Looking for interactions in the cosmological dark sector
  We study observational signatures of non-gravitational interactions between
the dark components of the cosmic fluid, which can be either due to creation of
dark particles from the expanding vacuum or an effect of the clustering of a
dynamical dark energy. In particular, we analyse a class of interacting models
($\Lambda$(t)CDM), characterised by the parameter $\alpha$, that behaves at
background level like cold matter at early times and tends to a cosmological
constant in the asymptotic future. In our analysis we consider both background
and primordial perturbations evolutions of the model. We use Cosmic Microwave
Background (CMB) data together with late time observations, such as the Joint
Light-curve Analysis (JLA) supernovae data, the Hubble Space Telescope (HST)
measurement of the local value of the Hubble-Lema\^itre parameter, and
primordial deuterium abundance from Ly$\alpha$ systems to test the
observational viability of the model and some of its extensions. We found that
there is no preference for values of $\alpha$ different from zero
(characterising interaction), even if there are some indications for positive
values when the minimal $\Lambda$(t)CDM model is analysed. When extra degrees
of freedom in the relativistic component of the cosmic fluid are considered,
the data favour negative values of $\alpha$, which means an energy flux from
dark energy to dark matter.
