The advent of third-generation (3G) gravitational-wave (GW) detectors opens new opportunities for multi-messenger observations of binary neutron star merger events, holding significant potential for probing the history of cosmic expansion. In this paper, we investigate the holographic dark energy (HDE) model by using the future GW standard siren data observed from the 3G GW detectors and the short $\gamma$-ray burst THESEUS-like detector joint observations. We find that GW data alone can achieve a relatively precise estimation of the Hubble constant, with precision of $0.2$–$0.6\%$, but its ability to constrain other cosmological parameters remains limited. Nonetheless, since the GW data can break parameter degeneracies generated by the mainstream EM observations, CMB + BAO + SN (CBS), GW standard sirens play a crucial role in enhancing the accuracy of parameter estimation. With the addition of GW data to CBS, the constraints on cosmological parameters $H_0$, c and ${\Omega }_{\mathrm{m}}$ can be improved by 63–$88\%$, 27–$44\%$ and 55–$70\%$. In summary, observations of GW standard sirens from 3G GW detectors could be pivotal in probing the fundamental nature of dark energy.

We present a multiwavelength study of the quasar SDSS J141945.50+524648.0 ( z = 1.1599), a member of the newly identified population of quasar-associated 2175 Å dust absorbers. Utilizing high-quality JWST/NIRCam observations from the SAPPHIRES survey and archival data, we analyzed the prominent 2175 Å extinction bump detected in this object. The extinction bump parameters are highly consistent with those of the Milky Way extinction curves, implying similar dust properties. Spectral analysis reveals a Mg II absorption doublet near the systemic velocity. A joint fit to the Mg II and Fe II absorption lines favors a partial-covering model, with the absorber fully covering the compact continuum source while only partially covering the broad-line emission region, yielding a covering factor of C _{f ,BEL} = 0.11 _−0.07 ^+0.12 . Together with the small velocity offset (Δ v ≈ 96 km s ⁻¹ ), these results favor an intrinsic absorber rather than a simple intervening system. Modeling of the infrared spectral energy distribution indicates a dusty torus viewing angle of θ ≈ 60 ₋₈ ⁺⁷ deg, and the double-peaked Pa α emission profile suggests an accretion-disk inclination of i = 63.7 _−7.8 ^+10.9 deg. The agreement between these independent geometric constraints suggests a “rim-penetrating” line of sight through the dusty torus. This configuration provides a self-consistent explanation for the infrared emission, continuum extinction, and associated 2175 Å extinction bump, although a contribution from the host-galaxy interstellar medium cannot be ruled out. If the absorber is indeed associated with the torus, the carbonaceous carriers of the 2175 Å feature may survive the intense active galactic nucleus radiation field through localized shielding in optically thick dusty clumps. These findings suggest that the detection of such extinction features is closely tied to the viewing geometry and dust distribution; if so, this would offer new insights into dust survival and destruction mechanisms in quasar environments.