When did time begin? Hint: It wasn't at the big bang

Our universe is expanding, so it must have been smaller in the past. Indeed, if we rewind our cosmological movie, we see the universe shrinking back almost to a point – the big bang – some 13.8 billion years ago. Is this when time began? Albert Einstein's general theory of relativity tells us that the backdrop of the universe is a fluid continuum, space-time, in which neither space nor time has an absolute meaning. What's more, at the big bang, space-time distorts into a point of infinite density called a singularity.