Galaxies aren't scattered randomly across space. They form groups, clusters, and superclusters, creating a vast cosmic web. This structure follows the cosmological principle, which says the universe looks the same everywhere on large scales.
Our Milky Way is part of the Local Group, a small cluster of about 50 galaxies. Beyond that, larger clusters and superclusters stretch across billions of light-years, separated by enormous voids. This layout shapes how galaxies evolve over cosmic time.
The Distribution of Galaxies in Space
Cosmological principle in large-scale structures
- Universe appears uniform and looks the same from every location when viewed on vast cosmic scales (homogeneous)
- Universe appears the same in all directions when observed on immense distances (isotropic)
- Cosmic microwave background (CMB) radiation supports the cosmological principle with its nearly uniform distribution across the sky, exhibiting only minor fluctuations
- Galaxies and galaxy clusters spread out evenly on scales exceeding 300 million light-years
- Cosmological principle enables cosmologists to create models of the universe assuming large-scale uniformity
Components of Local Group
- Local Group comprises a small cluster of galaxies, including our Milky Way
- Milky Way and Andromeda Galaxy (M31) dominate the Local Group as the two largest galaxies
- Triangulum Galaxy (M33), Large Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC) represent other significant members of the Local Group
- Local Group encompasses around 50 known galaxies, primarily dwarf galaxies
- Region of space approximately 10 million light-years in diameter contains the Local Group
Groups vs clusters vs superclusters
- Groups of galaxies:
- Contain up to 50 galaxies
- Span diameters under 10 million light-years (Local Group)
- Clusters of galaxies:
- Contain 50 to thousands of galaxies
- Span diameters from 10 to 30 million light-years (Virgo Cluster)
- Observations of galaxy clusters provide evidence for the presence of dark matter
- Superclusters of galaxies:
- Contain multiple galaxy clusters and groups, encompassing thousands of galaxies
- Span hundreds of millions of light-years (Laniakea Supercluster, which includes the Milky Way)
- Gravity binds groups, clusters, and superclusters together, forming the largest structures in the universe
Voids and cosmic structures
- Voids:
- Enormous regions of space with few or no galaxies
- Span hundreds of millions of light-years (Boötes Void)
- Filaments:
- Elongated, narrow, and relatively dense regions of galaxies
- Link galaxy clusters and superclusters
- Create a cosmic web-like structure
- Walls:
- Immense, flat structures made up of galaxies and galaxy clusters
- Span over a billion light-years in length (Sloan Great Wall)
- Cosmic web of filaments, walls, and voids with galaxy clusters and superclusters at their intersections characterizes the large-scale structure of the universe
Cosmic Expansion and Dark Energy
- Hubble's law describes the relationship between a galaxy's distance and its recession velocity
- Redshift of distant galaxies provides evidence for the expansion of the universe
- Dark energy is theorized to be responsible for the accelerating expansion of the universe
- Gravitational lensing helps astronomers study the distribution of matter in the universe, including dark matter
- Galaxy evolution is influenced by the large-scale structure and expansion of the universe