  The quark gluon plasma is formed in heavy-ion collisions, and it can be
described by solutions of relativistic hydrodynamics. In this paper we utilize
perturbative hydrodynamics, where we study first order perturbations on top of
a known solution. We investigate the perturbations on top of the Hubble flow.
From this perturbative solution we can give the form of the particle emitting
source and calculate observables of heavy-ion collisions. We describe the
source function and the single-particle momentum spectra for a spherically
symmetric solution.
