Big Bang Theory
The Big Bang theory is the prevailing cosmological model describing the observable universe as having expanded, and continuing to expand, from an extremely hot, dense initial state approximately 13.8 billion years ago — supported by three major independent lines of observational evidence: the recession of distant galaxies (Hubble's law), the cosmic microwave background radiation, and the observed abundances of light elements.
Curious
A common misconception is that the Big Bang was an explosion that occurred at a specific point within a pre-existing, empty space, with matter flying outward into that space. The actual picture, as understood by physicists, is different and considerably stranger: space itself has been expanding since the Big Bang, carrying galaxies apart from one another as it does — there was no pre-existing empty void for an explosion to happen "into," since space itself is what began expanding.
The evidence for this picture comes from several independent directions that all converge on the same basic story: distant galaxies are observed receding from us at speeds proportional to their distance (Hubble's law, directly implying past expansion); a faint, remarkably uniform microwave glow fills the entire sky in every direction (the cosmic microwave background, the cooled-down afterglow of the hot early universe); and the observed relative abundances of the lightest elements (hydrogen, helium, and trace lithium) match precisely what Big Bang nucleosynthesis predicts should have formed in the universe's first few minutes.
Exploring
| Symbol | Name | SI Unit |
|---|---|---|
| v | Recession velocity of a distant galaxy | m/s (often km/s) |
| H₀ | Hubble constant | km/s/Mpc |
| d | Distance to the galaxy | metre (often megaparsecs, Mpc) |
Deep Dive
Edwin Hubble's 1929 observation that distant galaxies' recession velocities are roughly proportional to their distance provided the first major direct observational evidence for cosmic expansion, building on Vesto Slipher's earlier (1910s) galaxy redshift measurements and on Georges Lemaître's independent 1927 theoretical proposal (based on Einstein's general relativity) of an expanding universe originating from what Lemaître called a "primeval atom." The term "Big Bang" itself was coined, somewhat mockingly, by astronomer Fred Hoyle during a 1949 radio broadcast — Hoyle favoured a rival "steady state" cosmological model and intended the phrase dismissively, though it was later adopted as the standard, neutral name for the theory once observational evidence increasingly favoured it over Hoyle's alternative.
Big Bang nucleosynthesis: Within the first few minutes after the Big Bang, as the universe cooled from its initial extreme temperature, protons and neutrons briefly fused into light nuclei — primarily hydrogen and helium-4, with small amounts of deuterium, helium-3, and lithium-7 — before the expanding universe cooled and thinned too much for further fusion to occur. The predicted relative abundances of these light elements depend sensitively on the universe's early conditions and match observed abundances with remarkable precision, providing one of the Big Bang theory's most quantitatively precise and independently verifiable confirmations.
The cosmic microwave background, discovered in 1964 and mapped with steadily increasing precision by subsequent satellite missions (COBE in the 1990s, WMAP in the 2000s, Planck in the 2010s), reveals tiny temperature fluctuations (of order 1 part in 100,000) across the sky — these fluctuations encode detailed information about the universe's very early structure and composition, and their precise statistical pattern provides strong, independent support for cosmic inflation (a very brief period of extremely rapid exponential expansion proposed to have occurred fractions of a second after the Big Bang, see the Codex's forthcoming Inflation entry) as well as precise measurements of the universe's overall geometry, age, and composition — including the striking finding that ordinary matter makes up only a small fraction of the universe's total mass-energy content, with dark matter and dark energy (see the Codex's forthcoming Dark Matter and Dark Energy entries) accounting for the great majority.
Concept Relationship Map
Requires: curved spacetime (general relativity), nuclear fusion (primordial nucleosynthesis)
Enables: cosmic microwave background [PENDING FULL ENTRY], inflation [PENDING FULL ENTRY], dark matter
Commonly confused with: picturing the Big Bang as an explosion into pre-existing empty space, rather than the expansion of space itself
Sources
- Hubble, E. (1929). Proceedings of the National Academy of Sciences, 15(3), 168–173.
- Penzias, A.A., Wilson, R.W. (1965). "A Measurement of Excess Antenna Temperature at 4080 Mc/s." The Astrophysical Journal, 142, 419–421.
- Planck Collaboration (2020). "Planck 2018 Results. VI. Cosmological Parameters." Astronomy & Astrophysics, 641, A6.