Discovery of Binary Black Holes Spin Could Shed Light on the Lifespan of Stars and General Relativity

According to Vijay Varma, a Klarman Postdoctoral Fellow in physics in the College of Arts and Sciences, the primary objective of gravitational wave astrophysics is to know and identify binary black holes spin.

By calculating the spin velocities and masses of binary black hole systems in which two super-compact astronomical objects orbit each other, using gravitational waves emitted as the objects merge, scientists can gain insight into deeper topics in astrophysics, including the lifespan of stars and general relativity.

In “New spin on LIGO-Virgo binary black holes,” issued in Physical Review Letters on April 29, 2021, Varma and his associates suggested a new approach of examining binary black holes by identifying each of their individual component black holes by their spins – rather than their masses – which leads to an enhanced analysis of the spins. The scientists used the new approach to study binary black hole data collected by the LIGO and Virgo gravitational wave detectors.

“Rather than trying to analyzing the spin of the lightest and heaviest of the two objects, as is normally done, we infer the attributes of the objects with the lowest and highest spin,” the scientist wrote. This refocuses on the black holes’ spins, rather than their masses, provides attention to spin analyses in binaries in which the masses of the two black holes are almost equal – “which appear to be the majority,” they wrote.

Their discovery possibly transforms the way researchers examine black holes, which give insight into general relativity and our understanding of the evolution of stars, among other deep mysteries.

black hole spins
An artist’s conception shows two combine black holes similar to those discovered by LIGO. Source: Aurore Simonnet/LIGO-Caltech-MIT-Sonoma State

“We understood that for arrangements where the two black holes in the binary have similar masses or close to same masses, it’s difficult to determine the spin,” Biscoveanu said. The group reframed the problem to look directly at the spin of the black hole with the lowest spin and the black hole with the highest spin.

Varma and his associates (lead author Sylvia Biscoveanu, Salvatore Vitale, and Maximiliano Isi, all from the Massachusetts Institute of Technology) were excited to continue this line of study while analyzing data from GW190521, a binary black hole system discovered by LIGO, a very fine machine which identifies gravitational waves from astronomical objects, including black holes. This system is impressive, the scientists said, because it is the most massive detected to date, and it also shows proof for a unique spin sign that hadn’t previously been observed.

“We are especially fascinated by systems that have spins as they provide a lot of astrophysical data that can explain to us how these binaries were produced in the first place,” said Varma, an expert on emerging ‘surrogate models’, which allow scientist to discover properties of black holes based on supercomputer simulations.

Black holes are astonishingly dense and heavy, Varma said, typically 10 to 30 times bulkier than the sun, sometimes heavier, but packed into space about the size of Hawaii.

Biscoveanu compared measuring the spin and the mass of a binary black hole approach to calculating the sweetness and the temperature of two juices. “You would calculate the sweetness of the sweetest juice and temperature of the chilled juice that you’re tasting,” she said. “You wouldn’t try to calculate the sweetness of the chilled juice because that’s a convoluted question, particularly if both of them are the same temperature.”

The scientist said that studying the fastest spinning black hole helps scientists to know more about individual binary black hole systems, or a whole group of binary black holes, such as those recognized via gravitational waves by the LIGO-Virgo collaboration.

“That has hints for how stars grow and produce black holes,” Varma said. “We can trace the initial stages of the growth and try to know the mysteries of black hole astrophysics.”

Journal Reference:

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

DOI: 10.1103/PhysRevLett.126.171103

The study was supported by the Sherman Fairchild Foundation, the Paul and Daisy Soros Fellowship, the Klarman Postdoctoral Fellowship in A&S, the National Science Foundation, and the NASA Hubble Fellowship.

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