Forecasting Dark-Energy Anisotropic Stress using gravitational-waves.

We confront the dark energy anisotropic stress using the usual cosmological probes namely cosmic microwave background radiation, baryon acoustic oscillations, latest pantheon sample of supernovae type Ia and then make use of the simulated gravitational waves standard sirens (GWSS) data from Einstein Telescope with an aim to examine the constraining power of the simulated GWSS data over the standard cosmological probes. Our analyses show that GWSS can give better constraints on the model parameters compared to the usual cosmological probes, but the viscous sound speed appearing due to the dark energy anisotropic stress, is totally unconstrained even after the inclusion of GWSS.