Overview
This lecture explains the Big Bang theory, how it describes the early universe's expansion, major supporting evidence, and competing or updated models about cosmic origins.
The Big Bang Theory Explained
- The Big Bang theory outlines the universe's history from a hot, dense origin about 14 billion years ago.
- Cosmologists agree the universe is expanding, cooling, and thinning out over time.
- The "Big Bang" can refer to the entire expansion or the specific moment of the universe's beginning.
- Debate remains about the exact details of the universe's first fraction of a second.
Evidence for the Big Bang
- Edwin Hubble discovered galaxies are moving away from us, suggesting an expanding universe.
- George Lemaitre proposed the idea of a "primeval atom" as the universe's origin.
- "Big Bang" as a term was coined by Fred Hoyle, originally to criticize the concept.
- Observable evidence includes the predicted ratios of light elements and the cosmic microwave background (CMB).
- The CMB is leftover light from when electrons and protons formed hydrogen, making it direct evidence of the Big Bang.
Modern Updates and Inflation Theory
- Big Bang theory originally implied a "singularity"—a state of infinite density and heat—which does not match observations.
- Alan Guth introduced inflation theory: the universe expanded extremely rapidly during its earliest moments.
- Inflation explains the smoothness and flatness of the CMB by stretching out initial irregularities.
- No definitive evidence of inflationary gravitational waves has yet been found in the CMB.
Alternative Theories and Open Questions
- The idea of a "multiverse" arises if inflation creates different regions with unique properties, but this is controversial.
- Some researchers propose alternatives like the "big bounce," where a prior contraction phase led to the current expansion.
- The exact starting conditions and detailed mechanisms of the universe's origin remain actively researched.
Key Terms & Definitions
- Big Bang Theory — theory that the universe began from a hot, dense state and has been expanding ever since.
- Cosmic Microwave Background (CMB) — faint microwave radiation, the oldest light observable, from when the universe cooled enough for atoms to form.
- Inflation Theory — proposes a period of extremely rapid expansion just after the universe began.
- Singularity — a hypothetical point with infinite density and temperature.
- Multiverse — concept that inflation creates multiple universes with different physical properties.
- Big Bounce — hypothesis that the universe’s expansion follows a prior contraction.
Action Items / Next Steps
- Review additional resources on inflation and CMB listed in the lecture.
- Read about the flatness problem and alternative cosmic origin models.
- Stay updated on new findings about gravitational waves and CMB data.