How Well Can We Really Determine the Scale of Inflation?
  A detection of primordial B-modes has been heralded not only as a smoking gun
for the existence of inflation, but also as a way to establish the scale at
which inflation took place. In this paper we critically reinvestigate the
connection between a detection of primordial gravity waves and the scale of
inflation. We consider whether the presence of additional fields and
non-adiabaticity during inflation may have provided an additional source of
primordial B-modes competitive with those of the quasi-de Sitter vacuum. In
particular, we examine whether the additional sources could provide the
dominant signal, which could lead to a misinterpretation of the scale of
inflation. In light of constraints on the level of non-Gaussianity coming from
Planck we find that only hidden sectors with strictly gravitationally strength
couplings provide a feasible mechanism. The required model building is somewhat
elaborate, and so we discuss possible UV completions in the context of Type IIB
orientifold compactifications with RR axions. We find that an embedding is
possible and that dangerous sinusoidal corrections can be suppressed through
the compactification geometry. Our main result is that even when additional
sources of primordial gravity waves are competitive with the inflaton, a
positive B-mode detection would still be a relatively good indicator of the
scale of inflation. This conclusion will be strengthened by future constraints
on both non-Gaussianity and CMB polarization.
