Climbing out of the shadows: building the distance ladder with black
  hole images
  In the era of precision cosmology it has became crucial to find new and
competitive probes to estimate cosmological parameters, in an effort of finding
answers to the current cosmological tensions/discrepancies. In this work, we
show the possibility of using observations of Super Massive Black Hole (SMBH)
shadows as an anchor for the distance ladder, substituting the sources usually
exploited for such purpose, such as Cepheid variable stars. Compared to the
standard approaches, the use of SMBH has the advantage of not needing to be
anchored with distance calibrators outside the Hubble flow since the shadows
physical size can be estimated knowing the mass of the SMBH. Furthermore, SMBH
are supposed to inhabit the center of all galaxies which, in principle, means
that we can measure the size of the shadows in any Supernova type Ia host
galaxy. Under the assumption that the mass of the SMBH can be accurately and
reliably estimated, we find that the Hubble constant can be constrained with a
$\approx10\%$ precision even considering current experimental design of
ground-based interferometers. By constructing a SMBH catalogue based on a
specific choice of the SMBH Mass Function (BHMF), we forecast the constraints
on the Hubble constant, finding that a precision of $\approx4\%$ may be within
reach of future interferometers.
