🌌Cosmology Unit 11 – The Large–Scale Structure of the Universe

Key Concepts and Definitions

• Large-scale structure refers to the patterns of galaxies and galaxy clusters on scales larger than individual galaxies, typically on the order of hundreds of millions of light-years
• Cosmic web describes the intricate, web-like arrangement of galaxies and galaxy clusters connected by filaments and separated by vast voids
• Dark matter plays a crucial role in the formation and evolution of large-scale structures, providing the gravitational scaffolding for baryonic matter to collapse and form galaxies
• Baryonic matter consists of ordinary matter made up of protons, neutrons, and electrons, which forms stars, galaxies, and other visible structures in the universe
• Voids are vast, underdense regions of the universe that contain few galaxies and are surrounded by the filaments and walls of the cosmic web
• Filaments are elongated, thread-like structures of galaxies and galaxy clusters that connect the nodes of the cosmic web
• Galaxy clusters are the largest gravitationally bound structures in the universe, containing hundreds to thousands of galaxies held together by gravity
• Superclusters are even larger structures composed of multiple galaxy clusters and groups, connected by filaments and extending over tens to hundreds of millions of light-years

Historical Context and Discoveries

• Early observations of the distribution of galaxies (1920s-1930s) hinted at the existence of large-scale structures beyond individual galaxies
• Fritz Zwicky's discovery of dark matter in the Coma Cluster (1933) provided early evidence for the presence of unseen matter influencing the motion of galaxies
• The first systematic galaxy surveys (1970s-1980s) revealed the presence of galaxy clusters, superclusters, and voids on scales of tens to hundreds of millions of light-years
  • The CfA Redshift Survey (1977) mapped the positions and velocities of galaxies, revealing a complex network of structures
  • The APM Galaxy Survey (1990) provided a detailed map of the distribution of galaxies over a large area of the sky
• The discovery of the Great Wall (1989), a vast sheet-like structure of galaxies extending over 500 million light-years, demonstrated the immense scale of the universe's largest structures
• The Sloan Digital Sky Survey (SDSS) (2000-present) has provided the most comprehensive map of the large-scale structure to date, covering over a quarter of the sky and measuring the positions and properties of millions of galaxies
• Observations of the cosmic microwave background (CMB) by COBE (1989), WMAP (2001), and Planck (2009) have revealed the seeds of large-scale structure in the early universe, supporting the theory of cosmic inflation

Observational Techniques and Tools

• Galaxy surveys use telescopes to map the positions and properties of galaxies across large areas of the sky, providing data for studying the large-scale structure
  • Surveys measure the redshifts of galaxies to determine their distances and construct 3D maps of the galaxy distribution
  • Examples include the Sloan Digital Sky Survey (SDSS), the 2dF Galaxy Redshift Survey, and the VIMOS Public Extragalactic Redshift Survey (VIPERS)
• Gravitational lensing is a technique that uses the distortion of background galaxy images by intervening matter to map the distribution of dark matter on large scales
  • Weak lensing surveys, such as the Dark Energy Survey (DES) and the Kilo-Degree Survey (KiDS), measure the subtle distortions in galaxy shapes caused by the gravitational influence of large-scale structures
• Cosmic microwave background (CMB) observations provide a snapshot of the early universe, revealing the initial conditions that gave rise to the large-scale structure
  • Satellites like COBE, WMAP, and Planck have mapped the tiny temperature fluctuations in the CMB, which correspond to the seeds of structure formation
• X-ray observations of galaxy clusters can detect the hot, diffuse gas that fills the space between galaxies, tracing the distribution of baryonic matter in the large-scale structure
  • Telescopes like Chandra, XMM-Newton, and eROSITA are used to study the X-ray emission from galaxy clusters and the warm-hot intergalactic medium (WHIM)
• Numerical simulations play a crucial role in understanding the formation and evolution of large-scale structures, modeling the complex interplay between dark matter, gas, and galaxies
  • Examples include the Millennium Simulation, the IllustrisTNG simulations, and the EAGLE project

