A Microscopic Limit on Gravitational Waves from D-brane Inflation

TL;DR

This paper establishes a robust, geometry-driven bound on the inflaton field range for warped D-brane inflation, translating it into a stringent limit on primordial gravitational waves via the Lyth bound. By analyzing warped throat geometries and volume constraints, the authors show that slow-roll warped brane inflation yields an unobservably small tensor fraction, and DBI inflation faces severe challenges to produce detectable tensors without compromising other observables. They further derive strong microscopic constraints for quadratic DBI inflation, tying five-form flux and compactification volume to the amplitude and non-Gaussianity of perturbations, with Planck-era implications. Overall, the work suggests that detecting primordial gravitational waves would strongly disfavour large classes of string inflation models, while motivating search for genuinely large-field string inflation constructions.

Abstract

We derive a microscopic bound on the maximal field variation of the inflaton during warped D-brane inflation. By a result of Lyth, this implies an upper limit on the amount of gravitational waves produced during inflation. We show that a detection at the level $r > 0.01$ would falsify slow roll D-brane inflation. In DBI inflation, detectable tensors may be possible in special compactifications, provided that $r$ decreases rapidly during inflation. We also show that for the special case of DBI inflation with a quadratic potential, current observational constraints imply strong upper bounds on the five-form flux.