We investigate the turnaround radius in the spherical collapse model, both in General Relativity and in modified gravity, in particular f (R) scenarios. The phases of spherical collapse are marked by the density contrast in the instant of turnaround δ t , and by the linear density contrast in the moment of collapse, δ c . We find that the effective mass of the extra scalar degree of freedom which arises in modified gravity models has an impact on δ t of up to ∼ 10%, and that δ c can increase by ∼ 1.0%. We also compute the turnaround radius, R t , which in modified gravity models can increase by up to ∼ 6% at z 0.

Reprinted with permission from the American Physical Society: Physical Review D, 95, 024018 c (2017) by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modied, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We use a spherical model and an extended excursion set formalism with drifting diffusive barriers to predict the abundance of cosmic voids in the context of general relativity as well as fðRÞ and symmetron models of modified gravity. We detect spherical voids from a suite of N-body simulations of these gravity theories and compare the measured void abundance to theory predictions. We find that our model correctly describes the abundance of both dark matter and galaxy voids, providing a better fit than previous proposals in the literature based on static barriers. We use the simulation abundance results to fit for the abundance model free parameters as a function of modified gravity parameters, and show that counts of dark matter voids can provide interesting constraints on modified gravity. For galaxy voids, more closely related to optical observations, we find that constraining modified gravity from void abundance alone may be significantly more challenging. In the context of current and upcoming galaxy surveys, the combination of void and halo statistics including their abundances, profiles and correlations should be effective in distinguishing modified gravity models that display different screening mechanisms.

Halo occupation distribution (HOD) models describe the number of galaxies that reside in different haloes, and are widely used in galaxy-halo connection studies using the halo model (HM). Here, we introduce and study HOD response functions R 𝒪 g that describe the response of the HODs to long-wavelength perturbations 𝒪. The linear galaxy bias parameters b 𝒪 g are a weighted version of b 𝒪 h + R 𝒪 g , where b 𝒪 h is the halo bias, but the contribution from R 𝒪 g is routinely ignored in the literature. We investigate the impact of this by measuring the R 𝒪 g in separate universe simulations of the IllustrisTNG model for three types of perturbations: total matter perturbations, 𝒪 = δ h ; baryon-CDM compensated isocurvature perturbations, 𝒪 = σ; and potential perturbations with local primordial non-Gaussianity, 𝒪 ∝ f NLϕ. Our main takeaway message is that the R 𝒪 g are not negligible in general and their size should be estimated on a case-by-case basis. For stellar-mass selected galaxies, the responses R 𝒪 g and R σ g are sizeable and cannot be neglected in HM calculations of the bias parameters b ϕ g and b σ g ; this is relevant to constrain inflation using galaxies. On the other hand, we do not detect a strong impact of the HOD response R 1 g on the linear galaxy bias b 1 g . These results can be explained by the impact that the perturbations 𝒪 have on stellar-to-total-mass relations. We also look into the impact on the bias of the gas distribution and find similar conclusions. We show that a single extra parameter describing the overall amplitude of R 𝒪 g recovers the measured b 𝒪 g well, which indicates that R 𝒪 g can be easily added to HM/HOD studies as a new ingredient.

Modified gravity (MG) theories aim to reproduce the observed acceleration of the Universe by reducing the dark sector while simultaneously recovering General Relativity (GR) within dense environments. Void studies appear to be a suitable scenario to search for imprints of alternative gravity models on cosmological scales. Voids cover an interesting range of density scales where screening mechanisms fade out, which reaches from a density contrast δ ≈ −1 close to their centers to δ ≈ 0 close to their boundaries. We present an analysis of the level of distinction between GR and two modified gravity theories, the Hu-Sawicki f (R) and the symmetron theory. This study relies on the abundance, linear bias, and density profile of voids detected in n-body cosmological simulations. We define voids as connected regions made up of the union of spheres with a mean density given by ρ v = 0.2 ρ m , but disconnected from any other voids. We find that the height of void walls is considerably affected by the gravitational theory, such that it increases for stronger gravity modifications. Finally, we show that at the level of dark matter n-body simulations, our constraints allow us to distinguish between GR and MG models with | f R0 | > 10 −6 and z S S B > 1. Differences of best-fit values for MG parameters that are derived independently from multiple void probes may indicate an incorrect MG model. This serves as an important consistency check.