Shadow of black holes at local and cosmological distances

TL;DR

This paper explores the shadows of black holes at both local and cosmological distances, analyzing how cosmic expansion affects their observed size and providing analytical and approximate methods for calculating shadow angular sizes.

Contribution

It introduces an exact analytical solution for black hole shadow size in de Sitter universe and an approximate method applicable to multicomponent universes.

Findings

Shadow size affected by cosmic expansion at large distances

Supermassive black holes at cosmological distances can have shadows comparable to nearby ones

Analytical and approximate methods for shadow size calculation are developed

Abstract

A brief illustrative discussion of the shadows of black holes at local and cosmological distances is presented. Starting from definition of the term and discussion of recent observations, we then investigate shadows at large, cosmological distances. On a cosmological scale, the size of shadow observed by comoving observer is expected to be affected by cosmic expansion. Exact analytical solution for the shadow angular size of Schwarzschild black hole in de Sitter universe was found. Additionally, an approximate method was presented, based on using angular size redshift relation. This approach is appropriate for general case of any multicomponent universe (with matter, radiation and dark energy). It was shown, that supermassive black holes at cosmological distances in universe with matter may give the shadow size comparable with the shadow size in M87, and in the center of our Galaxy.