Copyright Ars Technica
The Event Horizon Telescope only recently gave us the first images of the environment immediately surrounding a black hole. Since then, it has been boosting the resolution and filling in the details of an environment dominated by the most extreme gravity in the Universe. But which gravity are we talking about? Because of its incompatibility with quantum mechanics and our current inability to explain dark matter, people have proposed all sorts of variants of gravity that go beyond general relativity and clean up some of the physics’ awkwardness. It’s possible that the extreme environment near a black hole amplifies the differences among at least some of these hypotheses. So, a group of physicists decided to see whether any of those differences might be large enough that the next generation of telescopes might be able to rule out some potential replacements for relativity. Searching for subtlety Any replacement for general relativity faces an awkward challenge: General relativity does pretty well at explaining everything from the large-scale structure of the Universe to phenomena we can measure right here on Earth. So, any alternative theories would have to differ from relativity in very subtle ways that might be extremely difficult to detect. But the environment around a black hole might be extreme enough to amplify some of these subtle differences. Its gravity is so extreme that a rotating black hole drags bits of space time along with it (a phenomenon called frame dragging), forcing light to travel in a bendy path on its way to Earth. And this is highly dependent on gravity. “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,” the researchers write. “The accurate measurement structure of the photon rings and of their location represents the most compelling route to investigate gravity in the regime of strong but stationary curvature.” Right now, we probably don’t have the ability to detect these small changes in phenomena. However, that may change, as a next-generation version of the Event Horizon Telescope is being considered, along with a space-based telescope that would operate on similar principles. So the team (four researchers based in Shanghai and CERN) decided to repeat an analysis they did shortly before the Event Horizon Telescope went operational, and consider whether the next-gen hardware might be able to pick up features of the environment around the black hole that might discriminate among different theorized versions of gravity. Theorists have been busy, and there are a lot of potential replacements for general relativity out there. So, rather than working their way through the list, they used a model of gravity (the parametric Konoplya–Rezzolla–Zhidenko metric) that allows that isn’t specific to any given hypothesis. Instead, it allows some of its parameters to be changed, thus allowing the team to vary the behavior of gravity within some limits. To get a sense of the sort of differences that might be present, the researchers swapped two different parameters between zero and one, giving them four different options. Those results were compared to the Kerr metric, which is the standard general relativity version of the event horizon. Small but clear differences Using those five versions of gravity, they model the three-dimensional environment near the event horizon using hydrodynamic simulations, including infalling matter, the magnetic fields it produces, and the jets of matter that those magnetic fields power. The results resemble the sorts of images that the Event Horizon Telescope produced. These include a bright ring with substantial asymmetry, where one side is significantly brighter due to the rotation of the black hole. And, while the differences are subtle between all the variations of gravity, they’re there. One extreme version produced the smallest but brightest ring; another had a reduced contrast between the bright and dim side of the ring. There were also differences between the width of the jets produced in these models. That’s the good news. The bad news is that these differences will be hard to detect. “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].” Their overall conclusion is that the next generation of telescopes might make it possible to pick some of these up, but it won’t be easy. Basic variability in the black hole, due to differences in the amount of matter falling into its accretion disk, will cause variations that are larger than the ones seen in these models. So, at a minimum, the researchers say that we’ll need a lot of data gathered over multiple years to be able to control for the impact of this short-term variability. Even then, they think we’re going to need additional data, like polarization or spectral maps, to really start ruling anything out. So, we’re not quite at the point where we can turn event horizons into gravitational research labs. But we’re getting closer, and might get there through a coordinated observation campaign that combines the next generation of event-horizon-style telescopes with additional observations. Nature Astronomy, 2025. DOI: 10.1038/s41550-025-02695-4 (About DOIs).
