Turning Around along the Cosmic Web

TL;DR

This study investigates how the cosmic web environment influences the occurrence of bound violations in galaxy groups, using simulations to analyze turn-around radii and their relation to dark energy constraints.

Contribution

It demonstrates that the cosmic web environment affects the likelihood of bound violations, challenging the spherical collapse model's predictions and proposing a new method to constrain dark energy.

Findings

Average turn-around radii are below the spherical bound-limit.

Bound violations are possible for individual groups.

The cosmic web reduces the rarity of bound violations.

Abstract

A bound-violation designates a case that the turn-around radius of a bound object exceeds the upper limit put by the spherical collapse model based on the standard $\Lambda$CDM paradigm. Given that the turn-around radius of a bound object is a stochastic quantity and that the spherical model overly simplifies the true gravitational collapse which actually proceeds anisotropically along the cosmic web, the rarity of the occurrence of a bound violation may depend on the web environment. Assuming a Planck cosmology, we numerically construct the bound-zone peculiar velocity profiles along the cosmic web (filaments and sheets) around the isolated groups with virial mass $M_{\rm v}\ge 3\times 10^{13}\,h^{-1}M_{\odot}$ identified in the Small MultiDark Planck simulations and determine the radial distances at which their peculiar velocities equal the Hubble expansion speed as the turn-around radii of the groups. It is found that although the average turn-around radii of the isolated groups are well below the spherical bound-limit on all mass scales, the bound violations are not forbidden for individual groups and that the cosmic web has an effect of reducing the rarity of the occurrence of a bound violation. Explaining that the spherical bound limit on the turn-around radius in fact represents the threshold distance up to which the intervention of the external gravitational field in the bound-zone peculiar velocity profiles around the non-isolated groups stays negligible, we discuss the possibility of using the threshold distance scale to constrain locally the equation of state of dark energy .