Disformal transformations as a change of units
  A disformal transformation is a very useful tool to analyze a general
effective theory description of inflation. It can for example be used to set
the tensor sound speed to unity, such that the tensor power spectrum only
depends on the Hubble parameter. However, the disformal transformation has also
led to quite some confusion in recent literature. We hope to clarify that
confusion by pointing out that a disformal transformation is nothing else than
a change of units. We show how everything that can be achieved by a (possibly
time-dependent) disformal transformation, equally follows from the equivalent
change of units, up to all orders in perturbation theory. We also comment on
the sensitivity of the tensor power spectrum to a non-standard tensor speed of
sound.
