# Astrophysics and Gravitation

The Guelph Gravitation Group is led by Eric Poisson and Huan Yang; the group includes a number of graduate students and post-doctoral fellows. Our aim is to understand the nature of gravitation in its most extreme manifestations predicted by Einstein's theory of general relativity. This understanding has a theoretical component (how does one solve the equations to extract the phenomena?) and an observational component (how does one measure the phenomena?).

Our interests are in the theoretical aspects of gravitation at its interface with astrophysics; we aim to make predictions that guide the observers. Gravity is at its most extreme when bodies are massive and compact; we therefore study neutron stars and black holes, as well as more exotic objects such as boson stars and black strings.

The physics of compact objects involves more than just gravity; it is rich in other aspects such as magnetohydrodynamics, nuclear physics, and exotic field theories. We are interested in every aspect of the physics of compact objects, including their internal structure and the way they dynamically interact with companion bodies. An important component of our work is concerned with the ongoing efforts to measure gravitational waves using earth-based detectors (now operational) and space-based detectors (in development). Gravitational waves are produced when large masses are accelerated to high speeds; binary systems of compact objects are among the most promising sources.

With our work we aim to improve our understanding of such systems, and refine our predictions regarding the form that the gravitational-wave signals will take. Given the difficulty of integrating the Einstein field equations for these purposes, several avenues offer themselves. One can rely on approximations and develop pen-and-paper techniques to solve the equations. This approach is optimal when, for example, a binary system has a very small mass ratio, or when the orbital velocity is small compared with the speed of light. When the approximations fail, however, one must face the task of integrating the field equations on a supercomputer. Our work covers all these situations, and our individual web pages offer additional details.

We enjoy close associations with Perimeter Institute for Theoretical Physics and the Canadian Institute for Theoretical Astrophysics.