Theory-agnostic tests of gravity with black hole shadows

TL;DR

This paper investigates the robustness of theory-agnostic tests of gravity using black hole shadow observations, highlighting the importance of handling parameter degeneracies for reliable constraints.

Contribution

It introduces a generic metric framework to assess the robustness of parameter estimation from black hole shadows, considering current and future measurements.

Findings

Parameter degeneracies can mislead constraints if not properly managed.

Certain deviations from general relativity are more detectable with shadow measurements.

Future observations can robustly recover specific deviations.

Abstract

Observations of black hole shadows with the Event Horizon Telescope have paved way for a novel approach to testing Einstein's theory of general relativity. Early analyses of the measured shadow put constraints on theory-agnostic parameters typically used to study deviations from Einstein's theory, but the robustness of these constraints was called into question. In this letter, we use a generic theory-agnostic metric to study the robustness of parameter estimation with BH shadows, taking into consideration current measurements made with the Event Horizon Telescope and future measurements expected with the Event Horizon Imager. We find that the robustness issue is highly nuanced, and parameter constraints can be highly misleading if parameter degeneracy is not handled carefully. We find that a certain kind of deviation is particularly well suited for the shadow based analysis, and can be recovered robustly with shadow measurements in the future.