The Cosmic Web and Galaxy Clustering

On the largest scales, galaxies trace a web of dense clusters, threadlike filaments, and sheets surrounding enormous nearly empty voids.

Definition

The cosmic web is the large-scale pattern of matter, comprising clusters at the nodes, filaments and sheets connecting them, and intervening voids; galaxy clustering is the statistical study of how galaxies are spatially correlated, encoding the growth of structure and cosmological parameters.

Scope

This topic covers the morphology of the cosmic web of filaments, sheets, and voids, the statistical description of galaxy clustering through correlation functions and power spectra, the mapping of structure by redshift surveys, the baryon acoustic oscillation feature used as a cosmic ruler, and the relation between galaxies and the underlying dark matter.

Core questions

• What is the morphology of the cosmic web, and how does it arise?
• How is galaxy clustering quantified statistically?
• How do redshift surveys map large-scale structure?
• How does the baryon acoustic feature serve as a cosmic ruler?

Key theories

Statistical clustering

Peebles formalized the description of galaxy clustering through correlation functions and power spectra, linking the observed distribution of galaxies to the statistics of the underlying density field.

Origin of the cosmic web

Bond and collaborators showed that the web of filaments arises naturally from gravitational instability acting on the initial density field, with filaments bridging the peaks that become clusters.

Baryon acoustic oscillations

A preferred separation imprinted on matter by sound waves in the early universe appears as a feature in galaxy clustering, providing a standard ruler for measuring cosmic distances.

Clinical relevance

The cosmic web is the imprint of cosmic initial conditions and gravitational growth; measuring galaxy clustering constrains the matter content, the nature of dark energy, and the validity of gravity on the largest scales.

History

Peebles's clustering statistics laid the theoretical groundwork in the 1970s and 1980s. The 1986 de Lapparent, Geller, and Huchra slice revealed bubble-like voids and walls, the cosmic web's filamentary nature was explained in 1996, and the 2005 detection of baryon acoustic oscillations made clustering a precision cosmological probe.

Key figures

• P. James E. Peebles
• Margaret Geller
• John Huchra
• Daniel Eisenstein

Seminal works

• peebles1980
• delapparent1986
• eisenstein2005

Frequently asked questions

What are voids in the cosmic web?

Voids are vast, roughly spherical regions tens of millions of light-years across that contain very few galaxies. They make up most of the volume of the universe, with galaxies concentrated in the filaments and clusters that surround them.

How can the distribution of galaxies measure cosmology?

The way galaxies cluster reflects how gravity amplified tiny density variations from the early universe. Statistical measures of clustering, including a special preferred scale called the baryon acoustic feature, encode the amounts of matter and dark energy and the expansion history.