Flattened non-Gaussianities from the effective field theory of inflation
  with imaginary speed of sound
  Inflationary perturbations in multi-field theories can exhibit a transient
tachyonic instability as a consequence of their non-trivial motion in the
internal field space. When an effective single-field description is applicable,
the resulting theory is characterized by fluctuations that propagate with an
$imaginary$ speed of sound. We use the effective field theory of fluctuations
to study such a set-up in a model-independent manner, highlighting the
peculiarities and subtleties that make it different from the standard case. In
particular, perturbations feature exponentially growing and decaying modes
whose relative amplitude is undetermined within the effective field theory.
Nevertheless, we prove that in an interesting limit the dimensionless
bispectrum is in fact universal, depending only on the speed of sound and on
the cutoff scale that limits the validity of the effective theory. Contrary to
the power spectrum, we find that the bispectrum does not display an exponential
enhancement. The amplitude of non-Gaussianities in the equilateral
configuration is similar to the one of conventional models, but it is enhanced
in flattened configurations in a way that is ultraviolet sensitive.
