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