One of the most accurate determinations of the local Universe's expansion rate to date has been produced by an international team of astronomers. One of the biggest problems in contemporary cosmology is further complicated by the outcome. The study was published in Astronomy & Astrophysics.
Artist’s interpretation of the cosmic distance ladder. Image Credit: NSF NOIRLab
John Blakeslee, astronomer at NSF NOIRLab, supported by the U.S. National Science Foundation, is a member of the collaboration, and telescopes across two NSF NOIRLab Programs provided data.
Two radically different methods have been used by astronomers to try and gauge the universe's expansion pace. One approach is based on calculating the distances to nearby galaxies and stars. The other makes predictions about the current expansion rate under the standard model of cosmology using observations of the cosmic microwave background.
Although these two methods should provide the same outcome, they do not. While estimates from the early Universe provide a lower number, closer to 67 or 68, measurements based on the nearby Universe consistently show a higher expansion rate, about 73 kilometers per second per megaparsec.
The difference might seem small, but it is far beyond what statistical uncertainty can account for. This is known as the Hubble tension, and has now been noted in numerous independent investigations and methods.
By integrating decades of independent observations into a single, unified framework, an international team of astronomers has obtained the most precise direct measurement so far of the expansion rate of the nearby Universe. In this study, the H0 Distance Network (HODN) Collaboration reports a Hubble constant value of 73.50 ± 0.81 kilometers per second per megaparsec, corresponding to a precision of just over 1 %.
The study is the result of a large-scale community effort that began at the "What's under the H0od?" Breakthrough Workshop at the International Space Science Institute (ISSI) in Bern, Switzerland, in March 2025.
“This isn’t just a new value of the Hubble constant,” the collaboration notes, “it’s a community-built framework that brings decades of independent distance measurements together, transparently, and accessibly.”
This effort benefited from the expertise and observational data provided by NSF NOIRLab. The partnership includes astronomer John Blakeslee, Director of Research and Science Services at NSF NOIRLab. Data from telescopes at NSF Kitt Peak National Observatory (KPNO) in Arizona and NSF Cerro Tololo Inter-American Observatory (CTIO) in Chile, two NSF NOIRLab programs, are included in the study.
The overall outcome was strengthened by the integration of those data into a more comprehensive, cooperative framework that included both ground-based and space-based observatories.
The researchers built a “distance network” that links many overlapping methods for measuring distances across the local Universe, rather than relying on a single technique. These methods include observations of Type Ia supernovae, pulsating Cepheid variable stars, red giant stars with known brightness, and certain types of galaxies.
This approach enables multiple independent paths to the same result and, importantly, allows for a critical test: is the discrepancy driven by an error in any one method? The findings suggest that’s unlikely. Even when specific techniques are removed from the analysis, the overall result changes very little. Independent measurements also remain consistent with one another, reinforcing the reliability of the locally measured expansion rate.
“This work effectively rules out explanations of the Hubble tension that rely on a single overlooked error in local distance measurements,” the authors conclude. “If the tension is real, as the growing body of evidence suggests, it may point to new physics beyond the standard cosmological model.”
The standard model of cosmology, which describes how the universe has evolved since the Big Bang, underpins the lower expansion rate inferred from the early Universe. If that model is incomplete, for example, if it does not fully capture the behavior of dark energy, the existence of new particles, or possible modifications to gravity, its predictions for the present-day expansion rate would be affected.
In such a scenario, the Hubble tension might not necessarily be the result of measurement error, but rather proof that the current model of the Universe is lacking a crucial element.
In addition, the local distance network provides a strong foundation for future research. By making their data and methodologies publicly available, the collaboration has created a framework that others can build on with new observations. As next-generation observatories begin to deliver more precise measurements, astronomers hope to determine whether this discrepancy will ultimately be resolved, or continue to point toward new physics.
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Journal Reference:
H0DN Collaboration, et al. (2026) The local distance network: a community consensus report on the measurement of the Hubble constant at ∼1% precision’, Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202557993. https://www.aanda.org/component/makeref/?task=show&type=html&doi=10.1051/0004-6361/202557993.