Alignment of voids in the cosmic web

Abstract

We investigate the shapes and mutual alignment of voids in the large‐scale matter distribution of a Λ cold dark matter (ΛCDM) cosmology simulation. The voids are identified using the novel watershed void finder (WVF) technique. The identified voids are quite non‐spherical and slightly prolate, with axis ratios in the order of c : b : a≈ 0.5 : 0.7 : 1. Their orientations are strongly correlated with significant alignments spanning scales >30 h−1 Mpc. We also find an intimate link between the cosmic tidal field and the void orientations. Over a very wide range of scales we find a coherent and strong alignment of the voids with the tidal field computed from the smoothed density distribution. This orientation–tide alignment remains significant on scales exceeding twice the typical void size, which shows that the long‐range external field is responsible for the alignment of the voids. This confirms the view that the large‐scale tidal force field is the main agent for the large‐scale spatial organization of the cosmic web.

The Size, Shape, and Orientation of Cosmological Voids in the Sloan Digital Sky Survey

“…Perhaps not surprisingly, we find a slight majority of prolate voids (T average = 0.67±0.09) in agreement with the theoretical models of Platen et al (2008). Figure 5 shows the bivariate distribution of 1 versus 2 .…”

“…First, as with the above radius analysis, this result may be affected by cosmic variance, especially at low redshift. Second, using N-body simulations, Platen et al (2008) have shown that the ΛCDM model predicts that the orientation of voids in the distribution of dark matter haloes can be correlated for scales < 30 h −1 Mpc. On the other hand, they also find that this predicted void alignment becomes less significant for larger scales.…”

“…We have used the method of the best-fit ellipsoid similar to that used by Jang-Condell & Hernquist (2001) and Platen et al (2008). We use Monte Carlo methods to compute the shape tensor S:…”

Detection of the Velocity Shear Effect on the Spatial Distributions of the Galactic Satellites in Isolated Systems

“…It is worth mentioning here how the velocity shear field is different from the tidal shear field that was widely used as a tracer of the cosmic web in the previous approaches based on the classical cosmic web theory (e.g., Bond et al 1996;Hahn et al 2007; Platen et al 2008;Forero-Romero et al 2009;Lee et al 2009;Zhang et al 2009Zhang et al , 2013. The tidal shear field defined as the second derivative of the gravitational potential is basically a linear quantity, obtained under the implicit assumption that the peculiar velocity field is curl free, whereas the velocity shear field is an extension of the tidal shear field to the nonlinear regime where the velocity field develops a curl mode (i.e., vorticity).…”

CMB lensing and the WMAP cold spot

“…An order of magnitude estimate by [10] for a completely empty void (δ = −1) at z < ∼ 1 puts the comoving radius at ∼ 120h −1 Mpc. To put these dimensions in perspective, both observations [16,17] and numerical simulations [18,19] suggest that for δ ∼ −0.8 the typical void size tends to be around ∼ 10h −1 Mpc. This means that a ∼ 100 − 200h −1 Mpc void is extremely unlikely to form in the concordance cosmology.…”