Cold dark energy constraints from the abundance of galaxy clusters
  We constrain cold dark energy of negligible sound speed using galaxy cluster
abundance observations. In contrast to standard quasi-homogeneous dark energy,
negligible sound speed implies clustering of the dark energy fluid at all
scales, allowing us to measure the effects of dark energy perturbations at
cluster scales. We compare those models and set the stage for using non-linear
information from semi-analytical modelling in cluster growth data analyses. For
this, we recalibrate the halo mass function with non-linear characteristic
quantities, the spherical collapse threshold and virial overdensity, that
account for model and redshift dependent behaviours, as well as an additional
mass contribution for cold dark energy. We present the first constraints from
this cold dark matter plus cold dark energy mass function using our cluster
abundance likelihood, which self-consistently accounts for selection effects,
covariances and systematic uncertainties. We combine cluster growth data with
CMB, SNe Ia and BAO data, and find a shift between cold versus
quasi-homogeneous dark energy of up to $1\sigma$. We make a Fisher matrix
forecast of constraints attainable with cluster growth data from the on-going
Dark Energy Survey (DES). For DES, we predict $\sim$50$\%$ tighter constraints
on $\left(\Omega_\mathrm{m},w \right)$ for cold dark energy versus $w$CDM
models, with the same free parameters. Overall, we show that cluster abundance
analyses are sensitive to cold dark energy, an alternative, viable model that
should be routinely investigated alongside the standard dark energy scenario.
