  It is generally believed that in single-field slow-roll inflation, a large
tensor-to-scalar ratio $r > 0.1$ requires inflaton field values close to or
above the Planck scale. Recently, it has been claimed that $r > 0.15$ can be
achieved with much smaller inflaton field values $\Delta \phi < M_{Pl}/10$. We
show that in single-field slow-roll inflation, it is impossible to reconcile $r
> 0.1$ with such small field values, independently of the form of the
potential, and that the recent claim to the contrary is based on an invalid
approximation. We conclude that the result of the BICEP2 measurement of $r >
0.1$, if confirmed, truly has the potential to rule out small-field models of
single-field slow-roll inflation.

High Energy Physics - Phenomenology;Cosmology and Nongalactic Astrophysics;High Energy Physics - Theory