Remy Koskas | Searcharxiv

arXiv2024

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

In a recent paper, Alimi & Koskas highlighted in $w$CDM 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. However, this discovery raises crucial questions about the validity of this invariance in MG models. Thus, we examined whether the invariance observed by Alimi & Koskas 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 $w$CDM 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.

astro-ph.CO↗

arXiv2024

The shape of dark matter halos: a new fundamental cosmological invariance

We focus on the complex relationship between the shape of dark matter halos and the cosmological models underlying their formation. We used three realistic cosmological models from the Dark Energy Universe Simulation suite. They have significantly distinct cosmological parameters ($Ω_m$, $σ_8$, and $w$) but quasi-indistinguishable cosmic matter fields beyond the scale of DM halos. Firstly, we developed a robust method to measure the FoF halos shape, while avoiding numerical artifacts on shape measurements. These artifacts are generally induced by the presence of substructures (resolution-dependent) or by any spherical a priori in contradiction with the halos triaxiality. We obtain a marked dependence of the halos shape, both on their mass and on the cosmological model. However, when re-expressing the halos shape parameters in terms of the non-linear fluctuations of the cosmic matter field, the cosmological dependence disappears. This new fundamental cosmological invariance is a direct consequence of the matter field non-linear dynamics: as the universe evolves, the non-linear fluctuations of the cosmic field increase, driving the halos towards sphericity. The deviation from sphericity, measured by the prolaticity, triaxiality, and ellipticity of the dark matter halos, is therefore entirely encapsulated in the non-linear power spectrum. From this fundamental invariant relation, we can reconstruct with remarkable accuracy the non-linear variance of the cosmic matter field and, consequently, the non-linear power spectrum. We also re-find the $σ_8$ amplitude of the cosmological model. Our results highlight, not only the nuanced relationship between dark matter halo formation and the underlying cosmology, but also the potential of dark matter halo shape analysis as a powerful tool for probing the non-linear dynamics of the cosmic matter field.

astro-ph.CO↗