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 $\Lambda$CDM model. For a
flat $w$CDM model with growth of large-scale structure parameterized by the
redshift-dependent growth index $\gamma(z) = \gamma_0 + \gamma_1 z/(1+z)$ of
linear matter perturbations, combining void and cluster abundances in future
surveys with Euclid and the 4-metre Multi-Object Spectroscopic Telescope
(4MOST) could improve the Figure of Merit for ($w, \gamma_0, \gamma_1$) 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.
