Significant challenges for astrophysical inference with next-generation gravitational-wave observatories

The next generation of gravitational-wave observatories will achieve unprecedented strain sensitivities with an expanded observing band. They will detect O(10⁵) binary neutron star (BNS) mergers every year, the loudest of which will be in the band for ≈ 90 minutes with signal-to-noise ratios ≈ 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 ≈ 16 Hz —a part of the observing band that is critical for precise sky localization. We discuss potential paths toward solving this complex problem.