What Is the Big Bang Theory?

The theory describes the evolution of the universe from about a tiny fraction of a second after its beginning up to the present, roughly 13.8 billion years later. It says that everything was once far denser and hotter, and that space itself has been expanding.

It does not describe the moment of origin. The physics we have breaks down at extremely high densities, so what happened at the very start, or whether that question is even well formed, remains open. The theory is about what came after, and that part is very well tested.

Three independent observations support it strongly:

• •Distant galaxies are moving away from us, with speed proportional to distance
• •The cosmic microwave background, faint radiation left from when the universe cooled enough to become transparent
• •The measured proportions of hydrogen, helium and lithium, which match predictions from the first few minutes

This is the strongest single piece of evidence. About 380,000 years after the beginning, the universe cooled enough for atoms to form and light to travel freely. That light is still arriving, stretched by expansion into microwaves.

It was found accidentally in 1965 by radio astronomers who could not get rid of a persistent hiss in their antenna. Later satellites mapped it in detail, revealing tiny temperature variations that match what the theory predicts for the seeds of galaxies.

Several common images are wrong. The big bang was not an explosion at a point in existing space. It was an expansion of space itself, happening everywhere at once, so there is no centre to point to.

Nor are galaxies flying outwards through space like debris. The space between them is stretching. A useful mental image is dots on a balloon being inflated: every dot moves away from every other, and no dot is the centre.

Plenty. About 95 percent of the contents of the universe is dark matter and dark energy, neither of which is understood. There is also active debate over measurements of the expansion rate, where different methods give results that do not quite agree.

These are real open problems, not signs the framework is collapsing. The big bang theory is the best supported account we have of cosmic history, and the disagreements are about details within it rather than whether it is broadly right.

The name was coined by the astronomer Fred Hoyle during a radio broadcast in 1949, and Hoyle did not believe the theory. He favoured a rival steady state model in which the universe had no beginning and matter was continuously created.

Whether he intended the name as mockery is disputed, and he later said he was simply trying to draw a vivid contrast for listeners. Either way the label stuck to the theory it was meant to distinguish itself from, and the discovery of the cosmic microwave background in 1965 effectively ended the steady state model.

The big bang theory says the universe began hot and dense and has been expanding for 13.8 billion years, supported by galaxy motions, leftover radiation and the abundance of light elements, and it describes what happened after the beginning rather than the beginning itself.