Universe Expansion Rate: Old Stars Suggest Slower Pace

Cosmic Expansion Rate Debate Intensifies as Stellar Ages Offer New Clues

Astronomers are grappling with a persistent discrepancy in measuring the universe’s expansion rate, known as the Hubble constant. New research, focusing on the age of the oldest stars in the Milky Way, is adding another layer to the debate, potentially favoring a slower expansion rate than previously thought.

For years, two primary methods of calculating the Hubble constant have yielded conflicting results. Measurements based on observing stars and supernovae suggest a rate of approximately 73 kilometers per second per megaparsec – meaning that for every 3.26 million light-years further away a galaxy is, it appears to be receding 73 kilometers per second faster. However, measurements derived from the cosmic microwave background (CMB), the afterglow of the Big Bang, point to a slower rate of around 67 kilometers per second per megaparsec. This significant difference, exceeding 5 sigma statistical significance, has turn into known as the “Hubble tension.”

A team of researchers from Italy and Germany has approached the problem from a novel angle: establishing a minimum age for the universe based on the age of its oldest stars. The logic is straightforward – the universe cannot be younger than the stars it contains. The team analyzed data from the European Space Agency’s Gaia spacecraft, initially examining over 200,000 stars before focusing on 160 particularly ancient specimens whose ages could be determined with relative precision. Their findings, published in Astronomy & Astrophysics, indicate that these stars are, on average, approximately 13.6 billion years aged.

Accounting for the time elapsed after the Big Bang before the first stars formed – estimated to be several hundred million years – the researchers arrive at a minimum age for the universe of between 13.8 and 14 billion years. This age estimate aligns better with a slower expansion rate, closer to the 67 kilometers per second per megaparsec derived from CMB observations. A faster expanding universe, as suggested by supernova measurements, might not have allowed enough time for such ancient stars to form.

The research builds on previous perform attempting to resolve the Hubble tension. In 2024, researchers using data from the James Webb Space Telescope confirmed earlier Hubble Space Telescope measurements of the expansion rate using Cepheid variable stars and Type Ia supernovae, distance markers used to calculate cosmic distances. Webb’s observations of a gravitationally lensed supernova, nicknamed “H0pe,” also contributed to refining the Hubble constant measurement, though the tension remained.

Researchers at the Max Planck Institute for Astrophysics have also presented an independent determination of the Hubble constant using Type II supernovae, a different type of stellar explosion than Type Ia. Their findings, published in 2025, also support a higher Hubble constant value, contributing to the ongoing debate. A separate study, published in November 2024, explored using only “blue” Type Ia supernovae – those less affected by dust extinction – to refine the Hubble constant measurement, finding a lower value consistent with CMB data, but based on a smaller sample size.

Despite the new insights from stellar ages, researchers caution against drawing definitive conclusions. The age estimates are dependent on certain assumptions about the composition and evolution of the universe. The discovery of potentially even older stars could introduce further complexities. The discrepancy between the different measurement methods remains a significant challenge to the standard cosmological model.

The ongoing investigation into the Hubble constant highlights the limitations of current cosmological models and the potential need for “new physics” to explain the observed discrepancies. Further observations and refined measurement techniques, including those planned with upcoming major transient surveys, are crucial to resolving this fundamental question about the universe’s expansion and its ultimate fate.