Cluster–void Degeneracy Breaking: Dark Energy, Planck, and the Largest Cluster and Void

Abstract

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 complementary in scale, density, and nonlinearity, and are of particular interest for testing modified-gravity models.

Cluster-void degeneracy breaking: Modified gravity in the balance

“…Complementary parameter degeneracies arise between clusters and voids, because they have different sensitivity to structure growth vs. volume expansion with redshift, comoving linear scales, and orthogonal sensitivities between matter disperson σ and Ω m [10,Section 4.5]. While Euclid clusters are better, separately, at constraining structure growth, the void samples are better at constraining the shape of the matter power spectrum.…”

Fundamental physics with the Square Kilometre Array

“…We consider a flat wCDM cosmology (i.e., a CDM cosmology with a constant equation of state, w) with a modified gravity model described by a growth index γ (a) = γ 0 + γ 1 (1 − a) (Di Porto et al 2012). The void distribution is modelled following Sahlén et al (2016) and Sahlén & Silk (2018), here also taking into account the galaxy density and bias for each survey (Yahya et al 2015;Raccanelli et al 2016c). The results are shown in Figure 9.…”

“…The SKA2 void number counts could improve on this, down to σ (γ 0 ) = 0.07, σ (γ 1 ) = 0.15. Using the powerful degeneracy-breaking complementarity between clusters of galaxies and voids (Sahlén et al 2016;Sahlén & Silk 2018;Sahlén 2019), SKA2 voids + Euclid clusters number counts could reach σ (γ 0 ) = 0.01, σ (γ 1 ) = 0.07.…”

Cluster-void degeneracy breaking: Neutrino properties and dark energy

“…We predict cluster and void abundances adopting models and methodology developed in earlier work [3,5,22]. As in [5], we include scatter in cluster mass and void radius determinations, and also vary the characteristic void density contrast (through the parameter D v , see below).…”

“…For voids, we model the effect of neutrinos by extending the treatment in [3]. When neutrinos have nonzero mass, the neutrino density contributes to the dynamical evolution of a void, but does not have a significant density contrast on its own, except for voids larger than the neutrino free-streaming length [e.g.…”