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Einstein’s biggest prediction, first proven around a black hole

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Updated:04/18/2020 00: 14h

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Observations made with the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in the Atacama desert (Chile) have revealed for the first time that a star orbiting the supermassive black hole in the center of the Milky Way it moves as predicted by the Einstein’s general theory of relativity. Its orbit has rosette shape and not an ellipse, which was what Newton’s theory of gravity predicted. This result has been possible thanks to increasingly precise measurements for almost 30 years, which have allowed scientists to discover the mysteries of Sagittarius A *, the giant that lurks at the heart of our galaxy.

“Einstein’s General Relativity predicts that the attached orbits of one object around another are not closed, as in Newtonian gravity, but move forward on the plane of motion. This famous effect, first seen in the orbit of the planet Mercury around the Sun, was the first evidence in favor of General Relativity. One hundred years later, we have detected the same effect on the motion of a star orbiting the Sagittarius A * compact radio source in the center of the Milky Way, “says Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics (MPE) at Garching, Germany and the architect of the three-decade program that led to this result. “This observational advance strengthens the evidence that Sagittarius A * must be a supermassive black hole 4 million times the mass of the Sun,” he adds.

Very close to the giant

Located 26,000 light-years from the Sun, Sagittarius A * and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored regime of gravity. One of these stars, S2, heads toward the supermassive black hole at a closer distance of less than 20 billion km (120 times the distance from the Sun to Earth), making it one of the closest stars ever found in orbit around the massive giant. On its closest approach to the black hole, S2 rushes through space at nearly three percent the speed of light, completing an orbit once every 16 years.

Most stars and planets have a non-circular orbit, and therefore move closer and farther away from the object that they revolve around. In the case of S2, the location of its closest point to the supermassive black hole changes with each turn, so that the next orbit rotates relative to the previous one, creating a rosette shape. General relativity provides an accurate prediction of how much its orbit changes, and the latest measurements from this research exactly match the theory. This effect, known as the Schwarzschild precession, has never before been measured for a star around a supermassive black hole.

Invisible material

The study also helps scientists learn more about the vicinity of the supermassive black hole at the center of our galaxy. “Because S2 measurements follow general relativity very well, we can set strict limits on the amount of invisible material, such as distributed dark matter or possible smaller black holes, that is present around Sagittarius A *. This is of great interest to understand the formation and evolution of supermassive black holes »say Guy Perrin and Karine Perraut, the main French scientists on the project.

The research was carried out by an international team led by Frank Eisenhauer of the MPE with collaborators from France, Portugal, Germany and ESO. The team composes the GRAVITY collaboration, named after the instrument they developed for the VLT interferometer, which combines the light from the four 8-meter VLT telescopes into one super telescope (with a resolution equivalent to that of a 130-meter diameter telescope. ). The same team reported in 2018 another effect predicted by General Relativity: they saw the received light from S2 stretch to longer wavelengths when the star passed near Sagittarius A *. “Our previous result has shown that the light emitted by the star experiences general relativity. We have now shown that the star itself feels the effects of general relativity, “says Paulo García, a researcher at the Center for Astrophysics and Gravitation in Portugal and one of the main scientists of the GRAVITY project.

Space and time

With ESO’s upcoming Extremely Large Telescope, the team believes they could see much fainter stars orbiting even closer to the supermassive black hole. “If we’re lucky, we could capture stars close enough to actually feel the rotation, the spin, of the black hole,” says Andreas Eckart of the University of Cologne, another leading scientist on the project. This would mean that astronomers could measure the two quantities, spin and mass, that characterize Sagittarius A * and define the space and time around it. “That would be a completely different level of relativity testing again,” says Eckart.

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