The future ability to test theories of gravity with black-hole shadows

TL;DR

Future high-resolution black-hole imaging projects will significantly enhance our ability to test general relativity by distinguishing Kerr black holes from alternative solutions through detailed image analysis.

Contribution

This study uses advanced simulations to quantify how well future black-hole images can differentiate between Kerr and non-Kerr black-hole solutions.

Findings

Future missions can detect deviations from Kerr solutions if image mismatch exceeds 2-5%.

Percent-level image fidelity is sufficient to test strong-field gravity predictions.

Simulations show a large class of alternative black-hole solutions can be distinguished from Kerr.

Abstract

The horizon-scale images of supermassive black holes (BHs) by the Event Horizon Telescope Collaboration (EHT) have provided new opportunities to test general relativity and other theories of gravity. In view of future projects, such as the next-generation Event Horizon Telescope (ngEHT) and the Black-Hole Explorer (BHEX), having the potential of enhancing our ability to probe extreme gravity, it is natural to ask: \textit{how much can two black-hole images differ?} To address this question and assess the ability of these projects to test theories of gravity with black-hole shadows, we use general-relativistic magnetohydrodynamic and radiative-transfer simulations to investigate the images of a wide class of accreting BHs deviating from the Kerr solution. By measuring the mismatch between images of different BHs we show that future missions will be able to distinguish a large class of BHs solutions from the Kerr solution when the mismatch in the images exceeds values between $2\%$ and $5\%$ depending on the image-comparison metric considered. These results indicate future horizon-scale imaging with percent-level image fidelity can place meaningful observational constraints on deviations from the Kerr metric and thereby test strong-field predictions of general relativity.