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Researchers from the University of Bologna and the Leibniz Institute for Astrophysics Potsdam (AIP) and other Institutes have proposed a new way to address the “Hubble tension” by comparing estimates of the Universe’s age rather than its expansion rate.
Using precise stellar data, they determined ages for carefully selected very old Milky Way stars and found a most likely age of about 13.6 billion years. Under the assumption of the standard cosmological model, this age is inconsistent with the younger Universe implied by Cepheid- and Supernova-based expansion measurements, but it is compatible with the older age inferred from observations of the cosmic microwave background—thereby adding a new perspective to the ongoing Hubble tension debate.
One of the most debated questions in modern cosmology is the value of the Hubble constant, which measures how fast the Universe is expanding today. For years, the different traditional methods have provided inconsistent results, and despite many efforts, there is still no clear explanation. Since the Gaia space mission, our Milky Way has increasingly become a “close-up laboratory” for cosmology.
The study by the Università di Bologna and the Leibniz Institute for Astrophysics Potsdam (AIP) opens up an alternative approach to the methods used so far. Instead of searching for the discrepancies directly in the expansion rate, it was translated into a so-called age tension. Cosmological models link the current rate of expansion of the Universe directly to its age: a higher value of the Hubble constant implies a younger Universe, while a smaller value corresponds to an older one. The Hubble constant measurements currently in tension, based on measurements in the local Universe from Cepheids and Supernovae on the one hand and on the early Universe from the cosmic microwave background on the other, correspond to cosmic ages of about 13 and 14 billion years, respectively. But which of these two ages is the correct one?
The Universe cannot be younger than the oldest stars it contains. If the ages of the oldest stars in our Galaxy can therefore be measured with high precision, a robust lower limit on the age of the Universe can be established.
The project was initiated by an unusual collaboration between two research fields that have traditionally been separate: a cosmology group at the University of Bologna and a stellar archaeology group at the AIP. The work was based on an existing catalogue of stellar ages from a previous study from AIP, in which precise ages were measured by combining multiple pieces of information on the brightness, position, and distance of more than 200’000 stars in the Milky Way. A crucial element was the use of the third data release of the ESA Gaia mission, which provides exceptionally accurate parallaxes and spectra and thus improved stellar parameters for a large number of nearby stars.
From this extensive dataset, a carefully selected sample of the oldest stars with the most reliable age estimates was compiled. The focus was on quality over quantity, choosing only stars whose ages could be determined robustly by the StarHorse code and removing potential contaminants. The result: for the final sample of around one hundred stars, the most probable age is about 13.6 billion years. This is too old to be compatible with the age of the Universe inferred from Cepheids and Supernovae (unless other ingredients in the cosmological models are varied), but it aligns well with the cosmic age inferred from the cosmic microwave background.
“This project beautifully shows how combining expertise from different fields can open new windows on fundamental questions. Measuring the age of stars is, in itself, a complex challenge, but we now live in an era in which the quantity and quality of available data allow us to achieve unprecedented precision and, for the first time, statistically significant results. With the next Gaia data release on the horizon, stellar ages could become a fundamental anchor for cosmology.” says Elena Tomasetti from the Università di Bologna and first author of the study.
„With Gaia, the Milky Way has effectively become a near-field cosmology laboratory. We can now estimate stellar ages with unprecedented precision. The next breakthrough will be accuracy, anchoring the Galactic timeline with far greater certainty. The HAYDN mission concept, with AIP participation, aims to provide that decisive step.“ adds Cristina Chiappini from AIP.
Although these results are not yet conclusive due to remaining uncertainties in stellar age estimates, they provide an important independent constraint in the debate over the Hubble tension. At the same time, they highlight the potential of near-field cosmology and, in particular, the research at the AIP to tackle fundamental cosmological questions using the oldest “fossils” of the Milky Way. With the fourth Gaia data release, further significant progress is expected—and with it, even stronger constraints on the age of the Universe and the value of the Hubble constant.
https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202...
https://arxiv.org/abs/2509.02692
https://arxiv.org/abs/2402.00561
How old is the Universe
Copyright: Elena Tomasetti
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