The local universe might be expanding faster than ever, but it does not make sense
New research(opens in new window) supported by the EU-funded H1PStars and UniverScale projects has delivered one of the most precise measurements to date of how fast the universe is expanding. However, instead of unravelling a cosmological mystery, this result has only served to deepen it.
Precision through a unified framework
Up to now, astronomers have tried to measure the universe’s expansion rate using two entirely different strategies. One involves measuring distances to stars and galaxies in the universe to calculate how fast cosmic bodies are moving apart. The other uses measurements of the cosmic microwave background – the faint radiation from the Big Bang that fills all space in the observable universe – to predict what the expansion rate would be today based on the standard cosmology model. The two methods should lead to the same answer, but they do not. The first consistently points to a faster expansion rate of about 73 km per second per megaparsec, while the second produces a slower rate of 67 or 68 km. This difference – persistently encountered in multiple studies and known as the Hubble tension – might seem small, but it is too large to attribute to statistical uncertainty. To achieve greater precision, an international team of researchers combined decades of observations into a single, unified framework. Led by the H0 Distance Network Collaboration, this initiative produced the most precise direct measurement yet of the local universe’s expansion rate. Their study, published in the journal ‘Astronomy & Astrophysics’, reports a Hubble constant value of 73.50 ± 0.81 km per second per megaparsec, achieving a precision of just over 1 %. “This isn’t just a new value of the Hubble constant,” the collaboration reports in a recent ‘EurekAlert’ news release(opens in new window), “it’s a community-built framework that brings decades of independent distance measurements together, transparently and accessibly.” The different overlapping techniques used to measure cosmic distances included observations of pulsating Cepheid variable stars, red giant stars that shine with a known brightness, Type Ia supernovae and certain types of galaxies. This approach allowed researchers to cross-check measurements and led to the same overall result, strengthening confidence in the locally measured expansion rate.
So what does the Hubble tension mean?
“This work effectively rules out explanations of the Hubble tension that rely on a single overlooked error in local distance measurements,” the researchers observe. “If the tension is real, as the growing body of evidence suggests, it may point to new physics beyond the standard cosmological model.” The implications are far-reaching. The slower expansion rate derived from the early universe is based on the standard cosmological model, which describes how the universe has evolved since the Big Bang. If that model is incomplete, missing details about dark energy, new particles or changes in gravity, its predictions for the current expansion rate would be affected. In that case, the Hubble tension may not be a simple measurement error but rather evidence that our model of the universe is missing something. In other words, scientists might have to rewrite the rules of how the universe operates. The H1PStars (Measuring Hubble’s Constant to 1% with Pulsating Stars) project ended in March 2026. UniverScale (Sub-percent calibration of the extragalactic distance scale in the era of big surveys) ends in October 2027. For more information, please see: H1PStars project UniverScale project
