help supermassive black holes overcome this final parsec of separation and coalesce,” co-author Gonzalo Alonso-Álvarez, a postdoctoral fellow in the Department of Physics at the University of Toronto, said in a press statement. “Our calculations explain how that can occur, in contrast to what was previously thought.”
To simplify things (by a lot), when two SMBHs approach each other, they slowly lose energy via dynamical friction, otherwise known as gravitational drag. Similar to how spacecraft gain energy when on the receiving end of a planetary gravity boost, so too do stars and dust flung away from the two SMBHs take some energy with them. Eventually, SMBHs fling away everything that could provide any sort of dynamical friction and the two black holes form a orbital holding pattern about a parsec away from each other.
Previous studies have suggested that it’s possible a third black hole could mix things up, so a merger is possible. But that scenario can’t likely explain every merger, and other ideas include interactions with gas disks, recycling stellar material or, yes, interactions with dark matter. However, where those previous dark matter models suggested that this theoretical particle is also thrown from system, Alonso-Álvare and his team say that interactions between the dark matter particles themselves could sustain the dark matter halo and further degrade the orbits of both SMBHs, finally bridging the final parsec.
“The possibility that dark matter particles interact with each other is an assumption that we made,” Alonso-Álvarez says in a press statement, “an extra ingredient that not all dark matter models contain. Our argument is that only models with that ingredient can solve the final parsec problem.”
While LIGO forever changed astrophysics by detecting the first gravitational waves, the observatory is not up for the task of detecting waves emitted by these types of SMBH mergers, as they produce much longer wavelengths comparatively. But the existing Pulsar Timing Array, which networks pulsars throughout the galaxy into one big gravitational wave detector, or the upcoming Laser Interferometer Space Antenna (LISA) could have better luck.
For now, this dark matter model is yet another plausible solution to the pesky parsec problem, but we likely won’t know the real answer until we can see one of these mergers in action.
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Source Agencies
