2.1 Big Bang theory and cosmic evolution

The Big Bang theory explains the universe's origin and evolution. Evidence includes cosmic microwave background radiation, expanding universe, and light element abundance. These clues paint a picture of a universe that began as an infinitely dense point and expanded over billions of years.

Cosmic evolution unfolds in stages, from the initial singularity to the formation of galaxies and stars. Dark matter and energy play crucial roles in shaping the universe's structure and expansion. Cosmic inflation theory addresses early universe mysteries and supports the idea of a multiverse.

The Big Bang Theory and Cosmic Evolution

Evidence for Big Bang theory

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  The Cosmic Microwave Background · Astronomy View original

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• Cosmic microwave background (CMB) radiation
  • Uniform background radiation in all directions discovered by Penzias and Wilson in 1965
  • Remnant heat from early universe redshifted to microwave wavelengths (2.7 K)
• Expansion of universe
  • Observed by Hubble in 1920s, galaxies moving away from each other
  • Farther galaxies recede faster (Hubble's Law), universe expanding from single point
  • Expansion rate measured by Hubble constant ($H_0$) in km/s/Mpc
• Abundance of light elements
  • Universe composed of ~75% hydrogen and ~25% helium
  • Aligns with predictions from Big Bang nucleosynthesis in early universe
  • Heavier elements formed later in stars (stellar nucleosynthesis)

Stages of cosmic evolution

1. Big Bang (t = 0): Universe began as singularity with infinite density and temperature
2. Planck Epoch (t < $10^{-43}$ s): Quantum gravity effects dominant, four fundamental forces unified
3. Grand Unification Epoch (t ~ $10^{-43}$ to $10^{-36}$ s): Strong, weak, and electromagnetic forces unified
4. Inflationary Epoch (t ~ $10^{-36}$ to $10^{-32}$ s): Rapid exponential expansion of universe by factor of $10^{26}$
5. Electroweak Epoch (t ~ $10^{-32}$ to $10^{-12}$ s): Electromagnetic and weak forces separate, universe cools to $10^{28}$ K
6. Quark Epoch (t ~ $10^{-12}$ to $10^{-6}$ s): Quarks form hadrons (protons, neutrons), universe cools to $10^{13}$ K
7. Hadron Epoch (t ~ $10^{-6}$ to 1 s): Hadrons and antihadrons annihilate, leaving small excess of matter, universe cools to $10^{10}$ K
8. Lepton Epoch (t ~ 1 to 10 s): Leptons (electrons) and antileptons annihilate, leaving small excess of matter, universe cools to $10^9$ K
9. Photon Epoch (t ~ 10 s to 380,000 yr): Universe dominated by radiation, photons scatter off free electrons
10. Recombination (t ~ 380,000 yr): Electrons combine with nuclei to form neutral atoms (mainly hydrogen), universe becomes transparent to radiation, CMB emitted
11. Dark Ages (t ~ 380,000 to 400 Myr): No stars or galaxies yet formed, universe filled with neutral hydrogen
12. Reionization (t ~ 400 Myr to 1 Byr): First stars and galaxies form, their UV radiation reionizes neutral hydrogen in universe
13. Galaxy formation and evolution (t > 1 Byr): Galaxies continue to form and evolve through mergers and interactions, giving rise to diverse galaxy types (ellipticals, spirals, irregulars)
14. Star and planet formation (ongoing): Stars form within galaxies from collapsing gas and dust clouds, planets form around stars from protoplanetary disks

Dark matter and energy in universe

• Dark matter
  • Composes ~27% of universe's total mass-energy, does not interact with electromagnetic radiation
  • Detected through gravitational effects on visible matter (galaxy rotation curves, gravitational lensing)
  • Plays crucial role in formation and stability of galaxies and clusters, enables large-scale structure
  • Leading candidates are weakly interacting massive particles (WIMPs) and axions
• Dark energy
  • Composes ~68% of universe's total mass-energy, responsible for accelerating cosmic expansion
  • Acts as negative pressure counteracting gravity, exact nature remains unknown
  • Possible explanations include cosmological constant ($\Lambda$) and scalar fields (quintessence)
  • Discovered in 1998 from observations of distant supernovae, confirmed by CMB and galaxy clustering

Concept of cosmic inflation

• Period of rapid exponential expansion in early universe (t ~ $10^{-36}$ to $10^{-32}$ s after Big Bang)
• Proposed by Guth in 1980 to solve problems in standard Big Bang model:
  • Horizon problem: Explains uniformity of CMB temperature across sky
  • Flatness problem: Explains why universe appears to have flat geometry ($\Omega \approx 1$)
  • Magnetic monopole problem: Dilutes density of magnetic monopoles predicted by GUTs
• Driven by hypothetical inflaton field with negative pressure, causes space to expand faster than light
• Quantum fluctuations during inflation seeded formation of large-scale structures (galaxies, clusters)
• Predicts nearly scale-invariant spectrum of primordial density fluctuations, consistent with CMB observations
• Supports idea of multiverse, where inflation generates multiple universes with different properties
• Observational evidence from CMB polarization (B-modes) still being sought to confirm inflation