  It is remarkable that the primordial fluctuations as revealed by the CMB
coincide with what quantum fluctuations would look like if they were stretched
across the sky by accelerated cosmic expansion. It has been observed that this
same stretching also brings very small -- even trans-Planckian -- length scales
up to observable sizes if extrapolated far enough into the past. This
potentially jeopardizes later descriptions of late-time cosmology by
introducing uncontrolled trans-Planckian theoretical errors into all
calculations. Recent speculations, such as the Trans-Planckian Censorship
Conjecture (TCC), have been developed to avoid this problem. We revisit old
arguments why the consistency of (and control over) the Effective Field Theory
(EFT) governing late-time cosmology is not necessarily threatened by the
descent of modes due to universal expansion, even if EFT methods may break down
at much earlier times. Failure of EFT methods only poses a problem if late-time
predictions rely on non-adiabatic behaviour at these early times (such as is
often true for bouncing cosmologies, for example). We illustrate our arguments
using simple non-gravitational examples such as slowly rolling scalar fields
and the spacing between Landau levels for charged particles in slowly varying
magnetic fields, for which similar issues arise and are easier to understand.
We comment on issues associated with UV completions. Our arguments need not
invalidate speculative ideas like the TCC but suggest they are not required by
the present evidence.
