Universal Forces

Posted on Tuesday, March 10th, 2020

Colourful drawn image of black hole in the center of the Earth's Milky Way galaxy.
At the center of our Milky Way galaxy is a black hole with a mass four million times that of the Sun and the strongest gravitational field in our galaxy.

How gravity behaves around black holes can help us probe centers of galaxies.

The description of black holes could form the basis of a riddle: “I am billions of times heavier than the sun, but I am invisible. I experience gravity so strongly that even light—the fastest thing in the Universe—cannot escape. What am I?”

Albert Einstein would have known the answer: a black hole. Afterall, he had theorized about black holes in his 1916 Theory of Relativity. According to Einstein, gravitational forces are created when two black holes collide. Examining how gravity behaves around black holes can provide insight into astrophysical phenomena that we cannot directly observe. To that end, researchers can consider orbital resonance. This phenomenon occurs when two bodies—planets, stars, asteroids, or debris—circle, or orbit, a third body and influence each other through gravitational forces. There is a gap in our understanding, however, as most studies have not examined orbital resonance using Einstein’s relativistic approach, which considers space and time as dynamic entities.

University of Guelph physics professor, Huan Yang, and his colleagues are working to understand patterns of orbital resonance in a relativistic system. The team considered a scenario. Two stellar mass black holes, formed when the center of a large star collapses in on itself, are orbiting a supermassive black hole—a black hole that has an extremely large mass. In that scenario, gravity would be strong, and could change based on the distance between the three entities. Using mathematical functions, the team demonstrated that the gravitational forces between the two stellar-mass black holes would be steady and in sync at first, but as they move closer to the supermassive black hole, the forces would get stronger. The stellar-mass black holes may eventually merge with the supermassive black hole and could even leave an imprint on the outgoing gravitational waves. These imprints are like ripples in space and, amazingly, scientists can observe them using a laser gravitational-wave detector.

“Examining how gravity behaves in scenarios like this one provides a window into the centers of galaxies. Our study shows that it is important to incorporate relativistic corrections to model stellar orbits near galaxy-center supermassive black holes.”

This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Perimeter Institute for Theoretical Physics. Research at the Perimeter Institute is supported by the Government of Canada through the Department of Innovation, Science, and Economic Development Canada, and by the Province of Ontario through the Ministry of Economic Development, Job Creation and Trade.

Yang H, Bonga B, Peng Z, Li G. Relativistic mean motion resonance. Physical Review D. 2019 Dec 26. doi: 10.1103/PhysRevD.100.124056.

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