In 1929, working at the Mount Wilson Observatory with the largest telescope in the world, Edwin Hubble compiled a striking dataset. He had been measuring distances to galaxies (using Henrietta Leavitt's 1908 calibration of Cepheid variables) and the redshifts of their light. The pattern was unmistakable: the further away a galaxy was, the faster it was receding. Hubble's law, v = H₀ · d, had one explanation — the universe was expanding. Run the expansion backward in time, and at some finite past moment, everything was at a single point. The universe had a beginning. The moment has been called the Big Bang — a derisive term coined by Fred Hoyle in a 1949 BBC broadcast. Hoyle lost the argument; the term stuck.
The universe is expanding — not into anything, but intrinsically, with the spatial scale factor a(t) growing over time. Galaxy redshifts arise from this expansion: light emitted by a distant galaxy is stretched as it travels. Hubble's constant H₀ ≈ 67–73 km/s/Mpc sets the present rate. Run the expansion backward: a(t) → 0, density → ∞, temperature → ∞. The state at t = 0 is a singularity where general relativity breaks down. The first ~10⁻⁴³ seconds (Planck era) are inaccessible without quantum gravity. The standard Big Bang chronology runs: at 10⁻³⁵ s cosmic inflation (Guth, 1980) stretches a quantum-scale region to cosmic scales, explaining the observed flatness and uniformity and seeding primordial density fluctuations. At 10⁻⁶ s quarks bind into protons and neutrons. At 3 minutes Big Bang nucleosynthesis forms light nuclei (helium, deuterium, lithium) with predicted abundances matching observations to high precision. At 380,000 years recombination cools the universe enough for electrons to bind to nuclei; photons decouple and travel freely. The light emitted at this moment is still detectable today as the cosmic microwave background (CMB) — a faint 2.7 K microwave glow filling all of space, with minute temperature fluctuations (one part in 10⁵) that map the density structure at recombination. At 400 million years the first stars and galaxies form. At 13.8 billion years: now. The standard cosmological model — ΛCDM — fits a vast array of observations with just six parameters, even as the physical content of two of those parameters (dark matter, dark energy) remains fundamentally unknown.
Big Bang cosmology is settled science — no serious physicist doubts the broad outline. What remains unsolved: the nature of dark matter (~25% of the universe's energy density, observed gravitationally, never detected directly despite decades of search); the nature of dark energy (~70%, driving the acceleration of cosmic expansion discovered in 1998); the physical mechanism of inflation; the Hubble tension (different methods of measuring H₀ disagree by ~10%, possibly indicating new physics). CMB experiments (COBE, WMAP, Planck, future LiteBIRD) and galaxy surveys (DESI, Euclid, Rubin Observatory) continue to refine the picture. Gravitational-wave astronomy (LIGO, Virgo, KAGRA, future LISA) opens a new window on the very early universe.