🌌Cosmology Unit 7 – Dark Matter's Role in Cosmic Structure

What's Dark Matter Anyway?

• Mysterious, invisible substance that makes up ~85% of the matter in the universe
• Does not interact with electromagnetic radiation (light) making it difficult to detect directly
• Exhibits gravitational effects on visible matter, influencing the motion of galaxies and galaxy clusters
• Composed of as-yet-unidentified particles that are distinct from ordinary baryonic matter (protons, neutrons, electrons)
  • Leading candidates include weakly interacting massive particles (WIMPs) and axions
• Plays a crucial role in the formation and evolution of large-scale structures in the universe (galaxies, clusters, superclusters)
• Existence inferred from various observational evidence (gravitational lensing, galaxy rotation curves, cosmic microwave background)
• Different from dark energy, which is responsible for the accelerating expansion of the universe

The Cosmic Web: Dark Matter's Playground

• Large-scale structure of the universe characterized by a complex network of filaments, sheets, and voids
• Dark matter forms the backbone of this cosmic web, providing the gravitational scaffolding for the formation of galaxies and clusters
• Filaments are long, thin strands of dark matter that connect galaxies and clusters
  • Can extend for hundreds of millions of light-years
  • Serve as highways for gas and galaxies to flow along
• Sheets are planar structures of dark matter that can span vast distances
• Voids are vast, underdense regions nearly devoid of matter (both dark and baryonic)
  • Can have diameters of hundreds of millions of light-years
• Dark matter's gravitational influence shapes the cosmic web over billions of years, leading to the observed large-scale structure

How Dark Matter Shapes Galaxies

• Dark matter halos surround galaxies, extending far beyond the visible matter
• These halos provide the gravitational potential wells in which galaxies form and evolve
• Dark matter's gravitational influence affects the rotation curves of galaxies
  • Rotation curves remain flat at large radii, indicating the presence of dark matter
• Gravitational interactions between dark matter halos can lead to galaxy mergers and the formation of larger structures
• The distribution of dark matter within a galaxy influences its morphology and dynamics
  • Spiral galaxies have a more extended dark matter halo
  • Elliptical galaxies have a more concentrated dark matter distribution
• Dark matter substructure (smaller clumps within the main halo) can affect the formation and survival of satellite galaxies
• The interplay between dark matter and baryonic matter (gas, stars) shapes the properties of galaxies over cosmic time

Clusters and Superclusters: Dark Matter's Big Picture

• Galaxy clusters are the largest gravitationally bound structures in the universe, containing hundreds to thousands of galaxies
• Dark matter makes up ~80-90% of the total mass of galaxy clusters
• The deep gravitational potential wells created by dark matter hold clusters together
• Dark matter's gravitational lensing effect is most pronounced in galaxy clusters
  • Distorts and magnifies the light from background galaxies
  • Allows for the mapping of dark matter distribution within clusters
• Superclusters are even larger structures, consisting of multiple galaxy clusters and groups connected by filaments
• Dark matter's gravitational influence governs the formation and evolution of these immense structures over billions of years
• Studying clusters and superclusters provides insights into the nature of dark matter and the overall matter distribution in the universe

Detecting the Invisible: Observational Evidence

• Gravitational lensing: Dark matter's gravitational influence distorts the path of light from background sources
  • Strong lensing creates multiple images or Einstein rings around massive clusters
  • Weak lensing causes subtle distortions in the shapes of background galaxies
• Galaxy rotation curves: The flat rotation curves of spiral galaxies at large radii indicate the presence of dark matter halos
• Velocity dispersions in galaxy clusters: The high velocities of galaxies within clusters suggest a large amount of unseen mass (dark matter)
• Cosmic microwave background (CMB) anisotropies: The power spectrum of temperature fluctuations in the CMB is sensitive to the amount and nature of dark matter
• Bullet Cluster: A collision between two galaxy clusters where the dark matter (inferred from gravitational lensing) is separated from the hot gas (observed in X-rays)
• These observational techniques provide compelling evidence for the existence of dark matter, even though it cannot be directly detected

Computer Simulations: Modeling Dark Matter

• Numerical simulations play a crucial role in understanding the behavior and distribution of dark matter on various scales
• N-body simulations follow the gravitational interactions of a large number of dark matter particles over cosmic time
  • Enable the study of structure formation, from the cosmic web down to individual halos
• Hydrodynamical simulations include the effects of baryonic matter (gas, stars) in addition to dark matter
  • Allow for a more comprehensive understanding of galaxy formation and evolution
• Simulations test different dark matter models (cold, warm, self-interacting) and their impact on structure formation
• Comparison of simulation results with observational data helps constrain the properties of dark matter
• Examples of major simulation projects: Millennium Simulation, Illustris Project, EAGLE Project
• Simulations are continuously improving in resolution and physical complexity, providing valuable insights into the nature of dark matter

Dark Matter's Role in Galaxy Formation

• Dark matter halos serve as the gravitational scaffolding for galaxy formation
• Primordial density fluctuations in the dark matter distribution seed the formation of halos
• Gas falls into these halos, cools, and condenses to form stars and galaxies
• The properties of the dark matter halo (mass, concentration, substructure) influence the characteristics of the galaxies that form within them
• Hierarchical structure formation: Smaller halos merge to form larger ones, leading to the buildup of galaxies and clusters over time
• Feedback processes (supernovae, active galactic nuclei) shape the interplay between dark matter and baryonic matter
• The angular momentum of the dark matter halo affects the formation and morphology of the galaxy (disk vs. elliptical)
• Dark matter's gravitational influence regulates the gas supply for star formation and the overall evolution of galaxies

Unanswered Questions and Future Research

• The nature of dark matter particles remains unknown
  • Ongoing searches for WIMPs, axions, and other candidates using various experimental techniques (direct detection, indirect detection, particle colliders)
• The precise distribution of dark matter on small scales (within galaxies) is uncertain
  • Discrepancies between simulations and observations (cusp vs. core problem)
  • Potential solutions include self-interacting dark matter or baryonic physics effects
• The role of dark matter in the formation of the first stars and galaxies in the early universe
• The connection between dark matter and other cosmological mysteries (dark energy, cosmic inflation)
• Improving the sensitivity and resolution of observational techniques to better constrain dark matter properties
• Developing more advanced numerical simulations that incorporate complex physics and span a wider range of scales
• Exploring alternative theories of gravity as a possible explanation for the observed effects attributed to dark matter
• The study of dark matter remains a highly active and exciting field, with potential for groundbreaking discoveries in the coming years