Cosmology
Big Bang
The origin of the universe — 13.8 billion years ago, expansion from extremely hot dense state
The Big Bang is the prevailing cosmological model for the universe's origin. ~13.8 Gyr ago, universe began in an extremely hot, dense state and has been expanding ever since. Evidence: (1) cosmic microwave background — afterglow of Big Bang. (2) Hubble's law — distant galaxies receding. (3) Light element abundances (H, He, Li) — match Big Bang predictions. (4) Galaxy formation — structure builds from initial fluctuations. Inflation theory adds rapid early expansion. Universe was opaque until ~380,000 yr (recombination). After: transparent; first stars formed ~100-200 Myr.
- Age of universe13.8 ± 0.04 Gyr (Planck 2020)
- Initial conditionsHot, dense — temperature ∞ at t = 0
- Recombination~380,000 yr after BB; universe became transparent
- First stars~100-200 Myr after BB
- Hubble constant H₀67-73 km/s/Mpc (tension)
- Critical evidenceCMB, expansion, light elements, structure
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Why Big Bang matters
- Cosmology foundation. Universe's origin theory.
- CMB. Direct observational consequence.
- Element formation. Light element creation in early universe.
- Galaxy evolution. Initial conditions for structure.
- Particle physics. Probe high-energy regimes.
- Dark matter / DE. Cosmological context.
- Universe age. 13.8 Gyr framework.
Common misconceptions
- BB was an explosion. Expansion of space; not explosion in space.
- BB happened at a point. Everywhere uniformly.
- BB was 13.8 Gyr ago everywhere. Cosmic age — same everywhere.
- Universe started from nothing. Started from singularity; not "nothing."
- BB caused inflation. Inflation occurred ~10⁻³⁶ s after Big Bang.
- BB is not testable. Multiple lines of evidence confirm.
Frequently asked questions
What was before the Big Bang?
Question may not be well-defined. Big Bang theory describes evolution since t > 0. At t = 0: singularity (general relativity breaks down). Possible: quantum cosmology, multiverse, eternal inflation, cyclic models, quantum gravity. Current physics doesn't reach back to t = 0. Many proposals; no consensus.
Did the Big Bang happen at a place?
No "place." Universe was ALL hot/dense everywhere — not a single spot. Now expanding everywhere; not from any specific point. Cosmic microwave background fills sky uniformly — confirms isotropic origin. We're not at the center; there's no center.
How is age determined?
Multiple methods. (1) Hubble's law — H₀ from local universe → distance/velocity → expansion timescale. (2) CMB — Planck mission measured detailed structure → age. (3) Stellar ages — oldest globular clusters older than ~13 Gyr. All converge: 13.8 Gyr ± 0.04 Gyr.
What's "before recombination"?
Universe was opaque plasma — electrons interacted strongly with photons. ~400,000 yr after BB, T cooled to ~3000 K → electrons recombined with protons → first transparent atoms (hydrogen). Photons free-streamed → CMB. Pre-recombination: opaque; post: transparent.
How is BB confirmed?
(1) CMB — predicted 1948; detected 1964 (Penzias & Wilson). (2) Hubble's law — distant galaxies redshifted (1929). (3) Light elements — primordial nucleosynthesis predicts H, He, Li abundances; matches observations. (4) Galaxy evolution — structure formation traceable. (5) CMB anisotropies — match initial fluctuations.
What's inflation?
Period of rapid expansion (~10⁻³⁶ to 10⁻³² seconds after BB). Universe expanded by factor ~10²⁶. Solves: horizon problem (uniform CMB), flatness problem (universe geometric), monopole problem. Drives: scalar field at high energy. Quantum fluctuations during inflation become CMB anisotropies.
Will universe end?
Depends on dark energy behavior. Current understanding: cosmological constant. Universe expands forever; accelerates. After ~10⁵⁰⁰ years: dark, cold, isolated black holes. Heat death. Other scenarios: phantom DE → big rip; collapse via DE reversal — possible but unlikely. Most data favor heat death.