Gravitational Test Beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole

TL;DR

This paper demonstrates that black-hole shadow measurements from the EHT can test deviations from the Kerr metric beyond the first post-Newtonian order, providing new constraints on strong-field gravity.

Contribution

It analytically and numerically shows that shadow observations can constrain higher-order deviations from Kerr not accessible to weak-field tests.

Findings

Shadow measurements limit second post-Newtonian deviation parameters.

Deviations from Kerr can cause large shadow size differences.

Constraints are complementary to gravitational wave observations.

Abstract

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.