Testing the equivalence principle via the shadow of black holes

TL;DR

This paper investigates potential violations of the equivalence principle by analyzing black hole shadow deformations caused by hypothetical vector fields, proposing observational tests through multi-band imaging and existing data constraints.

Contribution

It introduces a phenomenological model linking equivalence principle violations to black hole shadow deformations via vector field couplings, and derives observational constraints.

Findings

Constraints on vector field coupling parameters from EHT data

Predicted wavelength-dependent shadow deformations

Potential for future multi-band observational tests

Abstract

We study the equivalence principle, regarded as the cornerstone of general relativity, by analyzing the deformation observable of black hole shadows. Such deformation can arise from new physics and may be expressed as a phenomenological violation of the equivalence principle. Specifically, we assume that there is an additional background vector field that couples to the photons around the supermassive black hole. This type of coupling yields impact on the way the system depends on initial conditions, and affects the black hole shadow at different wavelengths by a different amount, and therefore observations of the shadow in different wavelengths could constrain such couplings. This can be tested by future multi-band observations. Adopting a specific form of the vector field, we obtain constraints on model parameters from Event Horizon Telescope observations and measurements of gas/stellar orbits.