# Cold dark energy constraints from the abundance of galaxy clusters

## TL;DR

This paper investigates the constraints on cold dark energy with negligible sound speed using galaxy cluster abundance data, highlighting its clustering properties and potential to improve cosmological parameter estimates.

## Contribution

It introduces a new mass function calibration for cold dark energy models and demonstrates their distinguishability from standard models using cluster data.

## Key findings

- Cold dark energy models show up to 1σ shift from quasi-homogeneous models.
- Cluster data combined with other probes constrains dark energy parameters effectively.
- Forecasts indicate 50% tighter constraints on key parameters with DES data.

## Abstract

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.