Inflation in DBI models with constantγ

Abstract

Dirac–Born–Infeld scalar field theories which appear in the context of inflation in stringtheory in general have a field-dependent speed of sound. It is, however, possible to writedown DBI models which possess exact solutions characterized by a constant speedof sound different from unity. This requires that the potential and the effectiveD-brane tension appearing in a DBI action have to be related in a specific way. This paperdescribes such models in general and presents some examples with a constant speed of soundcs<1 for which the spectrum of scalar perturbations can be found analytically without resortingto the slow-roll approximation.