Constraints on Wrapped DBI Inflation in a Warped Throat

TL;DR

The paper investigates whether DBI inflation can produce observable cosmological signals within string-theory constructions by considering D-branes wrapped on cycles of warped throats (D5 and D7) instead of a simple D3-brane. By deriving bounds on the tensor-to-scalar ratio r and the background flux charge N, it shows that D3-brane inflation with large non-Gaussianity is incompatible with WMAP3 data, while wrapped D5/D7 scenarios either face severe Euler-number/tadpole constraints or offer some room for viability in certain throat regions (notably B2-dominant regions for D7). The analysis highlights that achieving observable gravitational waves in these setups requires substantial background charge or alternative relaxations (orbifolding, multi-throat configurations) and that backreaction considerations may be significant. Overall, the results constrain wrapped-brane DBI inflation but point to specific regions (especially D7 in B_{kl}-dominant regions) where compatibility with data and stringy limits might be achieved.

Abstract

We derive constraints on the tensor to scalar ratio and on the background charge of the warped throat for DBI inflation driven by D5- and D7-branes wrapped over cycles of the throat. It is shown that the background charge well beyond the known maximal value is required in most cases for DBI inflation to generate cosmological observables compatible with the WMAP3 data. Most of the results derived in this paper are insensitive to the details of the inflaton potential, and could be applied to generic warped throats.