Observable Chiral Gravitational Waves from Inflation in String Theory
  We consider gravitational wave production during inflation in type IIB string
theory, and the possibility of observable gravitational waves in small field
inflation. We show that the gauge field excitations on a set of coincident D7
branes, itself critical for moduli stabilization and hence intrinsic to
inflation in string theory, coupled with axion fields from bulk fluxes, can act
as a spectator sector during inflation. This results in a large production of
chiral gravitational waves, even for relatively small values of the axion-gauge
field coupling. We extend this to include a monodromy for the axion, and
demonstrate that in both cases an observable level of gravitational waves is
produced in small field inflation in string theory, with a spectrum that is
maximally chiral. Finally, we demonstrate the consistency with moduli
stabilization and with arbitrary (large or small field) inflationary dynamics
of the host model, considering as an explicit example Kahler Moduli Inflation.
