  The effects of plasticization of poly(methyl methacrylate) glass on the boson
peaks observed by Raman and neutron scattering are compared. In plasticized
glass the cohesion heterogeneities are responsible for the neutron boson peak
and partially for the Raman one, which is enhanced by the composition
heterogeneities. Because the composition heterogeneities have a size similar to
that of the cohesion ones and form quasiperiodic clusters, as observed by small
angle X-ray scattering, it is inferred that the cohesion heterogeneities in a
normal glass form nearly periodic arrangements too. Such structure at the
nanometric scale explains the linear dispersion of the vibrational frequency
versus the transfer momentum observed by inelastic X-ray scattering.
