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

Gravitational waves and 21-cm intensity mapping are emerging as complementary cosmological probes: GWs provide direct luminosity-distance measurements, while neutral-hydrogen IM traces the matter distribution in redshift space. The new analysis still awaits peer review, but it already lays out the central claim clearly.

The significance lies in cosmology operates at the edge of what current instruments can measure, where systematic errors and model assumptions are never trivial. Small discrepancies between independent measurements have historically pointed toward missing physics rather than simple calibration errors, and the ongoing tension in the Hubble constant is a live example of how a persistent disagreement between methods can reshape the theoretical landscape. Each new dataset that approaches this territory with independent systematics adds real information to a problem that has resisted easy resolution for more than a decade. Gravitational waves (GWs) and 21-cm intensity mapping (IM) are emerging as complementary cosmological probes: GWs provide direct luminosity-distance measurements, while. 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. 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%.

The relevance goes beyond one dataset because even small shifts in measured parameters can matter when the field is testing the limits of the standard cosmological model. The Lambda-CDM framework describes the observable universe with remarkable economy, but its success rests on two components, dark matter and dark energy, whose physical nature remains entirely unknown. Any credible measurement that tightens or loosens the constraints on those components moves the entire theoretical enterprise forward, regardless of whether the immediate result looks dramatic on its own terms.

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.

Because this is still a preprint, the result should be read with genuine interest and proportionate caution. Peer review is not a guarantee of correctness, but it is a process that forces authors to respond to technical criticism from specialists who have no stake in a particular outcome. Preprints that survive that process, often with substantive revisions, emerge with a stronger evidential base than the version that first appeared. Until that stage is complete, the responsible reading keeps uncertainty explicitly visible rather than treating the claims as established findings.

The next step is to see whether the effect survives when independent surveys, different calibration strategies and tighter control of systematic uncertainties enter the picture. Programmes such as Euclid, DESI and the Rubin Observatory will deliver datasets over the next several years that cover the same parameter space with largely independent methods. If the current signal persists through those tests, its theoretical implications will become impossible to set aside. Until peer review and independent follow-up address those open questions, skepticism is not a failure of appreciation for the work; it is part of how science decides what to keep.

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