Measuring primordial gravitational waves from CMB B-modes in cosmologies
  with generalized expansion histories
  We evaluate our capability to constrain the abundance of primordial tensor
perturbations in cosmologies with generalized expansion histories in the epoch
of cosmic acceleration. Forthcoming satellite and sub-orbital experiments
probing polarization in the CMB are expected to measure the B-mode power in CMB
polarization, coming from PGWs on the degree scale, as well as gravitational
lensing on arcmin scales; the latter is the main competitor for the measurement
of PGWs, and is directly affected by the underlying expansion history,
determined by the presence of a DE component. In particular, we consider early
DE possible scenarios, in which the expansion history is substantially modified
at the epoch in which the CMB lensing is most relevant. We show that the
introduction of a parametrized DE may induce a variation as large as 30% in the
ratio of the power of lensing and PGWs on the degree scale. We find that
adopting the nominal specifications of upcoming satellite measurements the
constraining power on PGWs is weakened by the inclusion of the extra degrees of
freedom, resulting in a reduction of about 10% of the upper limits on r in
fiducial models with no GWs, as well as a comparable increase in the error bars
in models with non-zero r. Moreover, we find that the inclusion of sub-orbital
CMB experiments, capable of mapping the B-mode power up to the angular scales
affected by lensing, can restore the forecasted performances with a
cosmological constant. Finally, we show how the combination of CMB data with
Type Ia SNe, BAO and Hubble constant allows to constrain simultaneously r and
the DE quantities in the parametrization we consider, consisting of present
abundance and first redshift derivative of the energy density. We compare this
study with results obtained using the forecasted lensing potential measurement
precision from CMB satellite observations, finding consistent results.
