**Causal Structure Of Gravitational Waves In Cosmology**

Despite being associated with particles of zero rest mass, electromagnetic and gravitational waves do not travel solely on the null cone in generic curved spacetimes. (That is, light does not always propagate on the light cone.) This inside-the-null-cone propagation of waves is known as the tail effect, and may have consequences for the quantitative prediction of gravitational waves from both in-spiraling binary compact stars/black holes and “Extreme-Mass-Ratio” systems. The latter consists of compact objects orbiting, and subsequently plunging into, the horizons of super-massive black holes astronomers now believe reside at the center of many (if not all) galaxies — including our own Milky Way galaxy. In the strong gravitational field near the black hole horizon, gravitational wave tails may induce a self-force on the orbiting compact body that needs to be modeled properly for an accurate prediction of the ensuing waveform.

Motivated by these considerations, I have been exploring techniques to understand the causal structure of scalar, electromagnetic and gravitational waves in curved spacetimes. I will describe my efforts to date, including the (small) discovery of a tail contribution to the gravitational wave memory effect in cosmology.