EHT tests of the strong-field regime of General Relativity

TL;DR

This paper demonstrates how Event Horizon Telescope shadow measurements of M87* can robustly constrain the black hole's strong-field geometry, highlighting the limitations of weak-field post-Newtonian constraints and exploring their relation to gravitational quasi-normal modes.

Contribution

It clarifies the robustness of shadow size constraints on black hole geometry and discusses the subtleties in deriving post-Newtonian limits from shadow measurements.

Findings

Shadow size provides robust constraints on the black hole's near-horizon geometry.

Post-Newtonian constraints from shadow data require careful interpretation and priors.

Shadow measurements and gravitational quasi-normal modes offer complementary insights.

Abstract

Following up on a recent analysis by Psaltis et al. [Phys. Rev. Lett. 125, 141104 (2020)], we show that the observed shadow size of M87$^*$ can be used to unambiguously and robustly constrain the black hole geometry in the vicinity of the circular photon orbit. Constraints on the post-Newtonian weak-field expansion of the black hole's metric are instead more subtle to obtain and interpret, as they rely on combining the shadow-size measurement with suitable theoretical priors. We provide examples showing that post-Newtonian constraints resulting from shadow-size measurements should be handled with extreme care. We also discuss the similarities and complementarity between the EHT shadow measurements and black-hole gravitational quasi-normal modes.