Turning Around along the Cosmic Web
  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 .
