Dr. Stephen GreenSenior Scientist
Quantum Gravity and Unified Theories
I have broad interests in gravitational physics, including astrophysics and gravitational wave observations, and more theoretical questions.
In astrophysics, I have recently become interested in using gravitational wave observations of neutron star binaries to extract information about their internal structure. Indeed, the deformability of the stars due to the tidal forces they exert on each other is sensitive to the equation of state of the interior, and this imprints on the waveform. There are various tidal effects at different multipole order, and those that couple to neutron star spin, they all appear to have universal behavior together with other internal-structure dependent properties. I would like to understand the origin of this behavior. I am also interested in gravitational-wave data analysis.
My more theoretical interests are in stability problems and nonlinear dynamics in general relativity, often in the context of asymptotically anti-de Sitter (AdS) spacetimes. These spacetimes are of holographic interest in high energy physics, and they can also exhibit complex nonlinear dynamics such as turbulence in the geometry. The latter occurs because asymptotically AdS spacetimes confine gravitational waves, reducing dissipation so nonlinear effects can take hold. A major question I have worked on is to determine the final state of small perturbations of AdS, a problem which is holographically dual to the thermalization of strongly coupled field theories. The interesting dynamics that occur may also manifest in the vicinity of near-extreme black holes, and I would like to study their implications for astrophysics.
For my PhD, I studied general relativistic effects in cosmology. I constrained possible nonlinear backreaction effects that had been suggested by others as an alternative to dark energy, and I demonstrated the effectiveness of Newtonian cosmological simulations as an approximation to general relativity.
I completed my PhD in physics at the University of Chicago in 2012. My dissertation, supervised by Robert Wald, constrained nonlinear backreaction effects in relativistic cosmology. After graduating, I became a CITA National Fellow at the University of Guelph, Canada, and later a postdoctoral researcher at the Perimeter Institute. At these places I worked on stability problems and turbulence in general relativity using both mathematical and numerical approaches. Since September 2017 I have been a postdoc at the AEI in Potsdam, joint between the Astrophysical and Cosmological Relativity and the Quantum Gravity and Unified Theories divisions.