Black hole shadow in the view of freely falling observers

TL;DR

This paper investigates how the shadow of a black hole appears to freely falling observers, revealing that they see a finite-sized shadow at the inner horizon and that the shadow's angular radius can increase with distance in certain black hole spacetimes.

Contribution

It provides a novel analysis of black hole shadows from the perspective of freely falling observers within general relativity, including new relations and examples.

Findings

Freely falling observers see a finite black hole shadow at the inner horizon.

In Schwarzschild-de Sitter spacetime, the shadow's angular radius can increase with distance.

The paper introduces aberration and redshift relations for such observations.

Abstract

First sketch of black hole from M87 galaxy was obtained by Event Horizon Telescope, recently. As appearance of black hole shadow reflects space-time geometry of black holes, observations of black hole shadow may be a promising way to test general relativity in strong field regime. In this paper, we focus on angular radius of spherical black hole shadow with respect to freely falling observers. In the framework of general relativity, aberration formulation and angular radius-gravitational redshift relation are presented. For the sake of intuitive, we consider parametrized Schwarzschild black hole and Schwarzschild-de Sitter black hole as representative examples. We find that the freely in-falling observers would observe finite size of shadow, when they go through inner horizon. For observers freely falling from the outer horizon of Schwarzschild-de Sitter black hole, we find that the angular radius of the shadows could increase even when the observers move farther from the black hole.