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Supermassive black hole confirms Einstein's general relativity

Astrophysicists are investigating whether the laws of physics will change over time and space. The theory of general relativity, well verified with weak gravitational fields, could not be valid not only when the fields are strong, as with a black hole supposed to be such, or simply in the past of the cosmos or even thousands even billions of light years from the Sun. ESO members therefore tested Einstein's theory of gravity with the central black hole in the Milky Way, as this ehealth explains. The Review was passed!

The Einstein's theory of general relativity is more than a century old. For the moment, it resists all experimental tests, as well as those made in the Solar system nowadays that by observing stars distant and in ancient times.

As expected, in May 2018, one of the stars the most famous in orbit around black hole central of the Milky Way ended up going to periaster, i.e. at the point closest to Sgr A * on this orbit. Called S2, it approached only about 16 light hours (120 times the Earth-Sun distance or four times the Sun-Neptune distance) from supermassive black hole four million masses that we think is near the center of our Galaxy. This also corresponds to a distance equivalent to almost 1,500 Schwarzschild rays of this black hole. S2 is found at this periosteum approximately every 16 years and, at that time, it traverses a portion of its elliptical orbit at almost 2.7% of the speed of light, or 8,000 km / s.

Sagittarius A *, the supermassive laboratory black hole

The astrophysicists relativists impatiently awaited this event. One could hope to see effects of the field of gravitation black hole Sagittarius A * – which are not described by the theory of Newton of gravitation, and perhaps not even completely by the theory ofEinstein – opening a window on a news physical.

The opportunity could not be missed to also probe the gravitational field of a black hole in a regime, where it is intense. This is why many researchers and engineers had seized it within the framework of the Gravity consortium led by the German Institute Max Planck for extraterrestrial physics (MPE) and involving the CNRS, the Paris Observatory – PSL, the Grenoble-Alpes University and several other French universities (as well as the University of Cologne and the Portuguese Center forastrophysics and gravitation). It was about being able to combine, by a method ofinterferometry, observations in theinfrared made by many of telescopes of Very Large Telescope ((VLT) of the'ESO to make the equivalent of a telescope of more than 100 meters in diameter, while analyzing the light using three instruments, Naco, Sinfoni and Gravity. The challenge was to be able to observe and measure the movements hour by hour of S2, with an accuracy of 50 microseconds of angle, which amounts to observing from the Earth a tennis ball placed on the Moon.

The first measurement of the spectral shift effect of a black hole

ESO has finally announced – via a press conference that accompanies the posting on arXiv of an article explaining the scientific results – that a culmination of 26 years of star observation around Sgr A * with its telescopes had been reached. Indeed, the theory of general relativity implies that the gravitational field of a star produces a red shift of light that it can emit, all the more important that it is massive or dense, and this, according to a precise law.

This is indeed what was observed with S2, and just as it was the case almost a century ago with the deflection of the light rays of stars by the Sun (observed and measured during the famous eclipse 1919), the measured effects cannot be explained with Newton's theory of gravitation. But they are on the contrary in full agreement, with the precision of the measurements reached, with Einstein's theory.

It is the first time that this offset effect has been measured for the gravitational field of a black hole. We knew him before, especially with white dwarfs (the first detection solid dates from 1954 with 40 Eridani B), and could be measured in the much weaker field of the Earth via the famous experience of Pound and Rebka.

This new success of the theory of general relativity should soon be followed by another, very likely. Indeed, the observations in progress should make it possible to observe the relativistic component of the precession of the peri-era of S2, the equivalent of the famous relativistic precession of the perihelion of Mercury. 16 years ago, although we did not have instruments as efficient as today, a previous passage into the peri-sphere of S2 was observed, thus allowing a comparison in progress with that of 2018.

© ESO