What we can learn from the spectral index of the tensor mode
  If the beginning of inflation is defined at the moment when the vacuum energy
of the inflaton starts to dominate, the energy density of the other fields at
that moment is (by definition) comparable to the inflaton. Although the
fraction will be small at the horizon exit due to the inflationary expansion,
they can alter the scale dependence of the spectrum. At the same time, velocity
of the inflaton field may not coincide with the slow-roll (attractor) velocity.
Those dynamics could be ubiquitous but can easily alter the scale dependence of
the spectrum. Since the scale dependence is currently used to constrain or even
exclude inflation models, it is very important to measure its shift, which is
due to the dynamics that does not appear in the original inflation model.
Considering typical examples, we show that the spectral index of the tensor
mode is a useful measure of such effect. Precise measurement of the higher
runnings of the scalar mode will be helpful in discriminating the source.
