Because mass bends the path of light, we can use observations of background sources of light to infer the presence of foreground masses, as well as their total mass and how it is distributed. Because most matter in the universe is dark, gravitational lensing has become an important astrophysical tool. I was lead author on the discovery of gravitational lensing by large-scale structure and on the first discovery of a galaxy cluster via lensing and the first use of tomography in lensing, and I continue to pursue these topics through the Deep Lens Survey and the planned Large Synoptic Survey Telescope described below. In fact, the merging cluster depicted above was discovered via lensing in the DLS.
I am currently focused on the Deep Lens Survey (DLS), a deep optical imaging survey of 20 square degrees of sky. As Co-PI, I am deeply involved in all aspects of the survey, from planning and coordinating over 100 nights of 4-meter telescope time, to data processing and algorithms, to final science analysis and public data release. A fun part of this is producing beautiful color images of the sky (more are available on the DLS website).
I am also involved in planning and design for the Large Synoptic Survey Telescope (LSST), an 8-m telescope with a 3-degree field of view which will repeatedly survey the sky beginning in 2015. For astronomers or physicists who would like to know more, I highly recommend the LSST Science Book. (Full disclosure: as co-chair of the LSST Weak Lensing Science Collaboration, I co-edited the weak lensing chapter.) For others, try the Public part of the LSST website.