  If inflation was preceded by a radiation era then at the time of inflation
there will exist a decoupled thermal distribution of gravitons. Gravitational
waves generated during inflation will be amplified by the process of stimulated
emission into the existing thermal distribution of gravitons. Consequently the
usual zero temperature scale invariant tensor spectrum is modified by a
temperature dependent factor. This thermal correction factor amplify the
$B$-mode polarization of the CMB by an order of magnitude at large angles,
which may now be in the range of observability of WMAP.
