a professor of Astronomy & Astrophysics at the Pennsylvania State university and a Sabbatical visitor at the CRA. He obtained a BSc degree in physics in 1987 from the University of London's Imperial College and a PhD degree in physics in 1992 from Columbia University. He joined the faculty in the department of Astronomy & Astrophysics at Penn State in 1998 after holding a postdoctoral position at the Space Telescope Science Institute in Baltimore and a Hubble Fellowship at the Department of Astronomy at the University of California, Berkeley. During the 2013-14 academic year he is a sabbatical visitor at the Kavli Institute for Theoretical Physics at the University of California Santa Barbara (Jul-Sep 2013), the Center for Relativistic Astrophysics at the Georgia Institute of Technology in Atlanta (Nov 2013-Mar 2014) , and the Department of Astronomy at the University of Washington in Seattle (Apr-Jul 2014)