Definition · Plain-language
Big Bang theory
The Big Bang theory is the prevailing cosmological explanation for the origin of the universe, which began in an extremely hot, dense state and has been expanding ever since.
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What the theory says
The Big Bang theory describes the universe beginning roughly 13.8 billion years ago in an extraordinarily hot, dense state, then expanding and cooling into everything we see today. A crucial and often-missed point is that the Big Bang was not an explosion of matter into pre-existing empty space; rather, space itself began expanding everywhere at once. As the universe cooled, energy condensed into particles, then atoms, and over billions of years gravity drew matter into stars, galaxies and larger structures. The theory rests on Einstein’s general relativity and is the foundation of modern cosmology.
The evidence
The Big Bang is accepted because three independent lines of evidence converge on it. First, distant galaxies are moving away from us, with farther ones receding faster — the expansion first measured by Edwin Hubble — implying everything was once together. Second, the cosmic microwave background is a faint, uniform glow of microwave radiation filling all space, the cooled afterglow of the hot early universe, predicted before it was found in 1965. Third, the observed proportions of the lightest elements, hydrogen and helium, match precisely what the theory predicts should have formed in the first few minutes. Together these make it the best-tested account of cosmic history.
What it does and does not explain
The Big Bang theory explains the universe’s evolution from a tiny fraction of a second after the beginning onward, but it does not claim to explain what, if anything, came “before”, nor what caused it — questions that lie at the frontier of physics. It is frequently misrepresented as a theory of how the universe came from nothing; in fact it describes how the universe developed from an early hot, dense state, not the ultimate origin. Open questions remain, including the nature of the dark matter and dark energy that dominate the cosmos and the details of the universe’s earliest instants.
Key facts
At a glance
- Definition: the leading explanation for the origin and expansion of the universe
- Age: the universe is about 13.8 billion years old
- Began as: an extremely hot, dense state that expanded and cooled
- Key point: space itself expanded; it was not an explosion into empty space
- Evidence: cosmic expansion, the cosmic microwave background, light-element abundances
- Does not explain: what came “before”, or the ultimate cause
Common misconceptions
What people often get wrong
Often heard: The Big Bang was an explosion of matter into empty space.
Actually: It was an expansion of space itself, happening everywhere at once. There was no pre-existing empty space for matter to explode into.
Often heard: The Big Bang theory explains how the universe came from nothing.
Actually: It describes how the universe evolved from an early hot, dense state onward. It does not claim to explain the ultimate origin or what, if anything, preceded it.
Often heard: The Big Bang is just a guess with no real evidence.
Actually: It is supported by several independent observations — cosmic expansion, the cosmic microwave background and the measured abundance of light elements — that together make it the best-tested cosmological theory.
Going deeper
