The massive black hole at the heart of the Milky Way is an ideal cosmic laboratory for all kinds of physical tests. Its extremely strong gravitational field influences the surrounding area and has an impact on the motion of stars passing by. Scientists at the Max Planck Institute for Extraterrestrial Physics (MPE) have now observed an effect that had been predicted by Albert Einstein with his general theory of relativity more than 100 years ago. The discovery of this gravitational redshift represents the climax of a 26-year-long observation campaign using telescopes of the European Southern Observatory (ESO) in Chile.
The closest supermassive black hole to the Earth lies 26,000 light-years away at the centre of the Milky Way. This gravitational trap, which has a mass four million times that of our Sun, is surrounded by a small group of stars orbiting around it at high speed. This region, with the strongest gravitational field in our galaxy, is the perfect place to explore gravitational physics, and particularly to test Einstein’s general theory of relativity.
In order to observe the galactic centre, the astronomers use sensitive instruments such as Gravity, Sinfoni and Naco. All these belong to the ESO’s Very Large Telescope, were constructed under the leadership of the Max Planck Institute for Extraterrestrial Physics (MPE), and scrutinise the sky in an infrared light. Now, researchers have turned their attention to a star called S2 and followed it on its orbit around the black hole, to which it came particularly close a few weeks ago.
The shortest distance between S2 and the black hole, on 19 May, was approximately 14 billion kilometres. Here, the star moved at a speed in excess of 25 million kilometres per hour – almost three percent of the speed of light. It takes about 15 years to complete its full orbit.
The scientists compared the position and velocity measurements from Gravity and Sinfoni, along with the measurements taken during previous observations of S2, with the predictions of Newtonian gravitational physics, the general theory of relativity and other theories of gravity. In fact, the new results are inconsistent with Newtonian predictions, although they are in excellent agreement with the predictions of the general theory of relativity.
“This is the second time that we have observed the close passage of S2 around the black hole in our galactic centre. But this time, because of much improved instrumentation, we were able to observe the star with unprecedented detail resolution,” explains Richard Genzel, Director of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching near Munich, and head of the international team of scientists.
The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole, making it appear red. And this change in the wavelength agrees precisely with that predicted by Einstein’s theory of general relativity. This is the first time that the researchers have observed this deviation from the predictions of the simpler Newtonian theory of gravity in the motion of a star around a supermassive black hole.
The team used the Sinfoni instrument to measure the velocity of S2 relative to Earth. Astronomers use Gravity to make extraordinarily precise measurements of the changing position of S2, and thus also of the shape of its orbit. In connection with what is known as interferometry, in which the light of several telescopes is merged and overlaid, Gravity creates extremely sharp images. On these images, the motion of the star can even be followed from night to night as it passes close to the black hole – 26,000 light-years from Earth.
“Our first observations of S2 with Gravity, about two years ago, already showed that with the galactic centre, we have the ideal black hole laboratory,” says Frank Eisenhauer from the Max Planck Institute in Garching, Principal Investigator of Gravity and Sinfoni. During the close passage, even the faint glow around the black hole could be detected on most of the images. “In this way, we could follow the star on its orbit to an extremely high degree of precision, and could ultimately provide evidence of the gravitational redshift in the spectrum of S2.”
More than 100 years after he published his paper setting out the equations of general relativity, Einstein has been proved right once more – in a much more extreme laboratory than he could have possibly imagined. “In the strong gravitational field around the black hole, we generally expect to see relativistic effects – but only when we are able to conduct sufficiently precise observations”, says Stefan Gillessen from the Max Planck Institute for Extraterrestrial Physics. “For this reason, we had to push the technology to the limits.”
Further measurements will follow, and are already expected to reveal another relativistic effect soon – a small rotation of the star’s orbit known as Schwarzschild precession – as S2 moves away from the black hole.