  We calculate the current power spectrum of the gravitational waves created at
the big bang (and later amplified by the different transitions during the
Universe expansion) taking into account the present stage of accelerated
expansion. Likewise, we determine the power spectrum in a hypothetical second
dust era that would follow the present one if at some future time the dark
energy, that supposedly drives the current accelerated expansion, evolved in
such a way that it became dynamically equivalent to cold dark matter. The
calculated power spectrum as well as the evolution of the density parameter of
the waves may serve to discriminate between phases of expansion and may help
ascertain the nature of dark energy.
