## TL;DR

The Big Bang theory (Georges Lemaître, 1927; term coined by Fred Hoyle, 1949) describes the universe's origin ~13.8 billion years ago from an infinitely hot, dense singularity. Evidence: cosmic microwave background radiation (CMB, discovered 1965), redshift of galaxies (Hubble, 1929), abundance of light elements (H, He).

## Core Explanation

Timeline: Planck epoch (first 10⁻⁴³ seconds, physics unknown), inflation (10⁻³⁶ to 10⁻³² seconds, rapid expansion), particle formation, nucleosynthesis (H, He, Li formed in first 3 minutes), 380,000 years: atoms form, CMB released (universe becomes transparent). Dark ages → first stars (~400 million years) → galaxies → present. Universe continues expanding at accelerating rate (dark energy, discovered 1998).

## Further Reading

- [The First Three Minutes (Steven Weinberg)](https://www.basicbooks.com/titles/steven-weinberg/the-first-three-minutes/9780465024377/)