Black hole shadow in an expanding universe with a cosmological constant

TL;DR

This paper analytically examines how a cosmological constant-driven expansion affects the observable shadow of a Schwarzschild black hole, revealing that the shadow's angular size approaches a finite limit at infinity.

Contribution

It provides an analytical calculation of the black hole shadow in a deSitter universe, accounting for cosmic expansion effects on an observer's measurements.

Findings

The shadow's angular radius shrinks to a finite value at infinity.

Cosmic expansion influences the apparent size of black hole shadows.

The analysis uses the Kottler spacetime model for embedded black holes.

Abstract

We analytically investigate the influence of a cosmic expansion on the shadow of the Schwarzschild black hole. We suppose that the expansion is driven by a cosmological constant only and use the Kottler (or Schwarzschild-deSitter) spacetime as a model for a Schwarzschild black hole embedded in a deSitter universe. We calculate the angular radius of the shadow for an observer who is comoving with the cosmic expansion. It is found that the angular radius of the shadow shrinks to a non-zero finite value if the comoving observer approaches infinity.