An international collaboration of astronomers has achieved the most precise direct measurement to date of the current expansion rate of the Universe. In a paper to be published in Astronomy & Astrophysics , the H0 Distance Network (H0DN) collaboration reports a value of the Hubble constant of H₀ = 73.50 ± 0.81 km s⁻¹ Mpc⁻¹, corresponding to a precision of just over 1%.
The study results from a broad community effort launched at the workshop "What's under the H0od? Towards Consensus on the local value of the Hubble Constant" , which took place at the International Space Science Institute (ISSI) in Bern in March 2025.
The full article will be published in Astronomy & Astrophysics. Once accepted, the analysis code will be made public via GitHub and the Astrophysics Source Code Library.
A network, not a single path
For nearly a century, astronomers have relied on the "distance ladder" to measure the Hubble constant, H0H_0H0, by calibrating increasingly distant cosmic objects through a sequence of interlocking steps. Albeit successful, this approach can allow uncertainties to propagate along a single dominant measurement path.
The H0DN Collaboration adopted a different strategy: a local distance network. It replaced a single ladder with a mathematical framework connecting many distance indicators simultaneously. The network links independent and overlapping distance probes - including Cepheid variables, the Tip of the Red Giant Branch (TRGB), Mira variables, megamasers, Type Ia and Type II supernovae, surface brightness fluctuations, the Tully-Fisher relation, and the Fundamental Plane - into one coherent analysis.
By explicitly accounting for correlations and shared uncertainties through full covariance weighting, the network allows the internal consistency of the entire system to be tested transparently for the first time.
