  All models of dynamical dark energy possess fluctuations, which affect the
number of galaxy clusters in the Universe. We have studied the impact of dark
energy clustering on the number of clusters using a generalization of the
spherical collapse model and the Press-Schechter formalism. Our statistical
analysis is performed in a 7-parameter space using the Fisher matrix method,
for several hypothetical Sunyaev-Zel'dovich and weak lensing (shear maps)
surveys. In some scenarios, the impact of these fluctuations is large enough
that their effect could already be detected by existing instruments such as the
South Pole Telescope, when its data is combined with WMAP and SDSS. Future
observations could go much further and probe the nature of dark energy by
distinguishing between different models on the basis of their perturbations,
not only their expansion histories.
