The most detailed simulations ever made of black holes solve a long-standing mystery

An international team has built the most detailed and highest resolution black hole simulation to date. The simulation proves the theoretical predictions about the nature of accretion disks – the material that orbits and eventually falls into a black hole – that has never been seen before.

The research will be published on June 5 in the Monthly Notices from the Royal Astronomical Society.

Among the discoveries, the team of astrophysicists specializing in computational computing from Northwestern University, the University of Amsterdam and the University of Oxford discovered that the The innermost region of an accretion disk aligns on the equator with its black hole.

This discovery solves a long-standing mystery, originally presented by physicist John Bardeen, Nobel laureate, and astrophysicist Jacobus Petterson in 1975. At the time, Bardeen and Petterson argued that a black hole in rotation would cause the inner region of an inclined accretion disc to align on the equatorial plane of its black hole.

After decades of global racing in search of the so-called Bardeen-Petterson effect, the team simulation showed that while the outer region of an accretion disk remains tilted, its inner region aligns on the black hole. A smooth chain connects the inner and outer regions. The team solved the problem by brightening the accretion disk to an unprecedented degree and integrating the magnetized turbulence that causes the accretion of the disk. Previous simulations have allowed for substantial simplification by merely approximating the effects of turbulence.

"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 the Northwest, who led the research. . "These details around the black hole may seem small, but they have a huge impact on what's going on in the galaxy as a whole, they control the rotation speed of the black holes and, therefore, what effect do the black holes have on their entire galaxies. "

Chekhovskoy is an assistant professor of physics and astronomy at the North West Weinberg College of Arts and Sciences and a member of CIERA (Center for Interdisciplinary Astrophysical Exploration and Research), a research center. equipped with specialized resources in the development of studies in astrophysics, focused on interdisciplinary relations. . Matthew Liska, a researcher at the Anton Pannenkoek Institute of Astronomy at the University of Amsterdam, is the first author.

"These simulations not only solve a 40-year-old problem, but they also demonstrated that, contrary to what we usually think, it is possible to simulate the brightest accretionary disks in full relativity. general, "said Liska. "This paves the way for a new generation of simulations that, I hope, will solve even more important problems related to light-activated disks."

Elusive alignment

Almost everything researchers know about black holes has been learned by studying accretion discs. Without the intensely bright ring of gas, dust and other stellar debris swirling around black holes, astronomers could not spot a black hole in order to study it. Accretion disks also control the rate of growth and rotation of a black hole. It is therefore essential to understand the nature of accretion discs to understand how black holes evolve and work.

"The alignment affects how accretion discs tighten their black holes," said Chekhovskoy. "This therefore has an impact on the evolution of the spin of a black hole over time and on the launch of outputs that have an impact on the evolution of host galaxies."

From Bardeen and Petterson to the present day, simulations have been oversimplified to find floor alignment. Two main problems have been an obstacle for computer astrophysicists. On the one hand, the accretion disks approach so much of the black hole that they move in a distorted space-time, which rushes into the black hole at an immense speed. To further complicate matters, the rotation of the black hole forces the space-time to revolve around him. Proper consideration of these two crucial effects requires general relativity, Albert Einstein's theory that predicts how objects affect the geometry of the space-time that surrounds them.

Second, astrophysicists did not have the computational power to account for magnetic turbulence or agitation inside the accretion disk. This agitation is what causes the particles of the disk to stand together in a circular shape and so that the gas eventually falls into the black hole.

"Imagine that you have this thin disc, and then you have to solve the turbulent movements inside the disc," Chekhovskoy said. "It's becoming a really difficult problem."

Without being able to solve these problems, computer scientists have not been able to simulate realistic black holes.

Break the code

To develop a code capable of performing accretion disk simulations titled around black holes, Liska and Chekhovskoy used graphics processing units (GPUs) instead of central processing units (CPUs). Extremely efficient for manipulating computer graphics and image processing, GPUs speed up the creation of images on a screen. They are much more efficient than processors for computer algorithms that process large amounts of data.

Chekhovskoy compares 1,000-horsepower GPUs and processors to a 1,000-horsepower Ferrari.

"Let's say you have to move into a new apartment," he said. "You will have to make a lot of trips with this powerful Ferrari because it does not fit in a lot of boxes, but if you could put a box on every horse, you could move everything in one go." It's the GPU of elements , each of which is slower than those of the CPU, but there are many. "

Liska has also added a method called adaptive mesh refinement, which uses a dynamic mesh, or grid, that changes and adapts to the flow of motion throughout the simulation. It saves energy and power of the computer by focusing only on specific blocks of the grid where movements occur.

GPUs dramatically accelerated simulation and adaptive mesh increased resolution. These improvements have allowed the team to simulate the thinnest accretion disk to date, with a height to radius ratio of 0.03. When the disk was simulated as thin, the researchers could see the alignment just next to 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.

Surprisingly, even with these incredibly thin accretion disks, the black hole still emitted powerful jets of particles and radiation.

"Nobody expects these discs to produce jets as thin as they are," Chekhovskoy said. "People were waiting for the magnetic fields that produce these jets to destroy those very thin discs, but they are there, and it actually helps us to solve the mysteries of observation."