A supercomputer helps solve a 45-year-old conundrum about spinning black holes.
Using a supercomputer and some custom-built code, an international team of researchers has created the "most detailed" simulation of a black hole proving a long-standing theory that has puzzled astrophysicists for 45 years.
The theory, first published in 1975, posited that the inner-most region of a spinning black hole would eventually align with the hole's equatorial plane. While that may seem somewhat confusing and inconsequential, how this region is warped by the black hole can have huge effects on entire galaxies.
The new research, published Wednesday in the journal Monthly Notices of the Royal Astronomical Society, details a simulated solution to the theory -- known as the Bardeen-Petterson effect -- and solves a problem co-lead author Sasha Tchekhovskoy says has "haunted the astrophysics community for more than four decades."
When we saw the very first image of a black hole on April 10, we weren't actually seeing the black hole. Black holes don't emit any visible light. Rather, the incredibly strong gravity of a black hole causes debris, gas and other particles to spin around its edges forming an "accretion disk." That's something we can see. Not only is that helpful for cosmic detectives to find and understand black holes but these disks are also responsible for the evolution and function of a black hole. Moreover, they can tell astrophysicists more about how black holes spin and potentially the radiation the comes from them.
The team, including the paper's first author Matthew Liska, used graphical processing units (GPUs) to develop their simulation's code.
"Once we created the code, we needed to find a large enough supercomputer to carry out the simulations," says Tchekhovskoy, who co-led the research. "The National Science Foundation supercomputer, Blue Waters, was just right for the task."
Blue Waters, an immensely powerful computer operating with 1.5 petabytes of memory, is housed at the University of Illinois at Urbana–Champaign, Illinois.
"We put a black hole inside of a computer and drop gas on it," says Tchekhovskoy. "Initially, the gas orbits around the black hole at a plane tilted relative to the black hole equator. However, over time the inner regions of the disk align with the equatorial plane, revealing the alignment.
The result may not be as immediately impressive as our first view of a black hole, but it is another first. Previously, astrophysicists studying the Bardeen-Petterson effect did not have access to enough computing power to adequately account for magnetic turbulence inside the accretion. With the supercomputer, researchers were able to simulate a more realistic black hole, with magnetic fields in place.
"The unique aspect of these simulations is their treatment of the magnetic fields, general relativistic effects and a cooling function at the same time," says Rebecca Nealon, theoretical astrophysicist at the University of Leicester not associated with the study. "Their results showing the Bardeen-Petterson effect while including these is an excellent confirmation of the general picture found in previous works."
The fields are a key factor in regulating how the accretion disk bends and falls into it, according to Tchekhovskoy. Ultimately, the researchers found that even for incredibly thin accretion disks, the proposed Bardeen-Petterson effect held up -- accretion disks did align to the black hole.
"The alignment of the disk with the black hole equatorial plane is the new finding of this work," explains Tchekhovskoy. "At larger radii, the disk is tilted relative to the equatorial plane of the black hole."
The next phase of the research will look at "radiation transport," says Tchekhovskoy. Essentially, the team will be able to predict what would happen to the particles of light that are produced during this process, giving astronomers a potential way to view the phenomenon via telescope.