On the slow roll expansion for brane inflation

Abstract

One possibility for identifying the inflaton in the framework of string theory isthat it is a D-brane modulus. This option involves a specific, non-canonical formof the kinetic energy—the Dirac–Born–Infeld action. This note investigates theapplicability of the slow roll approximation in inflationary models of this type.To this end the slow roll expansion of Liddle, Parsons and Barrow is derivedfor the case of the DBI action. The resulting slow roll conditions augment thestandard ones valid in the case of canonical kinetic terms. It is also shown that inDBI models inflation does not require that the potential dominate the energydensity.