A canonical quantization of potential perturbations of an ideal fluid in the Friedman cosmology is constructed. It is demonstrated that the spontaneous production of phonons during cosmological expansion is the cause of formation of the initial spectrum of adiabatic perturbations of matter in Friedman's isotropic universe. Long-wave density fluctuations during further evolution at the stage following hydrogen recombination in an expanding universe may lead to the formation of galaxies and galaxy clusters. The spectrum of the primordial density fluctuations is estimated and shown to be consistent with requirements imposed on the initial spectrum by the adiabatic theory of the origin of galaxies.