Measuring the Hubble constant with black sirens
  We investigate a recently proposed method for measuring the Hubble constant
from gravitational wave detections of binary black hole coalescences without
electromagnetic counterparts. In the absence of a direct redshift measurement,
the missing information on the left-hand side of the Hubble-Lema\^itre law is
provided by the statistical knowledge on the redshift distribution of sources.
We assume that source distribution in redshift depends on unknown
hyperparameters, modeling our ignorance of the astrophysical binary black hole
distribution. With tens of thousands of these "black sirens" -- a realistic
figure for the third generation detectors Einstein Telescope and Cosmic
Explorer -- an observational constraint on the value of the Hubble parameter at
percent level can be obtained. This method has the advantage of not relying on
electromagnetic counterparts, which accompany a very small fraction of
gravitational wave detections, nor on often unavailable or incomplete galaxy
catalogs.
