Frank Eisenhauer receives prestigious Gruber cosmology prize

May 17, 2022

The 2022 Gruber Cosmology Prize recognizes Frank Eisenhauer of the Max Planck Institute for Extraterrestrial Physics for the revolutionary design of instruments that collected seemingly irrefutable evidence for the existence of a black hole at the center of our Galaxy. The citation honors the “unprecedented and exquisite” precision of his instrumentation.

In 2018, the GRAVITY experiment traced the behavior of various phenomena near Sagittarius A*, or Sgr A*, a supermassive and therefore gravitationally voracious, invisible object near the center of our Galaxy. Thanks to Eisenhauer’s technical innovations, the GRAVITY team found that the orbit of stars and gas near the Galactic center matches theoretical predictions consistent with the existence of a black hole.  Eisenhauer’s mentor and longtime collaborator Reinhard Genzel shared the 2020 Nobel Prize in Physics with Andrea Ghez and Roger Penrose for his contributions to work on Sagittarius A* (Sgr A*).

GRAVITY had its basis in an earlier experiment on which Eisenhauer developed breakthrough technology in imaging spectroscopy—the measurements of how matter affects the absorption and emission of light. The instrument is part of the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the European Southern Observatory’s Very Large Telescope on Mount Paranal in Chile. It includes an adaptive optics system to correct the image blur from the Earth’s turbulent atmosphere developed under the lead of Henri Bonnet. In 2003, SINFONI began observing stars operating under the great gravitational influence of Sgr A* as they execute their exceedingly rapid, highly eccentric orbits.

Two years later the group of German and French researchers around Eisenhauer and Genzel, Stefan Gillessen, Pierre Lena, Guy Perrin, and Thibaut Paumard began discussing an opportunity to observe an upcoming event involving one of those stars, S2. Having measured a precise orbit of S2 after the first peri-passage in 2002, they could now calculate that in 2018 the star would reach the part of its orbit where it would again pass closest to Sgr A*, a distance of only 17 light-hours. Combining the observing power of all four 8-meter telescopes at Paranal (through a process called interferometry) meant that the experiment could achieve the necessary thousandfold improvement in sensitivity over earlier interferometers necessary to resolve the resulting relativistic effects.

“This project was seen by some as technically impossible,” wrote one nominator for this year’s Gruber Cosmology Prize. Advocates for the project, however, argued that even if the experiment didn’t reach its goals, any technological advances would have broader benefits. In particular, Tim de Zeeuw and Andreas Kaufer at the European Southern Observatory supported the bold project and the team around Julien Woillez to upgrade the interferometer. Eisenhauer’s designs did indeed wind up revolutionizing several kinds of instrumentation, including imaging detectors, laser metrology, and dual-beam operations.

The GRAVITY collaboration consisting of the Max Planck Institute for Extraterrestrial Physics, LESIA at the Paris Observatory, IPAG at the University Grenoble, the Max Planck Institute for Astronomy, the University of Cologne, the Center for Astrophysics and Gravitation in Portugal, and the European Southern Observatory completed the instrument with barely a year to spare, and the initial results, as one nominator wrote, “can only be described as astounding and ground-breaking for many fields of astrophysics.”

Those results include: precise measurements of Sgr A*’s general relativistic influence on S2; as well as observations of gas orbiting close to the “last stable orbit”—the point before which it succumbs to the gravitational tug of Sgr A* and disappears from sight forever. Together this data provides enough evidence to satisfy the astronomical community that Sgr A* is indeed a black hole.

Orbital motion of gas around the black hole

This visualization uses data from simulations of orbital motions of gas swirling around at about 30% of the speed of light on a circular orbit around the supermassive black hole Sagittarius A*.

Among GRAVITY’s other significant contributions to astronomy: A determination of the distance between the Sun and the Galactic center at a level of precision ten times greater than previous measurements (a calibration that other astronomers will use as a reliable first step in tracing the evolution of the universe on the largest scales). A test of Einstein’s general relativity using supermassive black holes at the highest level of precision to date.

As GRAVITY’s advocates hoped, Eisenhauer’s innovations in technology—the ones for which he is receiving the 2022 Gruber Prize in Cosmology—have changed astronomy beyond just the study of Sgr A*. Other astrophysicists have already begun using SINFONI and GRAVITY instrumentation to study distant star-forming galaxies, black holes at the centers of nearby galaxies, and planets orbiting stars within our own Galaxy.

GRAVITY is currently being upgraded with new adaptive optics, laser guide stars, and wide field capabilities. Called GRAVITY+, this project will soon boost optical interferometry to the next level, then also opening up the extra-galactic sky for highest resolution observations, and providing sharper and sharper images for the observation of exoplanets.

The development of SINFONI, GRAVITY and GRAVITY+ were and are made possible by the generous support from the Max Planck Society and the Max Planck Foundation – an independent, non-profit organization for private supporters of top research in the Max Planck Society.

Eisenhauer will receive the $500,000 award as well as a gold laureate pin at a ceremony that will take place on August 2 at the XXXIst General Assembly of the International Astronomical Union in Busan, Korea. Previous honors for Eisenhauer include the Tycho Brahe Medal of the European Astronomical Society for his leadership of the SINFONI and GRAVITY instruments, the Stern-Gerlach Medal of the German Physical Society for his pioneering work in high-resolution infrared astronomy, and the Jackson-Gwilt Medal of the Royal Astronomical Society for the development of astronomical instrumentation. He was elected to the French Academy of Sciences.

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