The structural elements of the cosmic web

TL;DR

This paper reviews methods for identifying the structural elements of the cosmic web, such as voids, filaments, and clusters, in observational data and simulations, emphasizing techniques that are parameter-free and scale-independent.

Contribution

It provides an overview of recent, parameter-free, scale-adaptive techniques for defining cosmic web structures and relating them to structure formation dynamics.

Findings

Effective methods for void, filament, and cluster identification are discussed.

Techniques that do not require fine-tuning are highlighted.

Structures identified relate directly to dynamical formation mechanisms.

Abstract

In 1970 Zel'dovich published a far-reaching paper presenting a simple equation describing the nonlinear growth of primordial density inhomogeneities. The equation was remarkably successful in explaining the large scale structure in the Universe that we observe: a Universe in which the structure appears to be delineated by filaments and clusters of galaxies surrounding huge void regions. In order to concretise this impression it is necessary to define these structural elements through formal techniques with which we can compare the Zel'dovich model and N-body simulations with the observational data. We present an overview of recent efforts to identify voids, filaments and clusters in both the observed galaxy distribution and in numerical simulations of structure formation. We focus, in particular, on methods that involve no fine-tuning of parameters and that handle scale dependence automatically. It is important that these techniques should result in finding structures that relate directly to the dynamical mechanism of structure formation.