2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/29928We develop a theory to describe the damping of elementary excitations of a Bose-condensed gas in the hydrodynamic regime for the thermal cloud. We discuss second sound in a spatially homogeneous gas and the lowest excitations of a trapped condensate.RevTeX, no figuresSoft Condensed MatterStatistical Mechanicstext