Standard cosmological models peg the universe's age at about 13.8 billion years, derived from cosmic microwave background studies and expansion acceleration data. Yet, a lingering puzzle in cosmology is stars that seem older than the cosmos itself—the so-called "age paradox." HD 140283, or Methuselah Star, exemplifies this, with estimates once reaching 14.27 billion years, prompting astrophysicists to scrutinize measurements and error margins.
For over a century, astronomers have tracked a peculiar star in the Libra constellation, roughly 190 light-years away. Racing across the sky at 1.3 million km/h, HD 140283—nicknamed Methuselah after the Bible's longest-lived figure—is among the universe's oldest known stars.
In 2000, using data from the European Space Agency's Hipparcos satellite, researchers estimated its age at 16 billion years—a startling figure. As Howard Bond, astronomer at Pennsylvania State University, noted, this clashed with the universe's 13.8-billion-year age from cosmic microwave background and expansion observations. "It was a very serious difference," he said.
The apparent age posed a fundamental issue: How could a star predate the universe? Methuselah, a metal-poor subgiant rich in hydrogen and helium but low in iron, likely formed before iron became widespread.
Bond's team revisited the estimate using 11 Hubble Space Telescope datasets from 2003–2011, captured by Fine Guidance Sensors for positions, distances, and energy outputs. Parallax, spectroscopy, and photometry enabled sharper accuracy.
"Distance was a key uncertainty for HD 140283," Bond explained. "Better parallax—tracking position shifts every six months from Earth's orbit—revealed luminosity, and thus age: brighter stars burn fuel faster and are younger."
Theoretical models also carried uncertainties, like core nuclear reaction rates and outer-layer element diffusion. The team factored in helium diffusion depleting core hydrogen, accelerating fuel use and lowering age estimates.
"Oxygen levels were crucial too," Bond added. HD 140283's elevated oxygen-to-iron ratio suggested formation after oxygen built up, further trimming its age.
Bond's group revised it to 14.46 billion years—down from 16 billion—but still seemingly older than the universe. An 800-million-year uncertainty margin, however, overlapped with cosmic timelines.
"All age estimates involve random and systematic errors," says physicist Robert Matthews of Aston University. "Overlapping error bars suggest no true conflict—resolving it requires stretching uncertainties to their limits."
Refinements continued: A 2014 study pegged it at 14.27 billion years. "With all uncertainties—observational and theoretical—the error bar spans 700–800 million years, encompassing 13.8 billion," Bond concluded.
What might make the universe seem younger? "Science often reveals a mix: overlooked observational errors plus theoretical gaps, like dark energy driving expansion," Matthews explains.
Resolving this demands precise expansion rates—the Hubble constant. Divergent measures (67 km/s/Mpc from cosmic background vs. 73 from Cepheids/supernovae) fuel debate. Gravitational waves from mergers could refine this over traditional methods.
Matthews posits varying dark energy might explain it, aligning with theories like causal set gravity. Enhanced Hubble data promises clarity on the universe's true age.
