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An international team of researchers has finally solved a long-standing cosmic mystery. To do this, she had to produce the most detailed black hole simulation ever created.
The mystery
Over time, black holes accumulate material that gravitates into orbit – and is eventually consumed by – the hole. These vortices of condemned matter are known as accretion disks. Almost everything we know about black holes comes from the study of accretion discs which, unlike the holes themselves, are bright and clearly visible. Accretion disks also control the rate of growth and rotation of a black hole.
In 1975, physicists John Bardeen and Jacobus Petterson claimed that the inner regions of inclined accretion discs would align with the equators of their black holes, opening a discussion in the astrophysics community that would take decades to resolve.
Nearly forty-five years later, a team of researchers from Northwestern University, the University of Amsterdam and Oxford University validated the "Bardeen-Petterson Alignment", showing that the outer regions inclined accretion disks remain inclined but their inner regions on the equatorial plane.
"This groundbreaking discovery of the Bardeen-Petterson alignment puts an end to a problem that has haunted the world of astrophysics for more than four decades," said Alexander Chekhovskoy of Northwestern University, co-director of research. "These details around the black hole may seem small, but they have a huge impact on everything that happens in the galaxy. They control the speed at which the black holes rotate and, as a result, the effect of black holes on their entire galaxies. "
The methods
Until now, black hole simulations were too simplified to analyze the alignment. The accretion disk alignment simulations had to integrate both the effects of deformed and rapidly rotating space-time, as well as the effects of magnetic turbulence inside the accretion disks. "Imagine that you have this thin disc. On top of that, you have to solve the turbulent movements inside the disc, "said Chekhovskoy. "It's becoming a really difficult problem."
To perform these simulations, Chekhovskoy and Matthew Liska (first author of the paper) used GPUs to better handle huge amounts of data and developed an adaptive mesh refinement, which uses a dynamic mesh that adapts to the movement throughout the simulation. They performed the simulation on the supercomputer Blue Waters at the National Center for Supercomputing Applications (NCSA). at the University of Illinois at Urbana-Champaign.
As the simulation was accelerated, the researchers thinned the simulated accretion disks to a height-to-radius ratio of 0.03, which allowed them – finally – to respect the alignment just beside the black hole. "The thinnest discs previously simulated had a height to radius ratio of 0.05, and it turns out that all the interesting things happen at 0.03," said Chekhovskoy.
About paper
The study discussed in this paper, titled "Bardeen-Petterson Alignment, Jets and Magnetic Truncation in GRMHD Simulations of Thin Inclined Accretion Disks" was published on June 5 in Monthly notices from the Royal Astronomical Society. The research was funded by the National Science Foundation, the Netherlands Organization for Scientific Research, the Royal Society and NASA.
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