An international collaboration of astronomers including Dr Eleonora Di Valentino and Dr Emre Özülker from the University of Sheffield 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 H0Distance 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, “The Local Distance Network: a community consensus report on the measurement of the Hubble constant at ∼1% precision,” is the outcome of a broad community effort launched at the ISSI Breakthrough Workshop “What’s under the Hood?", held at the International Space Science Institute (ISSI) in Bern in March 2025.
For nearly a century, astronomers have relied on the so-called “distance ladder” to measure the expansion rate of the Universe - the Hubble constant, H₀ - by calibrating increasingly distant cosmic objects through a sequence of interlocking steps. This method has delivered tremendous progress, but it also means that uncertainties can propagate along the chain, without the benefit of spreading risk or sharing the load. The H0DN Collaboration therefore adopted a broader mathematical framework, replacing a single measurement path with a Local Distance Network that links many distance indicators simultaneously.
Rather than depending on any single measurement path, the network connects a wide range of independent and overlapping distance indicators – 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 a single, coherent analysis.
Crucially, the network explicitly accounts for shared uncertainties and correlations between methods through full covariance weighting, allowing the consistency of the entire system to be assessed transparently for the first time.
“What makes this result exciting is that, for the first time, experts using many different ways to measure cosmic distances worked together to produce a consensus measurement of the Universe’s expansion rate. Even when we test different combinations of methods and astronomical objects, the expansion rate still comes out high, making it hard to explain the Hubble tension as a hidden systematic effect in local observations.” Dr Eleonora Di Valentino
Experts in distance measurements and cosmology, representing a wide range of institutions and methodological backgrounds, participated directly in the ISSI workshop, with additional contributors joining remotely. Before any calculations were carried out, participants voted on the set of first-rank distance indicators, gold standards for defining a baseline solution, along with predefined variants to test robustness.
The network analysis shows that:
- Independent distance indicators are mutually consistent within their stated uncertainties and with no outliers
- No single method or indicator dominates the final result.
- Removing or replacing key components – such as Cepheids, TRGB, or Type Ia supernovae – produces only minor changes in the inferred value of H₀.
To encourage scrutiny and reuse, the collaboration is releasing open-source software and data products, allowing anyone to reproduce the analysis, explore alternative assumptions, or incorporate future measurements as new data become available.
‘Different established ways of measuring the Universe’s current expansion rate continue to give incompatible answers. Our results suggest this tension is unlikely to be caused by how we model nearby astrophysical objects, and instead indicate that something fundamental may be missing from our current picture of the physics driving cosmic evolution.’ Dr Emre Özülker
Beyond delivering the most precise direct measurement of the Hubble constant to date, the Local Distance Network establishes a flexible and extensible framework for the future. With a flood of new observatories, improved calibrations, and additional geometric distance anchors becoming available, they can be integrated into the network to further refine our understanding of cosmic expansion and provide clues about the resolution of the Hubble tension.
The study also highlights the role of ISSI Bern in fostering open, collaborative, and methodologically rigorous science that bridges traditional disciplinary and institutional boundaries.
The full paper will appear in Astronomy & Astrophysics. Upon acceptance, the analysis code will be made publicly available via GitHub and the Astrophysics Source Code Library.
More information:
The International Space Science Institute (ISSI) is an Institute of Advanced Studies, where scientists from all over the world meet in a neutral, welcoming, and multi-disciplinary setting to discuss and publish about relevant and compelling topics related to four Disciplines: Astrophysics, Heliophysics, Planetary Science and Earth Science. ISSI’s mission is to advance science by facilitating scientific community interactions, meetings, discussions, and publications aimed at a deeper understanding of results from different space missions, ground-based observations, and theory. This is achieved through a broad portfolio of scientific opportunities that include: International Teams, Workshops, Working Groups, Fora, or visits of individual Visiting Scientists. For additional information related to ISSI and to the opportunities it offers, see: www.issibern.ch.
Original press release text by Fabio Crameri (ISSI) in collaboration with the authors.
ISSI Press Contact:
Fabio Crameri
ISSI Communication Scientist
Bern, Switzerland
fabio.crameri@issibern.ch