👽Galaxies and the Universe Unit 9 – Early Universe and Cosmic Background

Big Bang Theory Basics

• Proposes the universe began as an extremely hot, dense point ~13.8 billion years ago and has been expanding and cooling ever since
• Singularity represents the origin of space, time, matter, and energy as we know it
• Rapid expansion occurred within the first fraction of a second after the Big Bang (cosmic inflation)
• As the universe expanded, it cooled allowing for the formation of subatomic particles, atoms, stars, and galaxies
• Explains the observed expansion of the universe (Hubble's law) and the existence of the cosmic microwave background radiation
• Supported by multiple lines of observational evidence including the abundance of light elements, cosmic microwave background, and large-scale structure of the universe
• Does not address the cause of the Big Bang itself or what existed before it, which remains an open question in cosmology

Timeline of Early Universe

• Planck epoch (0 to 10^-43 seconds): Quantum gravity dominated, physics not well understood
• Grand unification epoch (10^-43 to 10^-36 seconds): Gravity separated from other fundamental forces, universe underwent cosmic inflation
• Electroweak epoch (10^-36 to 10^-12 seconds): Strong nuclear force separated from electroweak force, quarks formed
• Quark epoch (10^-12 to 10^-6 seconds): Quarks combined to form hadrons (protons and neutrons)
• Hadron epoch (10^-6 to 1 second): Hadrons and antihadrons annihilated, leaving a small excess of matter
• Lepton epoch (1 to 10 seconds): Leptons (electrons, positrons, neutrinos) dominated the mass of the universe
• Nucleosynthesis (10 seconds to 20 minutes): Protons and neutrons combined to form light elements (hydrogen, helium, lithium)
• Photon epoch (3 minutes to 240,000 years): Universe dominated by photons, matter and radiation decoupled forming the cosmic microwave background

Cosmic Inflation

• Brief period of exponential expansion in the early universe, lasting from ~10^-36 to 10^-32 seconds after the Big Bang
• Universe expanded by a factor of at least 10^26 in less than 10^-32 seconds
• Driven by a hypothetical scalar field called the inflaton, which had negative pressure and caused space to expand rapidly
• Solves several problems in standard Big Bang cosmology:
  • Horizon problem: Explains why distant regions of the universe have similar properties despite not being in causal contact
  • Flatness problem: Explains why the universe appears to have a flat geometry on large scales
  • Magnetic monopole problem: Dilutes the density of magnetic monopoles predicted by grand unified theories
• Quantum fluctuations during inflation are thought to be the seeds of large-scale structure (galaxies and clusters) in the universe
• Inflationary models predict a nearly scale-invariant spectrum of primordial density fluctuations, which has been observed in the cosmic microwave background

Formation of Primordial Elements

• Primordial nucleosynthesis occurred between 10 seconds and 20 minutes after the Big Bang when the universe had cooled enough for protons and neutrons to combine
• Main elements formed were hydrogen (H), deuterium (D), helium-3 (^3He), helium-4 (^4He), and a small amount of lithium-7 (^7Li)
• Relative abundances depend on the ratio of photons to baryons (protons and neutrons) in the early universe
• Predicted abundances match observations, providing strong evidence for the Big Bang model
  • ~75% hydrogen, ~25% helium-4 by mass
  • Trace amounts of deuterium, helium-3, and lithium-7
• Heavier elements (carbon, oxygen, etc.) were formed later in the cores of stars through stellar nucleosynthesis
• Primordial nucleosynthesis is one of the three pillars of the Big Bang theory, along with Hubble's law and the cosmic microwave background

