Inflating in a trough: single-field effective theory from multiple-field curved valleys

We examine the motion of light fields near the bottom of a potential valley in a multi-dimensional field space. In the case of two fields we identify three general scales, all of which must be large in order to justify an effective low-energy approximation involving only the light field, ℓ. (Typically only one of these — the mass of the heavy field transverse to the trough — is used in the literature when justifying the truncation of heavy …