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We evaluate our capability to constrain the abundance of primordial tensorperturbations in cosmologies with generalized expansion histories in the epochof cosmic acceleration. Forthcoming satellite and sub-orbital experimentsprobing polarization in the CMB are expected to measure the B-mode power in CMBpolarization, coming from PGWs on the degree scale, as well as gravitationallensing on arcmin scales; the latter is the main competitor for the measurementof PGWs, and is directly affected by the underlying expansion history,determined by the presence of a DE component. In particular, we consider earlyDE possible scenarios, in which the expansion history is substantially modifiedat the epoch in which the CMB lensing is most relevant. We show that theintroduction of a parametrized DE may induce a variation as large as 30% in theratio of the power of lensing and PGWs on the degree scale. We find thatadopting the nominal specifications of upcoming satellite measurements theconstraining power on PGWs is weakened by the inclusion of the extra degrees offreedom, resulting in a reduction of about 10% of the upper limits on r infiducial models with no GWs, as well as a comparable increase in the error barsin models with non-zero r. Moreover, we find that the inclusion of sub-orbitalCMB experiments, capable of mapping the B-mode power up to the angular scalesaffected by lensing, can restore the forecasted performances with acosmological constant. Finally, we show how the combination of CMB data withType Ia SNe, BAO and Hubble constant allows to constrain simultaneously r andthe DE quantities in the parametrization we consider, consisting of presentabundance and first redshift derivative of the energy density. We compare thisstudy with results obtained using the forecasted lensing potential measurementprecision from CMB satellite observations, finding consistent results.

Measuring primordial gravitational waves from CMB B-modes in cosmologies with generalized expansion histories