Modeling halo mass functions in chameleonf(R)gravity

Abstract

On cosmological scales, observations of the cluster abundance currently place the strongest constraints on f (R) gravity. These constraints lie in the large-field limit, where the modifications of general relativity can correctly be modeled by setting the Compton wavelength of the scalar field to its background value. These bounds are, however, at the verge of penetrating into a regime where the modifications become nonlinearly suppressed due to the chameleon mechanism and cannot be described by this linearized approximation. For future constraints based on observations subjected to cluster abundance, it is therefore essential to consistently model the chameleon effect. We analyze descriptions of the halo mass function in chameleon f (R) gravity using a mass-and environmentdependent spherical collapse model in combination with excursion set theory and phenomenological fits to N -body simulations in the ΛCDM and f (R) gravity scenarios. Our halo mass functions consistently incorporate the chameleon suppression and cosmological parameter dependencies, improving upon previous formalisms and providing an important extension to N -body simulations for the application in consistent tests of gravity with observables sensitive to the abundance of clusters.

Exploring the liminality: properties of haloes and subhaloes in borderlinef(R) gravity

“…Lombriser et al (2013) found that haloes in f(R) gravity can be well described by the NFW formula (equation 11). In this work, we have further confirmed this by explicitly checking the χ 2 goodness-of-fit, in which we found that equation (11) works almost equally well in GR and f(R) gravity (with marginally smaller χ 2 for haloes between ∼10 12 -10 13 h −1 M in f(R) gravity), though the concentration parameters can be different, as we shall show below.…”

“…In particular, the curves are smoother and better behaved when using M 200 , which may be because the FOF haloes are too irregular in their shapes and gravity is enhanced with different efficiency in different parts of the haloes, which can contaminate the screening effect expected for ideal spherical haloes (see e.g. Li & Efstathiou 2012;Li & Lam 2012;Lombriser et al 2013;Lombriser, Koyama & Li 2014, for more discussion about the expected behaviour of the f(R) halo mass function).…”

Thef(ℛ) halo mass function in the cosmic web

“…However, the difference between even |f R0 | = 10 −5 and ΛCDM is dwarfed by the difference between the theoretical volume fraction and the N-body result at all smoothing scales. This provides a quantitative basis for the assertion in [13] that there is insufficient evidence to warrant modelling the environemnt density using the f (R) equations. …”

“…A more accurate expression for the screening factor, which we will use in this paper, was derived in [13] from results in [15,26]. The gravitational force on a test-particle at the surface of a top-hat overdensity of radius R TH collapsing in an expanding background is given by…”

“…We provide an excursion-set-based approximation to the N-body results for the halo mass function. This synthesises two, major existing results: the unconditional mass function in f (R) calculated by [13][14][15] and the classification of halo statistics in the cosmic web by e.g. [16][17][18].…”