The Universe Isn't Getting Younger: Ancient Stars Rule Out "Early" Solutions ✦
An analysis of a sample of 155,600 stars in the Milky Way (using LAMOST spectroscopy and Gaia parallaxes) determined the age of the oldest star to be 13.73 billion years. Using the Bayesian MCMC method, scientists showed that this result aligns with the standard ΛCDM cosmological model, which predicts the age of the universe to be 13.8 billion years. This calls into question hypotheses that resolve the 'Hubble tension' through new physics before recombination — they require a younger universe (around 12.9 billion years). Imagine: the oldest star is like an hourglass, counting down the time since the Big Bang.
The standard cosmological model ΛCDM, where dark matter and dark energy act as the chief architects of the cosmos, has predicted astronomical data with Swiss-watch precision for decades. Its foundation was laid by three geniuses. Georges Lemaître proposed expansion from a super-dense "primeval atom." Edwin Hubble confirmed it with redshift. And Vera Rubin, with her galaxy rotation curves, convinced the world that the Universe is full of invisible matter. But one stubborn detail refused to fall into place: the value of the Hubble constant derived from the cosmic microwave background diverged from direct measurements in the local Universe. This Hubble tension has spawned numerous hypotheses. One of the boldest: in the early Universe, before recombination, an additional force accelerated expansion. The price of such an "early" solution? Rejuvenating the cosmos to a mere 12.9 billion years. But age has a strict arbiter—the ancient stars, whose thermonuclear clocks have been tirelessly ticking for 13.8 billion years.
The layered structure of the galaxy is like an ice core from Antarctica: each stellar layer preserves the chemical composition of its epoch. To reach the most ancient deposits, astronomers sifted through hundreds of thousands of samples, screening out recent "contamination." Using data from the spectroscopic survey LAMOST and astrometry from Gaia, they identified 155,600 subgiants in the vicinity of the Sun—stellar fossils depleted in iron and rich in alpha elements, the ashes of the first supernovae. Each star's age was determined by placing it on evolutionary tracks—a kind of "slow-motion timeline"—calibrated by its helium and carbon content.
The final age reconstruction using Markov chains is like a searchlight beam: at the boundary of 13.73 billion years, the distribution cuts off sharply. Adding 0.2 billion years for the birth of the first stars after the Big Bang gives us a Universe age of 13.93 billion years. This number aligns exactly with the ΛCDM prediction (13.8 billion years) and deviates from the "early" solutions to the Hubble tension by 0.8 billion years—a gap of over 3σ. Even relaxing the selection criteria, the astronomers could not push the limit below 13.31 billion years. Stellar fossils impose a veto on theories that rejuvenate the cosmos: this 0.8 billion years is longer than the age of the Solar System; it's impossible to be off by an entire planet's lifetime.
This result casts a shadow on a whole constellation of hypotheses that invoked exotic physics in the infancy of the cosmos—be it new particles, varying gravity, or extra dimensions. The resolution to the Hubble tension, it seems, is hidden not in the early but in the late Universe: in local voids, inhomogeneities, or cunning modifications of general relativity. And to find it, even more precise ages are needed. On the horizon are updated isochrones, asteroseismology that turns stars into tuning forks (their oscillations will reveal the age of their interiors), and the James Webb Space Telescope, capable of spotting the dim embers of the first low-mass stars. We are just beginning to leaf through the paleontological chronicle of the Milky Way, and each new layer narrows the gaps for imagination. Thus, step by step, fundamental cosmology gains its missing link.
🎯 The oldest star in this study is an ordinary subgiant with a mass of 0.8 solar masses. For nearly 13 billion years, it steadily burned hydrogen on the main sequence and only recently began to swell, turning into a red giant. Astronomers caught it on the threshold of stellar retirement—a moment that lasts millions of years, yet is fleeting by cosmic standards.
🎬 In Paul Anderson's novel "Tau Zero," a starship accelerates to near-light speed, and the crew witnesses billions of years of cosmic history. This study reminds us that even without extreme journeys, we can peer into the deep past—just by studying the stellar fossils scattered across the Milky Way.