Unified statistical framework delivers the most precise value for the Universe’s expansion rate and confirms Hubble tension at more than 5σ

A unified statistical framework delivers the most precise value for the Universe’s expansion rate and confirms Hubble tension at more than 5σ

An international team of cosmologists has created a unified method for determining the Hubble constant. The advance could help explain the accelerated expansion of the Universe, which cannot be accounted for by the standard cosmological model.

The Hubble constant and the measurement discrepancy

The Hubble constant defines the linear relationship between a galaxy’s distance and the speed at which it is moving away from an observer. This parameter makes it possible to estimate the present-day rate of the Universe’s expansion and calculate the time elapsed since the Big Bang, providing the basis for determining the Universe’s age.

Yet measurements of the Hubble constant face a major contradiction. Values derived from cosmological models of the early Universe differ substantially from those obtained through direct local astronomical observations. As Stefano Casertano, the study’s lead author from the Space Telescope Science Institute in Baltimore, explains, “the standard cosmological model predicts that the Hubble constant should be 10% lower than what we measure directly”. This discrepancy, known as the “Hubble tension”, exceeds five times the combined uncertainty of the models and measurements.

To investigate the source of this conflict, researchers need to remove inconsistencies among the Hubble constant measurements themselves. Numerous measurement techniques have been developed over recent decades, and although their results are similar, they are not identical. The various methods rely on different distance indicators and calibration stages, making it difficult to combine the data into a single statistically robust measurement.

A unified framework for the Hubble constant

In 2025, the International Space Science Institute (ISSI) in Bern held a dedicated workshop, where specialists considered the shared and independent elements of the different measurement methods. As study co-author Adam Riess of the Space Telescope Science Institute states, “we developed a statistical framework to combine all these measurements and identify potential inconsistencies”.

The work produced agreement on a single Hubble constant value compatible with the different methods. This result is the most precise measurement so far, reaching 1% accuracy for the first time. Casertano notes that “no individual measurement is critical to this result, and removing any component scarcely changes the value of the Hubble constant”.

Hubble tension exceeds five standard deviations

The new value increases the discrepancy with predictions based on early-Universe data, corresponding to tension of more than five standard deviations. Rather than reducing the conflict, it confirms its statistical significance.

As Riess emphasises, “confirming the Hubble tension makes it even more important to re-examine the foundations of the current cosmological model and search for new phenomena that could alter the evolution of the Universe”. The improved Hubble constant measurement will enable the most rigorous tests yet of the physics underlying this contradiction and bring researchers closer to resolving one of cosmology’s most profound mysteries.