  The lattice dynamics of the silica polymorph {\alpha}-cristobalite has been
investigated by a combination of diffuse and inelastic x-ray scattering and ab
initio lattice dynamics calculations. Phonon dispersion relations and
vibrational density of states are reported and the phonon eigenvectors analysed
by a detailed comparison of scattering intensities. The experimentally
validated calculation is used to identify the vibration contributing most to
the first peak in the density of vibrational states. The comparison of its
displacement pattern to the silica polymorphs {\alpha}-quartz and coesite and
to vitreous silica reveals a distinct similarity and allows for decisive
conclusions on the vibrations causing the so-called Boson peak in silica glass.
