Monica Grady: The universe is still expanding and cooling

From the Royal Institution Christmas Lecture by the meteorite scientist

Our journey starts a huge amount of time ago, when the universe came into existence. What happened then is described as a Big Bang. This was the instant when time and space began. All the material that makes up the universe was squashed into a pinprick. There were no dimensions, no length, width or height. And no fourth dimension, of time, which is why we cannot discuss what was there before the Big Bang; there was no "before". We don't understand what made the Big Bang happen; we don't know if we are the only universe, or if there are lots of them.

At the Big Bang, radiation expanded in every direction. The temperature was so hot, around 10 hundred thousand billion billion billion degrees, that atoms and electrons couldn't exist. By only a second after the Big Bang, the universe had inflated to about a light year across, and the temperature was "only" 10 billion degrees. Even though this is still pretty hot, electrons, protons and neutrons, the particles that make up atoms, had formed and were crashing into each other. After about a minute, the universe had expanded to four light years across, the temperature dropped to 1 billion degrees, and atoms of hydrogen and helium formed.

The universe is still expanding and cooling. It is the amount that it has expanded since the Big Bang that helps us to determine the size and age of the universe. We can see how much the universe has expanded by looking for where the oldest parts of the universe - the galaxies that are furthest away.

This lecture series will be broadcast on Channel 4 over the Christmas period