A Quiet Night in the Millihertz Sky? LISA Forecasts from LIGO/Virgo/KAGRA Populations

The high-mass black hole population inferred from ground-based observations can substantially affect the number of binaries detectable by the Laser Interferometer Space Antenna (LISA). We use population constraints based on data from the latest LIGO/Virgo/KAGRA Gravitational-Wave Transient Catalog (GWTC-5.0), as well as constraints from the O3 search for intermediate-mass black hole binaries, to forecast the number and masses of binary black holes detectable by LISA. Using the released GWTC-5.0 population posterior, we obtain $6.6^{+5.5}_{-2.2}$ expected detections with signal-to-noise ratio $>8$ in 4 years (median and $90\%$ credible interval). The alternative population models we consider use a broken power law with two or three Gaussian peaks for either the primary-mass distribution or a common mass distribution for both black holes. For the three-peak model with the common mass distribution, extrapolating from $300$ to $1000M_\odot$ roughly doubles the median expected number of LISA detections to $\sim14$. The upper end of its $90\%$ credible interval rises from tens to thousands of expected detections, although approximations may limit the reliability of the most extreme counts. For the reference GWTC-5.0 population, $39^{+22}_{-11}\%$ of the binaries detectable by LISA merge during the mission or within $10$ years after its end and are potential multiband sources. Essentially all of these mergers would be detectable by a reference Cosmic Explorer configuration if it is operating at merger. Ground-based measurements of the high-mass population can therefore inform both LISA forecasts and the prospects for observing the same binaries through inspiral and merger.