Large-Scale Structures Explained

• Galaxy clusters are the largest gravitationally bound structures in the universe, containing hundreds to thousands of galaxies, hot X-ray emitting gas, and large amounts of dark matter
  • Clusters typically have masses ranging from $10^{14}$ to $10^{15}$ solar masses and sizes of several megaparsecs (Mpc)
  • Examples include the Coma Cluster, the Virgo Cluster, and the Hercules Cluster
• Superclusters are even larger structures composed of multiple galaxy clusters and groups, connected by filaments and extending over tens to hundreds of millions of light-years
  • The Shapley Supercluster, the largest known structure in the local universe, spans over 400 million light-years and contains over 8,000 galaxies
  • Other notable superclusters include the Sloan Great Wall, the Hercules-Corona Borealis Great Wall, and the Saraswati Supercluster
• Filaments are elongated, thread-like structures of galaxies and galaxy clusters that form the backbone of the cosmic web, connecting clusters and superclusters
  • Filaments can extend over tens to hundreds of millions of light-years and have a width of a few megaparsecs
  • The Pisces-Cetus Supercluster Complex is an example of a prominent filamentary structure, spanning over 1 billion light-years
• Walls are vast, sheet-like arrangements of galaxies that can extend over hundreds of millions of light-years, often encompassing multiple superclusters and filaments
  • The Great Wall, discovered in 1989, is a prominent example of a wall structure, spanning over 500 million light-years
  • The BOSS Great Wall, discovered in 2016, is one of the largest known structures in the universe, with a length of over 1 billion light-years
• Voids are vast, underdense regions of the universe that contain few galaxies and are surrounded by the filaments and walls of the cosmic web
  • Voids typically have sizes ranging from tens to hundreds of millions of light-years and occupy a significant fraction of the universe's volume
  • The Boötes void, one of the largest known voids, has a diameter of approximately 330 million light-years

Theoretical Models and Simulations

• The Cold Dark Matter (CDM) model is the leading theoretical framework for understanding the formation and evolution of large-scale structures in the universe
  • CDM posits that the universe is dominated by cold, non-baryonic dark matter particles that interact primarily through gravity
  • The model successfully explains the observed large-scale structure, including the formation of galaxies, clusters, and the cosmic web
• The Cosmic Inflation theory proposes that the universe underwent a period of rapid exponential expansion in its early stages, providing the initial conditions for structure formation
  • Inflation explains the observed flatness, homogeneity, and isotropy of the universe on large scales
  • Quantum fluctuations during inflation are thought to be the seeds of the large-scale structure, giving rise to the tiny density perturbations that eventually grew into galaxies and clusters
• The Zeldovich Approximation is a simplified model that describes the early stages of structure formation, treating the universe as a fluid of cold, collisionless dark matter particles
  • The approximation predicts the formation of pancake-like structures, which later evolve into the filaments and walls of the cosmic web
• Numerical simulations, such as N-body and hydrodynamical simulations, play a crucial role in understanding the formation and evolution of large-scale structures
  • N-body simulations model the gravitational interactions of dark matter particles, tracing the growth of structure from the early universe to the present day
  • Hydrodynamical simulations incorporate the effects of gas physics, star formation, and feedback processes, providing a more complete picture of galaxy formation within the large-scale structure
• The Press-Schechter Formalism is a statistical approach to understanding the abundance and distribution of dark matter halos as a function of mass and redshift
  • The formalism predicts the number density of halos above a given mass threshold, which can be compared to observations of galaxy clusters and used to constrain cosmological parameters
• The Halo Model is a framework that describes the clustering of galaxies in terms of the properties of their host dark matter halos
  • The model assumes that all galaxies reside within dark matter halos and that the halo mass is the primary determinant of galaxy properties
  • The halo model has been successful in explaining the observed galaxy clustering and the connection between galaxies and their surrounding large-scale structure

Cosmic Web Components

• Nodes are the densest regions of the cosmic web, corresponding to the locations of massive galaxy clusters and superclusters
  • Nodes act as the intersection points of filaments and walls, where the concentration of dark matter and baryonic matter is the highest
  • Examples of nodes include the Coma Cluster, the Virgo Cluster, and the core of the Shapley Supercluster
• Filaments are the elongated, thread-like structures that connect the nodes of the cosmic web, forming a complex network of galaxies and galaxy clusters
  • Filaments are thought to be the primary channels for gas accretion onto galaxies and clusters, fueling star formation and galaxy growth
  • The Pisces-Cetus Supercluster Complex and the Sloan Great Wall are examples of prominent filamentary structures
• Walls are the vast, sheet-like arrangements of galaxies that often encompass multiple superclusters and filaments
  • Walls can extend over hundreds of millions of light-years and are among the largest structures in the universe
  • The Great Wall and the BOSS Great Wall are notable examples of wall structures
• Voids are the vast, underdense regions of the universe that contain few galaxies and are surrounded by the filaments and walls of the cosmic web
  • Voids occupy a significant fraction of the universe's volume and are thought to play a role in the evolution of galaxies and the large-scale structure
  • The Boötes void and the Local Void are examples of well-studied void regions
• The Warm-Hot Intergalactic Medium (WHIM) is the diffuse, highly ionized gas that resides in the filaments and walls of the cosmic web
  • The WHIM is thought to contain a significant fraction of the universe's baryonic matter and is heated to temperatures of $10^5$ to $10^7$ Kelvin by gravitational collapse and feedback processes
  • Observing the WHIM is challenging due to its low density and high temperature, but it has been detected through X-ray absorption lines and emission from highly ionized oxygen

