Cluster-void degeneracy breaking: Modified gravity in the balance

Combining galaxy cluster and void abundances is a novel, powerful way to constrain deviations from general relativity and the model. For a flat wCDM model with growth of large-scale structure parametrized by the redshift-dependent growth index (z) =₀+₁z/ (1+z) of linear matter perturbations, combining void and cluster abundances in future surveys with Euclid and the four-meter multiobject spectroscopic telescope could improve the figure of merit for (w, ₀, ₁) by a factor of 20 compared to individual abundances. In an ideal case, improvement on current cosmological data is a figure of merit factor 600 or more.