Black Holes 'Weigh' the Expansion Rate of the Universe express
Each merger of black holes produces gravitational waves — ripples in the fabric of the universe — predicted by Einstein and first detected with key contributions from Kip Thorne. Like strikes on a giant bell, they carry information about mass: most collisions cluster around three values — 10, 18, and 33 solar masses.
The expansion of the universe, discovered by Edwin Hubble, stretches this ringing, lowering the tones. By measuring the shift across 137 signals, scientists calculated the expansion rate: about 58 kilometers per second for every 3 million light-years (a megaparsec) — with an accuracy 21% better than previous methods. The main surprise: black holes of 33 solar masses were once exceptions, but now they play a loud note in the orchestra. The more mergers we record, the clearer the cosmic tuning fork sounds, and in the future, it might even allow us to hear the whisper of dark energy.
Original abstract
as written by the authors arXiv:2601.03347v2
Gravitational wave spectral sirens can provide cosmological constraints by using the shape of the binary black hole (BBH) mass distribution (MD). However, the precision and accuracy of these constraints depends critically on the capturing all the MD features. In this work, we analyze 137 BBH events from the latest GWTC-4.0 with a novel data-driven semiparametric approach based on \textsc{Bspline} that adaptively places knots around the most informative structures in the MD, while keeping the dimensionality of the parameter space moderate. Our flexible models resolve three distinct peaks at $$\sim10$$, $$18$$, and $$33\,\mathrm{M}_\odot$$ and are statistically preferred over standard parametric models, with Bayes factors up to 226. Because these features are correlated with $$H_0$$, the semiparametric model yields, under different prior assumptions, 12%-21% improvement in the precision of $$H_0$$ relative to parametric models, providing $$H_0 = 57.8^{+21.9}_{-20.6}\,\mathrm{km/s/Mpc}$$ in the best case. Our results demonstrate that capturing the full complexity of the BBH mass distribution is essential for realizing the cosmological potential of spectral sirens as gravitational wave catalogs continue to grow.
The lightest peak — 10 solar masses — is born from the collapse of an ordinary star. Black holes of 33 solar masses were considered a rarity, but now they are found more and more often, and they indeed set a powerful note in the symphony of mergers.