The Birth of the Universe

Scientists believe the universe began about 13.8 billion years ago in an event called the Big Bang. This was not an explosion in space — it was an expansion of space itself from an incredibly hot, dense single point. In less than a second, the universe grew from smaller than an atom to a vast, seething fireball of energy.

A common mistake is to imagine the Big Bang as a bomb going off somewhere in empty space. But there was no 'outside' for it to explode into — space, time, and everything else all began at that moment. It is better to picture the universe like the surface of a balloon being blown up, with every point moving away from every other point as the whole thing stretches.

How do we know the universe is expanding? In the 1920s the astronomer Edwin Hubble noticed that distant galaxies are racing away from us, and the further away a galaxy is, the faster it flees. If everything is flying apart today, then long ago it must all have been packed together — which points straight back to the Big Bang.

In the first three minutes after the Big Bang, the universe was hot enough for nuclear fusion to make the simplest elements: hydrogen and helium. These two gases are still the most common elements in the universe today. After about 380,000 years, the universe had cooled enough for atoms to form, and it became transparent — light could travel freely for the first time.