We use the continuous-time interaction expansion (CT-INT) quantum Monte Carlo
method to calculate the phonon spectral function of the one-dimensional
Holstein-Hubbard model at half-filling. Our results are consistent with a
soft-mode Peierls transition in the adiabatic regime, and the existence of a
central peak related to long-range order in the Peierls phase. We explain a
previously observed feature at small momenta in terms of a hybridization of
charge and phonon excitations. Tuning the system from a Peierls to a metallic
phase with a nonzero Hubbard interaction suppresses the central peak, but a
significant renormalization of the phonon dispersion remains. In contrast, the
dispersion is only weakly modified in the Mott phase. We discuss finite-size
effects, the relation to the dynamic charge structure factor, as well as
additional sum rules and their implications. Finally, we reveal the existence
of a discrete symmetry in a continuum field theory of the Holstein model, which
is spontaneously broken in the Peierls phase.Comment: 10 pages, 4 figure