The age of the Universe can be derived by measuring its current rate of expansion and then extrapolating backwards. In practice, however, we also have to know how that expansion rate may have changed through time, and this is dictated by the matter composition and energy density of the Universe. Fortunately, this information is embedded in the tiny temperature fluctuations found in the cosmic microwave background, the faint glow of light that fills the Universe with residual heat left over from the Big Bang. The latest estimate, in 2021, sets the age of the Universe at 13.797 billion years, using the so-called Lambda-CDM concordance model of cosmology.
This age may be a problem, however. A study from 2013 suggested that a particular star, HD 140283 (since dubbed the ‘Methuselah star’), could be older than this; 14.46 ± 0.8 billion years. Although other studies suggest a different age, some researchers have questioned whether it’s the age of the Universe that’s in error. The discovery of galaxies existing a mere 300 million years or so after the Big Bang, in an advanced state of evolution, also compounds this problem.