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Images of black holes are more than just fascinating visuals: they could serve as a “testing ground” for alternative theories of gravity in the future. An international team led by Prof. Luciano Rezzolla has developed a new method to examine whether black holes operate according to Einstein’s theory of relativity or other, more exotic theories. To that end, the researchers conducted highly complex simulations and derived measurable criteria that can be tested with future, even sharper telescopes. Over the next few years, this method could reveal whether Einstein’s theories hold true even in the most extreme regions of the universe.
Frankfurt. Black holes are considered cosmic gluttons, from which not even light can escape. That is also why the images of black holes at the center of the galaxy M87 and our Milky Way, published a few years ago by the Event Horizon Telescope (EHT) collaboration, broke new ground. “What you see on these images is not the black hole itself, but rather the hot matter in its immediate vicinity,” explains Prof. Luciano Rezzolla, who, along with his team at Goethe University Frankfurt, played a key role in the findings. “As long as the matter is still rotating outside the event horizon – before being inevitably pulled in – it can emit final signals of light that we can, in principle, detect.”
The images essentially show the shadow of the black hole. This finding now opens up the opportunity to closely examine the theories behind these extreme cosmic objects. So far, Einstein’s general theory of relativity is considered the gold standard in physics when it comes to the description of space and time. It predicts the existence of black holes as special solutions, along with all their peculiarities. This includes the event horizon, beyond which everything – including light – disappears. “There are, however, also other, still hypothetical theories that likewise predict the existence of black holes. Some of these approaches require the presence of matter with very specific properties or even the violation of the physical laws we currently know,” Rezzolla says.
Together with colleagues from Tsung-Dao Lee Institute Shanghai (China), the Frankfurt-based physicist introduced a new possibility to check such alternative theories in the journal “Nature Astronomy”. Until now, there has been no solid data to enable either the refutation or confirmation of these theories – something the researchers plan to change in the future by using shadow images of supermassive black holes.
“This requires two things,” Rezzolla explains. “On the one hand, high-resolution shadow images of black holes to determine their radius as accurately as possible, and on the other hand, a theoretical description of how strongly the various approaches deviate from Einstein’s theory of relativity.” The scientists have now presented a comprehensive description of how different types of hypothetical black holes diverge from relativity theory and how this is reflected in the shadow images. To investigate this, the team conducted highly complex three-dimensional computer simulations that replicate the behavior of matter and magnetic fields in the curved spacetime surrounding black holes. From these simulations, the researchers then generated synthetic images of the glowing plasma.
“The central question was: How significantly do images of black holes differ across various theories?” explains lead author Akhil Uniyal of the Tsung-Dao Lee Institute. From this, they were able to derive clear criteria that, with future high-resolution measurements, could often allow a decision to be made in favor of a specific theory. While the differences in images are still too small with the current resolution of the EHT, they systematically increase with improved resolution. To address this, the physicists developed a universal characterization of black holes that integrates very different theoretical approaches.
“One of the EHT collaboration’s most important contributions to astrophysics is turning black holes into testable objects,” Rezzolla emphasizes. “Our expectation is that relativity theory will continue to prove itself, just as it has time and again up to now.” So far, the results align with Einstein’s theory. However, the measurement uncertainty is still so high that only a few very exotic possibilities have been ruled out. For instance, the two black holes at the center of M87 and our Milky Way are unlikely to be so-called naked singularities (without an event horizon) or wormholes – just two of the many other theoretical possibilities that need to be checked. “Even the established theory must be continuously tested, especially with extreme objects like black holes,” the physicist adds. It would be groundbreaking if Einstein’s theory were ever proven invalid.
The EHT offers outstanding opportunities for such measurements. This collaboration of several large radio telescopes across the globe achieves a resolution equivalent to a telescope the size of Earth, for the first time enabling a sharp view into the immediate surroundings of black holes. In the future, additional telescopes on Earth are planned to be integrated into the EHT. Scientists are also hoping for a radio telescope in space, which would significantly improve the overall resolution. With such a high-resolution view, it would be possible to subject various theories about black holes to a rigorous test. As the newly presented study shows, this requires angular resolutions of less than one millionth of an arcsecond – comparable to viewing a coin on the Moon from Earth. While this exceeds today’s capabilities, it is expected to be achievable in a few years.
Prof. Dr. Luciano Rezzolla
Institute for Theoretical Physics
Goethe University Frankfurt
Tel: +49 (69) 798-47871
rezzolla@itp.uni-frankfurt.de
https://astro.uni-frankfurt.de/rezzolla/
Akhil Uniyal, Indu K. Dihingia, Yosuke Mizuno, Luciano Rezzolla: The future ability to test theories of gravity with black-hole shadows, Nature Astronomy (2025). https://doi.org/10.1038/s41550-025-02695-4
At the current resolution of telescopes, black holes predicted by different theories of gravity stil ...
Copyright: L. Rezzolla/Goethe University
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