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Over the last 15 years or so, computer simulations have become the primary tool that theoreticians have at their disposal to understand the formation of galaxies and of structure in the universe. The aim of the simulations is to take what we call initial conditions, which is an early state of the universe, and then see how that primeval, amorphous state evolves into an approximation of the universe we can compare with current observational surveys. Through these simulations we can arrive at an understanding of what the universe is made of, how it is structured, and how it came to be.
Computer-simulated universes are a very powerful tool because they allow you to produce material evidence for what various assumptions about the universe translate into, and then you can take this material evidence and compare it against reality. Because the universe is so complex, most mathematical treatments require many approximations and simplifications, so they are of limited applicability. Yet with a computer simulation you don’t need to make any of those approximations. You solve the equations in the full generality, so it's a very appealing activity for theoreticians to do.
In the classic Einsteinian view of the universe, everything is smooth at the beginning and stays smooth forever. That clearly is not what our universe is doing because today our universe is very inhomogeneous—it is broken up into islands that we call galaxies and galaxy clusters. If the universe had been entirely smooth, we wouldn’t be here to talk about it.
Instead, there must have been a small departure initially from this simplest assumption of a perfectly uniform universe. So the universe was not perfectly homogeneous either when it began or shortly after it began but, rather, it was slightly inhomogeneous. It had small regions where the density of matter was slightly higher than average and other regions where it was slightly lower than average. They were really tiny, these inhomogeneities, so tiny that for practical purposes it is hardly much of a departure from the simplest version of the theory. Yet tiny as they are to begin with, these inhomogeneities are very important because they are the seeds from which star clusters, galaxies and, eventually, human beings, will grow.
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