  Inflation predicts a stochastic background of gravitational waves over a
broad range of frequencies, from those accessible with cosmic microwave
background (CMB) measurements, to those accessible directly with
gravitational-wave detectors, like NASA's Big-Bang Observer (BBO), currently
under study. In a previous paper [Phys. Rev. D73, 023504 (2006)] we connected
CMB constraints to the amplitude and tensor spectral index of the inflationary
gravitational-wave background (IGWB) at BBO frequencies for four classes of
models of inflation by directly solving the inflationary equations of motion.
Here we extend that analysis by including results obtained in the WMAP
third-year data release as well as by considering two additional classes of
inflationary models. As often noted in the literature, the recent indication
that the primordial density power-spectrum has a red spectral index implies
(with some caveats) that the amplitude of the IGWB may be large enough to be
observable in the CMB polarization. Here we also explore the implications for
the direct detection of the IGWB.
