George Petros Efstathiou FRS (/ɛfˈstæθ.juː/; born 2 September 1955) is a British astrophysicist who is Professor of Astrophysics (1909) at the University of Cambridge and was the first Director of the Kavli Institute for Cosmology at the University of Cambridge from 2008 to 2016. He was previously Savilian Professor of Astronomy at the University of Oxford.