One Night to a Percent: Dark Sirens Reveal the Universe's Speed ✛
Predicted by Albert Einstein, gravitational waves are like beacons in the dark ocean of space. They flare up when black holes collide, sending ripples through the fabric of the Universe. A century ago, Edwin Hubble and Georges Lemaître noticed that distant galaxys are drifting away from us. Their speed is revealed by redshift — the stretching of light waves due to the expansion of space. But what is the true strength of this cosmic flow? The 2 main methods, one based on the glow of the cosmic microwave background and the other on stellar standards, differ by nearly 10%. Cosmology hit a dead end.
Now, gravitational beacons take the stage. When two black holes merge, detectors pick up a signal: this ripple carries the exact distance to the source. But most mergers produce no optical flash. They are dark sirens: they sound but do not shine. To learn the redshift of such a siren, astronomers have to guess: the source lies in one of thousands of galaxies that fall within the sky localization region. The method becomes statistical: all those galaxies are candidates, and by collecting their redshifts, we can reconstruct the expansion rate.
The researchers simulated a year of LIGO observations. In a numerical simulation based on the Uchuu-ν2GC catalog, they ran tens of thousands of mergers and selected 10 events with the best localization. Then they conducted a virtual spectroscopy campaign: measured the spectra of all candidate galaxies down to apparent magnitude r=19. Applying Bayesian inference, they obtained a median accuracy of the Hubble constant of 0.98%. The entire workload fits into 1 night on a DESI-class telescope. 4 hours for everything.
Such accuracy closes the gap between the cosmic microwave background and supernova measurements, resolving the Hubble tension. Moreover, the method is sensitive to modified gravity and the properties of dark energy. Future detectors like the Einstein Telescope will turn thousands of dark sirens into a fine cosmographic instrument. A century after Lemaître and Hubble first suspected the cosmic flow, gravitational beacons are ready to measure its exact speed. And it takes just 1 night at the telescope.
About 20% of these well-localized events fall in the Zone of Avoidance, where Milky Way interstellar dust blocks the view, requiring an extended campaign. For some events, shallower catalogs (r<19) turned up zero potential host galaxies, forcing a switch to deeper surveys or outright exclusion.