### The black hole counterparts’ final dance is not equal

*Rochester Institute of Technology scientists first performed a large mass ratio simulation black hole merged on Frontera.*

Solving the equations of general relativity for colliding black holes is not a simple problem.

Physicists began using supercomputers to work out a solution to this famously difficult problem in the 1960s. In 2000, there was no solution in sight, Kip Thorne, 2018 Nobel Prize winner and is one of the designers of LIGO, famous fish that there will be an observation about gravitational waves before reaching a numerical solution.

He lost that bet in 2005, Carlos Lousto, then at the University of Texas at Brownsville, and his team created a solution using the Lonestar supercomputer at the Advanced Computer Center. to Texas. (At the same time, groups now NASA and independent solutions sourced from Caltech.)

In 2015, when the Laser Interferometer Gravitational Wave Observatory (LIGO) first observed such waves, Lousto was shocked.

“It took us two weeks to realize this actually came from nature, not from importing our simulation,” said Lousto, now professor of mathematics at the Rochester Institute of Technology (RIT). as a test. “The comparison with our simulation is too clear. You can see with the naked eye that it is a fusion of two black holes.

Lousto returns again with a new relativity milestone, this time simulating black hole fusion where the mass ratio of the larger and smaller black hole is 128 to 1 – a scientific problem. at very high computational limits. His secret weapon: the Frontera supercomputer at TACC, the eighth most powerful supercomputer in the world and the fastest of any university.

His research with collaborator James Healy, supported by the National Science Foundation (NSF), has been published in *Letter of physical assessment* this week. It may take decades to confirm the results experimentally, but nonetheless serves as a computational achievement that will help drive the field of astrophysics forward.

“Modeling pairs of very different masses of black holes requires a lot of computation because it needs to be maintained accuracy Said Pedro Marronetti, program director in gravity physics at NSF. “The RIT team has performed the most advanced simulations in the world in the field, and each of them brings us closer to the observations that gravitational wave detectors will provide in the near future. . “

LIGO can only detect gravitational waves caused by intermediate and small mass black holes of approximately equal size. It would take 100 times more sensitive observatories to detect the type of merger that Lousto and Healy emulated. Their findings not only show what gravitational waves caused by a 128: 1 fusion will look like to an observer on Earth, but also the characteristics of how the last fused black hole will be. its final weight, rotation speed and recoil. This led to some surprises.

“These fused black holes could have a much greater rate than previously known,” said Lousto. “They can move at a speed of 5,000 km / second. They kick out of a galaxy and roam the universe. That is another interesting prediction.

The researchers also calculated gravitational waveforms – signals that will be felt near Earth – for such fusions, including their maximum frequency, amplitude, and luminosity. Comparing those values with predictions from existing scientific models, their simulations are within 2% of the expected results.

Previously, the largest mass ratio ever solved with high accuracy was 16 to 1 to eight times lower than in Lousto’s simulation. The challenge of simulating larger mass ratios is that it requires solving the dynamics of interactive systems at complementary scales.

Like computer models in many disciplines, Lousto uses a method called adaptive mesh refinement to obtain accurate models of the dynamics of interactive black holes. It involves placing black holes, the space between them and the distant observer (us) on a grid or mesh, and refining areas of the mesh with greater detail where needed.

Lousto’s group approached the problem with a method he compared with Zeno’s first paradox. By halving and halving the mass ratio while adding the level of refinement of the internal mesh, they can go from a 32: 1 black hole mass ratio to a 128: 1 undergone binary system. 13 orbits before fusion. On Frontera, it required seven consecutive months of computation.

“Frontera is the perfect tool for the job,” Lousto said. “Our problem required a high performance processor, communication and memory, and Frontera had all three.”

Simulation is not the end of the road. Black holes can have many different spins and configurations, affecting the amplitude and frequency of the gravitational waves their fusion produces. Lousto wants to solve the equations 11 more times to get the best possible first set of “patterns” to compare with future findings.

The results will help future Earth and space gravitational detector designers plan their devices. These include the advanced, third-generation ground-based gravitational wave detector, and the Laser Interferometer Space Antenna (LISA), targeted for launch in the mid-2030s.

Research could also help solve fundamental mysteries about black holes, such as how a number can grow so large – millions of times the mass of the Sun.

“The supercomputer helps us answer these questions,” Lousto said. “And problems inspire new research and pass the torch on to the next generation of students.”

See: “Exploring binary black hole fusion on small mass proportions through Zeno’s binary approach” by Carlos O. Lousto and James Healy, 5 November 2020, *Letter of physical assessment*.

DOI: 10.1103 / PhysRevLett.125.191102