Albert Einstein’s theory of General Relativity has passed a multitude of tests over the past century, but physicists remain unsatisfied. This is because evidence of it has never been found in a strong gravitational field , such as that of a supermassive black hole. But now a star surveillance team, on its way to a close encounter with the giant black hole at the center of our galaxy, has provided yet another piece of evidence confirming Einsten’s theory, which is more than 100 years old.
Observing stars near the black hole in the center of the Milky Way, known as Sagittarius A, is extremely difficult, given that it is 26,000 light years from Earth and enveloped in a cloud of gas and dust. Astronomers have to use large telescopes, capable of collecting infrared light, that can penetrate the dark.
For the past few decades, researchers have followed the paths of a group of stars as they pass around Sagittarius A at high speed. From these paths, it is possible to deduce not only the existence of the supermassive black hole, but also its mass: four million times that of the Sun.
But those price studies were not enough to verify General Relativity. Putting Einstein to the test requires tracking orbits with extreme precision to elucidate minute differences. The elliptical orbit of the closest star to Sagittarius A, called S2, brings it close to the black hole every 16 years, about four times the distance between the Sun and Neptune. During their last close encounter, in 2002, astronomers did not have the instruments to measure it accurately enough. But they are ready for the next screening in mid-2018.
Newton predicted that a star should follow the same elliptical orbit through space over and over again. But according to relativity, when the star swings close to the black hole, the orbit is slightly overshot, changing its center line. In an article published in the Astrophysical Journal , the team reports seeing both signs in their observations of the star S2: a shifting centerline and different shapes of orbit.
In 2018 the star S2 will get very close to the supermassive black hole, and it will be time to confirm this observation. If confirmed, it will be the first time that General Relativity effects have been measured in stars orbiting a supermassive black hole.
By then, the GRAVITY instrument, developed by a large international consortium led by the Max-Planck Institute for Extraterrestrial Physics (in Garching, Germany) and installed on the VLT interferometer, will be available to help measure the orbit much more accurately than the one that is currently achieved.
Not only is this instrument, which already makes high-precision measurements of the galactic center, is expected to clearly confirm general relativistic effects, but astronomers can also look for deviations from general relativity that reveal new physics. They will release the results of their observations next year.
References:
M. Parsa et al. Investigating the Relativistic Motion of the Stars near the Black Hole in the Galactic Center. Astrophysical Journal, August 2017.
Image: ESO / M. Parsa / L. Calçada.
