author={Martin Sahl'en and {\'I}{\~n}igo Zubeld{\'i}a and Joseph I. Silk},
  journal={The Astrophysical Journal Letters},
Combining galaxy cluster and void abundances breaks the degeneracy between mean matter density &OHgr; m ?> and power-spectrum normalization &sgr; 8 ?> . For the first time for voids, we constrain &OHgr; m = 0.21 ± 0.10 ?> and &sgr; 8 = 0.95 ± 0.21 ?> for a flat Λ CDM universe, using extreme-value statistics on the claimed largest cluster and void. The Planck-consistent results detect dark energy with two objects, independently of other dark energy probes. Cluster–void studies are also… 
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