👽Galaxies and the Universe Unit 7 – Cosmic Structure: Universe's Grand Design

Key Concepts and Terminology

• Cosmic microwave background (CMB) radiation leftover heat from the Big Bang permeating the universe
• Dark matter invisible matter detected through gravitational effects on visible matter and radiation
• Dark energy mysterious force causing the accelerating expansion of the universe
• Large-scale structure refers to the arrangement of galaxies and galaxy clusters on scales larger than individual galaxies
• Hubble's law describes the relationship between a galaxy's distance and its redshift, with more distant galaxies receding faster
• Cosmological principle states that the universe is homogeneous and isotropic on large scales
• Baryonic matter ordinary matter composed of protons, neutrons, and electrons, making up only ~5% of the universe's total energy density
• Redshift increase in the wavelength of light from an object moving away from the observer, used to measure cosmic distances

The Big Bang and Early Universe

• Big Bang theory proposes that the universe began as an infinitely dense point called a singularity and has been expanding and cooling ever since
• Inflation period of exponential expansion in the early universe, solving horizon and flatness problems
• Cosmic microwave background (CMB) radiation provides evidence for the Big Bang, with a nearly uniform temperature of 2.7 Kelvin
• Primordial nucleosynthesis formation of light elements (hydrogen, helium, and trace amounts of lithium) in the early universe
• Recombination epoch when the universe cooled enough for electrons to combine with nuclei, making the universe transparent to radiation
• Cosmic Dark Ages period after recombination when the universe was dark and neutral, before the formation of the first stars and galaxies
• Reionization era when the first stars and galaxies formed, ionizing the neutral hydrogen in the universe
• Cosmic structure formation began with quantum fluctuations in the early universe, which were amplified by inflation and grew through gravitational instability

Formation of Galaxies and Large-Scale Structures

• Gravitational instability process by which small density fluctuations in the early universe grew into galaxies and large-scale structures
• Dark matter halos provide the gravitational scaffolding for galaxy formation, with baryonic matter falling into these halos and forming stars
• Hierarchical structure formation model in which smaller structures (galaxies) form first and then merge to create larger structures (galaxy clusters and superclusters)
• Cosmic web large-scale arrangement of galaxies and galaxy clusters in a web-like pattern, with filaments, walls, and voids
• Jeans instability condition for a region of gas to collapse under its own gravity and form stars or galaxies, dependent on the region's mass and temperature
• Feedback processes (star formation, supernovae, and active galactic nuclei) regulate galaxy growth and shape their properties
• Environmental effects (ram-pressure stripping, tidal interactions, and mergers) influence galaxy evolution and morphology
• Epoch of galaxy formation began around 400 million years after the Big Bang, with the peak of star formation occurring at a redshift of z ~ 2 (10 billion years ago)

Types of Cosmic Structures

• Galaxies fundamental building blocks of the universe, containing stars, gas, dust, and dark matter
• Galaxy clusters largest gravitationally bound structures in the universe, containing hundreds to thousands of galaxies
• Superclusters groups of galaxy clusters connected by filaments, extending over 100 million light-years
• Filaments elongated structures of galaxies and galaxy clusters, forming the "skeleton" of the cosmic web
• Walls sheet-like arrangements of galaxies, typically found between filaments
• Voids vast regions of space containing few or no galaxies, occupying most of the volume of the universe
• Hubble sequence classification scheme for galaxies based on their morphology (elliptical, spiral, and irregular)
  • Elliptical galaxies smooth, ellipsoidal shape with little gas and dust, and older stellar populations
  • Spiral galaxies disk-like structure with spiral arms, containing gas, dust, and ongoing star formation
  • Irregular galaxies lack a well-defined structure, often due to interactions or mergers with other galaxies

