A Microscopic Limit on Gravitational Waves from D-brane Inflation

TL;DR

This paper establishes a fundamental limit on gravitational wave production in D-brane inflation models, linking microscopic field variations to observable tensor-to-scalar ratios and constraining flux parameters.

Contribution

It derives a microscopic bound on inflaton field variation in warped D-brane inflation, connecting it to gravitational wave limits and observational constraints.

Findings

Detection of r > 0.01 would falsify slow roll D-brane inflation.

Detectable tensors in DBI inflation require rapid decrease of r during inflation.

Current constraints impose strong bounds on five-form flux in quadratic potential DBI models.

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.