A global collaboration reaches 1% precision on the local expansion rate of the Universe

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, “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 H₀od?”, held at the International Space Science Institute (ISSI) in Bern in March 2025.

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. While remarkably 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 that replaces 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.

“This approach shifts the focus from defending individual methods to understanding how the full ecosystem of distance measurements fits together,” says Licia Verde (ICREA–ICCUB), member of the H0DN Collaboration and Scientific Director of the ICCUB.

Robust, transparent, and community-driven

Nearly 40 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.

Putting the Hubble tension in context

The new local measurement remains in significant disagreement with values inferred from observations of the early Universe — such as the cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) — when interpreted within the standard ΛCDM cosmological model. The discrepancy reaches approximately 5–7 standard deviations, reinforcing what is known as the Hubble tension.

Researchers from ICCUB (Institute of Cosmos Sciences of the University of Barcelona), including Licia Verde and Héctor Gil‑Marín (ICCUB–IEEC), brought to the collaboration extensive expertise in cosmology, galaxy surveys, BAO, and the CMB — precisely the observations that underpin the other determination of the Hubble constant.

“Our contribution was not to champion one side of the tension,” explains Héctor Gil‑Marín, “but to rigorously assess systematic uncertainties, compare datasets, and examine whether the discrepancy points to new physics or to unresolved measurement issues.”

Because ICCUB researchers are not directly involved in producing the distance‑ladder observations used in H0DN, they were able to play a distinct role within the collaboration.

“That external perspective allowed us to help uncover hidden assumptions and act, when needed, as neutral arbiters,” adds Verde. “We represented the viewpoint of the cosmology and early‑Universe community within a collaboration focused on local measurements.”

Rather than identifying a single problematic measurement, the distance‑network result strengthens the case that the tension cannot be explained by a lone overlooked systematic in the local distance scale.

A foundation for future precision cosmology

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

“This work shows that explanations invoking a single overlooked systematic in local distance measurements are increasingly difficult to sustain,” the authors conclude. “If the tension reflects real physics, it may indicate new ingredients beyond the standard cosmological model or require a reassessment of early-Universe inferences.”

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.

Original press release text by Fabio Crameri (ISSI) in collaboration with the authors.

Reference:

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H0DN Collaboration, Casertano, S. et al. (2026). The Local Distance Network: A community consensus report on the measurement of the Hubble constant at ∼1% precision. Astronomy & Astrophysics (forthcoming).
https://doi.org/10.1051/0004-6361/202557993