Abstract:  Since Hubble and Lamaitre's discovery, direct measurements of the scale factor of the universe as a function of time a(t) have played central roles in understanding the evolutionary history of our universe. While such measurements for the late time universe are achieved through the standard candles, in this talk we show that, for the primordial universe, it can be achieved through the standard clocks. These primordial standard clocks are classically or quantum-mechanically oscillating massive fields, generically present during the primordial epoch. We identify new observable signals, induced by these primordial standard clocks, that allow us to directly measure a(t) and thus distinguish in a model-independent way the scenario of cosmic inflation from its alternatives. We forecast the observability of these signals in future experiments.