Probing the cosmic opacity from Future Gravitational Wave Standard
  Sirens
  In this work, using the Gaussian Process, we explore the potentiality of
future gravitational wave (GW) measurement to probe cosmic opacity through
comparing its opacity-free luminosity distance (LD) with the opacity-dependent
one from type Ia supernovae (SNIa). GW data points are simulated from the third
generation Einstein Telescope, and SNIa data are taken from the Joint Light
Analysis (JLA) or Pantheon compilation. The advantages of using Gaussian
Process are that one may match SNIa data with GW data at the same redshift and
use all available data to probe cosmic opacity. We obtain that the error bar of
the constraint on cosmic opacity can be reduced to $\sigma_{\epsilon}\sim
0.011$ and $0.006$ at $1\sigma$ confidence level (CL) for JLA and Pantheon
respectively in a cosmological-independent way. Thus, the future GW
measurements can give competitive results on the cosmic opacity test.
Furthermore, we propose a method to probe the spatial homogeneity of the cosmic
transparency through comparing the reconstructed LD from the mock GW with the
reconstructed one from SNIa data in a flat $\Lambda$CDM with the Gaussian
Process. The result shows that a transparent universe is favored at $1\sigma$
CL, although the best-fit value of cosmic opacity is redshift-dependent.
