# Black hole shadow as a standard ruler in cosmology

## TL;DR

This paper proposes using supermassive black hole shadows as a standard ruler in cosmology, enabling independent measurements of the Hubble constant and cosmic expansion history through future observations.

## Contribution

It introduces a novel method to utilize black hole shadows as a cosmological tool, linking astrophysical observations with fundamental cosmological parameters.

## Key findings

- Shadow size can constrain the Hubble constant at low redshift.
- High-redshift black hole shadows can probe cosmic expansion history.
- Shadow measurements can estimate black hole masses assuming known cosmology.

## Abstract

Advancements in the black hole shadow observations may allow us not only to investigate physics in the strong gravity regime, but also to use them in cosmological studies. In this paper, we propose to use the shadow of supermassive black holes as a standard ruler for cosmological applications assuming the black hole mass can be determined independently. First, observations at low redshift distances can be used to constrain the Hubble constant independently. Secondly, the angular size of shadows of high redshift black holes is increased due to cosmic expansion and may also be reachable with future observations. This would allow us to probe the cosmic expansion history for the redshift range elusive to other distance measurements. Additionally, shadow can be used to estimate the mass of black holes at high redshift, assuming that cosmology is known.