A new cosmological probe using super-massive black hole shadows
  We study the prospects of using the low-redshift and high-redshift black hole
shadows as new cosmological standard rulers for measuring cosmological
parameters. We show that, using the low-redshift observation of the black hole
shadow of M87$^\star$, the Hubble constant can be independently determined with
a precision of about $13\%$ as $H_0=70\pm 9$ km s$^{-1}$ Mpc$^{-1}$. The
high-redshift observations of super-massive black hole shadows may accurately
determine a combination of parameters $H_0$ and ${\Omega_{\rm m}}$, and we show
by a simple simulation that combining them with the type Ia supernovae
observations would give precise measurements of the cosmological parameters.