Dark Matter's Role

• Dark matter is the dominant component of matter in the universe, making up approximately 85% of the total matter content
  • Dark matter interacts primarily through gravity and does not emit, absorb, or scatter light, making it difficult to detect directly
  • The presence of dark matter is inferred from its gravitational effects on visible matter, such as the rotation curves of galaxies and the motion of galaxies within clusters
• Dark matter plays a crucial role in the formation and evolution of large-scale structures, providing the gravitational scaffolding for baryonic matter to collapse and form galaxies and clusters
  • In the early universe, dark matter began to collapse under its own gravity, forming a network of filaments and halos that served as the seeds for structure formation
  • Baryonic matter, attracted by the gravitational potential wells created by dark matter, followed the dark matter distribution and gave rise to the visible structures we observe today
• The distribution of dark matter on large scales is traced by the cosmic web, with the highest concentrations found in the nodes and filaments
  • Gravitational lensing studies have revealed the presence of dark matter filaments connecting galaxy clusters, providing direct evidence for the cosmic web
  • The Bullet Cluster is a famous example of a merging cluster system where the separation of dark matter and baryonic matter has been observed through gravitational lensing
• Dark matter halos are the extended, roughly spherical regions of dark matter that surround galaxies and galaxy clusters
  • The properties of dark matter halos, such as their mass and concentration, play a significant role in determining the properties of the galaxies that form within them
  • The halo mass function, which describes the number density of halos as a function of mass, is a key prediction of the Cold Dark Matter model and has been confirmed by observations
• The nature of dark matter remains one of the greatest mysteries in cosmology, with several candidate particles proposed, such as Weakly Interacting Massive Particles (WIMPs), axions, and sterile neutrinos
  • Efforts to detect dark matter directly, through experiments like XENON, LUX, and PandaX, have placed stringent limits on the properties of dark matter particles
  • Indirect detection methods, such as searching for gamma-ray emission from dark matter annihilation in galaxy clusters or the Galactic Center, provide complementary constraints on dark matter properties

Current Research and Open Questions

• The nature of dark matter and dark energy remains a major open question in cosmology, with ongoing efforts to constrain their properties and identify their physical origin
  • Experiments like XENON, LUX, and PandaX aim to directly detect dark matter particles through their interactions with atomic nuclei
  • Surveys like DES, KiDS, and HSC are using gravitational lensing to map the distribution of dark matter on large scales and constrain the properties of dark energy
• The role of baryonic physics in shaping the large-scale structure is an active area of research, with simulations and observations probing the impact of star formation, feedback, and gas accretion on the cosmic web
  • Hydrodynamical simulations like IllustrisTNG, EAGLE, and Horizon-AGN are incorporating increasingly realistic models of galaxy formation and evolution within the large-scale structure
  • Observations of the circumgalactic medium (CGM) and the intergalactic medium (IGM) are providing insights into the gas content and chemical enrichment of the cosmic web
• The search for the earliest galaxies and the reionization of the universe is a frontier in observational cosmology, with telescopes like ALMA, JWST, and SKA aiming to probe the first billion years of cosmic history
  • The formation and evolution of the first galaxies are thought to be closely tied to the large-scale structure, with the first stars and galaxies forming in the densest regions of the cosmic web
  • The reionization of the universe, when the first galaxies ionized the neutral hydrogen in the IGM, is a key milestone in the history of the universe and is influenced by the distribution of matter on large scales
• The study of cosmic voids and their role in the evolution of the universe is an emerging field, with voids potentially holding clues to the nature of dark energy and modified gravity theories
  • Voids are the largest structures in the universe and are sensitive probes of cosmology, as their properties depend on the expansion history and growth of structure
  • The void size function, which describes the number density of voids as a function of size, is a promising tool for constraining cosmological parameters and testing alternative gravity models
• The connection between the large-scale structure and the cosmic microwave background (CMB) is a key area of research, with the CMB providing a snapshot of the early universe and the seeds of structure formation
  • The Sunyaev-Zeldovich (SZ) effect, which imprints the signature of galaxy clusters on the CMB, is a powerful probe of the large-scale structure and the properties of dark energy
  • The integrated Sachs-Wolfe (ISW) effect, which arises from the interaction between CMB photons and the evolving gravitational potential of the large-scale structure, provides a test of dark energy and modified gravity theories
• The ultimate fate of the universe and the role of the large-scale structure in its evolution is a fundamental question in cosmology, with the accelerating expansion due to dark energy pointing towards a cold, dark future
  • The continued growth and evolution of the cosmic web depend on the nature of dark energy and the properties of dark matter
  • Simulations and observations are exploring the long-term evolution of the universe and the potential for the large-scale structure to be a fossil record of cosmic history