Origin & Evolution of Universe and Solar System – Key Notes

Investigating the Universe

• Cosmology: scientific study of the physical universe; major progress since Einstein’s $1917$ static model
• Astronomy: study of extraterrestrial objects/phenomena
• Data sources
  • Ground/space telescopes (e.g.
    Hubble)
  • Space probes (e.g.
    Voyager $1$ & $2$ — first human‐made objects in interstellar space)
• Key instruments
  • Telescope (first patent $1608$, improved by Galileo $1609$)
  • Voyager $1$: launched $1977$, crossed heliopause $2012$, carries Golden Record

Major Theories on the Origin of the Universe

• Big Bang Theory (standard model) — universe began from a singularity $13.8\,\text{billion years}$ ago, then expanded & cooled
• Steady State Theory — universe expands yet maintains constant density via continuous matter creation; now disfavoured
• Cyclic Model — universe undergoes endless sequences of contraction (pre-bang) and expansion (post-bang)
• Inflation Model — very early rapid exponential expansion solves flatness, horizon, homogeneity problems
• Multiverse Model — our observable universe is one of many distinct, possibly diverse, universes

Big Bang Theory: Basics & Evidence

• Proposer: Georges Lemaître
• Timeline highlights
  • <10^{-6}\,\text{s}: quarks form protons & neutrons
  • $\approx 3\,\text{min}$: nuclei of $^1!H$ and $^4!He$ created
  • $380\,000$ years: electrons captured → first atoms (mainly H, He)
• Key evidence
  • Cosmic Microwave Background (CMB): relic radiation at $2.725\,\text{K}$ (discovered by Penzias & Wilson)
  • Spectral redshift of galaxies → expansion (Hubble-Lemaître law)
  • Higgs boson confirms mechanism giving mass to particles (supports particle‐physics underpinning)

Alternative / Complementary Cosmological Models

• Steady State: perfect cosmological principle (same in space & time); ruled out by CMB & galaxy evolution
• Cyclic: big bang as transition, not beginning; structure seeded during slow contraction phase
• Inflation: universe grew by factor >10^{26} in <10^{-32}\,\text{s}; makes observable region homogeneous before rapid expansion
• Multiverse: ensemble of causally disconnected universes; arises in inflationary, quantum, and philosophical frameworks

Origin of the Solar System: Competing Theories

• Protoplanetary Theory (McCrea) — Sun & planets formed simultaneously from same condensation
• Capture Theory (Woolfson) — pre-existing Sun gravitationally captures matter torn from passing protostar → planets
• Modern Laplacian Theory — rotating nebula forms rings; dust drag slows core → Sun, rings aggregate → planets
• Solar Nebula Theory (Cameron, widely accepted)
  • Giant molecular cloud collapsed $\approx4.57\,\text{Ga}$ ago, perhaps triggered by supernova shock
  • Central mass became proto-Sun; surrounding accretion disk → planetesimals → planets, moons, minor bodies

Milky Way, Stars & Sun (Context)

• Milky Way: barred spiral, our solar system in Orion Spur
• Stellar evolution (simplified)
  • Interstellar cloud → protostar → main sequence → red giant / supergiant → planetary nebula / supernova → white dwarf / neutron star / black hole
• Sun: G-type yellow dwarf, central energy source of solar system

Quick-Recall Questions

• List universe-origin theories: Big Bang, Steady State, Cyclic, Inflation, Multiverse
• Big Bang formulator: Georges Lemaître
• Light shift terms: redshift (moving away), blueshift (moving toward)
• Solar-system-origin theories: Protoplanetary, Capture, Modern Laplacian, Solar Nebula