Dark Matter and Dark Energy's Role

• Dark matter accounts for ~27% of the universe's total energy density, providing the gravitational scaffolding for cosmic structure formation
• Evidence for dark matter includes galaxy rotation curves, gravitational lensing, and the motion of galaxies within clusters
• Cold dark matter (CDM) model proposes that dark matter consists of slow-moving, non-relativistic particles, which explains the observed large-scale structure of the universe
• Dark energy accounts for ~68% of the universe's total energy density, causing the accelerating expansion of the universe
• Evidence for dark energy comes from observations of distant supernovae, the CMB, and the large-scale structure of the universe
• Cosmological constant (Λ) simplest explanation for dark energy, representing a constant energy density throughout space and time
• Dark energy's effect on cosmic structure suppresses the growth of structures on large scales and influences the ultimate fate of the universe
• Modified gravity theories attempt to explain the accelerating expansion of the universe without invoking dark energy, by modifying Einstein's theory of general relativity

Observational Techniques and Evidence

• Hubble Space Telescope (HST) has provided detailed images of distant galaxies and cosmic structures, enabling studies of galaxy evolution and morphology
• Sloan Digital Sky Survey (SDSS) has mapped the 3D positions of millions of galaxies, revealing the large-scale structure of the universe
• Cosmic microwave background (CMB) experiments (COBE, WMAP, Planck) have measured the temperature and polarization of the CMB, providing evidence for the Big Bang and constraining cosmological parameters
• Gravitational lensing distortion of light from distant sources by intervening mass, used to map the distribution of dark matter and study the mass of galaxy clusters
• Redshift surveys measure the redshifts of galaxies to determine their distances and map the 3D distribution of galaxies in the universe
• Sunyaev-Zel'dovich effect distortion of the CMB by high-energy electrons in galaxy clusters, used to detect and study distant clusters
• Ly-α forest absorption features in the spectra of distant quasars caused by neutral hydrogen gas, used to study the distribution of gas in the early universe
• 21 cm line emission from neutral hydrogen, used to study the epoch of reionization and the distribution of gas in the early universe

Current Theories and Models

• ΛCDM (Lambda Cold Dark Matter) model standard cosmological model, incorporating dark energy (Λ) and cold dark matter (CDM) to explain the observed properties of the universe
• Inflation theory proposes a period of exponential expansion in the early universe, solving horizon and flatness problems and providing a mechanism for generating primordial density fluctuations
• Hierarchical structure formation model describes the formation of galaxies and large-scale structures through the merger and accretion of smaller structures
• Cosmic web theory explains the observed filamentary structure of the universe as a result of gravitational instability and the interplay between dark matter and baryonic matter
• Galaxy formation and evolution models incorporate various physical processes (gas accretion, star formation, feedback, and mergers) to explain the observed properties and diversity of galaxies
• Dark matter models (WIMPs, axions, sterile neutrinos) attempt to explain the nature and properties of dark matter particles
• Dark energy models (cosmological constant, quintessence, modified gravity) aim to explain the accelerating expansion of the universe and the nature of dark energy
• Cosmological simulations (Millennium, Illustris, Eagle) use large-scale computer simulations to study the formation and evolution of galaxies and cosmic structures in a cosmological context

Unanswered Questions and Future Research

• Nature of dark matter identifying the particle(s) that make up dark matter and understanding their properties and interactions
• Nature of dark energy determining the cause of the accelerating expansion of the universe and distinguishing between different dark energy models
• Epoch of reionization studying the formation of the first stars and galaxies and their role in reionizing the universe
• Galaxy formation and evolution understanding the detailed physical processes that govern galaxy growth and shape their properties over cosmic time
• Role of supermassive black holes investigating the formation and growth of supermassive black holes and their impact on galaxy evolution through feedback processes
• Cosmic magnetism exploring the origin and evolution of magnetic fields in galaxies and the intergalactic medium
• Cosmic dawn observing the formation of the first stars and galaxies using next-generation telescopes (JWST, SKA)
• Precision cosmology using future surveys (Euclid, LSST, WFIRST) to constrain cosmological parameters and test theories of dark matter and dark energy
• Gravitational wave astronomy using gravitational waves to study the merger of black holes and neutron stars, and to probe the early universe
• Multimessenger astronomy combining observations from different messengers (electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays) to study cosmic phenomena and test fundamental physics