  Scalar modes are considered for the propagation of gravitational waves in
cosmological background taking into account of thermal radiation and near
cosmological singularities. In particular, we point out that the contribution
of thermal radiation can heavily affect the dynamics of gravitational waves
giving enhancement or dissipation effects both at quantum and classical level.
These effects are considered both in General Relativity and in modified
theories like $F(R)$ gravity. The possible detection and the disentanglement of
the various gravitational modes on the stochastic background are also
discussed.
