Research Interests

My research interests center on observational cosmology: What is the large-scale structure of our universe and how did it form? How much dark matter is there in the universe and where is it located? And will the universe expand forever -- is it "closed" or "open"? In trying to resolve these questions, I have been using clusters of galaxies, the largest known bound systems in the universe (each constituting a "close family" of hundreds of galaxies), as powerful tracers of the structure, evolution, and mass-density of the universe. In collaboration with students and postdoctoral fellows, I have used clusters of galaxies to trace the large-scale structure of the universe (just as mountain peaks trace large mountain-chains on Earth), revealing the common existence of unexpectedly large structures (superclusters), extending to scales of hundreds of millions of light-years. We have also determined the abundance and clustering properties of the clusters as a function of their mass and luminosity, and showed, by comparison with extensive computer simulations of the universe, that the space distribution, abundance, and clustering properties of clusters of galaxies provide some of the most critical constraints on cosmological models. We have used masses of galaxies and clusters to "weigh" the universe, suggesting that most of the dark matter in the universe is located in huge dark halos around galaxies, but without significant amount of dark matter in "voids", where there is no or little light. Most recently, I have used observations of the evolution of the cluster abundance with time, revealing how massive structures grew at earlier times in the life of the universe; these data led us to determine cosmological parameters, including the mass-density of the universe. I showed that all the above measures portray a consistent picture of the universe and place stringent constraints on cosmology. All the results suggest that we live in a low-density universe, with only a fraction of the mass needed to halt its expansion. Thus, the Universe may expand forever.

Membership Type


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Primary Section

Section 12: Astronomy

Secondary Section

Section 13: Physics