Lower bound on the radii of black-hole shadows

Shahar Hod

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arXiv · 2603.03421

Lower bound on the radii of black-hole shadows

Shahar Hod

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Abstract

The non-linearly coupled Einstein-matter field equations predict the existence of shadows with well-defined boundaries around black holes. We prove that, in spherically symmetric hairy black-hole spacetimes whose matter fields satisfy the weak energy condition, the radii of these shadows are bounded from below by the dimensionless relation $r_{\text{sh}}/r_{\text{H}}\geq 3\sqrt{3}/2$, where $r_{\text{H}}$ is the horizon radius of the central hairy black hole. The characteristic shadow of the (bald) Schwarzschild black-hole spacetime saturates the analytically derived lower bound.

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Shahar Hod. 2026-03-03. Lower bound on the radii of black-hole shadows. https://arxiv.org/abs/2603.03421

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