Turnaround density evolution encodes cosmology in simulations

Abstract

Context. The mean matter density within the turnaround radius, which is the boundary that separates a nonexpanding structure from the Hubble flow, was recently proposed as a novel cosmological probe. According to the spherical collapse model, the evolution with cosmic time of this turnaround density, ρta(z), can be used to determine both Ωm and ΩΛ, independently of any other currently used probe. The properties of ρta predicted by the spherical collapse model (universality for clusters of any mass, value) were also shown to persist in the presence of full three-dimensional effects in ΛCDM N-body cosmological simulations when considering galaxy clusters at the present time, z = 0. However, a small offset was discovered between the spherical-collapse prediction of the value of ρta at z = 0 and its value measured in simulations. Aims. In this letter, we explore whether this offset evolves with cosmic time; whether it differs in different cosmologies; whether its origin can be confidently identified; and whether it can be corrected. Specifically, we aim to examine whether the evolution of ρta can be used to distinguish between simulated universes with and without a cosmological constant. Methods. We used N-body simulations with different cosmological parameters to trace the evolution of the turnaround density ρta with cosmic time for the largest dark matter halos in the simulated boxes. To this end, we analyzed snapshots of these simulations at various redshifts, and we used radial velocity profiles to identify the turnaround radius within which we measured ρta. Results. We found an offset between the prediction of the spherical collapse model for ρta and its measured value from simulations. The offset evolves slightly with redshift. This offset correlates strongly with the deviation from spherical symmetry of the dark matter halo distribution inside and outside of the turnaround radius. We used an appropriate metric to quantify deviations in the environment of a structure from spherical symmetry. We found that using this metric, we can construct a sphericity-selected sample of halos for which the offset of ρta from the spherical collapse prediction is zero, independently of redshift and cosmology. Conclusions. We found that a sphericity-selected halo sample allows us to recover the simulated cosmology, and we conclude that the turnaround density evolution indeed encodes the cosmology in N-body simulations.