Galaxy Clusters and Large-Scale Structure
Galaxies are not scattered at random but organized into groups, clusters, filaments, and voids that together form the cosmic web, the largest structures in the universe.
Definition
Large-scale structure is the organization of matter in the universe into a web of galaxy clusters, filaments, sheets, and voids; galaxy clusters are the largest gravitationally bound systems, containing hundreds to thousands of galaxies embedded in hot gas and dark matter.
Scope
This area covers galaxy groups and clusters as the most massive bound systems, the hot intracluster gas that dominates their visible mass, the cosmic web of filaments and voids traced by galaxy clustering, and the ways the dense cluster environment transforms the galaxies within it.
Sub-topics
Core questions
- How are galaxies organized into groups, clusters, and the cosmic web?
- What is the composition and mass budget of galaxy clusters?
- How does the large-scale distribution of galaxies trace the underlying matter?
- How does environment transform the galaxies within dense structures?
Key theories
- Clusters as dark-matter-dominated systems
- Zwicky's measurement of galaxy motions in the Coma cluster implied far more mass than the visible galaxies provide, an early indication of dark matter that clusters still constrain today.
- Hierarchical growth of clusters
- Clusters form at the nodes of the cosmic web by the merging and accretion of smaller groups along filaments, the culmination of hierarchical structure formation.
- Clustering as a cosmological probe
- The statistical clustering of galaxies, described by correlation functions, encodes the growth of structure and the cosmological parameters, as developed by Peebles.
Clinical relevance
Galaxy clusters and large-scale structure are powerful cosmological laboratories: their abundance and growth constrain dark matter and dark energy, their gravitational lensing weighs invisible mass, and the cosmic web is the arena in which galaxies form and evolve.
History
Zwicky's 1937 study of the Coma cluster first revealed missing mass in clusters. Abell catalogued clusters in the 1950s, and redshift surveys from the 1980s onward, together with theoretical work by Peebles and others, mapped the cosmic web and made large-scale structure a precision cosmological tool.
Key figures
- Fritz Zwicky
- P. James E. Peebles
- George Abell
- Margaret Geller
Related topics
Seminal works
- zwicky1937
- peebles1980
- kravtsov2012
Frequently asked questions
- What is the cosmic web?
- The cosmic web is the large-scale pattern in which galaxies and dark matter are arranged: dense clusters connected by filaments, separated by vast nearly empty voids. It is the imprint of how gravity amplified tiny initial density variations.
- What holds a galaxy cluster together?
- Gravity binds a cluster, but the visible galaxies supply only a small part of the needed mass. Most of the mass is dark matter, with hot intracluster gas contributing more visible mass than the galaxies themselves.