Overview
The lecture explores the origins and evolution of the universe, discussing the Big Bang, cosmic inflation, the formation of matter, and theories regarding the universe's fate.
Light, Distance, and the Past
- The further we look into space, the further back in time we see due to light's finite speed.
- Light from Andromeda shows it as it was 2.5 million years ago; the earliest detected light is from when the universe was a few hundred thousand years old.
- The Cosmic Microwave Background (CMB) is ancient light from 380,000 years after the Big Bang.
The Big Bang and Cosmic Inflation
- The universe began from a hot, dense state about 13.8 billion years ago.
- Early universe was opaque until photons could travel freely; CMB marks this event.
- Inflation theory: universe rapidly expanded, smoothing out irregularities and stretching space.
- Alan Guth's model: universe expanded exponentially in less than a second.
Entropy, Time, and Order
- Entropy is the spread and loss of usable energy, defining the direction of time.
- Lower entropy marks the past; higher entropy marks the future.
- Despite apparent order, the universe’s evolution reflects increasing entropy.
- The observed uniformity and flatness of the universe lead to the horizon and flatness problems.
Theories on Universe's Origin and Fate
- Pre-Bang models: universe in a ‘false vacuum’ or hibernation before a trigger event.
- Eternal inflation: endless creation of 'pocket universes' with different constants.
- Cyclic models: universe undergoes infinite cycles of expansion (Big Bang) and contraction (Big Crunch).
- Fate may include heat death (entropy), Big Rip (dark energy tearing apart), or endless cycles.
Early Universe Timeline
- Planck Epoch: first 10^-43 seconds; size of a Planck length, extreme temperature.
- Grand Unification: separation of gravity from other forces, energy ~10^28 eV.
- Particle Era: formation of fundamental particles, Higgs field gives mass, matter-antimatter imbalance forms.
- Nucleosynthesis: after 3 minutes, nuclei like hydrogen and helium form.
- First molecules (helium hydride) and neutral atoms allow light to travel freely at 380,000 years.
Dark Matter and Dark Energy
- Dark matter outweighs normal matter 6:1; exerts gravitational pull but does not emit light.
- Dark energy began to dominate the universe's expansion ~5-6 billion years ago; its nature remains unknown.
Key Terms & Definitions
- Cosmic Microwave Background (CMB) — Ancient light from the early universe, now in microwave spectrum.
- Entropy — Measure of disorder or spread-out energy in a system.
- Inflation — Rapid early expansion of the universe.
- False Vacuum — Metastable state before the universe transitioned to a lower energy state.
- Planck Epoch — Earliest period of the universe, up to 10^-43 seconds after the Big Bang.
- Grand Unification — Era when fundamental forces began to separate.
- Nucleosynthesis — Process forming atomic nuclei from protons and neutrons.
- Dark Matter — Unseen matter detected by gravitational effects.
- Dark Energy — Unknown force driving accelerated expansion of the universe.
Action Items / Next Steps
- Review inflation, CMB, and early universe sections in textbook.
- Prepare notes on entropy, cosmic cycles, and fate of the universe for discussion.