I am a Professor in the Department of Physics & Astronomy at the University of Pennsylvania.  I have worked on instrumentation and a little bit of theory, but mostly focussed on astronomical image processing, data analysis, and statistics.


Weak Gravitational Lensing: Light is bent by gravity, so if we had some wallpaper behind a big pile of mass, our view of this wall paper is be distorted – just like looking through a bumpy sheet of glass. Faraway galaxies can serve as this wallpaper. By careful examination of the shapes of many of these galaxies, we can make a map of all the matter between us and the distant galaxies. This even includes the dark matter which greatly outweighs all the normal atomic matter in the Universe, but is otherwise invisible!

Highlights of many years of work in this field with my students and collaborators include the first and the most accurate measurements of the dark matter “lumpiness” in the Universe using this weak gravitational lensing method.

Trans-Neptunian Objects: Pluto is now known to be just one of thousands of icy bodies orbiting the Sun beyond Neptune. But finding them isn’t easy – they are very faint, and they hide among the billions of faint stars and galaxies on the sky. These TNOs can be found by looking for the faint dots that move slowly across the sky, roughly 1 degree per year or less. The orbital arrangements and sizes of these TNOs reveal much about the history of the Solar System. In particular, that the early Solar System must have been much different than the present ones, with the giant planets in different locations, and maybe even some additional large planets that are hiding very far away or have escaped entirely.

Our TNO searches have yielded the first definitive evidence of a sharp decrease in the number of TNOs beyond 50 AU from the Sun, the discovery of the faintest TNO ever found, and the detection of over 300 TNOs using the Dark Energy Survey.

Check the Projects menu for current and past research projects and instruments that I have worked on.


This research has been made possible by generous support from the National Science Foundation, the Department of Energy Office of Science, and NASA. Support for our participation in DES has been provided by the UPenn School of Arts & Sciences.

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