The energy-momentum tensor of perturbed Friedmann universes in the longitudinal gauge (depending on only one gravitational potential) is obtained in order to clarify the physical meaning of two important cases: (1) conformally static perturbations (when the potential is independent of time), and (2) nonstatic perturbations in the case where the potential allows a particular separation of time and space coordinates. The statement according to which the longitudinal gauge allows a description of high-density-contrast regions is analyzed. In the conformally static case we suggest interpreting the energy-momentum tensor as representing a set of particles in gravitational interaction, suitable for describing the post-recombination epoch. It is suggested that the second case be interpreted as a plasma in equilibrium with radiation, suitable for describing the period just before the recombinat... [Llegir més ...]