Describe the Big Bang theory of the origin of the universe. In your answer, include what the universe was like at the start and how it has changed over time.

The Big Bang theory states that the universe began from an extremely small, hot and dense state – a singularity – about 13.8 billion years ago. From this point it has been expanding, so the average distance between galaxies is increasing. As the universe expands it cools, so the temperature of the cosmic background radiation has fallen from millions of kelvin to the present 2.7 K. The relic radiation – the cosmic microwave background – is a key piece of evidence for the Big Bang. The age of the universe, measured from the expansion rate and the CMB, is therefore about 13.8 billion years, showing that the universe is very old.