Scientists forecast how well future gravitational-wave events with light flashes—so-called bright sirens—could measure the Hubble constant H0, the present expansion rate of the Universe. They simulated binary neutron star mergers that the planned Einstein Telescope would see. When a merger also has an electromagnetic counterpart (for example a short gamma-ray burst or a kilonova), astronomers can identify the host galaxy and read off its redshift. Combining that redshift with the distance measured from the gravitational waves gives an independent estimate of H0.

The team built mock catalogs with different numbers of events and different redshift mixes. They used the real event GW170817 as a reference bright siren and analysed results inside a flat Lambda Cold Dark Matter cosmology (flat ΛCDM). Their main finding is simple and practical: most of the power to measure H0 comes from low-redshift events. Once sources lie beyond about z ≃ 1 (where z is a measure of distance and look-back time), adding more events brings little improvement. At higher redshift the distance depends more strongly on the total matter density (labelled Ωm), which creates a degeneracy: the same distance can be explained by different combinations of H0 and Ωm.

Because of that degeneracy, if bright sirens are used alone the numbers needed for tight H0 measurements are large. The authors estimate that roughly 90 low-redshift bright sirens would be required to reach an uncertainty σH0 ≃ 1 km s−1 Mpc−1, and about 45 to reach σH0 ≃ 2 km s−1 Mpc−1. If external information is added to break the H0–Ωm trade-off, the requirements fall sharply. Including mock baryon acoustic oscillation (BAO) data—distance measurements from the large-scale distribution of galaxies, here modelled after future SKA Observatory results—reduces the needed sample to about 20 bright sirens for 1 km s−1 Mpc−1 precision and about 15 for 2 km s−1 Mpc−1.