Evolution of spherical overdensities in holographic dark energy models

Abstract

In this work we investigate the spherical collapse model in flat FRW dark energy universes. We consider the Holographic Dark Energy (HDE) model as a dynamical dark energy scenario with a slowly time-varying equation-of-state (EoS) parameter w de in order to evaluate the effects of the dark energy component on structure formation in the universe. We first calculate the evolution of density perturbations in the linear regime for both phantom and quintessence behavior of the HDE model and compare the results with standard Einstein-de Sitter (EdS) and ΛCDM models. We then calculate the evolution of two characterizing parameters in the spherical collapse model, i.e., the linear density threshold δ c and the virial overdensity parameter ∆ vir . We show that in HDE cosmologies the growth factor g(a) and the linear overdensity parameter δ c fall behind the values for a ΛCDM universe while the virial overdensity ∆ vir is larger in HDE models than in the ΛCDM model. We also show that the ratio between the radius of the spherical perturbations at the virialization and turn-around time is smaller in HDE cosmologies than that predicted in a ΛCDM universe. Hence the growth of structures starts earlier in HDE models than in ΛCDM cosmologies and more concentrated objects can form in this case. It has been shown that the non-vanishing surface pressure leads to smaller virial radius and larger virial overdensity ∆ vir . We compare the predicted number of halos in HDE cosmologies and find out that in general this value is smaller than for ΛCDM models at higher redshifts and we compare different mass function prescriptions. Finally, we compare the results of the HDE models with observations.

Model selection and constraints from holographic dark energy scenarios

“…We focus our analysis at sub-horizon scales, where the results of Pseudo Newtonian dynamics are well consistent with those of General Relativity (GR) paradigm (see Abramo et al 2007). In this context, two different scenarios have been studied in literature (Armendariz-Picon et al 1999;Garriga & Mukhanov 1999;Armendariz-Picon et al 2000;Erickson et al 2002;Bean & Doré 2004;Hu & Scranton 2004;Abramo et al 2007Abramo et al , 2008Bean & Doré Bal;Abramo et al 2009;Basilakos et al 2009a;de Putter et al 2010;Pace et al 2010;Akhoury et al 2011;Sapone & Majerotto 2012;Pace et al 2012;Batista & Pace 2013;Dossett & Ishak 2013;Batista 2014;Basse et al 2014;Pace et al 2014b,a,c;Malekjani et al 2015;Naderi et al 2015;Mehrabi et al 2015a,b,c;Nazari-Pooya et al 2016;Malekjani et al 2017). In the first scenario the DE component is homogeneous and only the corresponding non-relativistic matter is allowed to clump, while in the second scenario the whole system clusters (both matter and DE).…”

Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations

“…Here ρcrit is the critical density ρcrit ≡ 3H 2 /4πG and ∆vir is a parameter that depends on cosmology. For instance, ∆vir ≈ 178 in an Einstein de Sitter cosmology (see Naderi et al (2015) for a review). More generally, in the literature, people prefer to make use of radii definitions that are independent of cosmology and refer to R∆ as the radius that includes an over-density of ∆ · ρcrit.…”

Spherical collapse model and cluster number counts in power-lawf(T) gravity

“…The spherical collapse model (Gunn & Gott 1972) is a simple but still a useful tool utilized to investigate the growth of bound systems in the universe through gravitational instability (Peebles 1993). It is well known that the main quantities of the SCM, such as the linear overdensity parameter δc and the virial overdensity ∆vir, are affected by the presence of dark energy (Lahav et al 1991 Malekjani et al 2015;Naderi et al 2015). Here our aim is to extent SCM within the f (T ) cosmological scenario, in order to derive the non-linear structure formation in such models and study the differences with the corresponding predictions of the usual ΛCDM cosmology.…”