  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.
