  The cosmological stochastic gravitational-wave background produced by the
mildly non-linear evolution of density fluctuations is analyzed, in the frame
of an Einstein-de Sitter model, by means of a fully relativistic perturbation
expansion up to second order. The form of these
gravitational-instability-induced gravitational waves is gauge-dependent. In
the synchronous gauge, where the second-order expansion is most easily carried
out, the transverse and traceless tensor modes which are produced also contain
a Newtonian and post-Newtonian piece, whose interpretation as gravitational
waves is non-trivial. A more direct physical understanding of this background
is obtained in the so-called Poisson gauge, where it is seen to consist of a
constant term plus an oscillating piece whose amplitude decays inside the
Hubble radius.
