Effect of intermediate Minkowskian evolution on CMB bispectrum
  We consider a non-inflationary early Universe scenario in which relevant
scalar perturbations get frozen out at some point, but then are defrosted and
follow a long nearly Minkowskian evolution before the hot era. This
intermediate stage leaves specific imprint on the CMB 3-point function, largely
independent of details of microscopic physics. In particular, the CMB
bispectrum undergoes oscillations in the multipole l space with roughly
constant amplitude. The latter is in contrast to the oscillatory bispectrum
enhanced in the flattened triangle limit, as predicted by inflation with
non-Bunch--Davies vacuum. Given this and other peculiar features of the
bispectrum, stringent constraints imposed by the Planck data may not apply. The
CMB 3-point function is suppressed by the inverse duration squared of the
Minkowskian evolution, but can be of observable size for relatively short
intermediate Minkowskian stage.
