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 .