BIG BANG THEORY

The Big Bang theory is the leading scientific explanation for how the universe began and developed. It says the universe started about 13.8 billion years ago from an extremely hot, dense state, and has been expanding and cooling ever since.
What it says

• Everything we see (all matter, energy, space and time) was once packed into an incredibly small, hot, dense state.
• It wasn’t an explosion in space. Space itself expanded, and it still is.
• In the first fractions of a second the universe grew extremely fast (called inflation), then cooled enough for particles to form.
• After a few minutes, protons and neutrons combined into the first light atoms: hydrogen, helium and a little lithium.
• About 380,000 years later, the universe cooled enough for atoms to form and light could travel freely. That light is still visible today.
• Over hundreds of millions of years, gravity pulled gas together into the first stars and galaxies, and later into planets.

Main evidence

1. Expanding universe: Edwin Hubble found in the 1920s that distant galaxies are moving away from us, and the farther they are, the faster they recede.
2. Cosmic microwave background: a faint glow of leftover radiation fills the whole sky. It was discovered in 1965 and matches what the theory predicted.
3. Light elements: the observed amounts of hydrogen and helium in the universe match the theory’s predictions.
4. Galaxy patterns: the large-scale structure of galaxies fits how the theory says they should have grown.