Iconic first image of black hole holds up to scrutiny

A new independent reanalysis of the Event Horizon Telescope's 2017 observations of the supermassive black hole at the center of galaxy M87 has reproduced the distinctive ring-like structure first published in 2019, according to coverage from New Scientist, Phys.org, BigGo Finance, and Nature. The validation comes from a technique called interferometric closure phases, which extracts information about the source structure without relying on the full imaging pipeline that produced the original picture. Researchers applied this method to the same Very Long Baseline Interferometry dataset collected by the global EHT array in April 2017 and found that the closure-phase data alone requires a ring-shaped brightness distribution with a diameter of approximately 42 microarcseconds, consistent with the shadow predicted by general relativity for a 6.5-billion-solar-mass black hole. The result addresses long-standing questions about whether the ring could be an artifact of the sparse uv-coverage, the choice of imaging algorithms, or the regularization parameters used in the original reconstruction.

Coverage emphasizes that this is the first time the ring has been confirmed using an entirely different observable — closure phases — that is immune to the calibration and imaging choices that shaped the 2019 release. Nature reports that the independent analysis was led by a subset of the EHT collaboration using a technique developed specifically for this validation, while Phys.org notes the method bypasses the need for amplitude calibration, which has historically been the largest source of systematic uncertainty in VLBI. New Scientist highlights that the closure-phase signal shows a clear non-zero signature at the spatial frequencies corresponding to the ring diameter, a pattern that cannot be produced by a centrally concentrated source without a central depression. BigGo Finance frames the finding as a vindication of Einstein's general relativity in the strong-field regime, noting that the measured ring size matches theoretical predictions to within the observational uncertainties.

The context matters because the 2019 image became a cultural and scientific touchstone, yet critics pointed out that the EHT's sparse array — effectively a handful of telescopes linked across the globe — left large gaps in the Fourier plane, forcing imaging algorithms to interpolate. Several papers since 2019 have tested the robustness of the ring using different priors, regularization schemes, and even synthetic data challenges, but all relied on the same amplitude and phase data fed into imaging pipelines. The new work uses only closure phases, which are formed by summing phases around triangles of telescopes and are therefore unaffected by station-based gain errors. Nature notes that the analysis also incorporates data from the 2018 observing campaign, which added the Greenland Telescope and the IRAM NOEMA array to the network, improving uv-coverage and providing an independent epoch. The consistency across epochs and observables strengthens the case that the shadow is astrophysical rather than instrumental.

What to watch next includes the upcoming publication of the full closure-phase analysis in a peer-reviewed journal, the application of the same technique to the EHT's observations of Sagittarius A* at the center of the Milky Way, and the integration of data from the next-generation EHT (ngEHT) which will add more antennas, wider bandwidth, and simultaneous multi-frequency observations starting in the late 2020s. Coverage does not specify a timeline for the Sgr A* closure-phase results, but Nature indicates the collaboration is already extending the method to the 2017 and 2018 Sgr A* datasets. The ngEHT upgrades aim to turn snapshot images into movies of accretion dynamics, and the validated closure-phase technique will provide a model-independent baseline for those future studies.