Observing Gravitational Radiation with QSO Proper Motions and the SKA

The history of astronomy has been that of our measurement of photons from astrophysical objects and, more recently, the early universe (the CMB). Analogously, the universe is thought to be suffused by gravitational radiation — a background from sources in the early universe (e.g., inflation) and “foregrounds” from the gravitational interaction of massive astrophysical objects, such as the massive black holes in the centers of galaxies. This gravitational radiation perturbs spacetime, altering path-lengths and lensing light rays, throughout the universe. The small amplitude of the perturbations (strains of h ∼ 10) makes these effects very difficult to observe. In this section, we examine the time-dependent “gravitational lensing” which results in apparent proper motions of distant galaxies, potentially observable by the SKA.