Gravitational waves refine cosmic expansion rate express
Measuring the universe’s expansion is like timing a race when the track stretches. Neutron star mergers act as standard sirens: their gravitational waves ring out with a known “volume,” so fainter signals mean greater distance.
A new study paired the siren from merger GW170817 with a galaxy map tracing matter, including dark matter (invisible mass first detected by Vera Rubin). The result: the Hubble constant (named after Edwin Hubble) is 67.9 km/s per megaparsec, so a galaxy 3.26 million light-years away recedes at that speed — with just 6.4% uncertainty.
This value eases a cosmic dispute: the early universe’s afterglow suggests slower expansion, while nearby exploding stars point to faster. The new number falls between them, highlighting the strength of cross-checking methods. Remarkably, the spacetime ripples from GW170817 jiggled our detectors by less than a proton’s width, yet clocked the universe’s growth.
Original abstract
as written by the authors arXiv:2601.04774v1
We present a new measurement of the Hubble constant ($$H_0$$) resulting from the first joint analysis of standard sirens with weak gravitational lensing and galaxy clustering observables comprising three two-point correlation functions (3$$\times$$2pt). For the 3$$\times$$2pt component of the analysis, we use data from the Dark Energy Survey (DES) Year 3 release. For the standard sirens component, we use data from the Gravitational-Wave Transient Catalog 4.0 released by the LIGO-Virgo-KAGRA (LVK) Collaboration. For GW170817, the only standard siren for which extensive electromagnetic follow-up observations exist, we also use measurements of the host galaxy redshift and inclination angle estimates derived from observations of a superluminal jet from its remnant. Our joint analysis yields $$H_0 = 67.9^{+4.4}_{-4.3}$$~km~s$$^{-1}$$~Mpc$$^{-1}$$, a $$6.4\%$$ measurement, while improving the DES constraint on the total abundance of matter $$\Omega_m$$ by $$22\%$$. Removing the jet information degrades the $$H_0$$ precision to $$9.9\%$$. The measurement of $$H_0$$ remains a central problem in cosmology with a multitude of approaches being vigorously pursued in the community aiming to reconcile significantly discrepant measurements at the percent-level. In light of the impending new data releases from DES and LVK, and anticipating much more constraining power from 3$$\times$$2pt observables using newly commissioned survey instruments, we demonstrate that incorporating standard sirens into the cosmology framework of large cosmic surveys is a viable route towards that goal.
The term 'standard siren' echoes 'standard candle,' but while candles use light, sirens use gravitational waves, a clever twist on an old cosmic yardstick.