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

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "Cosmological Inference with Gravitational-Wave Dark Sirens and 21-cm Intensity Mapping".

Jocelyn: Here's the gist of the paper. The synergy between gravitational waves and 21-cm intensity mapping offers a promising avenue for precise measurements of the Hubble constant, H0, by providing direct luminosity-distance measurements from gravitational waves combined with matter distribution tracing from neutral-hydrogen intensity mapping Synergy of Probes Gravitational waves provide direct luminosity-distance measurements while neutral-hydrogen intensity mapping traces the matter distribution in redshift space, producing large-scale three-dimensional maps of the 21-cm signal This synergy offers a promising avenue for precise measurements of the Hubble constant, H0 Within a standard-siren framework, the authors perform a joint cosmological and population analysis using mock gravitational-wave detections and mock 21-cm intensity maps Impact of Observational Limitations The study investigates how observational and instrumental limitations in both the gravitational-wave and intensity-mapping data affect the H0 constraints Key observational effects include: "Beam convolution most severely weakens the constraints" "Instrumental noise can induce multimodality in the posterior, potentially biasing the inferred parameters" "Observational effects in the 21-cm surveys substantially weaken the constraints, although they remain tighter than those obtained without 21-cm information (spectral-siren analysis)" Performance Forecasts The results show that including idealised HI maps can substantially improve the cosmological inference relative to the spectral-siren case, yielding a sub-percent measurement of H0 and a 99% reduction in its uncertainty When beam convolution, instrumental noise, partial sky coverage, and foreground cleaning are incorporated, the improvement is reduced to 61% for MeerKLASS-UHF and 53% for SKA-Mid1 The individual impact of these observational limitations is moderate, particularly for SKA-Mid1, although they can introduce multimodality in the posterior distributions Key Findings on Constraints The uncertainty in gravitational-wave parameter measurements is a dominant limitation on the cosmological constraints When this uncertainty is neglected, the inclusion of 21-cm information enables sub-percent constraints on H0 The final results show that observational effects to produce realistic forecasts reduce the cosmological information carried by the maps, substantially diminishing the relative impact of GW parameter uncertainties Mitigation and Future Directions The authors suggest that a promising direction for further modelling these effects would be to quantify their impact on the temperature-weights, and subsequently on the redshift priors, from a simulation-based systematic study They also note that the importance of well-measured, and in particular well-localised, GW detections will increase with third-generation detectors such as the Einstein Telescope (ET) and Cosmic Explorer (CE) The paper concludes by suggesting that a joint multi-tracer analysis combining HI intensity maps, galaxy catalogues, or other large-scale structure tracers could provide a more robust redshift prior than any single tracer alone The inclusion of the HI field into a hierarchical Bayesian framework allows for the construction of an informative line-of-sight prior for the redshift of BBH sources The results demonstrate that HI IM can provide significant additional information on H0 and serve as a complementary probe for GW cosmology The final comparison combines these two competing effects and should be interpreted with that caveat in mind References Afroz S., Mukherjee S., 2024, MNRAS, 534, 1283 Alonso D., et al., 2015, MNRAS, 447, 400 Baker T., et al., 2026, Cosmology from Synergies Between SKAO Surveys and Gravitational Wave Observations (arXiv:2606.26011) Battye R.

Vera: A., Davies R.

Jocelyn: D., Weller J., 2004, MNRAS, 355, 1339 Battye R.

Vera: A., et al., 2013, MNRAS, 434, 1239–1256 Beirnaert F., Dálya G., Ghosh A., 2025, MNRAS, 542, 3346–3353 Belgacem E., et al., 2018, Phys.

Paper summary: Vera: So, to recap, this paper lays out how to use gravitational waves as standard sirens—which give us a direct measurement of luminosity distance—and pair that up with twenty-one-cm intensity mapping from MeerKAT and SKA-Mid <ref:2610.31081#pg1>.

Jocelyn: The core thesis is about combining these two probes for cosmological inference, aiming for a precise measurement of the Hubble constant, H0. They use mock gravitational-wave detections and mock HI intensity maps to test this setup under a standard siren framework.

Subrahmanyan: The main point they are making is that this combination offers a promising avenue because GWs give us distance directly, and twenty-one-cm mapping gives us the matter distribution in redshift space <ref:2610.31081#pg1>.

Vera: They’re specifically looking at the impact of observational limitations in both data sets on the resulting H0 constraints. They test how things like beam convolution and instrumental noise affect the inferences we draw from these two sources.

Jocelyn: The study shows that while ideal HI maps can lead to sub-percent constraints, real observational effects substantially weaken those results when you look at realistic conditions.

Subrahmanyan: The paper also highlights that the uncertainty in measuring the gravitational-wave parameters themselves is a dominant limitation on the final cosmological constraints.

Vera: But they argue that if you ignore that GW parameter uncertainty, then adding the HI information allows for sub-percent constraints on H0. It’s a trade-off between two different sources of error.

Jocelyn: And they show how much those observational limitations matter by comparing the results against spectral sirens, which is what we get if you only use GWs without the HI data.

Subrahmanyan: So, the implication here is that while this combination of probes is powerful for probing H0, we need to be very careful about modeling all these instrumental and observational effects accurately.

Conclusion: Vera: Thinking about the title, it really sums up what they are doing: using gravitational waves and twenty-one-cm intensity mapping to make cosmological inferences <ref:2610.31081#pg1,and 21-cm intensity mapping>. It’s a very specific approach tied to the upcoming LVK network and SKA Observatory.

Jocelyn: The authors are clearly focused on setting realistic forecasts for how precise we can get with these future instruments, considering all the practical limitations like noise and beam effects.

Subrahmanyan: What this means in simple terms is that this joint analysis shows that twenty-one-cm intensity mapping isn't just an interesting side project; it’s a valuable complementary probe for GW cosmology when it comes to measuring H0 <ref:2610.31081#pg1>.

Vera: It proves that the synergy between these two probes can lead to much tighter constraints on H0 than either method could achieve on its own, provided we handle the observational caveats correctly.

Jocelyn: It’s showing that even with limitations in real-world data, including HI information significantly improves our ability to constrain the Hubble constant.

Subrahmanyan: The paper confirms that this dual approach is a viable path forward for using gravitational waves and radio observations together to study the expansion history of the universe.