Quantum Primordial Standard Clocks
  In this paper, we point out and study a generic type of signals existing in
the primordial universe models, which can be used to model-independently
distinguish the inflation scenario from alternatives. These signals are
generated by massive fields that function as standard clocks. The role of
massive fields as standard clocks has been realized in previous works. Although
the existence of such massive fields is generic, the previous realizations
require sharp features to classically excite the oscillations of the massive
clock fields. Here, we point out that the quantum fluctuations of massive
fields can actually serve the same purpose as the standard clocks. We show that
they are also able to directly record the defining property of the scenario
type, namely, the scale factor of the primordial universe as a function of time
a(t), but through shape-dependent oscillatory features in non-Gaussianities.
Since quantum fluctuating massive fields exist in any realistic primordial
universe models, these quantum primordial standard clock signals are present in
any inflation models, and should exist quite generally in
alternative-to-inflation scenarios as well. However, the amplitude of such
signals is very model-dependent.
