  The statistical uncertainty in measuring the primordial density perturbations
on a given comoving scale is dictated by the number of independent regions of
that scale that are accessible to an observer. This number varies with cosmic
time and diminishes per Hubble volume in the distant past or future of the
standard cosmological model. We show that the best constraints on the initial
power spectrum of linear density perturbations are accessible (e.g. through
21-cm intensity mapping) at redshifts z~10, and that the ability to constrain
the cosmological initial conditions will deteriorate quickly in our cosmic
future.
