Big Bang Theory
The Big Bang Theory is the prevailing cosmological model describing the origin and evolution of the universe, proposing that all matter, energy, space, and time originated from an extremely hot, dense singularity approximately 13.8 billion years ago. The rapid expansion from this primordial state led to cooling, the formation of fundamental particles, and eventually atoms, stars, and galaxies. Evidence supporting this model includes the observed expansion of the universe, the cosmic microwave background radiation, and the abundance of light elements like hydrogen and helium.
Key Epochs in Big Bang Cosmology
| Epoch | Time After Big Bang | Temperature (K) | Key Event |
|---|---|---|---|
| Planck Epoch | < 10⁻⁴³ s | > 10³² | All forces unified |
| Quark Epoch | 10⁻¹² – 10⁻⁶ s | 10¹⁵ | Quarks and gluons form |
| Nucleosynthesis | 3 min – 20 min | 10⁹ | Protons and neutrons fuse |
| Recombination | ~380,000 yr | 3000 | Atoms form; CMB released |
| Reionisation | ~150–800 Myr | ~100 | First stars ignite |
| Present | 13.8 Gyr | 2.73 | Observable universe today |
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Related Terms
Cosmic Microwave Background
The Cosmic Microwave Background (CMB) is the thermal electromagnetic radiation permeating the entire observable universe, representing the afterglow of light released approximately 380,000 years after the Big Bang when the universe cooled enough for protons and electrons to combine into neutral hydrogen atoms. It is observed today as a nearly uniform blackbody radiation at a temperature of approximately 2.725 K, with tiny temperature fluctuations of about 1 part in 100,000 that encode the seeds of large-scale cosmic structure. The CMB is considered one of the strongest pieces of evidence for the Big Bang model and provides precise measurements of cosmological parameters.
Cosmic Inflation
Cosmic inflation is a theoretical period of exponentially rapid expansion that the universe underwent approximately 10⁻³⁶ to 10⁻³² seconds after the Big Bang, during which it expanded by a factor of at least 10²⁶ in linear size. Proposed by Alan Guth in 1980 and refined by Andrei Linde and others, inflation elegantly resolves three major cosmological problems: the horizon problem (why the CMB is so uniform), the flatness problem (why the universe appears spatially flat), and the magnetic monopole problem. Quantum fluctuations during inflation are thought to be the seeds of all large-scale structure, stretched to cosmological scales.
Expanding Universe
The expanding universe is the observational discovery that the fabric of space itself is stretching over time, causing galaxies that are not gravitationally bound to each other to recede from one another at velocities proportional to their separating distances. First confirmed observationally by Edwin Hubble in 1929 through measurements of galaxy redshifts, this expansion is described by the Friedmann–Lemaître–Robertson–Walker (FLRW) metric in general relativity. The rate of expansion, parameterised by the Hubble constant H₀ ≈ 67–73 km/s/Mpc, has been measured to be accelerating due to dark energy.
The term "Big Bang" was coined by British astronomer Fred Hoyle in 1949 during a BBC radio broadcast, originally as a somewhat derisive label for the theory proposed by Georges Lemaître (1927) and later developed by George Gamow. "Big" derives from Old English "byċġe" and "Bang" is onomatopoeic, describing the explosive expansion.