Simply put: we don’t know. We estimate when the Big Bang happened by extrapolating backwards from observations, using cosmological theory. The current uncertainty in that estimation (currently 13 billion years) is about 20 million years – nowhere near accurate enough to pinpoint a 24-hour period.
To complicate things further: time passes differently in different places. The ‘age of the Universe’ is based on the way time flows on average, but for people near a big mass like a planet – or those moving with a different-from-average speed – time will pass differently. This means counting backwards day-by-day from different parts of the Universe will leave you with different answers.
Asked by: Dileep Bagnall, Lancashire
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Jon is a Professor of Physics at University College London. He works on the ATLAS experiment at the CERN Large Hadron Collider.