Big Bang theory

If the universe is expanding today, then yesterday everything was a little closer together, and the day before that, closer still. Keep rewinding and the galaxies merge, the gas heats, and you reach a moment when the whole observable universe was packed into an unimaginably hot, dense state. The Big Bang theory is the well-tested account that the universe began about 13.8 billion years ago from exactly such a hot, dense state and has been expanding and cooling ever since.

Two misconceptions need clearing up first. The Big Bang was not an explosion of matter into pre-existing empty space — it was the expansion of space itself, happening everywhere at once, with no centre and no edge. And the theory does not actually claim to describe the very first instant or a 'creation from nothing'; it describes the universe from a tiny fraction of a second onward, where the known physics is reliable. As the universe expanded and cooled, it passed through clear stages: a hot soup of particles; the forging of the lightest atomic nuclei in the first few minutes (Big Bang nucleosynthesis); about 380,000 years later, cooling enough for atoms to form and light to fly free (leaving the cosmic microwave background); then hundreds of millions of years of darkness before the first stars and galaxies lit up.

The Big Bang theory is the foundation of modern cosmology because it rests on three independent pillars of evidence that all point the same way: the observed expansion (Hubble's law), the cosmic microwave background glow predicted decades before it was found, and the precise cosmic abundances of hydrogen, helium, and lithium that nucleosynthesis explains. No rival theory accounts for all three. It is honest to say the theory leaves the ultimate origin open, but as a description of the universe's hot, expanding history, the evidence is overwhelming.