The Effective Strength of Gravity, the Scale of Inflation (and how KK gravitons evade the Higuchi Bound)

For any given momentum transfer, gravitational interactions have a strength
set by a characteristic scale $M_*$ inferred from amplitudes calculated in an
effective theory with a strong coupling scale $M_{**}$. These are in general
different from each other and $M_{\rm pl}$, the macroscopic strength of gravity
as determined by (laboratory scale) Cavendish experiments. During single field
inflation, $M_*$ can differ from $M_{\rm pl}$ due to the presence of any number
of (hidden) universally coupled species between laboratory and inflationary
scales. Although this has no effect on dimensionless (i.e. observable)
quantities measured at a fixed scale such as the amplitude and spectral
properties of the CMB anisotropies, it complicates the inference of an absolute
scale of inflation given any detection of primordial tensors. In this note we
review and elaborate upon these facts and address concerns raised in a recent
paper.