Why is matter in the universe so unevenly distributed after the big bang?

Answer

This question touches on a very important research issue.

The main reason why there are structures in the universe is gravity, which tries to bring matter together. Important here is what we call gravitational instability. Suppose that in a large cloud there is a small area where the mass density is greater than elsewhere. A particle on the edge of this area will be attracted to that area: because it feels more gravitational pull from that direction. The result: the area contracts, and the density contrast grows. And so on, until a star is formed, say.

Now you ask: where does that density difference come from? The answer is that you cannot avoid density contrasts: there is no such thing as perfect homogeneity. And the property that gravity is unstable (a small contrast is magnified) implies that structure must arise.

In addition to matter – which we see structured in (clusters of) galaxies – the universe also consists of radiation. It had to be in the big bang theory, because if everything was close together, it must have been very hot, and heat produces radiation. Now radiation cannot be structured very well. This ‘cosmological background radiation’, the afterglow of the Big Bang, has been found since 1965 and fills the entire sky.

An important research item in astronomy today is how matter has structured itself from the fairly homogeneous primordial soup into what we see today. In the beginning – when it was so hot that the matter was charged – the radiation held the matter in its grip through electromagnetic interactions. These are so much more intense than gravity that density differences do not grow. When matter and radiation finally decouple (after 400,000 years), they each go their own way: the radiation remains homogeneous and cools down to what we see today as the cosmic background radiation; matter begins to form structures. How all this happened, and why the result ended up being what we see today, is the great story of contemporary astronomy.