An international team of researchers has finally solved a long-standing cosmic mystery – and to do it, they needed to produce the most detailed black hole simulation ever created.
Over time, black holes pool matter that orbits – and eventually is consumed by – the hole. These whirlpools of doomed matter are known as accretion disks. Nearly everything known about black holes has been derived from study of accretion disks, which – unlike the holes themselves – are bright and clearly visible. Accretion disks also control how quickly a black hole grows and rotates.
In 1975, physicists John Bardeen and Jacobus Petterson posited that the inner regions of tilted accretion disks would align with their black holes’ equators, starting a discussion in the astrophysical community that would take decades to solve.
Nearly forty-five years later, a team of researchers from Northwestern University, the University of Amsterdam and the University of Oxford has validated the “Bardeen-Petterson alignment,” showing the outer regions of tilted accretion disks remain tilted, but their inner regions align on the equatorial plane.
“This groundbreaking discovery of Bardeen-Petterson alignment brings closure to a problem that has haunted the astrophysics community for more than four decades,” said Northwestern University’s Alexander Tchekhovskoy, co-lead of the research. “These details around the black hole may seem small, but they enormously impact what happens in the galaxy as a whole. They control how fast the black holes spin and, as a result, what effect black holes have on their entire galaxies.”
Until now, black holes simulations were too simplified to analyze the alignment. Simulations of accretion disk alignment needed to incorporate both the effects of warped – and rapidly spinning – space-time, as well as the effects of magnetic turbulence inside the accretion disks. “Imagine you have this thin disk. Then, on top of that, you have to resolve the turbulent motions inside the disk,” Tchekhovskoy said. “It becomes a really difficult problem.”
To carry out these simulations, Tchekhovskoy and Matthew Liska (first author of the paper) used GPUs to better-handle the massive amounts of data and implemented adaptive mesh refinement, which uses a dynamic mesh that adapts to movement throughout the simulation. They ran the simulation on the Blue Waters supercomputer at the National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign.
With the simulation accelerated, the researchers thinned the simulated accretion disks until they reached a height-to-radius ratio of 0.03, which allowed them — at long last — to observe the alignment right next to the black hole. “The thinnest disks simulated before had a height-to-radius ratio of 0.05, and it turns out that all of the interesting things happen at 0.03,” Tchekhovskoy said.
About the paper
The study discussed in this article, “Bardeen-Petterson alignment, jets and magnetic truncation in GRMHD simulations of tilted thin accretion discs,” was published on June 5th in the Monthly Notices of the Royal Astronomical Society. The research was supported by the National Science Foundation, the Netherlands Organisation for Scientific Research, The Royal Society and NASA.