Testing Dynamical Dark-Energy Model with Standard Sirens

Bright standard sirens from the Einstein Telescope (ET) provide an independent probe of cosmic expansion. In this article, we test whether future ET observations can constrain a dynamical dark-energy scenario inspired by DESI DR2. Using the best-fitting 29-bin dark-energy equation of state as the fiducial cosmology, we generate mock catalogs of 100, 500, 1000, and 3000 binary-neutron-star standard sirens with known host redshifts, and reconstruct the equation of state using both GW-only data and combined analyses with DESI DR2 BAO, BBN, and a compressed CMB constraint. The effective numbers of constrained modes are $N_{\mathrm{e}\mathrm{f}\mathrm{f}}\sim 4-5$ (GW-only) and $5-6$ (combined), with information concentrated in a few correlated modes. The GW3000 catalog yields a departure from $\mathrm{\Lambda }$CDM exceeding $5\sigma$, and combined analyses reach similar significance for $O({10}^3)$ events. Comparing flat $\mathrm{\Lambda }$CDM with its Chevallier-Polarski-Linder extension using GW data alone gives significances of $3.3\sigma$ and $6.3\sigma$ for 1000 and 3000 events, rising to $7.7\sigma$ and $11.0\sigma$ with external data. These results indicate that a few thousand ET bright standard sirens could provide a sensitive test of departures from $\mathrm{\Lambda }$CDM.