Big Bang
A physical theory describing how the universe expanded from an initial state of extremely high density and temperature approximately 13.8 billion years ago. The most widely accepted cosmological model for the origin and evolution of the universe. Connects directly to dark-matter, dark-energy, and stellar-nucleosynthesis in the aphelion knowledge graph.
Core Idea
The Big Bang does not describe an explosion into pre-existing space — it describes the expansion of space itself from an ultra-hot, ultra-dense initial state. Extrapolating backward using known physics, the universe was in an extraordinarily hot and dense condition; extrapolating further reaches a singularity where general relativity breaks down, requiring an as-yet-undeveloped theory of quantum gravity.
Timeline of the Early Universe
| Time | Event |
|---|---|
| 0 – 10⁻⁴³ s | Planck epoch — all four fundamental forces unified; characteristic scale ~10⁻³⁵ m; temperature ~10³² °C |
| ~10⁻³⁷ s | Cosmic inflation — exponential expansion driven by a phase transition; universe grows from ~4×10⁻²⁹ m to ~0.9 m; quantum fluctuations frozen in as seeds of large-scale structure |
| ~10⁻¹² s | Electroweak symmetry breaking; electromagnetic and weak forces separate |
| ~10⁻⁶ s | Quarks and gluons combine into protons and neutrons; matter-antimatter asymmetry (baryogenesis) leaves 1 baryon per 10⁸ annihilation events |
| ~3 minutes | Big Bang nucleosynthesis (BBN) — neutrons and protons fuse into deuterium, helium-4, lithium-7; ~75% H, ~25% He by mass |
| ~50,000 years | Matter energy density exceeds radiation density |
| ~380,000 years | Recombination — universe cools enough for electrons and nuclei to form neutral atoms; previously opaque universe becomes transparent; photons of this era = cosmic microwave background |
| ~100–400 million years | First stars and galaxies form from dark-matter-seeded density fluctuations |
Four Pillars of Observational Evidence
- Hubble's Law (1929) — galaxies recede at speeds proportional to their distance; Edwin Hubble's observation of redshifts implied an expanding universe
- Cosmic Microwave Background (CMB, 1964) — uniform 2.7 K thermal radiation permeating all of space; the afterglow of recombination; confirmed the hot dense early universe
- Big Bang nucleosynthesis — predicted abundances of H, He, D, Li match observations to high precision
- Large-scale structure — galaxy distribution and clustering match predictions from inflation-seeded density fluctuations growing under dark matter gravity
Connection to Dark Universe
- dark-matter — required to explain how gravity amplified small density fluctuations into galaxies; without it, the universe's large-scale structure doesn't form correctly
- dark-energy — discovered via Type Ia supernova distances showing accelerating expansion; not predicted by the original Big Bang model; causes the universe to expand faster over time rather than slowing down
- The standard model incorporating all three — ordinary matter (5%), dark matter (27%), dark energy (68%) — is the Lambda-CDM model
Open Questions
- The origin of the initial state itself (before Planck epoch) is unknown
- Baryon asymmetry — why matter dominated over antimatter by the observed 1:10⁸ ratio
- The precise nature of inflation (the inflaton field, energy scale)
- Whether the universe is finite or infinite