Black-hole Movies Reveal Extreme-Lensing Signatures Via Two-Point Image Correlation of Brightness Fluctuations

Black holes warp space and time so dramatically that they bend light, creating multiple images of objects near their event horizons, and a team led by Barbora Bezděková and Shahar Hadar from the University of Haifa, alongside George Wong from the Institute for Advanced Study and Princeton University, and Maciek Wielgus from the Instituto de Astrofísica de Andalucía, now demonstrates how to detect these subtle distortions. The researchers investigate the correlations within simulated black-hole ‘movies’ to reveal the telltale signs of extreme gravitational lensing, a phenomenon where light bends and magnifies objects behind the black hole. Their work shows that while these lensing signatures remain hidden in standard black-hole images and light curves, they become clearly visible when analysing the correlations between different points within the simulated images, offering a promising new technique for future black-hole imaging campaigns. This achievement paves the way for a deeper understanding of gravity in its most extreme form and motivates further investigation into the instruments and analytical methods needed to resolve these faint, yet crucial, signals.

Researchers investigated simulated black hole images to reveal telltale signs of extreme gravitational lensing, a phenomenon where light bends and magnifies objects behind the black hole. Their work shows that these lensing signatures, while hidden in standard black hole images, become clearly visible when analysing correlations between different points within the simulated images, offering a promising new technique for future black hole imaging campaigns. This achievement paves the way for a deeper understanding of gravity in its most extreme form and motivates further investigation into the instruments and analytical methods needed to resolve these faint signals.

Event Horizon Resolution Limits and Feasibility

The Event Horizon Telescope is pushing the boundaries of observational astronomy to achieve resolution comparable to the size of a black hole’s event horizon, allowing scientists to resolve features very close to it. Achieving this requires high sensitivity to detect the faint signals emitted from this region, a feat accomplished through Very Long Baseline Interferometry, which combines signals from telescopes around the world. Long observing times are also crucial, as the EHT requires multiple observing runs over several years to build up sufficient data for imaging. While the EHT has already successfully imaged the supermassive black holes in M87 and Sgr A*, improvements in sensitivity, resolution, and observing time are still needed to probe the physics near the event horizon in more detail, with next-generation VLBI representing a future step forward.

Simulating Black Hole Lensing with Ray Tracing

Scientists developed a novel methodology to probe extreme gravitational lensing around black holes by analysing time-dependent images, or “movies”, generated through sophisticated computational modelling. The study began with a general relativistic magnetohydrodynamic simulation, a complex numerical representation of the plasma surrounding a black hole, to create a realistic baseline for image generation. Researchers then employed ray tracing, a technique that simulates the paths of light as it interacts with the curved spacetime around the black hole, to produce a movie depicting the black hole’s appearance as viewed by a distant observer. This movie was then blurred to mimic the expected angular resolution achievable by current and next-generation terrestrial very-long-baseline interferometric arrays, simulating realistic observational conditions. The core innovation lies in applying the two-point correlation function to analyse fluctuations in brightness across the black hole movie, revealing subtle correlations induced by the time delays between direct and indirectly lensed photons. Researchers demonstrated that this approach exploits the temporal evolution of the black hole’s appearance to extract information about the surrounding spacetime geometry, offering a complementary technique to spatially resolving the photon ring with future space-based interferometers.

Brightness Fluctuations Reveal Black Hole Lensing

Scientists have demonstrated a novel method for probing the extreme gravitational lensing effects around black holes, achieving a breakthrough in detecting subtle signatures previously obscured by observational limitations. The work centres on analysing fluctuations in brightness within simulated black hole “movies”, generated by tracing the path of light around a rotating black hole with a mass of 6. 5 billion times that of the Sun. Researchers utilized a high-resolution general relativistic magnetohydrodynamic simulation to model the accretion flow around the black hole, accurately capturing the complex interplay of gravity and electromagnetism.

The team focused on the two-point correlation function of brightness fluctuations, measuring how brightness variations at different positions and times within the movie are related. This revealed that even with the expected resolution of next-generation terrestrial very-long-baseline interferometric arrays, such as the next-generation Event Horizon Telescope, the signatures of extreme lensing are clearly visible in this fine-grained correlation function. To isolate the lensing signatures, scientists compared the results to simulations where light was assumed to travel at infinite speed, and to simulations excluding photons that had been bent around the black hole, confirming that the observed correlations originate from the multi-path propagation of light due to the black hole’s intense gravity.

Lensing Signatures Detected In Brightness Fluctuations

This research demonstrates that subtle signatures of extreme gravitational lensing around black holes can be detected through detailed analysis of brightness fluctuations in simulated images. Scientists developed a method focusing on the two-point image correlation function, which examines how brightness variations relate across different points in an image, and across time delays. This approach proves superior to traditional methods that average brightness over larger areas, as those methods can obscure the very signals researchers seek. The team successfully identified lensing signatures in simulations mirroring observations expected from next-generation very-long-baseline interferometric arrays, even when those simulations incorporated realistic limitations in image resolution. This suggests that upcoming black hole imaging campaigns may be able to probe the extreme gravitational environment near black holes by analysing correlations in brightness fluctuations, rather than relying solely on direct imaging. Future work will focus on improving the realism of the simulations and investigating how the number of observational baselines affects the detectability of these correlations.