  In this thesis, we have reviewed how the expansion of the Universe amplifies
the quantum vacuum fluctuations, and how the relic GWs spectrum is related with
the scale factor. We have evaluated the spectrum in different scenarios: a
model of expanding universe with an era dominated by mini black holes and
radiation right after the inflation and a model with an era of accelerated
expansion right after the radiation era. Next, we have applied the generalized
second law of thermodynamics to the latter model. Finally, we have extended the
GSL study to a single stage universe model dominated by dark energy (either
phantom or not).
