Standard Clock in Primordial Density Perturbations and Cosmic Microwave
  Background
  Standard Clocks in the primordial epoch leave a special type of features in
the primordial perturbations, which can be used to directly measure the scale
factor of the primordial universe as a function of time a(t), thus
discriminating between inflation and alternatives. We have started to search
for such signals in the Planck 2013 data using the key predictions of the
Standard Clock. In this Letter, we summarize the key predictions of the
Standard Clock and present an interesting candidate example in Planck 2013
data. Motivated by this candidate, we construct and compute full Standard Clock
models and use the more complete prediction to make more extensive comparison
with data. Although this candidate is not yet statistically significant, we use
it to illustrate how Standard Clocks appear in Cosmic Microwave Background and
how they can be further tested by future data. We also use it to motivate more
detailed theoretical model building.
