Scientists suggest that advanced telescopes could detect subtle differences in black hole images to evaluate alternative gravity theories. The Event Horizon Telescope's successors might reveal variations from general relativity in extreme gravitational environments. However, detecting these differences will require extensive data collection over years.
The Event Horizon Telescope has recently provided the first images of black hole surroundings, enhancing resolution in regions dominated by extreme gravity. This progress prompts questions about the nature of gravity, given general relativity's incompatibilities with quantum mechanics and its failure to explain dark matter. Researchers propose that black hole environments could amplify subtle differences in alternative gravity theories, potentially allowing next-generation telescopes to rule out some models.
A team of four physicists from Shanghai and CERN revisited their pre-Event Horizon Telescope analysis to assess future capabilities. They focused on rotating black holes, where frame dragging bends light paths. As the researchers note, “General relativity predicts that the image of such a region will consist of a series of nested ring-like images where each ring is distinguished by the number of half-orbits [of the black hole] that photons make before reaching the observer.” Measuring photon rings could probe strong, stationary curvature.
Using the parametric Konoplya–Rezzolla–Zhidenko metric, the team varied two parameters from zero to one, creating four gravity variants alongside the standard Kerr metric from general relativity. Hydrodynamic simulations modeled the three-dimensional environment, including infalling matter, magnetic fields, and jets. The resulting images show asymmetric bright rings, with one side brighter due to rotation. Differences emerged: one variant produced the smallest but brightest ring, another reduced contrast between sides, and jet widths varied.
“Differences are obviously present in the images,” the researchers conclude, “but these are also rather small even when considering the most extreme deviations from [general relativity].” Black hole variability from accretion disk changes will complicate detection, necessitating multi-year observations. Additional data like polarization or spectral maps may be essential to discriminate theories. A coordinated campaign with next-generation Event Horizon Telescope upgrades and space-based instruments could advance gravitational research.
The study appears in Nature Astronomy (2025). DOI: 10.1038/s41550-025-02695-4.