Unique Fingerprints of Alternatives to Inflation in the Primordial Power
  Spectrum
  Massive fields in the primordial universe function as standard clocks and
imprint clock signals in the density perturbations that directly record the
scale factor of the primordial universe as a function of time, a(t). A
measurement of such signals would identify the specific scenario of the
primordial universe in a model-independent fashion. In this Letter, we
introduce a new mechanism through which quantum fluctuations of massive fields
function as standard clocks. The clock signals appear as scale-dependent
oscillatory signals in the power spectrum of alternative scenarios to
inflation.
