Testing the predictions of general relativity near the Milky Way’s central black hole

For the first time, astronomers have clearly detected the effects predicted by general relativity for the orbit of a star around the central supermassive black hole of our home galaxy. The measurements took advantage of the closest approach of one particular star to the black hole in May 2018. The required high precision was provided by the GRAVITY instrument at ESO’s Very Large Telescope, which exploits the wave properties of light to allow for highly accurate relative positions of point sources.

Bild vergrößern Simulation von Sternbahnen rund um das supermassereiche Schwarze Loch im Zentrum der Milchstraße. Simulation von Sternbahnen rund um das supermassereiche Schwarze Loch im Zentrum der Milchstraße. Bild: ESO/L. Calçada/spaceengine.org Bild: ESO/L. Calçada/spaceengine.org

The central black hole of our home galaxy, the Milky Way, is one of the best-studied black holes in the cosmos. Detailed information about the black hole’s mass and compactness has come from studies of stars moving in the galactic center, orbiting the black hole. Now, a team of astronomers led by Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics) and including astronomers Wolfgang Brandner and Thomas Henning from the Max Planck Institute for Astronomy (MPIA) has published the most detailed such motion study yet.

Using the GRAVITY instrument co-developed at MPIA for the European Southern Observatory’s Very Large Telescopes, the astronomers were able to monitor the orbit of the star S2 during the closest possible approach of that star to the black hole. The result clearly shows the effects of Einstein’s general relativity on the star’s orbit – the first definite detection of such relativistic properties of a stellar orbit around a black hole.

A compact object in the center of our galaxy

To the best of current knowledge, supermassive black holes in the centers of galaxies are the rule, not the exception. When astronomical telescopes became sufficiently powerful in the mid-1990s, a research groups led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics and a few years later a group led by Andrea Ghez at UCLA independently began to track the motion of stars near the galactic center. Their data showed clearly that these stars were orbiting an object with a mass of about 4 million times the mass of the Sun. The object is almost invisible using ordinary light, but had been detected earlier as the radio source “Sagittarius A*” (pronounced ``Sagittarius A star’’) by radio astronomers.

Data from stars which pass fairly closely to that central mass have ruled out all other known astronomical objects (such as very compact star clusters) which could provide the mass. To the best of our knowledge, the central object is indeed a black hole, as described by Einstein’s theory of general relativity: a region with a mass concentrated so compactly that not even light can escape that region’s gravitational pull; matter and light can fall in, but what has fallen in cannot get out again. The radio waves are thought to be emitted by a plasma disk of matter orbiting the black hole before falling in.

This defining property of a black hole makes it impossible to test predictions of general relativity for the inside of a black hole. The study that has been published now does the next best thing: testing the predictions for gravitational effects in the direct vicinity of the black hole, and in particular the deviations of general relativity’s predictions from the predictions of classical, Newtonian gravity.

Astronomical geometry with light waves

For detecting the effects of general relativity, the timing had to be just right. The orbit of the best-observed star orbiting the black hole, called S2, is somewhat eccentric – not a circle, but an elongated ellipse. At the star’s closest approach to the black hole, called the orbit’s pericenter or peribothron (the latter from the Greek bothrosfor hole or pit), the speed of S2 is highest, reaching values of about 7650 kilometers per second, which corresponds to 2.6% of the speed of light.

Every 16 years (the orbital period of S2), this close passage provides particularly favorable conditions for observing relativistic effects, which are most pronounced at high speeds and at an object’s closest approach to a mass. The most recent close approach was on May 19, 2018, affording a rare opportunity to astronomers. Genzel says: "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 resolution." Compared with the previous observations of a closest approach in 2002, the accuracy has increased by a factor of more than 10.

Even under these favorable conditions, the relativistic effects are comparatively small, and thus require considerable observational sophistication. As a key instrument, the astronomers used GRAVITY, an instrument designed with exactly this particular application in mind. GRAVITY can combine near-infrared light from all four 8-meter-telescopes of the European Southern Observatory’s Very Large Telescope (VLT) in Chile in a way that makes use of the wave properties of light, using a technique known as interferometry. One particular advantage of interferometry is that it allows astronomers to determine the relative position of two point sources of light with extreme precision.

Using GRAVITY, the astronomers were able to track the closest approach of S2 to the black hole with a precision of better than 30 micro-arc seconds – equivalent to tracking the relative position of two candles on the Moon with an accuracy of better than 6 centimeters. Wolfgang Brandner (MPIA), Co-Investigator for the GRAVITY project, says: “GRAVITY is so sensitive that we can detect infrared radiation from matter in close orbit around the black hole with less than five minutes of exposure times – that is what allows for these highly precise position measurements.” MPIA was responsible for GRAVITY’s Adaptive Optics (AO) systems. Those systems mitigate the effect of Earth’s atmosphere on the light from distant objects, a necessary prerequisite for the interferometric measurements.

For the analysis, the GRAVITY data was combined with data from two additional VLT instruments: the SINFONI spectrograph, which traces how S2 moves directly towards us or away from us, and larger-scale images from the NACO instrument, which was also co-developed and built at MPIA, and which has traced stellar orbits around the galactic center since 2001.

General relativity passes another test

The results were unambiguous: Classical Newtonian gravity cannot explain the observed orbit of S2 near the pericenter. Instead, the observations clearly show the combined effects of both the fast motion of S2 and the black hole’s gravitational field on the orbital dynamics (specifically, time dilation for moving objects as predicted by special relativity and the gravitational redshift predicted by general relativity’s equivalence principle).

Assuming there are no significant masses between S2 and the central black hole, the observations confirm the general relativistic prediction for the orbit to within plus/minus 15% – well within the observational uncertainty. (Light-emitting masses which could alter the situation can be excluded by the observations, although an unseen stellar black hole remains a theoretical possibility.)

Thomas Henning, director at the MPIA and co-author of the article reporting the results, says: “This is an excellent example for the impact of observational astronomy. Before GRAVITY, observations at this level of precision would have been impossible. With this impressive result, the GRAVITY instrument is exceeding our wildest expectations.”

Observations of our galaxy’s central black hole with GRAVITY and other instruments are continuing. By 2020, the researchers hope to be able to detect an additional effect predicted by general relativity: the Schwarzschild precession; a slow rotation of the entire elliptical orbit around the black hole. For the star S2, that rotation amounts to about 0.2 degrees per orbit. They also hope to find out more about the matter circling, and about to fall into, the black hole – the matter whose weak glow in the near-infrared has made GRAVITY’s direct position comparison possible in the first place.

Background information

The results have been accepted for publication in the journal Astronomy & Astrophysics as an article by the GRAVITY Collaboration: "Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole’’.

arXiv preprint

Additional images (including high-resolution versions) and video material can be found in ESO's version of the press release:

Press release by the Max Planck Institute for Extraterrestrial Physics:

The MPIA researchers involved were Wolfgang Brandner, Thomas Henning, Stefan Hippler, Sarah Kendrew (now at ESA), Martin Kulas, Rainer Lenzen, Eric Müller (now at ESO), José Ramos, Ralf-Rainer Rohloff, Joel Sanchez-Bermudez and Silvia Scheithauer as part of the GRAVITY Collaboration.

The measurements were made by an international team led by Reinhard Genzel (Max Planck Institute for Extraterrestrial Physics in Garching, Germany) and collaborators around the world, at the Paris Observatory–PSL, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA – Centro de Astroﬁsica e Gravitação and ESO.

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