You must obey these three laws to join the group.


My scientific work has been on theoretical problems in cosmology with ties to observations. Four personae have been diagnosed for my multiple personality syndrome, although my graduate students and postdocs may report (and have induced) more identities:

  • The Smooth Side: During the first ten million years or so after the big bang, the fluctuations in matter and radiation in the universe remain small enough that they can be viewed as tiny ripples imprinted on a smooth background. The universe at this stage is well described by the linear cosmological perturbation theory, which I have investigated in some depth. This theory is specified by the coupled Einstein, Boltzmann, and fluid equations and is the foundation for many calculations in cosmology, e.g., the matter fluctuation power spectra and the temperature variations imprinted on the cosmic microwave background.

  • The Lumpy Side: A major challenge in theoretical cosmology today is in understanding how small initial fluctuations in matter and radiation in the smooth universe grow under gravitational instability into highly collapsed objects in the lumpy universe. I have been involved in designing and running large numerical N-body and hydrodynamical codes to simulate and study the nonlinear growth of dark matter and galaxies.

    My interests are in developing new analytical techniques to better understand and model the nonlinear structure in the universe. The questions I am tackling include the phase-space evolution of galaxy host halos and the origin of their density profiles. This has led me into the deep trenches of the 90-year-old Fokker-Planck and dissipation-fluctuation theories.

  • The Dark Side: The nature of dark matter remains one of the most intriguing unsolved mysteries in astronomy. My interest has evolved from cold, to hot, to warm, and back to cold (but never lukewarm) dark matter, and has recently extended to the "dark energy." Our investigations involve making detailed theoretical predictions for the impact of dark matter and energy on structure formation. By comparing these with observational results from, e.g., the high redshift universe and gravitational lenses, we can obtain constraints on the nature and abundance of dark matter and energy.

  • The Bright Side: I occasionally manage to climb out of the dark and lumpy worlds. During these sunny moments (or rather, moonless nights), I have collaborated with observers on a number of projects using telescopes such as Palomar and Keck. The projects include dynamical studies of superclusters, gravitational lensing studies of quasars and galaxy clusters, properties of distant galaxies, and weighing supermassive black holes.

    Graduate Students (to keep off the street)


    Postdocs (to dine and wine)


    Former Team Members (and other diligent students who brightened my days or ate cookies at group meetings)


    • Michael Boylan-Kolchin (PhD 2006, postdoc at UC Irvine)
    • Jun Zhang (Columbia PhD 2006, TAC Fellow 2006-09, postdoc at UT Austin)
    • Tzu-Ching Chang (Columbia PhD 2003, TAC Fellow 2003-06, faculty at ASIAA)
    • Alice Shapley (Caltech PhD 2003, Miller Fellow 2003-05, UCLA Professor)
    • James McBride (Senior Thesis 2009)
    • Brad Hagan
    • David Rusin (UPenn)
    • Shwetabh Singh (UPenn)
    • Nick Sarbu (UPenn)