Breaking the cosmological invariance of the dark-matter halo shape as a new probe of modified gravity

Laboratoire Univers et Théories (LUTH), CNRS, Observatoire de Paris, PSL Research University, 5 place Jules Janssen, 92195 Meudon, France

^⋆ Corresponding authors; remy.koskas@obspm.fr; jean-michel.alimi@obspm.fr

Received: 15 August 2024
Accepted: 18 November 2024

Abstract

In a recent paper, we highlighted in wCDM models derived from general relativity (GR) (with Dark Energy Universe numerical simulation data), a cosmological invariance of the distribution of dark-matter (DM) halo shapes when expressed in terms of the nonlinear fluctuations of the cosmic matter field. This paper shows that this invariance persists when tested on numerical simulations performed with a different N-body solver, and that it is also robust to adding massive neutrinos to the cold DM component. Furthermore, this discovery raises crucial questions about the validity of this invariance in MG models. Thus, we examined whether it remains robust in the case of Hu & Sawicki model using DUSTGRAIN-pathfinder numerical simulations. By comparing the results of advanced numerical simulations in these different theoretical frameworks, we found significant deviations from the invariance observed in the framework of wCDM models of GR. These deviations suggest that the gravitation’s nature significantly influences the DM halos’ shape. We then interpreted this departure from the GR models’ invariance as a manifestation of the scalar-field screening effect corresponding to such f(R)-type theories. This one modifies the sphericization process of DM halos during their formation, precisely because the critical mass at which this scalar field becomes non-negligible is the mass at which the deviation appears. To this extent, the departure from cosmological invariance in DM halos’ shape is a cosmological probe of the nature of gravity, and the mass scale at which it appears can be used to estimate the f_R0 parameter of such theories.