Gravitational waves carry their own distances, and cosmology has been waiting for them to break the tie. A compact-binary merger's waveform encodes its luminosity distance absolutely, no ladder, no calibration, no standard candle assumptions: pair that with a redshift from an electromagnetic counterpart and the Hubble constant follows from first principles. The method debuted with GW170817, the binary neutron star merger whose kilonova pinned it to NGC 4993, yielding H0 = 70 (+12/-8) km/s/Mpc (LIGO-Virgo 2017): elegantly independent and statistically toothless, the error bars straddling both camps of the Hubble tension. The years since have been slower than forecast: no second counterpart-bearing merger has matched GW170817's quality, dark-siren analyses (statistical host association without counterparts) have crept the constraint toward 68-to-70 with wide uncertainty, and the promised percent-level siren cosmology remains a next-decade deliverable awaiting detector upgrades and luckier skies.
The stakes meanwhile sharpened: sirens are the designated tiebreaker precisely because their systematics overlap neither ladder nor CMB, so whichever camp they land in inherits powerful independent support. The subtlety the single-number framing ignores is that sirens are local: counterpart events at tens to a few hundred megaparsecs sample the same local volume as the distance ladder, so a siren H0 near 73 would not adjudicate between "the ladder is wrong" and "the local volume genuinely expands faster," while sirens reaching beyond a few hundred megaparsecs would map the transition, if there is one.
The standing is a tiebreaker still loading: one golden event, a slow accumulation of dark sirens, A+ era sensitivity arriving, and the genuine discriminating power, an H0 measured as a function of distance and environment, not yet tapped.