Implications of the scalar tilt for the tensor-to-scalar ratio
  We investigate the possible implications of the measured value of the scalar
tilt $n_s$ for the tensor-to-scalar ratio $r$ in slow-roll, single-field
inflationary models. The measured value of the tilt satisfies $n_s -1\sim
1/N_*$, where $N_* \sim 60$ is the number of $e$-folds for observationally
relevant scales. If this is not a coincidence and the scaling holds for
different values of $N$, it strongly suggests that either $r$ is as big as
$10^{-1}$ (a possibility in tension with the recent data), or smaller than
$10^{-2}$ and exponentially dependent on $n_s$. A large region of the
($n_s$,$r$) plane is not compatible with this scaling.
