  In the framework of the single-field slow-roll inflation, we derive the
Hamiltonian of the linear primordial scalar and tensor perturbations in the
form of time-dependent harmonic oscillator Hamiltonians. We find the invariant
operators of the resulting Hamiltonians and use their eigenstates to calculate
the adiabatic Berry phase for sub-horizon modes in terms of the
Lewis-Riesenfeld phase. We conclude by discussing the discrepancy in the
results of Pal et. al [Class. Quant. Grav. 30 (2013) 12] for these Berry
phases, which is resolved to yield agreement with our results.
