  A macroscopic Coulomb system of identical charged particles with or without a
compensating background charge can evolve maintaining spatial homogeneity and
isotropy that mimic the cosmological evolution of a universe with repulsive
gravity. Here we study dynamics of small perturbations on the background of the
corresponding Hubble flow by analyzing its normal modes of vibrations.
Arbitrary disturbance of the flow can be resolved into two electro-acoustic,
two vortical, and one entropic modes whose dynamics is investigated.
Specifically, in the zero pressure or long-wavelength limits perturbations of
density and velocity evolve in a manner that is independent of the form of the
initial disturbance. The same conclusion applies to vortical perturbations of
the velocity for arbitrary pressure while entropic perturbations are advected
by the Hubble flow. Without the background charge the underlying Hubble flow
describes a Coulomb explosion whose stability with respect to small
disturbances is also demonstrated.
