Scalar and Tensor Perturbations in DHOST Bounce Cosmology
  We investigate the bounce realization in the framework of DHOST cosmology,
focusing on the relation with observables. We perform a detailed analysis of
the scalar and tensor perturbations during the Ekpyrotic contraction phase, the
bounce phase, and the fast-roll expansion phase, calculating the power spectra,
the spectral indices, and the tensor to-scalar ratio. Furthermore, we study the
initial conditions, incorporating perturbations generated by Ekpyrotic vacuum
fluctuations, by matter vacuum fluctuations, and by thermal fluctuations. The
scale invariance of the scalar power spectrum can be acquired by introducing a
matter contraction phase before the Ekpyrotic phase or invoking a thermal gas
as the source. The DHOST bounce scenario with cosmological perturbations
generated by thermal fluctuations proves to be the most efficient one, and the
corresponding predictions are in perfect agreement with observational bounds.
Especially the tensor-to-scalar ratio is many orders of magnitude within the
allowed region since it is suppressed by the Hubble parameter at the beginning
of the bounce phase.
