Voids in modified gravity: excursion set predictions

Abstract

We investigate the behavior of the fifth force in voids in chameleon models using the spherical collapse method. Contrary to Newtonian gravity, we find the fifth force is repulsive in voids. The strength of the fifth force depends on the density inside and outside the void region as well as its radius. It can be many times larger than the Newtonian force and their ratio is in principle unbound. This is very different from the case in halos, where the fifth force is no more than 1/3 of gravity. The evolution of voids is governed by the Newtonian gravity, the effective dark energy force and the fifth force. While the first two forces are common in both ΛCDM and chameleon universes, the fifth force is unique to the latter. Driven by the outward-pointing fifth force, individual voids in chameleon models expand faster and grow larger than in a ΛCDM universe. The expansion velocity of the void shell can be 20% to 30% larger for voids of a few Mpc/h in radius, while their sizes can be larger by ∼ 10%. This difference is smaller for larger voids of the same density. We compare void statistics using excursion set theory; for voids of the same size, their number density is found to be larger in chameleon models. The fractional difference increases with void size due to the steepening of the void distribution function. The chance of having voids of radius ∼ 25 Mpc/h can be 2.5 times larger. This difference is about 10 times larger than that in the halo mass function. We find strong environmental dependence of void properties and population in chameleon models. The differences in size and expansion velocity with GR are both larger for small voids in high density regions. In general, the difference between chameleon models and ΛCDM in void properties (size, expansion velocity and distribution function) are larger than the corresponding quantities for halos. This suggests that voids might be better candidates than halos for testing gravity.

The lensing and temperature imprints of voids on the cosmic microwave background

“…those with the chameleon screening mechanism, voids are expected to be emptier than their GR counterparts. The dark matter profile of voids can therefore provide powerful test for modified gravity (Clampitt, Cai & Li 2013;Barreira et al 2015;Cai, Padilla & Li 2015;Lam et al 2015). Our measurement suggests that it is possible to do this with CMB lensing.…”

Evolution of density and velocity profiles of dark matter and dark energy in spherical voids

“…The various aspects of their formation and distribution of sizes and amplitudes have been developed by Sheth & van de Weygaert (2004) in the context of hierarchical scenarios of the large-scale structure formation. Investigations of last few years show that parameters of voids can be probe for cosmology and gravity theories since the void density profile, form, intrinsic dynamics, statistical properties and their dependences on voids sizes are sensitive to models of dark energy and modified gravity (Biswas et al 2010;Bos et al 2012;Li et al 2012;Jennings et al 2013;Dai 2015;Li & Zhao 2009;Clampitt et al 2013;Cai et al 2015;Hamaus et al 2015;Falck 2016). That is why the voids are extensively investigated using modern catalogues of galaxies as well as N-body simulations (Gottloeber et al 2003;Sutter et al 2014b,c,d;Woltak et al 2016;Demchenko et al 2016).…”

Revealing modified gravity signals in matter and halo hierarchical clustering

“…This is reflected in a more efficient transfer of matter from underdense to ovedense regions, which results in cosmically depressed regions, like cosmic voids, to be characterised by lower densities [see e.g. 51,107,108]. Due to mass conservation the average density remains the same as in the GR case, hence the first affected clustering characteristic is of the second-order, thus the variance of the density field.…”