  We theoretically and observationally investigate different choices of initial
conditions for the primordial mode function that are imposed during an epoch
preceding inflation. By deriving predictions for the observables resulting from
several alternate quantum vacuum prescriptions we show some choices of vacua
are theoretically observationally distinguishable from others. Comparing these
predictions to the Planck 2018 observations via a Bayesian analysis shows no
significant evidence to favour any of the quantum vacuum prescriptions over the
others. In addition we consider frozen initial conditions, representing a
white-noise initial state at the big-bang singularity. Under certain
assumptions the cosmological concordance model and frozen initial conditions
are found to produce identical predictions for the cosmic microwave background
anisotropies. Frozen initial conditions may thus provide an alternative
theoretic paradigm to explain observations that were previously understood in
terms of the inflation of a quantum vacuum.
