A Direct Probe of the Evolutionary History of the Primordial Universe
  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. 
Xingang Chen, Mohammad Hossein Namjoo, Yi Wang
astro-ph.CO gr-qc hep-ph hep-th