One of the most stubborn problems in modern cosmology has just been sharpened to an uncomfortable new clarity. An international collaboration of cosmologists has published a unified measurement of the Hubble constant — the number that describes how fast the universe is expanding today — achieving a precision of approximately 1% for the first time. The result, published in Astronomy & Astrophysics, does not resolve the so-called Hubble tension. It confirms it, more decisively than ever.
What the Hubble constant actually tells us
When astronomers observe distant galaxies, a clear trend emerges: the farther a galaxy is from us, the faster it appears to be receding. This remarkably consistent relationship is encoded in a single number, the Hubble constant (H₀), which tells us how fast the universe is expanding at the present moment. From H₀, cosmologists can estimate the time elapsed since the Big Bang — our current best estimate for the age of the universe flows directly from this value.
The problem is that there are two fundamentally different ways to measure it, and they persistently disagree.
An enigma that refuses to go away
When researchers derive the Hubble constant from the standard cosmological model — using observations of the early universe, particularly the cosmic microwave background — they obtain a value that is systematically lower than what is measured through direct, local astronomical observations of nearby galaxies and standard candles such as Cepheid variable stars and Type Ia supernovae.
«The prevailing cosmological model predicts that the Hubble constant should be 10% smaller than what we measure directly,» explained lead author Stefano Casertano of the Space Telescope Science Institute in Baltimore. «This is known as the Hubble tension, and the difference is more than five times the combined uncertainty of both models and measurements.»
This mismatch points to physics that cannot yet be fully explained within the current theoretical framework — and before researchers can probe its causes, they first need to ensure that the direct local measurements themselves are internally consistent.
Over the past few decades, numerous independent methods have emerged for measuring H₀ locally: geometric distances, masers, parallax, Cepheid stars, the Tip of the Red Giant Branch (TRGB), surface brightness fluctuations, Type Ia supernovae, and more. While broadly similar, these techniques rely on different distance indicators and calibration chains, making it statistically difficult to combine them into a single rigorous value.
A workshop that changed the approach
In 2025, the International Space Science Institute (ISSI) in Bern convened a landmark workshop, bringing together every leading expert in the field. The goal was to systematically map which parts of these measurements overlap, which are truly independent, and how they reinforce one another.
«We then developed a statistical framework to properly combine all of these measurements together, and to identify any possible inconsistencies,» explained co-author Adam Riess, also at the Space Telescope Science Institute.
The result: unprecedented precision, persistent disagreement
The collaboration succeeded in reaching a consensus on a single value for H₀ compatible across all their different methodologies. «This is the most precise measurement to date, achieving 1% accuracy for the first time,» Casertano noted. «We also found that no single measurement or thread is critical to this result; any component can be eliminated entirely, and the value of the Hubble constant remains essentially unchanged.»
This robustness is significant. It means the result is not an artifact of any particular instrument, calibration choice, or distance indicator. The unified value is stable.
And yet, that stability cuts both ways. The new measurement sharpens the discrepancy with early-universe predictions to a tension exceeding five standard deviations — a threshold that in physics is generally considered definitive evidence of a real effect rather than statistical noise. Rather than reducing the disagreement, the improved precision has confirmed it.
What comes next
«Confirming the Hubble tension makes it even more important for us to reexamine the foundations of the current cosmological model, and to identify any new phenomena that might modify the evolution of the universe,» said Riess.
Whether the resolution lies in new physics beyond the standard model — dark energy that evolves with time, early dark energy injected before recombination, new relativistic species, or something not yet imagined — remains open. What is no longer open is whether the tension is real.
It is. And now it is measured to 1% precision.
The work was published as The Local Distance Network: A community consensus report on the measurement of the Hubble constant at ~1% precision, in Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202557993. Also available on arXiv: 10.48550/arxiv.2510.23823.
© 2026 SKYCR.ORG — Homer Dávila Gutiérrez. All rights reserved. Reproduction, total or partial, of the text, images or any other content of this article is prohibited without prior written authorization from the author. The scientific information presented is based on the original publication: Casertano et al. (2026), «The Local Distance Network,» Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202557993.
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