Astronomers are baffled by new measurements of the age of the universe which appear to suggest it's younger than some of the stars it contains.
According to their best estimates, the universe is about 13.8 billion years old - but there's a star relatively near to Earth, HD 140283, which appears to be 14.5 billion years old.
"It's a riddle of cosmic proportions: how can the universe contain stars older than itself?" physicist Robert Matthews wrote for The National, an English-language newspaper published in UAE.
"That's the conundrum now facing astronomers trying to establish the age of the universe - and its resolution could spark a scientific revolution."
Dr Matthews, a member of the UK's Royal Astronomical Society, said recent measurements of the cosmic background radiation left over after the Big Bang very precisely show the universe to be 13.8 billion years, plus or minus about 20 million years.
But in 2013, measurements suggested HD 140283 - dubbed the 'Methuselah star' after the biblical figure said to have lived to the ripe old age of 969 - was about 14.5 billion years ago, based on its extremely low metal content.
"Astronomers now know it contains very little iron - which means it must have been formed before this element became common in the universe," wrote Dr Matthews. "And that implies HD 140283 must be almost as old as the universe itself."
But 14.5 billion is a bigger number than 13.8 billion - so what's going on? Well, NASA's estimate of HD 140283's age is imprecise - it could be about 800 million years younger, or possibly older, than 14.5 billion. But that leaves only a slim chance its age is lower than 13.8 billion.
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Dr Matthews says new research into gravitational waves could resolve the paradox, but it's still early days.
"In the meantime, theorists have been busy dreaming up new physics that might solve the problem," he wrote.
One is a revival of an idea thought up by the man who coined the phrase 'Big Bang', Fred Hoyle. Sir Fred used the term disparagingly, believing instead matter was constantly being created and the universe had no beginning, having existed forever.
Dr Matthews says most scientists would consider that a "more mysterious" state of affairs than the current explanation for the origin of matter, despite its flaws.
"But now, theorists are looking again at such 'crazy' ideas, because the age problem is back with a vengeance."