  We report the results of inelastic neutron scattering experiments performed
with triple-axis spectrometers to investigate the low-temperature collective
dynamics in the ordered phase of uranium dioxide. The results are in excellent
agreement with the predictions of mean-field RPA calculations emphasizing the
importance of multipolar superexchange interactions. By comparing neutron
scattering intensities in different polarization channels and at equivalent
points in different Brillouin zones, we show the mixed
magneto-vibrational-quadrupolar character of the observed excitations. The high
energy resolution afforded by the cold triple-axis spectrometer allowed us to
study in detail the magnon-phonon interaction giving rise to avoided crossings
along the $[00\xi]$ reciprocal space direction.
