#2217 - Brian Cox

5201.839 - 5228.877 Brian Cox

So in this young universe, 380,000 years after the Big Bang, that's only hydrogen and helium gas, basically, and a bit of lithium, some of the lighter elements, but basically hydrogen and helium. So you've got an almost smooth, almost featureless universe then. But these little density fluctuations are very important because as the universe expanded and cooled, they collapsed to form the galaxies.