How did the universe begin?

The most popular theory is the Big Bang Matter, energy, space and time all started 13.7 billion years ago Nobody knows what caused the Big Bang, or even if it had a “cause”

At first the universe was infinitely hot and dense, smaller than a single atom In an instant, the universe expanded very quickly Then it settled down to a slower rate of expansion, cooling all the time After a few minutes it had cooled enough for protons to form

It took another years to cool enough for hydrogen and helium atoms to form The universe became transparent and filled with light This is as far back as astronomers will ever be able to see, even with the biggest space telescopes [Image by NASA]

Hydrogen and helium were the raw ingredients that were available for making stars form Heavier atoms were made inside stars Then more stars were born, many of them with planets like our solar system These smudges, taken by the Hubble space telescope, are galaxies forming in the early universe [Image by NASA]

Whichever way you look out into space, you see galaxies speeding away from us The light from these galaxies is stretched out towards the red end of the spectrum Measuring this red-shift tells us how fast they are moving The further away they are, the faster they are moving away So the universe was once much smaller and denser [Image by NASA]

We can measure faint microwave (heat) radiation coming from space This image shows the cosmic microwave background radiation arriving from all directions (the colour is false) It’s the after-glow from the Big Bang, when the universe must have been incredibly hot billions of years ago It’s existence was predicted from Big Bang theory before it was discovered WMAP [CERN]

Light elements like hydrogen make up most of the matter in the universe, with smaller proportions of heavier elements This ties in nicely with what the Big Bang theory predicts we should find

Like any idea in science, the Big Bang theory is only as good as the evidence that supports it So far it has stood up to everything we’ve thrown at it But there are big gaps in our understanding The LHC experiments recreate conditions 1/100th of a billionth of a second after the Big Bang Nobody is sure what we will discover… [CERN]