  It is always interesting to investigate how well can a future experiment
perform with respect to others (present or future ones). Cosmology is really an
exciting field where a lot of puzzles are still unknown. In this article we
consider a generalized dark energy (DE) scenario where anisotropic stress is
present. We constrain this generalized cosmic scenario with an aim to
investigate how gravitational waves standard sirens (GWSS) may constrain the
anisotropic stress, which according to the standard cosmological probes,
remains unconstrained. In order to do this, we generate the luminosity distance
measurements from $\mathcal{O} (10^3)$ mock GW events which match the expected
sensitivity of the Einstein Telescope. Our analyses report that, first of all,
GWSS can give better constraints on various cosmological 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.
