  Since Hubble and Lamaitre's discovery of the expanding universe using
galaxies till the recent discovery of the accelerating universe using standard
candles, direct measurements of the evolution of the scale factor of the
universe a(t) have played central roles in establishing the standard model of
cosmology. In this letter, we show that such a measurement may be extended to
the primordial universe using massive fields as standard clocks, providing a
direct evidence for the scenario responsible for the Big Bang. This is a short
and non-technical introduction to the idea of classical and quantum primordial
standard clocks.
