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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi mathvariant="script">O</a:mi> <a:mo stretchy="false">(</a:mo> <a:msup> <a:mn>10</a:mn> <a:mn>5</a:mn> </a:msup> <a:mo stretchy="false">)</a:mo> </a:math> binary neutron star (BNS) mergers every year, the loudest of which will be in the band for <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mo>≈</f:mo> <f:mn>90</f:mn> </f:math> minutes with signal-to-noise ratios <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mo>≈</h:mo> <h:mn>1500</h:mn> </h:math> . 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 <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mo>≈</j:mo> <j:mn>16</j:mn> <j:mtext> </j:mtext> <j:mtext> </j:mtext> <j:mi>Hz</j:mi> </j:math> —a part of the observing band that is critical for precise sky localization. We discuss potential paths toward solving this complex problem.