Bayesian reconstruction of the inflaton's speed of sound using CMB data
  We update the search for features, due to transient reductions in inflaton's
speed of sound, in the Cosmic Microwave Background (CMB) angular power spectrum
using Planck 2018 temperature, polarization and lensing data. We develop a new
methodology to test more flexible templates to reconstruct the reduction of the
speed of sound based on Gaussian Processes. We formally derive a dynamical
prior for the shape of the reduction using a \textit{maximum-entropy} approach
to ensure the physical conditions of the model are satisfied. The posterior
allows for one or more consecutive reductions, fitting apparent features in the
CMB power spectra in multipoles from a few tens to $\ell\simeq 2000$. As
expected, these fits are not statistically favored with respect to the
$\Lambda$CDM model. The methodology derived here allows for the inclusion of
additional data sets (in particular, Large Scale Structure data), which in
principle will increase the statistical significance of the reconstruction of
the inflaton's speed of sound.
