Shadow Implications: What does measuring the photon ring imply for gravity?

TL;DR

Measuring the photon ring of black holes like M87* provides insights into gravity, but the constraints are limited; this paper analyzes what such measurements can and cannot reveal about spacetime.

Contribution

The paper introduces a nonperturbative, nonparametric method to characterize shadow size constraints, clarifying their implications for testing gravity theories.

Findings

Shadow size measurements have limited constraining power on gravity models.

A new nonparametric approach to analyze shadow data is proposed.

Implications for current and future EHT observations are discussed.

Abstract

With the imaging and characterization of the horizon-scale images of M87* and Sgr A* by the Event Horizon Telescope (EHT), it has become possible to resolve the near-horizon region of astrophysical black holes. As a result, there has been considerable interest in the implications of the measurement of the shadow size, i.e., the asymptotic photon ring. We explore the general implications of such a measurement, identifying what is and, more importantly, is not constrained by such measurements, with applications to EHT and future instruments. We consider a general spherically symmetric metric, which effectively applies for a polar observer (appropriate for M87*) in the slow rotation limit. We propose a nonperturbative, nonparametric spacetime-domain characterization of shadow size and related measurements that makes explicit the nature and power (or lack thereof) of shadow-size-based constraints, and facilitates comparisons among observations and targets.