Cosmological constraints on non-adiabatic dark energy perturbations
  The exact nature of dark energy is currently unknown and its cosmological
perturbations, when dark energy is assumed not to be the cosmological constant,
are usually modeled as adiabatic. Here we explore the possibility that dark
energy might have a nonadiabatic component and we examine how it would affect
several key cosmological observables. We present analytical solutions for the
growth rate and growth index of matter density perturbations and compare them
to both numerical solutions of the fluid equations and an implementation in the
Boltzmann code CLASS, finding that they all agree to well below one percent. We
also perform a Monte Carlo analysis to derive constraints on the parameters of
the nonadiabatic component using the latest cosmological data, including the
temperature and polarization spectra of the Cosmic Microwave Background as
observed by Planck, the Baryon Acoustic Oscillations, the Pantheon type Ia
supernovae compilation and lastly, measurements of Redshift Space Distortions
(RSD) of the growth rate of matter perturbations. We find that the amplitude of
the nonadiabatic pressure perturbation is consistent with zero within
$1\sigma$. Finally, we also present a new, publicly available, RSD likelihood
for MontePython based on the "Gold 2018" growth rate data compilation.
