Cosmological Inference with Gravitational-Wave Dark Sirens and 21-cm Intensity Mapping: Forecasts for LIGO-Virgo-KAGRA with MeerKAT and SKAO

Abstract

Gravitational waves (GWs) and 21-cm intensity mapping (IM) are emerging as complementary cosmological probes: GWs provide direct luminosity-distance measurements, while neutral-hydrogen (HI) IM traces the matter distribution in redshift space. Their synergy offers a promising avenue for precise measurements of the Hubble constant. Within a standard-siren framework, we perform a joint cosmological and population analysis using $\mathcal{O}(150)$ mock GW detections with SNR > 12, as projected for one year of observations at LVK O5 sensitivity. We combine these with mock HI intensity maps from MeerKLASS and SKA-Mid, and study how observational limitations in both the GW and IM data affect the $H_0$ constraints. We find that uncertainty in GW parameter measurements is a dominant limitation on the cosmological constraints. Neglecting this uncertainty, the inclusion of HI information enables sub-percent constraints on $H_0$. We also find that observational effects in the HI surveys weaken the constraints, although they remain tighter than those obtained without HI information (spectral sirens). Despite accounting for HI IM observational limitations, including HI information reduces the $H_0$ uncertainty relative to the spectral-siren results by 61% for MeerKLASS and 53% for SKA-Mid. Because our results are based on a single realisation, they should be interpreted in light of possible realisation-to-realisation variability. Beam convolution most severely weakens the constraints. Instrumental noise can induce multimodality in the posterior, potentially biasing the inferred parameters. The results are sensitive to foreground under-cleaning and over-cleaning. Partial sky coverage has a relatively small impact. In the most realistic scenario, which accounts for both GW and HI IM limitations, we obtain $H_0$ precision estimates of 10% and 15% for MeerKLASS and SKA-Mid, respectively.