Why do you think your paper is highly cited?

This paper concentrates on what the dark matter distribution in the universe should be like if our currently favored theory of universal structure formation, known as "cold dark matter", is correct. The work provides statistically complete predictions for the density of dark matter around galaxies and galaxy clusters as a function of space and time in the universe.

One of the main challenges facing our current paradigm for structure formation is that measurements of galaxy rotation curves and other observations seem to indicate that there is too little mass in dark matter at the centers of galaxies compared to what is predicted. So our work on dark matter densities provides a reasonably stable target for cosmologists who are interested in testing the cold dark matter theory (and its variants) against observations. An understanding of how dark matter is distributed is also fundamental for modeling the sizes and rotation speeds of galaxies, as well as calculating the way in which dark matter and galaxies cluster together in space.

Yes, we have described a new discovery, but interestingly enough it is a discovery made by running a high-resolution simulation of a theory that has existed for some time. The results of our calculations were somewhat surprising. Namely, we discovered that the distribution of dark matter in "halos" around galaxies is expected to vary significantly from object to object. We also realized that the density distribution of these dark matter halos should vary substantially as a function of time. These results are useful to those interested in modeling how galaxies form, how fast they are rotating, and how they cluster together in space. In addition, we tried to present our results in a simple and concise manner that would be useful to as many people as possible.

It was mainly a technical advance that made this work possible. Very high-resolution simulations that modeled the gravitational collapse of structure in the universe became available. This was the work of Andrey Kravtsov and Anatoly Klypin among others. The simulations allowed us to model a cosmological volume and to recover the way in which dark matter should be distributed on scales as small as galaxies—a factor of over 30,000 in resolution.

Most cosmologists believe that a large fraction of the matter in the universe is in the form of "cold dark matter". This theoretical material cannot cool or emit light like ordinary matter, but does feel the force of gravity. We believe that each galaxy is surrounded by a large, extended "halo" of dark matter that is held together by gravity. If the dark matter theory is right then we can (in principle) predict how much dark matter there should be at the center of each halo (or galaxy). Our paper works out these predictions, and finds some interesting results. Namely, we came to the realization that the dark matter density is different for halos that collapsed at different times. Halos that collapsed ten billion years ago will be much denser than those that collapsed relatively recently. Thus the density of a galaxy might tell us about when it was formed. In addition, the dark matter content of a galaxy might vary depending on its size, its color, or whether it looks like a disk (like our Milky Way) or a spheroid (like the Virgo cluster galaxy M87).

Dr. James Bullock, Postdoctoral Fellow
Ohio State University