Extragalactic Proper Motions: Gravitational Waves and Cosmology

Extragalactic proper motions can reveal a variety of cosmological and local phenomena over a range of angular scales. These include observer-induced proper motions, such as the secular aberration drift caused by the solar acceleration about the Galactic Center and a secular extragalactic parallax resulting from our motion with respect to the cosmic microwave background rest frame. Cosmological effects include anisotropic expansion, transverse peculiar velocities induced by large scale structure, and the real-time evolution of the baryon acoustic oscillation. Long-period gravitational waves can deflect light rays, producing an apparent quadrupolar proper motion signal. We review these effects, their imprints on global correlated extragalactic proper motions, their expected amplitudes, the current best measurements (if any), and predictions for Gaia. Finally, we describe a possible long-baseline ngVLA program to measure or constrain these proper motion signals. In most cases, the ngVLA can surpass or complement the expected end-of-mission performance of the Gaia mission.