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H Diagram showing (a) top-down and (b) bottom-up scenarios for galaxy formation.

has made it possible to simulate the evolution of the universe and galaxy formation under various conditions. The results of the computer simulations can then be compared with the large body of data. If a particular simulation can reproduce the distributions of galaxies and their large-scale motions, then it is possible that the physical ideas that went into that simulation might be important in the real universe. However, to date, no single simulation has been able to produce structure on all of the scales that we see.

Part of the problem involves dark matter. We have already seen that there is more dark matter than luminous matter around us. In clusters of galaxies, the domination of dark matter is even greater. Since galaxy formation is initiated by gravitational attraction, most of that attraction will be provided by most of the matter. This means that, even though we see galaxies and clusters as luminous objects, their formation is governed by dark matter. Therefore, to make a successful computer simulation of galaxy formation, we must start with the right type of dark matter. However, we have said that we don't know what the dark matter is. We don't even know if the dark matter in individual galaxies is the same as that in clusters of galaxies.

While this may seem like an insurmountable problem, theoreticians have managed to turn it around. They first realized that, even if we don't know the details of the dark matter, there are probably classes that can be treated as a whole. For example, if the dark matter in individual galaxies is in the form of Jupiter mass objects, then, from the point of view of gravity, it doesn't matter whether we have a certain number of Jupiter mass objects or ten times as many objects whose mass is one-tenth that of Jupiter. The point would be that we were dealing with ordinary matter, but not in quantities large enough to form luminous stars.

By analyzing the various possibilities, theoreticians have been able to group the dark matter into two general types, according to how it behaves. The two types are called cold dark matter and hot dark matter. These don't have to do with the temperature, but with how the material behaves. An example of hot dark matter would be neutrinos, if they had even a very small rest mass. We will talk about what particles these might actually be in Chapter 21, when we talk about the earliest times in the universe. For now, we only need to know that these produce different types of structures.

So we have gone from vague notions of top-down and bottom-up scenarios to asking a very specific question: What is the dark matter that dominates galaxy formation? The hope is that we can answer that question by comparing the simulations with the data. Unfortunately, at this point, neither one works completely. The cold dark matter is good at producing the small-scale structures (the galaxies) but not the large-scale structures (the clusters and superclusters, or the 'bubbles'). The hot dark matter is just the opposite. It appears that cold dark matter does a better job of describing the structures we see, provided we add some modifications that come from general relativity, which we will discuss in Chapter 20.

Even when we correctly identify the dark matter, and show how galaxies and clusters can grow from some small density enhancements in the early universe, we still haven't addressed the issue of where those initial enhancements came from. To do that we must look at the history of the universe, the field called cosmology, and we do that in Chapters 20 and 21.

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