Constraints on the interacting vacuum -- geodesic CDM scenario
  We investigate an interacting dark sector scenario in which the vacuum energy
is free to interact with cold dark matter (CDM), which itself is assumed to
cluster under the sole action of gravity, i.e. it is in free fall (geodesic),
as in $\Lambda$CDM. The interaction is characterised by a dimensionless
coupling $q_{\rm V}$ that we constrain using cosmic microwave background data
from the Planck 2015 data release, along with baryon acoustic oscillation,
redshift space distortion and Type Ia supernova measurements. We present the
full linear perturbation theory of this interacting scenario and use MCMC
sampling to study five different cases: two cases in which we have $\Lambda$CDM
evolution in the distant past, until a set redshift $z_{\rm trans}$, below
which the interaction switches on and $q_{\rm V}$ is the single sampled
parameter, with $z_{\rm trans}$ fixed at $z_{\rm trans}=3000$ and $z_{\rm
trans}=0.9$ respectively; a case where we allow this transition redshift to
vary along with $q_{\rm V}$; a case in which the vacuum energy is zero for
$z>z_{\rm trans}$ and then begins to grow once the interaction switches on; and
the final case in which we bin $q_{\rm V}(z)$ in four redshift bins to
investigate the possibility of a dynamical interaction, reconstructing the
redshift evolution of the function using Gaussian processes. We find that, in
all cases where the high redshift evolution is not modified, the results are
compatible with a vanishing coupling, thus finding no significant deviation
from $\Lambda$CDM.
