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The Big Bang
13.8 billion years ago· Event· Universe & Earth
Well documentedSolid records, physical evidence or scientific consensus.
The universe began expanding from an extremely hot, dense state about 13.8 billion years ago.
See it on the History map →What happened
The Big Bang was not an explosion into empty space; space itself began stretching. In the first minutes the universe was a dense soup of particles, hot enough to fuse hydrogen into helium and a trace of lithium. It has been expanding and cooling ever since. What came before, if anything, is unknown. The Curious Science map covers the physics in depth.
Why it matters
It is the starting point for everything on this map: all matter, energy, space and time trace back to it.
Think of it like
Like raisins in rising bread dough: every raisin moves away from every other, and there is no center.
How we know
Georges Lemaître's 1927 prediction and Edwin Hubble's 1929 measurements that galaxies are rushing apart; the cosmic background light found in 1965; and the measured hydrogen-to-helium ratio all match the theory.
Still debated
What caused it, whether 'before' has any meaning, and what dark matter and dark energy are remain open questions.
Inside this (3)
Connections (2)
▸The Big Bang cooled enough to form First atoms and the cosmic background lightWell documented
The connection
The young universe was too hot for atoms: electrons moved freely and scattered light. As space expanded, it cooled. After about 380,000 years it was cool enough for electrons to bind to nuclei, forming neutral atoms and releasing the light now seen as the cosmic microwave background.
When and how
Throughout the universe, about 13.8 billion years ago. The leftover light was predicted by Ralph Alpher and Robert Herman, working with George Gamow, in 1948.
How we know
The background light has exactly the heat pattern the theory predicts, measured at 2.725 degrees above absolute zero by the COBE satellite in the early 1990s.
Debates and open questions
Why the early universe was so smooth, with only tiny ripples, is usually explained by 'inflation', a brief early growth spurt that remains unproven.
▸The Big Bang made the hydrogen and helium that fueled First stars and galaxiesWell documented
The connection
In its first few minutes, the Big Bang produced a mix of about three-quarters hydrogen and one-quarter helium, plus a trace of lithium. That was nearly all the matter available, so the first stars formed entirely from it, fusing hydrogen into helium for energy.
When and how
Across the universe: the elements formed within the first 20 minutes or so; the first stars ignited 100–200 million years later.
How we know
Pristine gas clouds, barely touched by stars, contain about 25 percent helium by mass, matching Big Bang calculations begun by Alpher and Gamow in 1948 and refined in the 1960s.
Debates and open questions
Theory predicts about three times more lithium than old stars actually show; this 'lithium problem' remains unsolved.