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The next generation of gravitational-wave observatories will achieve unprecedented strain sensitivities with an expanded observing band. They will detect $\mathcal{O}\left({10}^5\right)$binary neutron star (BNS) mergers every year, the loudest of which will be in the band for $\approx 90$minutes with signal-to-noise ratios $\approx 1500$. We show that subtleties arising from the rotation of the Earth and the free-spectral range of gravitational-wave interferometers dramatically increases the complexity of next-gen BNS signals compared to the one-minute signals seen by LIGO-Virgo. Reduced-order quadrature, a compression method currently relied upon to speed up the most expensive BNS calculations, may no longer be effective in determining the astrophysical parameters of next-gen BNS signals. We carry out reduced-order inference on a simulated next-gen BNS signal taking into account the Earth’s rotation and the observatories’ free-spectral range. We show that reduced-order modeling becomes impractical, and the full problem becomes computationally infeasible, when we include data below $\approx 16\mathrm{Hz}$—a part of the observing band that is critical for precise sky localization. We discuss potential paths toward solving this complex problem. 

Observed evolution of the total mass distribution with redshift is crucial to testing galaxy evolution theories. To measure the total mass distribution, strong gravitational lenses complement the resolved dynamical observations that are currently limited to z ≲ 0.5. Here we present the lens models for a pilot sample of seven galaxy-scale lenses from the ASTRO3D Galaxy Evolution with Lenses (AGEL) survey. The AGEL lenses, modeled using HST/WFC3-F140W images with Gravitational Lens Efficient Explorer (GLEE) software, have deflector redshifts in the range 0.3 < z defl < 0.9. Assuming a power-law density profile with slope γ, we measure the total density profile for the deflector galaxies via lens modeling. We also measure the stellar velocity dispersions (σ obs) for four lenses and obtain σ obs from SDSS-BOSS for the remaining lenses to test our lens models by comparing observed and model-predicted velocity dispersions. For the seven AGEL lenses, we measure an average density profile slope of ‑1.95 ± 0.09 and a γ–z relation that does not evolve with redshift at z < 1. Although our result is consistent with some observations and simulations, it differs from other studies at z < 1 that suggest the γ–z relation evolves with redshift. The apparent conflicts among observations and simulations may be due to a combination of (1) systematics in the lensing and dynamical modeling; (2) challenges in comparing observations with simulations; and (3) assuming a simple power law for the total mass distribution. By providing more lenses at z defl > 0.5, the AGEL survey will provide stronger constraints on whether the mass profiles evolve with redshift as predicted by current theoretical models.