Cosmic Microwave Background

• Relic radiation from the early universe, observed in all directions of the sky with a nearly uniform temperature of 2.725 K
• Formed ~380,000 years after the Big Bang when the universe had cooled enough for electrons to combine with protons to form neutral hydrogen atoms (recombination)
• Before recombination, photons were constantly scattered by free electrons, making the universe opaque
• After recombination, photons could travel freely through space, making the universe transparent
• As the universe expanded, the wavelength of these photons was stretched (redshifted) into the microwave part of the electromagnetic spectrum
• Cosmic microwave background has a near-perfect blackbody spectrum, as predicted by the Big Bang model
• Tiny fluctuations in temperature (~1 part in 100,000) correspond to density variations in the early universe that seeded the formation of galaxies and large-scale structure
• Provides a snapshot of the universe at the time of recombination and is one of the strongest pieces of evidence for the Big Bang theory

Observational Evidence

• Hubble's law: Galaxies are receding from us with velocities proportional to their distance, implying an expanding universe
  • Measured using redshift of galaxies' spectra
• Cosmic microwave background: Relic radiation from the early universe with a near-perfect blackbody spectrum at 2.725 K
  • Discovered by Penzias and Wilson in 1965
• Abundance of light elements: Relative abundances of hydrogen, deuterium, helium-3, helium-4, and lithium-7 match predictions from Big Bang nucleosynthesis
  • Measured using absorption lines in spectra of distant quasars
• Large-scale structure: Distribution of galaxies and clusters on large scales matches predictions from inflationary models with dark matter and dark energy
  • Mapped using galaxy surveys (Sloan Digital Sky Survey, 2dF Galaxy Redshift Survey)
• Baryon acoustic oscillations: Imprint of sound waves in the early universe on the distribution of galaxies, providing a standard ruler to measure the expansion history
• Type Ia supernovae: Used as standard candles to measure distances, revealing the accelerating expansion of the universe due to dark energy

Key Discoveries and Experiments

• Hubble's law (1929): Edwin Hubble measured the distances and redshifts of galaxies, discovering the expansion of the universe
• Cosmic microwave background discovery (1965): Arno Penzias and Robert Wilson detected the cosmic microwave background using a radio telescope, providing strong evidence for the Big Bang
• COBE satellite (1989-1993): Measured the blackbody spectrum and anisotropies of the cosmic microwave background with high precision
  • Spectrum matches predictions of the Big Bang model
  • Anisotropies are the seeds of large-scale structure
• WMAP satellite (2001-2010): Measured the temperature and polarization anisotropies of the cosmic microwave background with higher resolution than COBE
  • Provided precise measurements of cosmological parameters (age, composition, and geometry of the universe)
• Planck satellite (2009-2013): Measured the temperature and polarization anisotropies of the cosmic microwave background with even higher resolution than WMAP
  • Provided the most accurate measurements of cosmological parameters to date
  • Placed tight constraints on inflationary models
• Sloan Digital Sky Survey (2000-present): Mapped the 3D positions of millions of galaxies, quasars, and stars, revealing the large-scale structure of the universe
  • Measured baryon acoustic oscillations, providing an independent probe of the expansion history

Implications for Modern Cosmology

• The Big Bang theory is the foundation of modern cosmology, describing the origin and evolution of the universe
• Observations support a universe dominated by dark matter and dark energy
  • Dark matter: Non-baryonic matter that interacts gravitationally but not electromagnetically, necessary to explain galaxy rotation curves and large-scale structure
  • Dark energy: Unknown form of energy with negative pressure, responsible for the accelerating expansion of the universe
• Inflationary models provide a mechanism for generating primordial density fluctuations that seeded the formation of galaxies and large-scale structure
  • Predicts a nearly scale-invariant spectrum of fluctuations, which has been observed in the cosmic microwave background
• The universe appears to have a flat geometry on large scales, consistent with predictions from inflation
• The Big Bang model does not address the initial singularity or the cause of the Big Bang, which remains an open question
• Theories of quantum gravity (string theory, loop quantum gravity) attempt to describe the physics of the early universe and the initial singularity
• The study of the early universe and the cosmic microwave background continues to be an active area of research in cosmology, with ongoing and future experiments (CMB-S4, LiteBIRD, PICO) aiming to probe the physics of inflation and the nature of dark matter and dark energy