Clearing the Brush: The Last Stand of Solo Small Field Inflation
  By incorporating both the tensor-to-scalar ratio and the measured value of
the spectral index, we set a bound on solo small field inflation of $\Delta
\phi / m_{Pl} \geq 1.00 \sqrt{r/0.1}$. Unlike previous bounds which require
monotonic $\epsilon_V,$ $|\eta_V| <1$, and 60 e-folds of inflation, the bound
remains valid for non-monotonic $\epsilon_V$, $|\eta_V| \gtrsim 1$, and for
inflation which occurs only over the 8 e-folds which have been observed on the
cosmic microwave background. The negative value of the spectral index over the
observed 8 e-folds is what makes the bound strong; we illustrate this by
surveying single field models and finding that for $r \gtrsim 0.1$ and 8
e-folds of inflation, there is no simple potential which reproduces observed
CMB perturbations and remains sub-Planckian. Models that are sub-Planckian
after 8 e-folds must be patched together with a second epoch of inflation that
fills out the remaining $\sim 50$ e-folds. This second, post-CMB epoch is
characterized by extremely small $\epsilon_V$ and therefore an increasing
scalar power spectrum. Using the fact that large power can overabundantly
produce primordial black holes, we bound the maximum energy level of the second
phase of inflation.
