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Black Hole Spin Discovery could shed light on the general theory of relativity and the lifespan of stars

One of the main goals of gravitational wave astronomy is to understand and characterize black hole binary spins, according to Vijay Varma, a Klarman Postdoctoral Fellow in Physics at the College of Arts and Sciences.

By measuring the masses and rotational speeds of binary black hole systems in which two of the super-compact astronomical objects orbit each other, using gravitational waves emitted as the objects converge, researchers can gain insight into larger questions in astrophysics, including the general relativity and the lifespan of stars.

In “New spin on LIGO-Virgo binary black holes,” published in Physical Review Letters on April 29, 2021, Varma and colleagues proposed a new way to study binary black holes by identifying each of their individual constituent black holes by of their spins – rather than their mass – which leads to a better measurement of the spins.

The researchers applied the new method to analyze black hole binary data collected by the gravitational wave detectors LIGO and Virgo.

“Rather than trying to identify the spin of the heaviest and lightest of the two objects, as is usually done, we infer the properties of the objects with the highest and lowest spin,” the researchers wrote.

This reorientation to the spins of the black holes, rather than their mass, gives new importance to spin measurements in binaries in which the masses of the two black holes are nearly equal – “which appear to be the majority,” they wrote.

Their finding may change the way scientists study black holes, which provide insights into general relativity and our understanding of the evolution of stars, among other big questions.

“We realized that for systems where the two black holes in the binary are of equal mass or nearly equal mass, it is difficult to measure spin,” Biscoveanu said.

The team reformulated the question to look directly at the spin of the black hole with the highest spin and the black hole with the lowest spin.

Varma and collaborators (lead author Sylvia Biscoveanu, Maximiliano Isi and Salvatore Vitale, all from the Massachusetts Institute of Technology) were inspired to continue this line of research while studying data from GW190521, a binary black hole system detected by LIGO, a very sensitive instrument that detects gravitational waves from astronomical objects, including black holes.

This system is particularly interesting, the researchers said, because it is the most massive to have been detected to date, and it also shows evidence for a unique spider signature that had not been seen before.

“We are particularly interested in systems with spins because they contain a lot of astrophysical information that can tell us how these binaries were formed in the first place,” said Varma, an expert in developing ‘surrogate models’ that allow researchers to determine characteristics of black holes based on supercomputer simulations.

Black holes are incredibly massive and dense, Varma said, usually 10 to 30 times the mass of the sun, sometimes heavier, but packed into space the size of Hawaii.

Biscoveanu compared measuring the mass and spin of a binary black hole system to measuring the temperature and sweetness of two juices. “You would measure the temperature of the coldest juice you taste and the sweetness of the sweetest juice,” she said.

“You wouldn’t try to measure the sweetness of the coldest juice, because that’s a complicated question, especially if they are both at the same temperature.”

The researchers said that by inquiring about the fastest-spinning black hole, researchers could learn more about individual binary black hole systems, or an entire population of binary black holes, as observed through gravitational waves by the LIGO-Virgo collaboration.

“That affects the way stars evolve and form black holes,” Varma said. “We can go back to the earlier stages of evolution and try to understand the secrets of black hole astrophysics.” Reference:

“New Spin on LIGO-Virgo Binary Black Holes” by Sylvia Biscoveanu, Maximiliano Isi, Salvatore Vitale and Vijay Varma, April 29, 2021, Physical Review Letters.