Curing inflationary degeneracies using reheating predictions and relic
  gravitational waves
  It is well known that the inflationary scenario often displays different sets
of degeneracies in its predictions for CMB observables. These degeneracies
usually arise either because multiple inflationary models predict similar
values for the scalar spectral index $n_{_S}$ and the tensor-to-scalar ratio
$r$, or because within the same model, the values of $\lbrace n_{_S}, r
\rbrace$ are insensitive to some of the model parameters, making it difficult
for CMB observations alone to constitute a unique probe of inflationary
cosmology. We demonstrate that by taking into account constraints on the
post-inflationary reheating parameters such as the duration of reheating
$N_{_{\rm re}}$, its temperature $T_{_{\rm re}}$ and especially its equation of
state (EOS), $w_{_{\rm re}}$, it is possible to break this degeneracy in
certain classes of inflationary models where identical values of $\lbrace
n_{_S}, r \rbrace$ can correspond to different reheating $w_{_{\rm re}}$. In
particular, we show how reheating constraints can break inflationary
degeneracies in the T-model and the E-model $\alpha$-attractors. Non-canonical
inflation is also studied. The relic gravitational wave (GW) spectrum provides
us with another tool to break inflationary degeneracies. This is because the GW
spectrum is sensitive to the post-inflationary EOS of the universe. Indeed a
stiff EOS during reheating $(w_{_{\rm re}} > 1/3)$ gives rise to a small scale
blue tilt in the spectral index $n_{_{\rm GW}} = \frac{d\log{\Omega_{_{\rm
GW}}}}{d\log{k}} > 0$, while a soft EOS $(w_{_{\rm re}} < 1/3)$ results in a
red tilt. Relic GWs therefore provide us with valuable information about the
post-inflationary epoch, and their spectrum can be used to cure inflationary
degeneracies in $\lbrace n_{_S}, r\rbrace$.
