  Propagation of gravitational and acoustic plane waves in a flat universe
filled with a general relativistic, homogeneous and isotropic, spatially flat
continuum is studied. The continuum is described by analogues of
nonrelativistic characteristics, namely energy per particle, pressure and Lame
coefficients, and considered in the comoving proper-time gauge. For all modes
with the given wave covector, differential equations governing the time
dependence of the amplitudes are derived. In particular, longitudinal acoustic
waves are described, in analogy with the nonrelativistic theory, by two coupled
first-order equations. As an example, plane waves in a stiff ultrarigid
continuum are considered.
