An international collaboration of astronomers including IUCAA faculty Prof. Anupam Bhardwaj, has made the most precise direct measurement to date of how fast the Universe is expanding. The H_o Distance Network (H0DN) Collaboration reports a value of the Hubble constant - the number that describes the Universe's present expansion rate - with just over 1% precision.

The study to be published in Astronomy & Astrophysics reports a value of:


A Megaparsec is a unit astronomers use to measure very large cosmic distances and equals about 3.26 million light-years (1 light year ~ 9.5 trillion kilometers). The new H_o measurement shows that for every megaparsec of distance, a galaxy’s speed away from us increases by about 73.5 kilometers per second. In our expanding universe, more distant galaxies move away faster than nearby galaxies. Achieving this percent-level precision of H_o helps astronomers better estimate the age, size, and future evolution of the universe, and improve our understanding of dark energy – the mysterious force accelerating cosmic expansion.

The work is the result of a broad community effort, launched at a workshop at the International Space Science Institute (ISSI) in Bern, Switzerland in March 2025.

A robust, transparent network, not a single path

For nearly a century, astronomers have relied on the so-called “distance ladder” to measure the expansion rate of the Universe. This traditional method is a sequence of interlocking steps where nearby distance tracers are connected to very distant cosmic objects. This method has delivered tremendous progress, but relies on a chain of measurements. If one link has hidden errors, it can affect the final value of the Hubble constant.

The H0DN Collaboration therefore adopted a Local Distance Network that links many distance indicators simultaneously. Instead of relying on one chain or measurement, the network connects a wide range of independent and overlapping distance indicators — including Cepheid and Mira variable stars, the Tip of the Red Giant Branch (TRGB) stars, exploding stars (Supernovae), geometric megamaser distances, surface brightness fluctuations in galaxies, the Tully–Fisher and the Fundamental Plane relations. The distance network is analogous to a subway map with many routes connecting the same destination.

By connecting all these methods in one single, coherent analysis, the collaboration ensured that no single technique controls the final answer. Even when key methods are removed or replaced, it produces only minor changes in the inferred value of H_o.

“This isn't just a new number for H_o,” the collaboration notes. “It's a community-built framework that brings decades of independent distance measurements together, transparently and accessibly.”

Nearly 40 experts in distance measurements and cosmology, representing a wide range of institutions and methodological backgrounds, participated in the ISSI workshop and contributed to the study. To encourage scrutiny and reuse, the collaboration is releasing open-source software and data products publicly so that anyone can check or improve the analysis.

Implications for the Hubble tension

With its unprecedented precision and internal consistency, the new measurement adds weight to one of the biggest puzzles in modern cosmology: the Hubble tension. The direct distance-ladder based measurements of the Hubble constant are consistently higher than the value inferred from observations of the early Universe. The H0DN collaboration reports a value that is nearly 10% (7 standard deviations) larger than the recent determinations based on the cosmic microwave background under the assumption of the standard ΛCDM cosmological model.

Because this new result combines many independent methods, it makes it increasingly difficult to explain the tension as a simple measurement mistake. “This work effectively rules out explanations of the Hubble tension that rely on a single overlooked error in local distance measurements.” the authors explain.

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. As new observatories and improved measurements become available, they can easily be integrated into the network. This approach will further refine our understanding of cosmic expansion and provide clues about the resolution of the Hubble tension.

“If the tension is real — as the growing body of evidence suggests — it may point to new physics beyond the standard cosmological model.” the authors conclude.

IUCAA scientist Prof. Anupam Bhardwaj, a member of the H0DN collaboration, explains – “Stars such as Cepheids and Miras play a vital role in connecting different steps of the Distance Network. At IUCAA, we are working on determining accurate luminosity scales of all types of stars that are used as distance indicators. These stellar luminosity scales are central to the Local Distance Network leading to a percent-level precise Hubble constant value.” Prof. Bhardwaj is also leading an international team of ISSI on the “EXPANDING Universe” project, with a goal to determine accurate and precise independent values of the Hubble constant.

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The full paper, “The Local Distance Network: a community consensus report on the measurement of the Hubble constant at ~1% precision, will appear in Astronomy & Astrophysics”. Upon acceptance, the analysis code will be made publicly available via GitHub and the Astrophysics Source Code Library.

Links:

• Science paper: https://doi.org/10.1051/0004-6361/202557993
• ISSI Workshop: https://workshops.issibern.ch/hubble-constant/
• PR-graphic (in various formats): https://s-ink.org/local-distance-network
• Original ISSI press release: https://www.issibern.ch/hubble-constant-press-release/

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