  In the primordial universe, fields with mass much larger than the mass-scale
of the event-horizon (such as the Hubble parameter in inflation) exist
ubiquitously, and can be excited from time to time and oscillate quickly around
their minima. These excitations can induce specific patterns in density
perturbations, which record the time dependence of the scale factor of the
primordial universe, thus provide direct evidence for the inflation paradigm or
its alternatives. Such effects are conventionally averaged out in theoretical
and data analyses, but can be accessible for experiments targeting on density
perturbations with high multipoles.
